Laser Synthesis and Processing of Colloids: Fundamentals and

Review pubs.acs.org/CR

Laser Synthesis and Processing of Colloids: Fundamentals and Applications Dongshi Zhang, Bilal Gökce, and Stephan Barcikowski* Technical Chemistry I and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Universitaetsstrasse 7, 45141 Essen, Germany ABSTRACT: Driven by functionality and purity demand for applications of inorganic nanoparticle colloids in optics, biology, and energy, their surface chemistry has become a topic of intensive research interest. Consequently, ligand-free colloids are ideal reference materials for evaluating the effects of surface adsorbates from the initial state for application-oriented nanointegration purposes. After two decades of development, laser synthesis and processing of colloids (LSPC) has emerged as a convenient and scalable technique for the synthesis of ligand-free nanomaterials in sealed environments. In addition to the high-purity surface of LSPC-generated nanoparticles, other strengths of LSPC include its high throughput, convenience for preparing alloys or series of doped nanomaterials, and its continuous operation mode, suitable for downstream processing. Unscreened surface charge of LSPCsynthesized colloids is the key to achieving colloidal stability and high affinity to biomolecules as well as support materials, thereby enabling the fabrication of bioconjugates and heterogeneous catalysts. Accurate size control of LSPC-synthesized materials ranging from quantum dots to submicrometer spheres and recent upscaling advancement toward the multiple-gram scale are helpful for extending the applicability of LSPC-synthesized nanomaterials to various fields. By discussing key reports on both the fundamentals and the applications related to laser ablation, fragmentation, and melting in liquids, this Article presents a timely and critical review of this emerging topic.

CONTENTS 1. Introduction 1.1. Overview 1.2. Status of Specific LSPC Fields 1.3. Scope of the Current Review 2. Principal Laser Synthesis Methods 2.1. Laser Ablation in Liquids (LAL) 2.1.1. Plasma and Bubble Dynamics 2.1.2. Nanoparticle Formation 2.1.3. Reactive Laser Ablation in Liquids (RLAL) 2.2. Laser Fragmentation in Liquids (LFL) 2.3. Laser Melting in Liquids (LML) 3. Size Control 3.1. Laser Modulation 3.2. In Situ Size Control by Surface Chemistry 4. Upscaling 4.1. Laser Parameters for LAL Upscaling 4.2. Material Geometry for LAL Upscaling 4.3. Liquid Properties and Process Parameters for LAL, LFL, and LML Upscaling 5. Material, Process, Liquid, and Laser Parameters 5.1. Material Properties 5.1.1. Composition 5.1.2. Shape 5.2. Process Parameters 5.2.1. Processing Time 5.2.2. Downstream Processing 5.3. Liquid Properties 5.3.1. Composition © 2017 American Chemical Society

5.3.2. Solid Additives 5.3.3. Concentration 5.3.4. Layer Thickness 5.3.5. Flow 5.3.6. Viscosity, Temperature, Pressure, and Supercritical Fluids 5.4. Laser Parameters 5.4.1. Pulse Duration 5.4.2. Laser Wavelength 5.4.3. Laser Fluence and Focusing 5.4.4. Spatial Laser Pulse Delivery 5.4.5. Repetition Rate 6. Applications 6.1. Magnetism 6.2. Optics 6.2.1. Semiconductor Optics 6.2.2. Carbon Dots 6.2.3. Doping, Upconversion 6.2.4. Optical Limiters 6.3. Plasmonics 6.4. Biology 6.4.1. Toxicity 6.4.2. Bioconjugates 6.4.3. Antibacterial and Ion Release 6.4.4. Micronization and Solubilization of Drugs 6.4.5. In Vitro Application

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Received: July 19, 2016 Published: February 13, 2017 3990

DOI: 10.1021/acs.chemrev.6b00468 Chem. Rev. 2017, 117, 3990−4103

Chemical Reviews 6.4.6. In Vivo Application 6.5. Analytical Chemistry (SERS, SEIRA, and LDIMS) 6.6. Catalysis and Energy 6.6.1. Reference Materials, Model Reactions 6.6.2. Supported Catalysts 6.6.3. Water Oxidation (Splitting) 6.6.4. Photocatalyst 6.6.5. Hydrogen Fuel Cell 6.6.6. Biofuel Cell and Glucose Oxidation 6.6.7. Solar Cell 6.6.8. Friction Reduction 6.7. Environmental Protection 7. Summary of Advantageous Properties Correlated to Application Prospects 8. Outlook and Challenge Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References

Review

This increased attention is highlighted by the growth of citations in the past decade obtained by searching the ISI Web of Science with the keywords “laser” and “nanoparticle” (Figure 1a). The underlying reason for this rapid advancement is the steadily increasing output power of pulsed laser systems, enabling an increased yield of NMs through laser synthesis (Figure 1b) and significantly reducing the investment cost per laser power (Figure 3b). As compared to laser ablation synthesis in air, which may endanger researchers’ health by causing workplace contamination with aerosols,13 a liquid is a relatively safe medium for laser synthesis and processing (Figure 1c) and has the advantage of confining the resulting NMs. In contrast to pulsed laser deposition,14−16 in which thin films or nanostructures are formed in place and immobilized on a substrate via vacuum techniques, liquid processing facilitates the transfer of NMs to different substrates and requires simple processing apparatus (often just a solid plate and a vial of water). The laser synthesis and processing of colloids (LSPC, Figure 2a) can be classified into three methodologies: laser ablation in liquids (LAL), laser fragmentation in liquids (LFL), and laser melting in liquids (LML). Overall, the field is interdisciplinary, involving chemistry, physics, and related subdisciplines (Figure 1d). Since the pioneering reports of Patil et al.17 on LAL and of Fojtik and Henglein on the laser synthesis of colloids,18,19 LAL has been demonstrated to be a scalable20−22 and versatile method for NP synthesis. The derivative LSPC techniques of LML and LFL allow precise size control of the synthesized colloidal NMs over a wide range from several micrometers23 to ∼1 nm,24 where gold particles have been studied most intensively because of their tunability from 400 to 4 nm11 or even smaller, approaching gold atom cluster sizes.25 Today, the chemical industry emphasizes the development of sustainable production processes,26 and thus sustainable NM synthesis is highly desirable. In this regard, it is worth mentioning that this technique conforms to the 12 principles of “green chemistry”27,28 and benefits from the elimination of the particle surface-blocking effect of chemical ligands or the residues of reducing agents. Because no molecular precursors are required for laser-based synthesis, the energy demand and reagent costs are lower. Target raw materials (e.g., powder, bulk, wire, and particle-containing solutions) for LSPC-based NP production are often 5−10 times less expensive than commonly used metal salts or metal−organic precursor compounds.29 A simple economic consideration of cumulative synthesis costs of laser ablation synthesis as compared to wet chemical synthesis is shown in Figure 3a. Because of the high cost of laser systems, the investment costs for LAL are typically higher than for chemical reduction, where the centrifuge required for nanoparticle purification is the most expensive investment. However, in majority, the running costs consist of material and labor costs. As labor costs are identical for both techniques if permanent assistance is assumed, the difference in material costs is the driving factor and leads to the different slopes of the lines in Figure 3a. Typically 1 g of a gold foil costs approximately 100 Euro, while 1 g of the precursor gold(III) chloride costs almost twice as much. Accordingly, there should be a break-even point, after which LAL is more cost-efficient than chemical reduction. This in turn leads to the conclusion that higher productivities (section 4) are necessary to catch up with wet chemical synthesis economically. The main cost advantage of LAL is derived from lower educt (bulk vs chemical precursor) costs. Of course, there are more dimensional effects involved in a proper comparison, such as concentration and batch size effects, as shown in a recent

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1. INTRODUCTION 1.1. Overview

The past decade has witnessed the fast expansion of nanotechnology and nanoscience and their extensive application in almost every field related to industry.1−4 Associated therewith, the urgent global demand for diverse nanomaterials (NMs) has stimulated rapid advances in the development of both chemical and physical nanoparticle (NP) synthesis techniques. Conventional chemical synthesis methods as a means to synthesize inorganic NP colloids are based on a precursor reaction or ligand exchange and are negatively affected by the inevitable contamination of the nanoproduct solution, resulting in aggregation,5,6 background noise in analytical chemistry,7 toxicity,8 and catalyst deactivation.9 Thus, in biomedical or catalysis applications, the chemical synthesis of stable and “pure” NP dispersions always requires extra purification procedures to eliminate excess of surfactants and residual reactants before being processed toward functional NMs. Given that particles coated by different ligands may have different biomedical performances,10 “naked” NPs are favored to be used to unveil the ligands’ effect by comparative studies.11,12 To improve the NPs’ purity and to develop functional particles starting from scratch, increasing attention has been devoted to exploiting novel techniques to fabricate purely electrostatically stabilized NPs. Moreover, current research and industry efforts require scalable and reproducible NMs. To celebrate the entry into the “photon century”, the United Nations dedicated the year 2015 as the year of light. Industry workers and researchers have been urged to advance initiatives exploring the nearly limitless future applications of light. Nanotechnology, in turn, is one of the most vibrant areas of research and economy in the 21st century. The integration of nanotechnology with “light” has created a new active field, termed “laser synthesis of colloids”, which has attracted increasing attention worldwide because of its extensive applicability in energy science, catalysis, optics, and biomedicine. 3991


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Figure 1. Prospects of laser-based colloid synthesis. (a) Citations per year in the past decade determined by a ISI Web of Science search using the keywords “laser” and “nanoparticle”. (b) The development of the available average power of pulsed laser systems over time. Solid lines are fits to the data points to guide the eye. (c) Schematic illustration of laser synthesis of colloids based on the two key enabling technologies: photonics and nanotechnology. (d) Overview of the subdisciplines of chemistry and connected disciplines involved in laser synthesis research.

study by Jendrzej et al.30 Therefore, in the long run, despite the initial capital investment required to obtain the laser system, the mass-specific production costs of the synthesized NMs will be lower than those of the chemical reduction method if the cumulative NP productivity reaches the break-even point in Figure 3a. To this end, laser investment costs at a given laser power or throughput will continue to decrease as laser systems become more powerful (Figure 1b) and the price-to-laser power ratio decreases (Figure 3b). Additionally, the 100% yield relative to the solid educt mass and no waste of reagents may save costs for waste treatment and disposal as compared to chemical synthesis methods and represent environmental benefits intrinsic to LSPC. For research purposes, milligram-scale NPs are often required for characterization of solid-state materials. For industrial applications, colloids are becoming relevant at the multiple-gram-scale, which recently became accessible by LSPC.22,31 Because the processing parameters (e.g., bulk target, solvent and solutes, and system temperature and pressure) and laser parameters (e.g., wavelength, pulse duration, pulse energy, repetition rate, and number of laser pulses) can be flexibly adjusted, a library of NMs covering nearly the entire periodic table can be produced.32 In addition to the crystalline spheres that are usually generated, soluble, reactive, or supersaturated seed concentrations during LSPC may lead to the formation of nonspherical NMs through ripening, and the resulting morphologies include fractal,33−35 hexagonal,36 flower-like,37,38 football-like,23 fullerene-like,39 leaf-like,40 nanocube,41−45 nanowire,46−49 nanospindle,50,51 nanoribbon,52 hollow,53−60 core− shell,61−65 necklace,66 nanotruffle,67 nanosheet,68−70 tubular,71 and nanodisk.72 These features of LSPC have recently stimulated product commercialization by start-ups on at least three

continents, including Particular GmbH in Germany and inhouse spin-offs at IMRA in the USA and Hamamatsu Nanotechnology in Japan. As compared to chemically synthesized NPs, the advantages of laser-generated NPs include higher grafting density, higher conjugation efficiency, higher electroaffinity toward charged biomolecules, and (heterogeneous) catalyst functionality (Figure 2b). 1.2. Status of Specific LSPC Fields

Since the first report of LAL, many articles have been published on this topic. In addition to five themed journal issues that addressed specific aspects of both laser-based NP formation mechanisms and NP applications,29,74−77 several groups have published reviews of specific LSPC fields. For example, Yang et al. reviewed LAL-induced functional nanostructures with metastable phases and novel shapes,78 and this group was the first to review79 and publish a book80 on novel LAL-produced nanocrystals and their corresponding formation mechanisms. Zeng et al.,81 Amendola and Meneghetti,28,32 Yan and Chrisey,82 as well as Besner and Meunier83,84 contributed comprehensive insights on mechanistic scenarios involving LAL. LFL was first demonstrated by Kamat85 and significantly advanced by the groups of Hashimoto86−88 and Meunier.89,90 Koshizaki et al. described the principles underlying the influence of laser parameters on the size of LML-synthesized NMs.91 Tsuji et al. briefly introduced the impact of polyvinylpyrrolidone (PVP) on LAL-obtained NPs and described how these NPs were altered by laser postirradiation.92 Sasaki et al. summarized the preparation of metal oxide-based NMs using nanosecond (ns) pulsed LAL.93 Majima and co-workers reviewed photoinduced synthetic strategies to obtain metal NPs under different conditions.94 3992


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Figure 2. (a) Classification scheme of LSPC according to the typical sequence of increased laser fluence (LML, LFL, and LAL). (b) (left) Ligand- or reactant-coated NP with different molecules representative of wet-chemical NP synthesis. (right) Laser-synthesized ligand-free NP and the multiple advantages they provide.

Figure 3. Key scaling figures for LAL. (a) Absolute costs as a function of cumulative NP mass yield for the discontinuous synthesis of 1 g of colloidal gold by pulsed laser ablation in water and chemical reduction (the Turkevich method) (sum of 3-mg batches). (b) Relative price (=laser price/laser power) of capital investment per laser power (ns and ps pulsed lasers). Reprinted with permission from ref 73. Copyright 2014 Aachener Kolloquium für Lasertechnik.

LSPC methodology for the laser synthesis of colloids32,82,95 and do not address all three (LAL, LFL, LML) methodologies.

Tveryanovich et al. summarized multiple factors that could influence the properties of nanodispersed materials and thin films prepared by LAL and laser ablation in air.95 In addition, design approaches for experimental liquid flow configurations96 and their use in synthesizing complex composite/compound NPs,97,98 approaches to manipulate the size of noble metal NPs,28 origin of carbon allotropes fabricated by LSPC,99 techniques for femtosecond (fs) laser ablation synthesis of colloids,100 and the types of fabricated NMs101 have also been specifically reviewed. Liz-Marzán and co-workers recently reviewed the reshaping and assembly of gold NPs using pulsed lasers.102 Barcikowski et al. briefly introduced the current trends and some emergent topics in this field29,75,103 and the use of sizecontrolled metallic/alloy NPs made by LSPC as ligand-free reference materials for nanotoxicological assays.11 Zhang et al. described the approaches for rapid prototyping of NP−polymer composites.104,105 Following a different approach, Gökce and coworkers published a handbook describing the practical aspects of LSPC to facilitate the entry of new researchers into this field.106 The applications of LSPC-generated NMs in optics, biology, and catalysis have been shortly reviewed in brief sections.81,100,107 Ma’s group recently reviewed the use of LSPC-synthesized metal nanoparticles for catalytic applications,108 and Stratakis et al. briefly reviewed laser-generated NP-based photovoltaics.109 Although Rao et al. summarized the applications of LALsynthesized NPs in nonlinear optics, SERS, and antibacterial activities in detail,110 yet most of the literature they cited are based on their works using ultrafast LAL, not representative for all laser systems. Additionally, most reviews focus on only one

1.3. Scope of the Current Review

This Review covers all three LSPC methodologies (LAL/LFL/ LML) to provide insight regarding their similarities and differences. For example, LAL is often accompanied by unintended LFL. If a high liquid layer is used for LAL, the fluence gradient as indicated in Figure 2a will lead to LFL or LML along the beam path. This Review is intended to describe the following: the chemical reactions between laser-excited solids and the liquids confining their energy dissipation during synthesis, how to tailor the chemical and physical properties of colloids by adjusting various parameters, and how to apply these colloids in a variety of fields (see Figure 4, in which the orange sections indicate topics highlighted in this Review). Some aspects of LSPC are intentionally described from different perspectives (e.g., laser parameters for size control or upscaling) to emphasize the coupling of various parameters within the laser−liquid− target system. In the following, we first introduce the three main laser synthesis methods (LAL, LFL, and LML) with a focus on NM formation mechanisms and the dynamic processes involved. Subsequent sections illustrate the strategies and difficulties in size control and the NM productivity of LSPC. We then review the influential factors of targets, processing, liquid, and laser parameters, which are crucial factors in controlling the physical and chemical properties of the synthesized NMs. In this section, we also present a panorama of the integration abilities of LSPC 3993


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the material/liquid. If Au0 is ablated in water, a chemical reaction takes places, and a minor fraction of Au+ surface atoms is obtained (Table 4). However, the reactive nature is not dominant. An example of RLAL is the reaction that takes place when Ti0 is ablated in water, and the dominant synthesis products are Ti3+ and Ti4+ oxides.111 In this section, the three main laser synthesis and processing methods (LAL, LFL, and LML) are introduced, and some typical examples and a corresponding discussion of the chemical reactions involved are provided. 2.1. Laser Ablation in Liquids (LAL)

Material ablation by lasers is one of the predominant processes in laser machining and is well-known for cutting, drilling, and microstructuring under atmospheric conditions with the surface of an ablated workpiece being the object of interest.112 Microand nanomanufacturing using ultrafast lasers is a versatile route for advanced materials processing and constitutes the basis of an emerging international field.113−115 The use of liquid environments has also been a focus of laser machining with regard to controlling the surface quality116 and ablation efficiency.117−119 Immersing a workpiece into a liquid or employing a liquid film has many advantages, such as lowering the heat load on the workpiece, producing debris-free treated surfaces,116 confining the vapor and plasma, and increasing the shock pressure on the surface.120,121 As a result, LAL has long been used in laser cleaning117 and laser shock peening.122 In contrast to laser machining (in which the target surface is the main product), when LAL is used to synthesize colloids, the ablated materials dispersed into the liquid become the product, as shown in Figure 6. Generally, a fraction of the ablated material is inevitably

Figure 4. Table of contents of this Review, which contains five sections addressing fundamentals and synthesis determinants and a sixth section describing seven emerging fields in which LSPC has applications, including magnetism, optics, plasmonics, biology, analytical chemistry, catalysis and energy, and environmental protection.

with other techniques and external stimulus resources to develop new classes of NMs. In the Applications, the existing applications of LSPC-generated NMs (including magnetism (section 6.1), optics (section 6.2), plasmonics (section 6.3), biology (section 6.4), analytical chemistry (section 6.5), catalysis and energy (section 6.6), and environmental protection (section 6.7)) are thoroughly discussed to provide a more comprehensive view of the field, as shown in Figure 4. In the final sections, we briefly summarize the unique advantages of LSPC and its applicable fields, followed by statements on the challenges and prospects of LSPC.

2. PRINCIPAL LASER SYNTHESIS METHODS LSPC can be classified methodologically between nanotechnology and laser application in liquids, and it can be subdivided into laser ablation (synthesis) in liquid (solution) (LAL/LASIS) and laser processing of colloids (LPC) methods (Figure 5). LASIS/

Figure 6. Scheme of LAL. A laser beam interacts with a target material, leading to the formation of a plasma (not shown) and a cavitation bubble (shown here at its initial size) in which the ablated matter condenses. After the cavitation bubble collapses, NPs are dispersed into the liquid. Figure 5. Classification of LSPC methods.

chemically oxidized or reduced at the surface of the NPs during LAL (see section 5), for example, the oxidized noble metal NP surface123 or NPs reduced by oxygen vacancies.124 These defects in the synthesized colloids are essential to provide surface charge and thus electrostatic stabilization. Volume or surface defects are also often beneficial for potential applications, such as optics (section 6.2) or catalysis and energy (section 6.6). However, the major mass fractions of the colloids are often preserved with the same chemical composition as the bulk targets, including alloys,125,126 binary,127 and even ternary materials.128 Therefore, LAL is universally considered to be a physiochemical combined top-down (macroscopic solid targets) and bottom-up (initially

LAL includes pulsed LAL (PLAL) and continuous-wave LAL (CLAL). LPC can, in turn, be categorized into LFL and LML. A clear classification of the individual experiments and simulations reported in the literature is expected to help the community understand the important methodological features. Of course, there are some emerging topics, such as reactive laser ablation or fragmentation or melting in liquid (RLAL/RLFL/RLML), in which both the solid and the liquid dominantly react after excitation by a laser beam. The reactive nature of the process depends on the intention of the experiment and the reactivity of 3994


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Figure 7. Plasma and bubble dynamics: (a) The main stages occurring during the relaxation of the laser−target-liquid system for each laser pulse. Reprinted with permission from ref 143. Copyright 2014 Royal Society of Chemistry. (b−m) Cavitation bubble phenomenon during LAL: images and two-dimensional (2D) sketches of the temporal evolution of a cavitation bubble for single-pulse irradiation. Times in % of lifetime (definition: initial laser−solid interaction until second bubble collapses) are approximates rather than precise values and depend on the solid, the liquid, and the laser parameters. Shadowgraph images: (c) Reprinted with permission from ref 144. Copyright 2008 Elsevier. (e) Reprinted with permission from ref 145. Copyright 2015 Elsevier. (l) Reprinted with permission from ref 146. Copyright 2010 International Union of Pure and Applied Chemistry.

When a laser pulse with an intensity >109 W/cm2 interacts with a target immersed in a liquid (Figure 7b), a plasma is generated on the target surface (Figure 7c). This plasma is composed of highly ionized or atomic species with an initial density of about 1020 cm−3 (depending on the laser pulse energy) originating from the target material,147 and thus its emission can be spectroscopically probed.148,149 Note that plasma emission spectroscopy is sensible only to emitting species, which are not necessarily the majority of species in the ablation plume. For example, it has been reported for laser ablation in air that emitted species consist of both atomic and ionic species, and their ratio depends on plasma temperature.150 In addition, overlapping of some peaks in the spectra may also take place.151 Therefore, extracting quantitative information about the kinetics of chemical reactions in plasma is complex, and may not represent the whole mass emitted during LAL. Also, ballistic emission has been reported in the literature on ablation of liquid at the liquid/air boundary directly ejecting species out of the bubble’s phase boundary.152 The duration of the plasma is generally in the range of tens of ns153 to a few μs143 for each laser pulse and depends on the surrounding environment (e.g., liquid composition) and laser parameters as demonstrated by the Park group (e.g., intensity or pulse trains).153 Amans and co-workers reported that the electron density decays from 1018−1019 cm−3 at 200 ns by 1 order of magnitude after 800 ns, and an exponential fit of the electron density decay can be extrapolated to a value of 1014 cm−3 after 2 μs.143 Time-resolved imaging of the plasma revealed that increasing the laser energy from 50 to 150 mJ increases the plasma lifetime from 80 to 420 ns (Figure 8a) during LAL of a silver target in water.153 The addition of an electrolyte (e.g., NaCl) further extends the plasma lifetime (Figure 8a) because of

formed plasma, atoms, and clusters) method with initial processes governed by laser plasma and cavitation physics.129,130 Typical laser beam parameter requirements for LAL are wavelengths from ultraviolet131 (UV) (if the liquid allows UV transmission) and visible132 (vis) to near-infrared133 (NIR, where most liquids are transparent), a typical laser fluence of approximately 0.1−100 J/cm2, and pulse durations from fs134 to picosecond135 (ps), nanosecond (ns),136 microsecond137 (μs), and millisecond138 (ms) regimes, extending to continuous-wave (CW) lasers.139−142 Note that CLAL is currently limited to second-scale processing times because of target-heating-induced liquid boiling. Pulsed lasers are typically used for LAL, and their detailed effects are discussed in section 5.4. In the following, we first introduce the plasma and cavitation bubble dynamics to analyze their roles in particle formation and discuss which parameters can influence their properties, thus providing an overview to interpret the complex physicochemical processes occurring during LAL. We then address where NPs form during LAL and why the size distribution is seldom monodisperse in the absence of additional measures. In the last subsection, possible chemical reactions linked to LAL (RLAL) are summarized to present options for synthesizing different classes of NMs, including novel materials that are difficult to obtain using conventional chemical methods. 2.1.1. Plasma and Bubble Dynamics. Distinctive events related to the plasma and cavitation bubble that occur during LAL are summarized in Figure 7. These events can be classified into three stages: the plasma phase, the gas phase (or cavitation bubble phase), and a phase in which particles are dispersed and interact (or react) with liquid molecules after bubble collapse (Figure 7a).143 3995


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The plasma’s temperature and pressure can be as high as thousands of kelvins (Figure 9e) and hundreds of pascals.129 During plasma decay, the energy is transferred to the liquid, producing a layer of vapor with roughly the same volume as the plasma (Figure 7d). Approximately 1 μs after the first material expansion-related shockwave, a second shockwave parallel to the target is observed (Figure 7e). This shockwave usually propagates at a speed of approximately 1500 m/s in water145 and 1200 m/s in acetone157 and undergoes anisotropic expansion along the directions parallel and perpendicular to the target surface.158 Thus, at least two different physicomechanical processes contribute to the initial process. Tanabe et al. identified the second parallel wave as a sound wave generated during the shockwave reflection.145 They based their conclusion on the fact that a sound wave undergoing a roundtrip in the target would produce planar soundwaves (Figure 7e). Nguyen et al. studied the influence of the shockwaves on the target by photoelastic imaging techniques and found that the number of photoelastic fringes depends on the pulse energy and the laserinduced shockwave.159 Tamura et al. investigated the relationship between the evolution of the plasma and the formation of a cavitation bubble and found that the plasma and bubble coexist in the early stage (already during the ns pulse), as shown in Figure 8b,c.130 The reason why both the bubble and the plasma exhibit asymmetric cloud-like shapes is not fully understood yet because the cavitation bubble forms a sharp quasi-hemispherical boundary in the liquid μs later. However, this result indicates that the ablated species (ions, atoms, and atom clusters) may interact with the surrounding liquid and undergo early chemical reactions inside the cavitation bubble. During this period, fast plasma quenching occurs, and the vapor layer grows into a cavitation bubble whose initial size is similar to that of the laser focus (Figure 7c−e). This initial state is schematically shown in Figure 8d. Most probably, the supercritical vapor phase is the location where reported chemical reactions such as galvanic replacement,160 water splitting,161,162 or oxidation143,155 occur. The vapor encloses a region that initially includes liquid matter. It is assumed that in this region solid crystallization occurs leading to atom clusters163,164 as well as primary particles,165 but also droplets163 are formed here that lead to secondary particles.165 Soon after, the cavitation bubble then undergoes a periodic evolution of further expansion and shrinkage until its collapse (Figure 7f−m), after which it rebounds once or more (at least the first collapse generates another hemispherical shockwave). Depending on the various influential factors, which will be described in section 5, different phenomena may arise during this process. Tsuji et al. observed a shockwave accompanied by an ejection of light-scattering objects along the laser incident axis at approximate 1% of the lifetime.166 A similar observation with more distinct particle ejection was reported by Sasaki at approximately 10% of the lifetime.146 However, through comparison with single pulse LAL, Tanabe et al. recently concluded that these “jets” only occur when the liquid medium includes NPs before the pulse arrives or when multiple pulses are used, which is consistent with the “jet” being aligned with the laser beam’s incident axis.145 Thus, these jets could be caused by either spallation167,168 or the microbubbles formed during LFL (or shockwave propagation heating) of the NPs generated by the previous ablative pulse.145 After the cavitation bubble reaches its maximum height at approximately 20% of its lifetime and proceeds through a quasi-hemispherical stationary phase (Figure 7g), it begins to shrink. The shrinking first bubble is almost perfectly hemispherical (Figure 7h), whereas the second

Figure 8. Early phase of LAL. (a) Time-resolved images of a plasma plume induced by the laser ablation of a silver target in deionized water at energies of 50, 100, and 150 mJ/pulse and in NaCl solution at an energy of 100 mJ/pulse. Adapted with permission from ref 153. Copyright 2015 American Institute of Physics. (b) Plasma emission image and (c) shadowgraph image of LAL of a Cu target in water with a 30 ns pulse. In the initial state of bubble expansion (about 25 ns after laser pulse, 5 ns ICCD gating), an asymmetric and cloud-shaped cavitation bubble is evident. Reprinted with permission from ref 130. Copyright 2015 The American Institute of Physics. (d) Schematic showing the phases involved in the initial stages of the cavitation bubble: A phase of liquid matter containing atom clusters, primary and secondary particles, and a (supercritical) vapor phase in which chemical reactions occur.

enhanced Bremsstrahlung emission from the electrolytes and the abundant Ag+, H3O+, and OH− ions and electrons generated during LAL.153 De Giacomo et al. extended the plasma lifetime using double-pulse LAL (DP-LAL; Figure 9a).149 Because of the effective recombination of ions with electrons and Bremsstrahlung emission, plasma decay is often accompanied by continuum emission (gray dots in Figure 9b),154 which should be differentiated from the signal of LAL-generated species for plasma analysis, for example, C2 species154 during LAL of graphite in water (▲ in Figure 9b) or Al and AlO species during LAL of a Al2O3 target in water (Figure 9c).143 Consequently, to calculate the species’ density, the continuum background must be removed. On the basis of the AlO rovibronic emission between the B2∑+ and X2∑+ states in the range of 448−528 nm and the AlI 2P0−2S doublet emission at 394/396 nm in the plasma spectra (Figure 9c), Lam et al. successfully observed the alteration of the density ratio between the Al atoms and AlO molecules over time (Figure 9d)155 and used it to analyze the kinetics of the chemical reactions inside the cavitation bubble leading to the formation of Al2O3.143 Sakka et al. observed that atoms disappear on a time scale of 1−10 μs, indicating that the chemical reactions may last for several μs.156 Note that this time range of chemical reactions is far shorter than the lifetime of the cavitation bubble. These plasma spectroscopy results are valuable for determining where and when the chemical reaction of the ablated matter with the liquid constituents occurs, and thereby predicting the possible products of LAL. 3996


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Figure 9. Plasma decay and reaction: (a) Temporal evolution of the ionization degree (ionic density/(ionic density + atomic density)) of single-pulse (SP) and double-pulse (DP) laser-induced plasmas of a Ti target under water. Reprinted with permission from ref 149. Copyright 2011 American Chemical Society. (b) Continuum emission (gray ■) and C2 molecular emission superimposed upon the underlying continuum emission (▲). Reprinted with permission from ref 154. Copyright 2009 Elsevier. (c) Spectra of molecular AlO and Al emissions from LAL-generated plasma and (d) their density ratio over time. Adapted with permission from ref 143. Copyright 2014 Royal Society of Chemistry. Reprinted with permission from ref 155. Copyright 2015 American Chemical Society. (e) Temporal evolution of plasma continuum intensities and plasma temperatures as determined by a Planck-like distribution during the ablation of Ti and Cu bulk targets immersed in water. Reprinted with permission from ref 129. Copyright 2015 Elsevier.

Figure 10. (a) Temporal evolution of the cavitation bubble radius after the initial pulse ablation at pulse energies of 100 mJ (□) and 50 mJ (△). Reprinted with permission from ref 145. Copyright 2015 Elsevier. (b) Maximum cavitation bubble height as a function of laser fluence. Reprinted with permission from ref 169. Copyright 2015 Royal Society of Chemistry. (c,d) Simulation of the temperature (c) and pressure (d) of the gas vapor inside the cavitation bubble as a function of time at four different liquid pressures (P = 1, 4, 20, and 146 atm). Reprinted with permission from ref 149. Copyright 2011 American Chemical Society.

shrinking of the first bubble is reported,169 with inward jet, and this effect could be linked to the wetting angle of the initial

(rebound) bubble is neither hemispherical nor symmetric throughout its evolution (Figure 7i,j). Sometimes asymmetric 3997


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Figure 11. Interior of the LAL-induced cavitation bubble. Height and mass scans of the cavitation bubble at maximum expansion. (a) SAXS measurement configuration, (b) sketch of the relevant species in a laser-induced cavitation bubble during its largest extension, and (c) experimental results of primary particles and secondary particles (larger spheres or agglomerates) at different heights. Dashed lines indicate the relative bubble thickness. Reprinted with permission from ref 183. Copyright 2012 American Institute of Physics. (d) Diameters of primary and secondary particles found inside laser-induced cavitation bubbles at different times and bubble heights. Second dashed red line marks collapse, and the horizontal bar at approximately the first dashed line marks the detector gating time. Reprinted with permission from ref 186. Copyright 2013 Royal Society of Chemistry. (e) X-ray radiography density mapping of the first, second, and third bubbles, confirming the inhomogeneous internal density distribution in the second and third bubbles. Adapted with permission from ref 165. Copyright 2015 Nature Publishing Group. The experiments mentioned above were performed by ns lasers (6 ns183,186 and 10 ns165) with pulse energies in the range of 10−20 mJ.

bubble,165 indicating that shrinking symmetry may depend on target wettability and temperature. The dynamics during the final collapse is shown in Figure 7j,k. Shockwaves precede each sufficiently intense collapse (Figure 7k). The subsequent “collapse” and “rebound” of the bubble are often asymmetric, exhibiting irregular shapes and, sometimes, division into “daughter bubbles”, as shown in Figure 7l,m. Some authors have observed stable spherical bubbles (Figure 7l) that remain attached to the target surface at the end of the bubble lifetime;146 however, in many cases the cavitation bubble vanishes completely (Figure 7m). The lifetime of the first cavitation bubble depends on the laser fluence,145 as shown in Figure 10a. Because the ablated mass scales linearly with the laser fluence,22 one would expect it to be proportional to the heat available to vaporize a certain volume of the liquid. Accordingly, the bubble volume387 scales with fluence (Figure 10b),169 and at the same laser fluence, the bubble volume may increase with the liquid’s compressibility134 and decrease with the liquid’s density.144 Note that bubble sizes are not always on the millimeter (mm) scale;387 indeed, bubbles generated by ps lasers (with typically lower pulse energies) are at least 1 order of magnitude smaller than those induced by high pulse energy ns lasers.22,31,170 LAL is often described as a “high temperature and high pressure” process. This terminology is closely related to the initial plasma state rather than the cavitation bubble, although the NPs are located inside the “cold” cavitation bubble vapor most of the time. The initial high pressure, high temperature, and high density of the plasma and its fast cooling period provide a favorable thermodynamic environment for the formation of some NMs in high-temperature environments (e.g., MoC171)

and for fast freezing of metastable phase NMs (e.g., hexagonal close-packed Ni,172 AuFe,173,174 AgFe67 solid solutions), which are difficult to obtain by conventional chemical methods. However, this is only one-half of the picture. In contrast, abundant reports on low-temperature phases derived from LAL have also been reported, for example, cubic boron nitride,175 TiO2 with rutile and anatase phases,111,158,176 and Al2O3 with αand γ-phases.177,178 De Giacomo developed a model to simulate the pressure and the temperature inside the cavitation bubbles as a function of time, based on two assumptions: (1) that the mass and momentum conservation equations are valid in the liquid phase and (2) that the vapor pressure inside the bubble is balanced by the pressure on the bubble wall facing the liquid.149,179 These results indicate that only the initial (tens or hundreds of ns129,143,180) and collapse moments are “hot” (>1000 K) as extracted from calculations that are based on the Rayleigh Plesset model.181 However, please note this model does not account for the compressibility of the liquid and fails to describe the bubble dynamics after the collapse of the first bubble as suggested by recent reports of Sasaki182 and Plech.183 On the basis of studies of laser-induced spherical bubbles in liquid,184 the Gilmore model185 might be better suited to describe the cavitation bubble dynamics occurring during LAL. In contrast, during bubble expansion and compression, the temperature inside the cavitation bubble is only hundreds of kelvins (Figure 10c).149 Recently, these simulation results were experimentally verified.129 Regarding the bubble pressure, similar to the temperature evolution, high pressures inside the bubble are reached only initially. After several μs have passed, the pressure decreases to nearly that of the external liquid (Figure 10d).149 3998


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Figure 12. (a) Bimodal size distribution of gold NPs prepared by fs laser ablation. Reprinted with permission from ref 191. Copyright 2005 Springer. (b) Different fluence thresholds using a laser beam with a Gaussian profile, including the ablation threshold, annealing threshold, and modification threshold. Reprinted with permission from ref 192. Copyright 2002 Springer. (c) Crater generated by fs laser ablation of silicon in ethanol with a Gaussian beam profile and (d) deduced ablated mass as a function of the laser fluence with a Gaussian profile. Adapted with permission from ref 193. Copyright 2010 Elsevier.

≥40 to >60 nm after the first bubble collapse (Figure 11d). The findings to date indicate that the excited species from the ablated substrate are in direct contact with the liquid vapor, which may trigger chemical reactions, and that NPs are already confined inside the cavitation bubble. However, most spectroscopic studies on LAL bubble dynamics only address the first bubble, and thus at least two additional bubble effects must be investigated. The plasma-heated supercritical water phase and the cold bubble vapor may provide a perfect environment for NP crystallization; however, the temperature and pressure peak during bubble collapse creates conditions that may reset the NP structure and composition that have formed before the bubble collapse. Moreover, bubble collapse itself may mechanically ablate materials with a mechanism completely different from that of laser ablation.188,189 Also, cavitation collapse may generate an additional plasma from the liquid or solid.190 Thus, although significant progress has been made in probing the interior of the first bubble, a missing link related to the structure and composition of the NPs found outside the bubble in the free liquid remains. X-ray videography of the first and second cavitation bubble rebound (second and third bubbles, Figure 11e) has been performed to provide part of this missing information.165 In contrast to the homogeneity of the X-ray transmission image of the first bubble, the second bubble exhibits an interestingly bright (less dense) hemisphere center. Furthermore, the third bubble has an inhomogeneous discrete density distribution. The discovery of mass emission occurring during the third bubble phase also hints at the complexity of the LAL-induced cavitation and NP release process. The formation of primary and secondary NPs (a bimodal size distribution, which is often found after performing LAL11) is not fully understood. A typical bimodal size distribution of gold NPs obtained by fs LAL in water is shown in Figure 12a.191 The authors attributed this phenomenon to multiple factors, including target thermal heating by the plasma and bubbleinduced mechanical erosion.191,194 Nichols et al. reported that thermal vaporization and explosive boiling mechanisms are responsible for the bimodal size distribution of Pt NPs obtained by ns LAL.195 In addition to pulse duration-related mechanisms, the formation of this pronounced bimodal distribution could be associated with the beam intensity distribution on the target

Therefore, the internal environment of the bubble is relatively cold rather than hot, and the terminology of “high pressure and high temperature” is only applicable to the initial plasma and final collapse phases. From a quantitative perspective, this potentially misleading simplified terminology should not be used to describe particle formation because the temperature and pressure oscillate over several orders of magnitude in the gas phase. Therefore, accurate in situ detection techniques and thermodynamically theoretical modeling163 are essential to gain insight into the formation of molecular inorganic intermediates and the crystal phase transformation of NPs over time. 2.1.2. Nanoparticle Formation. Understanding where, when, and how NPs form and grow is of great importance to control at least the size of the LAL-synthesized particles. Zhigilei et al. theoretically demonstrated that the condensation of clusters from the material vapor might be the main particle formation mechanism (assuming the absence of chemical reactions to simplify molecular dynamics (MD) simulation) (Figure 13b,c).163 Using small-angle X-ray scattering (SAXS), Plech and co-workers revealed the dispersion of nanoscale particles inside the cavitation bubble, as shown in Figure 11a−d.165,183,186 The primary (40 nm spheres or agglomerates) are dominant in the center and upper parts of the cavitation bubble, and there is no indication that they penetrate into the liquid before the bubble collapses (Figure 11b−d). The (mass-weighted) diameter of the large particle fraction significantly and transiently increases from 3999


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Figure 13. Simulated LAL phase diagrams: (a) Time-space phase diagram for fs ablation of gold in water with local intensity of I0 = 5 × 1013 W cm−2. Reprinted with permission from ref 168. Copyright 2013 Royal Society of Chemistry. (b) Atomistic MD simulated contour plots of the initial spatial and temporal evolution of the lattice temperature in the direction away from the laser (40 fs, effective fluence of 40 mJ/cm2) ablated 20 nm Ag film substrate in water. (c) The enlarged view of the atomic structure of two crystalline nanoparticles located in the upper part of the mixing region. (b,c) Reprinted with permission from ref 163. Copyright 2016 Elsevier. (d) Size histogram showing the predicted number frequency of particles obtained by LAL at an effective laser fluence of 70 mJ/cm2 1, 2, 3 ns after the laser pulse. Adapted with permission from ref 163. Copyright 2016 Elsevier.

finite element (cavitation) simulation of LAL at time scales from several ns to hundreds of μs. Unfortunately, the optimal settings for gaining fundamental information are opposite to those optimized for high yield, in which high-repetition rate lasers are often applied with temporal pulse delays shorter than the cavitation bubble lifetime, causing an interaction between the pulses and the bubble and colloidal particles, shielding the incoming beam. 2.1.3. Reactive Laser Ablation in Liquids (RLAL). Lam et al. demonstrated that ablated species react with (dissolved molecular or water-bound) oxygen delivered by the liquid on the time scale of plasma cooling.155 Because of the conditions that arise during plasma generation and decay, photoionization of the target materials and photothermal decomposition of the solvent occur simultaneously, creating an excited environment for reactions to form carbides,198 oxides,199 nitrides,200 sulfides,43 dopants,111,201 or defects202 within the particles. These material− liquid reactions are sometimes accompanied by a reaction between the species ablated from a multielement target, for example, the formation of MoO3, MoO3−x, and MoS2 NPs by LAL of a MoS2 target in water.203 Because of the high density of atoms and ions during the initial phase of LAL, a series of reactions may take place within a short time scale, including the possibility of an in situ catalytic reaction. For example, da Cunha et al. found that the Pb atoms or ions generated during LAL of a Pb target in alcohols may act as catalysts to decompose alcohol into CO2 and H2O, which then react with the synthesized PbO particles to form hydrocerussite (Pb3(CO3)2(OH)2).36 Additional attention should be directed toward photochemical reactions occurring directly in the liquid (e.g., FeOCl nanosheets formed during LAL in FeCl3 aqueous solutions), in which the Au NPs generated from the target ablation contribute to the physical adsorption on the FeOCl nanosheets, as demonstrated by the Cai group.68 The chemical reaction rate and the phase of the nanoproducts are correlated with the ratio of the active species and their collision rate. Therefore, because of plasma quenching, reaction products (e.g., SiC) and the relevant elemental species (e.g., Si) may nucleate and grow into particles, as verified by highresolution transmission electron microscopy.204 Moreover, the temperature/pressure differences in different local regions may trigger several reactions, thereby leading to the formation of mixed-phase NPs. Supersaturated concentrations of differentphase nuclei may allow collision and growth into a mixed-phase particle (e.g., anatase and rutile TiO2)205 or a polycrystalline

surface (Figure 12b, which describes the transmission beam) in terms of three different excitation regimes and three corresponding thresholds.192 The highest fluence regime in the center spot is defined as the ablation area, the lower-fluence regime ring corresponds to the approximate annealing area, and the outermost area is termed the modification area. Although these definitions were deduced from fs laser ablation of silicon in air, they also apply to LAL,22,193 as shown in Figure 12c. Accordingly, the ablated morphology can be classified into two or three regions of different ablated mass density (Figure 12d), which may influence the nucleation and growth of the NPs and result in the formation of differently sized NPs. Povarnitsyn et al. simulated the ablation process and predicted that smaller particles form in the liquid−gas region (l+g zone, Figure 13a [orange]), that the larger NPs originate in the molten layer (l zone, Figure 13a [yellow]), and that both regions depend on the laser fluence.168 Recently, Zhigilei and co-workers theoretically predicted that during LAL the liquid will be transformed into a supercritical state, and that therefore an expanding low-density metal−water mixing region will form (Figure 13b).163 Materials in different regions are “incubators” for particle formation at different sizes. The authors predict that in this initial phase of LAL < 3 ns a trimodal size distribution (Figure 13c) consisting of atom clusters (1−2 nm), nanoparticles (some nm) and large droplets (tens of nm) exists.163 Interstingly, the prediction of atom clusters agrees well with the experimental findings of Jendrzej et al.164 Although transforming a Gaussian beam profile into a tophat196 or Bessel beam197 profile can vary the size distribution, monomodal colloidal particles remain difficult to obtain practically by varying the laser parameters during LAL. Note that the optical components (axicons) that create Bessel beams distribute the laser pulse energy in a larger trapezoidal column along the beam path, making it difficult to attribute the laser intensity distribution effects exclusively to LAL. Additionally, larger volumes of the colloid are excited at higher fluences, resulting in LFL of NPs synthesized by previous pulses. Thus, studying single pulses could facilitate a better understanding of the influence of the lateral intensity distribution on the particle size. Further understanding of the fundamentals of LAL could be achieved by decreasing the gating times of spectroscopic or X-ray imaging techniques, performing single pulse experiments to probe the NP size, and conducting cavitation shadowgraphy and molecular dynamics (MD) (particle nucleation and reaction) or 4000


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Figure 14. (a) Schematic diagram showing the formation of PtCo NPs via LAL of Co in aqueous K2PtCl4 solution and galvanic replacement reaction (GRR)-LAL based on the GRR between Co and PtCl42− ions and (b) the Pt atomic ratio (%) distribution across the line scan of a PtCo alloy NP; TEM and the electron energy loss spectroscopy (EELS) mapping images are shown in the inset. Adapted with permission from ref 160. Copyright 2016 Elsevier. (c) Layered seed growth of AgGe microspheres during continuous LAL of Ag-doped Ge wafers in water. Adapted with permission from ref 23. Copyright 2015 Nature Publishing Group. (d) Effect of thin film layer sequence during LAL of bilayers on the FeAu NP alloy composition. A topmost layer of Fe leads to Fe-rich AuFe alloy NPs (upper row), while an Au topmost layer leads to low-Fe AuFe alloy NPs (lower row). The topmost layer can be considered as a “sacrificial” layer that protects the underlying material from interaction with oxidizing species in the vapor phase. Adapted with permission from ref 208. Copyright 2016 Elsevier.

the alloying degree could be controlled by finely adjusting the pH value (for CoOx etching) and Pt2+ concentration. In detail, the chemical reduction of the dissolved noble metal cations is hypothesized to happen at the bubble’s “boundary phase” (Figure 14a). Note that this boundary most likely is not the outer phase boundary of the bubble during its expanded volume phase (Figure 11, Figure 7g), but rather in the (supercritical) vapor phase and mixing region during the initial phase of LAL (Figure 8d). This mixing region, atomistically simulated on the ns time scale in Figure 13b, is in contact with an underneath layer of liquid metal droplets. Amendola et al. compared LAL of a thin film bilayer with different sequences (Au on Fe vs Fe on Au) coated on glass (Figure 14d).208 Interestingly, this allowed the formation of AuFe alloy NPs with larger Fe content when using the Fe/Au/glass instead of the Au/Fe/glass targets. They hypothesized that the upper layer acts as a “sacrificial” layer that reacts with the liquid molecules, protecting the underlying material from interaction with reactive oxygen species in the vapor phase. Hence, RLAL may also be used to produce a variety of core−shell NMs by adding specific ions to the solutions to initialize galvanic reactions during LAL. Further examples include the generation of metal/oxide (e.g., TiO2@Ag by LAL of a Ti target in AgNO3 solution) nanocomposites212 and semiconductor (e.g., Si@metal (e.g., Au, Ag, Au−Ag, Pd, and Pt)) core−shell structures by LAL of a silicon target in different salt solutions.213,214 Another finding is the formation of footballlike AgGe microparticles as large as 7 μm following layered seed growth triggered by a rapid GRR between Ge atoms/clusters and Ag+ ions during laser ablation of silver-coated germanium targets (Figure 14c).23 LAL is a continuous process that allows the GRR to proceed so that the AgGe spheres can grow into microscale rather than stay on the nanoscale.23 As suggested by these experimental examples, RLAL offers the advantage of manipulating the NP composition using reactive targets, solvents, or additives. The drawback of RLAL is that it is associated with complex chemical processes because, as is well-known in photochemistry, ions can be reduced directly by laser irradiation,215 resulting in parallel reactions (target ablation,

NP.206 However, the mechanisms by which nuclei evolve into particles and different structures remain under debate for LALsynthesized NMs. Thanh et al. summarized five principal nucleation−growth mechanisms that can lead to chemical colloid formation,207 including the LaMer mechanism, Ostwald ripening and digestive ripening, the Finke−Watzky two-step mechanism, coalescence and orientated attachment, and intraparticle growth. In the case of LAL, the concentration of local nuclei can be kept supersaturated by continuously adding materials through repetitive “seed and feed” emission, which may lead to prolonged particle coalescence and ripening (Figure 14c).23 The solvent decomposition fraction remains difficult to quantify, and precisely predicting and controlling the reaction during LAL is challenging. To estimate the main educt ratio in the gas phase (during cavitation), the density of the ablated matter (7 × 1015 ± (5 × 1015) atoms per pulse) relative to the liquid molecule abundance ((1−1.5) × 1017) may be helpful.209 This ratio indicates that the gas-phase reaction is unlikely to be limited by the liquid molecules’ kinetic availability and that species dissolved in the liquid may contribute to the final NP composition, even at low (e.g., micromolar) concentrations. Increasing attention has been focused on the addition of ions or surfactants to the liquid to let them react with the excited species from the ablated target in the bubble’s gas phase or adsorb onto the particle surface to induce anisotropic growth. For example, CdS can be obtained by reactive LAL of a sulfur target in a CdCl2 solution because of the reaction of Cd2+ from CdCl2 salt and S2− from the ablated target.210 A further example was reported by Thareja et al., who demonstrated that (CH3COCH2)− ions generated through LAL-induced acetone decomposition could bind to zinc-coordinated sites to prevent the dehydration of zinc hydroxides from evolving into ZnO NPs during Zn-LAL.211 The GRR-LAL is an emerging route for novel NM synthesis. For instance, Hu et al. successfully synthesized PtCo nanoalloys by LAL of a cobalt target in aqueous K2PtCl4 salt solution (Figure 14a) and found that the alloy consisted of an outer shell that was more enriched in Pt than the inner core (Figure 14b), which is highly relevant for catalytic application.160 They discovered that 4001


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outside of the focal volume are melted instead of fragmented.219 Thus, LML side products are frequently found in the processed samples obtained via LFL with short focal distances and must be removed by postprocessing treatment. These side products can be avoided by either minimizing the liquid layer limiting the beam path to the focal Rayleigh length (e.g., using a liquid jet setup)218 or by applying high-pulse energy lasers that do not require focusing (but may be limited by a low repetition rate, and thus lead to lower yield). Size selectivity can be achieved by LFL because the energy absorption cross-section of the product is often smaller than that of the particulate educt.220 However, continuous educt−product mixing is inherent to the LFL method, and therefore physisorption of the product on the educt may lower the efficiency and reproducibility. Typical laser requirements for LFL are wavelengths from the UV to NIR regime, pulse durations from the fs to the ns regime, and laser fluences above the material-dependent threshold, for example, >1−100 mJ/cm2.221,222 Because the extinction of the laser wavelength strongly depends on the NP material (particularly for plasmonic particles in the VIS range), either selective LFL is caused or broadband extinction may even allow the simultaneous LFL of NP mixtures. Thus, as an alternative, a white light filament (supercontinuum) can also be used to fragment dispersed particles efficiently, and this process can be initiated by highenergy pulses in the fs regime.223,224 As mentioned previously, the laser intensity must remain below the optical breakdown threshold,225 and consequently the window of laser parameters that can fragment plasmonic particles in a fs supercontinuum is narrow, as reported by the Meunier group.226,227 On the basis of theoretical calculations and experimental confirmation, the mechanisms of photothermal evaporation and Coulomb explosion are believed to be responsible for the size reduction during LFL,96 as illustrated in Figure 16. However, the the heat transfer from a NP to a liquid causes the liquid to reach its spinodal temperature (which is much higher than its boiling point), and this in turn leads to the formation of a nanobubble around the particle (Figure 16a).229 Hence, it cannot be excluded that photothermal vaporization already starts during conditions expected to purely form nanobubbles (which are thermal isolators). Accordingly, Figure 16a does not show any temporal evolution during LFL but is rather meant to show the factors (pulse energy, pulse duration, and initial particle properties) that influence the dominant mechanism. The photothermal vaporization LFL mechanism was proposed in 1999 by Koda and co-workers, who investigated the size reduction of Au NPs by ns laser irradiation.230 Size reduction through surface evaporation is thought to initiate when the particle temperature exceeds the boiling point of the bulk material. The degree of size reduction depends on the laser fluence, irradiation period, and number of pulses. Generally, the higher is the laser fluence and the longer is the irradiation period, the smaller the particles will be.231 Amendola et al. observed that increasing the ns-pulsed laser fluence from 88 to 442 mJ cm−2 decreased the average diameter of Au NP products from 24 to 4.6 nm via LFL.232 Lower laser energy (relative to the boiling temperature of the solid in water) has also been reported to enable size reduction when the formation of a vapor layer of material that is approximately the educt particle size after laser irradiation is considered.233 Subsequent cooling of these vaporized atoms leads to their condensation into significantly smaller particles. Through computational simulations using the two-temperature model coupled to a surface evaporation mechanism, Hashimoto and co-workers clearly demonstrated

plasma−solvent reaction, solvent decomposition, and ion redox reaction) that are challenging to control. Overall, in most cases, the chemical composition or surface chemistry of LAL-synthesized NPs is not completely identical to that of the raw material because of the possible reaction between the ablated materials and the liquid during LAL. Because the compositions of the material and liquid determine the chemical reactions, the influence of each requires thorough investigation; the results to date for the material and liquid are summarized in sections 5.1 and 5.3, respectively. However, note that it still lacks strong evidence indicating where and when the reactions occur. The literature suggests that reactions may occur during each phase of LAL: plasma cooling209 or bubble expansion151 or after bubble collapse.160 Because the ablated matter is confined in the bubble,183,186 after the bubble collapse, the dispersed NPs may adsorb liquid molecules and undergo liquid-phase reactions. In contrast, inside the bubble, plasma physics and gas-phase chemistry will dominate NP crystallization and composition, and thus agents designed for RLAL may be useful if they are effective in both the gas and the liquid phases. Here, LSPC often involves liquid-phase reagents, but concepts applied in the mature field of chemical gas-phase synthesis216,217 could support further developments in RLAL. 2.2. Laser Fragmentation in Liquids (LFL)

Laser fragmentation of microparticle powder suspensions or NP colloids is induced by the NPs’ absorption of laser energy (Figure 15). To initiate LFL in a liquid, commercial solid-state lasers are

Figure 15. Scheme of LFL. An initially micrometer- or nanometer-sized (not necessarily spherical) particle suspension or colloid is fragmented by a laser beam into smaller NPs.

often combined with focusing optics to generate a laser focal spot with a diameter of several tens to hundreds of micrometers, similar to that of LAL, in which a deviation of the focus position leads to a pronounced change in the laser fluence. Thus, the efficiency of LFL is quite sensitive to focus shift.218 In general, LFL efficiency (mass-specific downsizing throughput) strongly increases with the laser fluence,218 and the maximum efficiency lies just below the optical breakdown of the liquid. Therefore, strong focusing is required unless high-pulse energy lasers are available. However, high-numerical aperture lenses cause not only higher fluence but also higher fluence gradients within the irradiated volume. Because of the fluence gradient along the beam path volume in the liquid, NPs in this volume and areas 4002


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Figure 16. Schematic illustration of the size-reduction and fragmentation mechanisms of plasmonic NPs in a colloidal solution. (a) Nanobubble formation, photothermal evaporation, Coulomb explosion. (b) Bathtub profile of the fluence threshold for bubble formation as a function of NP diameter and LFL pulse duration. Adapted with permission from ref 220. Copyright 2015 American Chemical Society. (c) Beam attenuation along the optical path in the solution as a function of the distance from the target for laser energies of 5, 10, 15, and 20 mJ/pulse. The irradiated volume as a function of the distance from the target is shown by the dashed line. Reprinted with permission from ref 228. Copyright 2016 Institute of Physics.

increasing temperature within the duration of the excitation pulse.234 This dependence is crucial, because the refractive index of the medium also changes dramatically with the temperature (fluence) as was shown by Werner and Hashimoto.86 This means that the refractive index of the NP cannot be assumed as a constant but is subject to dynamic changes during the time lapse of excitation. Further, the heat transfer to the liquid is often not accurately considered.86 These important refinements have to be considered when energy balancing during LFL is performed. A fundamental understanding of these laser-induced nanobubbles is all the more imporant because nanobubble creation is an emerging field with numerous applications, for example, photoacoustic imaging and cancer therapy or drug delivery via the optoporation of living cells by transient pore generation upon the laser excitation of gold NPs seeded on the cell membrane.229,236−238 The second mechanism for LFL is the Coulomb explosion model (Figure 16a), which states that electrons are ejected from the solid educt (photochemical bleaching) to generate ionized NPs that are immediately fragmented because of internal charge repulsion. According to the liquid-drop model for Coulomb explosion,239 a multiply charged particle becomes unstable when the disruptive Coulombic force exceeds the attractive cohesive force. The criterion for the Coulomb explosion is expressed by

the viability of NP size reduction below the material boiling temperature.234 Heat transfer from a NP colloid to a liquid causes the liquid to reach its spinodal temperature and bubble formation around the particle,229 which provides an incubation atmosphere in which the emitted fragments can coagulate and coalescence into small particles. As a result, NP evaporation may occur at ambient pressure. Recently, Hashimoto reported that the Au NP surface evaporation threshold is close to the bubble formation threshold, suggesting that bubble nucleation will not inhibit NP evaporation, but instead actually facilitate it.235 Consequently, a polydisperse colloid containing a significant fraction of smaller NPs is often observed during incomplete LFL. It should be noted that bubble generation depends on both the pulse duration and the laser fluence but is minimally influenced by the laser wavelength (UV or green for LFL of AuNPs).220 Metwally et al. calculated that the fluence threshold is significantly higher for ns lasers than for fs lasers with respect to Au NPs, especially when the particle size is below 50 nm (Figure 16b).220 Thus, ultrashort pulses offer a clear advantage in LFL, particularly when pulse durations are shorter than the electron−phonon coupling time of the NP material (typically a few ps). However, please note that in this calculation the transient temperature effect is neglected even though the absorption cross section (which is antiproportional to the fluence threshold for nanobubble formation) decreases with 4003


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Figure 17. (a−d) TEM images and corresponding size distributions of 60 nm Au NPs before (a,c) and after 60 min (b,d) of laser irradiation at 1 kHz and an excitation wavelength of 400 nm. Reprinted with permission from ref 86. Copyright 2006 American Chemical Society. (e) Decay of the extinction at 700 nm after 355 nm laser irradiation under nearly identical conditions and different SDS concentrations: 2 × 10−2 M (●), 6 × 10−3 M (○), and 2 × 10−3 M (▼). Reprinted with permission from ref 243. Copyright 2006 American Chemical Society. (f) Mean NP size after laser fragmentation as a function of the initial concentration of Si microcolloids. Reprinted with permission from ref 244. Copyright 2013 Royal Society of Chemistry.

the “fissility”, which is defined as X ≡ EC/2ES, where EC is the Coulombic energy and ES is the surface energy, and is given for metals by X = (q2/n)/(16πr3wsσ/e2) with the charge state q, the number of particle atoms n, the Wigner−Seitz radius rws, the surface tension σ (both rws and σ are temperature-dependent quantities), and the elementary charge e.240 For gold nanoparticles, the fissility parameter is given by X = 0.9 q2/n.241 Nanoparticles dissociate into smaller entities when X ≥ 1 (Rayleigh limit). For 0.3 < X < 1, both evaporation and Coulomb explosion competitively occur, and for X < 0.3 only evaporation takes place.242 Coulomb explosion becomes relevant at high laser intensities and short wavelengths as was shown for LFL of Au NPs by Hartland et al.85 By monitoring the transient bleaching and the increase of maximum absorbance as a function of laser intensity, the authors showed that the photoejection of electrons may turn into a multiphotonic process and that the charge state has a nonlinear dependence on the excitation laser intensity.85 Similarly, Mafuné’s group observed gradual “bleaching” absorption signals (Figure 17e) during ns laser irradiation of Au NPs using ns transient absorption spectroscopy243 and found that this phenomenon is related to the solvated electrons.241 By analyzing the correlation of the size-reduction extent with the charge state after laser irradiation at different SDS concentrations243 and different laser wavelengths (e.g., 355 and 532 nm),241 they concluded that Coulomb explosion is responsible for the size reduction of the highly charged NPs. Because photothermal evaporation may also occur in the case of ns laser irradiation, competition between Coulomb explosion and photothermal evaporation together with the threshold dependence on the ratio of the Coulomb energy to the surface energy of the particle were assumed.243 Further insight on how Coulomb explosion occurs was provided by theoretical simulations. For example, Zhigilei et al.

proposed that the absorption of pulsed laser beams by microparticles causes the particles to burst from inside because of pressure and temperature gradients and that this process is influenced by the pulse duration (Figure 18).245 Recently, on the

Figure 18. Simulation of LFL. Snapshots from MD simulations of laser irradiation of individual particles (∼100 nm) versus deposited laser energy. The results obtained at 500 ps after the end of the laser pulses are shown for pulse durations of 15 and 300 ps. Adapted with permission from ref 245. Copyright 1998 Elsevier.

basis of the two-temperature model, Delfour and Itina demonstrated the applicability of Coulomb explosion to fs laser LFL and theoretically confirmed that high laser fluence is required to trigger Coulomb explosion; otherwise, only NP reshaping occurs.246 Similarly, Fahdiran et al. reported energization-dependent fragmentation in terms of Eo/Ecoh, where Eo is the energy input per atom in the irradiated spheres, and Ecoh is the cohesive energy.247 In the case of low energization (Eo/Ecoh = 0.63), abundant voids appear inside the sphere and lead to splitting of the spheres, whereas for high energization 4004


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Figure 19. Temporal LFL cascade of events: (a) Schematic of the Coulomb explosion mechanism of fs laser-induced fragmentation. Reprinted with permission from ref 86. Copyright 2011 American Chemical Society. (b) Routes to final LFL products: concentration-driven coalescence [Reprinted with permission from ref 248. Copyright 2014 American Chemical Society], aggregation [Reprinted with permission from ref 249. Copyright 2015 Royal Society of Chemistry], and dispersion or adsorbate stabilization [left, Reprinted with permission from ref 250. Copyright 2013 Nature Publishing Group; right, Reprinted with permission from ref 251. Copyright 2002 American Chemical Society].

Figure 20. Simulated ablation and bisection mechanisms for LFL. (a) Calculated nanoparticle diameter distribution functions at laser intensities of 4/3 × 107 W cm−2, 2 × 107 W cm−2, 8/3 × 107 W cm−2, 10/3 × 107 W cm−2, and 11/3 × 107 W cm−2. Reprinted with permission from ref 252. Copyright 2013 Institute of Physics. (b) Calculated weight distribution functions of Al particle sizes at different moments of time. Reprinted with permission from ref 253. Copyright 2014 Elsevier.

ps, because of the thermionic electron emission and subsequent hot electron relaxation, the lattice temperature of the plasmonic NPs initially increases to 700 K, causing Au NP reshaping below the melting point. After the liquid NP charge exceeds the Rayleigh instability threshold, Coulomb explosion occurs, and the NPs split into many nanodroplets that eventually separate (or reaggregate). The entire process lasts ∼100 ps and is dependent on the employed materials and the laser fluence. Two novel mechanisms have recently been proposed by Shafeev and cowokers,252,253 deducing from the numerical solution of the kinetic equation for distribution function, which is in accordance with the particle size evolution recorded by analytical disc centrifugation (ADC) analysis during experiments. The first one is the detachment of smaller fragments from a molten nanoparticle (Figure 20a), similar to the scenario of

(Eo/Ecoh = 1.86), sphere fragmentation occurs uniformly without void formation.247 Numerical insights were recently provided by Delfour and Itina regarding how the size of Au NPs determines the occurrence probability of either Coulomb explosion or photothermal evaporation.246 Coulomb explosion tends to occur when Au NPs are below a well-defined size R* (R* = 30 nm for a 400 nm, 150 fs laser), whereas photothermal evaporation takes place when the particles are larger.246 Hashimoto’s group verified thermionic emission during fs LFL of gold NPs based on in situ extinction spectroscopy and transient absorption spectroscopy and obtained very small particles with diameters of 3.5 nm (Figure 17a−d).86 On the basis of their experimental data, they constructed a Coulomb explosion model and estimated the time scale for each process, as shown in Figure 19a.86 At times of 3−10 4005


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ablation of the particles upon laser irradiation.252 The other is bisection of larger particles (Figure 20b) induced by the

changing the particulate educt particle size.258 These defect sites weaken the material’s cohesion, possibly enhancing the fragmentation efficiency. Lau et al. compared fragmentation with ultrasonication and found that LFL (at 5−70 kJ/g) leads to a decrease of the hydrodynamic diameter, while it was not possible to reduce the size by ultrasonication (up to 20 000 kJ/ g).218 Although the LFL process mainly comprises physical effects, it is sometimes accompanied by chemical reduction,259 chemical oxidation,25,260 or phase transformation.231 Singh et al. reported that LFL of nickel powders (1−5 μm) in an oxidizing environment (e.g., water) led to the formation of spherical and spiral-like Ni@NiO core−shell structures.261 Kawasaki indicated that reductive LFL of CuO in acetone generated elemental Cu NPs or Cu@Cu2O NPs and that this process depended on the solvent (e.g., acetone and diethyl ketone) because of the different high-temperature cooling periods.260 They also presented the reductive LFL of Ag2O micropowders into AgNPs (10−20 nm in diameter).262 In a follow-up study in which the solid educt’s chemistry was systematically varied, Schaumberg et al. demonstrated that LFL of Cu3N powders in ethyl acetate led to the formation of elemental Cu NPs.248,259 Interestingly, under nearly identical LFL conditions (except for the educt particle size), fragmentation of the CuI microparticles altered neither the oxidation state of Cu nor the composition, thereby yielding CuI NPs.259 Surface chemistry also usually changes during LFL. For partially oxidized silicon particles, Švrček et al. depicted three alteration stages affecting Si NP surface chemistry during LFL, including Si−dimer cleavage and OH/OH− condensation with H atoms, by analyzing photoluminescence (PL) and fourier transform infrared spectroscopy (FTIR) transmission spectra.263 Through these stages, oxidation occurs increasingly close to the Si core, resulting in strained bonds and defects on the surface that can provide nonradiative paths to exciton recombination. Nakamura et al. recently found that LFL of porous Si powders in organic solvents (1-octene) generates organic moleculepassivated Si NPs with improved PL quantum efficiencies (20− 23%) relative to the educt (1−3%).264 Thus, the particles are also involved in chemical reactions during LFL, similar to LAL, which alters the particle composition and surface chemistry and expands LFL toward RLFL. The creation of oxygen vacancies during LFL can be regarded as a partial chemical reduction process. These defects can be introduced into the nanoproducts during LFL of semiconductor materials, as elucidated by Cai and co-workers265 and Lau et al.218 for ZnO. Introducing abundant topological defects and vacancies into the synthesized semiconductor NPs during LFL enhances the charge transport in bulk heterojunctions. Also, LFL of electrochemically etched Si nanocrystals in water can transform NP surface wettability from hydrophobic to hydrophilic,263 allowing the easy functionalization of the NPs with water-soluble polymers to enhance the external quantum efficiency. However, defects may be disadvantageous for certain applications in optics, such as lighting (electroluminescence) or phosphors. During the fragmentation of K2SiF6:Mn4+ phosphor particles, the PL quantum efficiency decreased, and the authors attributed this phenomenon to the increased nonradiative recombination channels resulting from the formation of defects.266 Decreasing the NP size of noble metals during LFL requires the delivery of surface charge to achieve NP repulsion and colloidal stability and therefore benefits from the presence of an oxidative additive.267 Surface defects on noble metal NPs after LFL increase the surface

oscillation of nanodroplets with a frequency of ∼ σ /ρr 3 (where σ is surface tension, r is particle radius, and ρ is particle mass density). Here, the radii of the particles will follow the sequence of r → 2−1/3r → 2−1/3r.253 These two mechanisms have been validated experimentally by laser irradiation of Au NPs and Al powders, respectively.252,253 The size of the LFL-generated products depends on the laser parameters and the solid educt properties.221 For high pulse energies and ultrashort pulse durations, the resulting laser intensities may induce self-focusing (sensitized by the presence of NPs223), optical breakdown, and liquid vaporization effects, which influence the overall laser energy input to the dispersed particles. Such nonlinear optical effects reduce the energy absorbed by particles in the laser beam path, even if the pulse energy emitted from the laser source is increased. These effects must be considered when working close to the relevant fluence and intensity thresholds. Tuning the wavelength to the materials’ surface plasmon resonance (SPR) band or the semiconductors’ interband absorption has successfully been employed.21,254,255 Intartaglia et al. demonstrated that LFL of Si particles was much more pronounced for 355 nm ps laser pulses than for those at 1064 nm because the absorbance at the former wavelength was higher, and, as a result, the size of silicon NPs was dramatically reduced to 3 nm.21 Interestingly, Kabashin and co-workers reported that the size of the crystalline product Si NPs could be controlled by changing the initial educt concentration (Figure 17f).244 This finding suggests a coalescence-based formation mechanism during LFL, emphasizing the importance of the particulate educt concentration. LFL may create a liquid that is supersaturated with NP fragments in which the collision frequency of the fragments is significantly increased, allowing them to coalesce into larger particles248 or aggregate into long chains249,253 (Figure 19b). Thus, the final size distribution of the colloid reflects the equilibrium between fragmentation by laser pulses and NP growth via coalescence or aggregation. If a minimal size is desired, the educt particle concentration must not exceed the critical local concentration that favors the coalescence or aggregation of fragmented seeds with neighboring fragments. To inhibit coalescence and aggregation, either dilution or capping agents are good options for LFL to obtain welldispersed, small NPs (Figure 19b).250,251 Very often, small monodisperse NPs are found in the supernatant after LFL, not representing a high mass fraction. In contrast, higher concentrations promote higher throughput (see section 4). Furthermore, higher seed concentrations favor the chemistry of seed-growth and ripening, resulting in high variation and large particles composed of ultrasmall building blocks. Accordingly, NPs generated by LFL are spherical, unless dissolution, subsequent chemical reactions (e.g., resulting from surface adsorbates), or external fields cause morphological or geometrical changes256,257 during the ripening of the supersaturated colloids. For microparticle or submicroparticle educt, the irradiated material is often not entirely vaporized, and, as a result, the size of the solid educt gradually decreases in a linear fashion, depending on the educt mass-specific laser energy input (kJ/g).218 One method to increase the LFL efficiency of microscale educt was described by Wagener et al., who combined LFL with mechanical pretreatment of microparticles in a stirredmedia mill to introduce defect sites in the educt particles without 4006


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charge density.268 Thus, LFL of noble metal NPs can be regarded as oxidative LFL.25 Overall, LFL may lead to oxidation (e.g., Au → Au(+)25), partial reduction (e.g., CuO → Cu with traces of oxides depending on the solvent248) and full reduction (Ag2O → Ag262), or the preservation of educt stoichiometry (e.g., CuI259). Although systematic research on the determinants of redox-LFL remains challenging, the predictive factors of dominant pathways during RLFL include the redox potential of the liquid25 and the educt composition248,259 but not yet the laser parameters. However, the latter are important for LFL efficiency (see section 5.1.1). The downsizing of colloids to particle sizes ≪20 nm clearly benefits from ultrashort pulse durations as these require far lower threshold fluences. Additionally, the required ultrashort-pulsed fluence values are nearly independent of the particulate educt size when it is less than 100 nm. In contrast, the bathtub threshold fluence profile220 (Figure 16b) for ns pulses could allow the selective fragmentation of NPs with sizes in the intermediate range (approximately 50 nm) because 20−60 nm AuNPs have lower fluence thresholds for bubble formation. However, laser attenuation in liquid must also be considered. Attenuation can reach up to 50% after 300 s of laser processing,228 which means that the volume that is irradiated creates mild (low fluence) LML conditions (Figure 16c) and may explain why a 20−60 nm NP mode byproduct is often detected, particularly at high liquid levels (high irradiated volume). Hence, a thin liquid layer is required to achieve LFL without LML. Note that the model presented in Figure 16c is only qualitatively (not quantitatively) reliable because heat transfer to the liquid and bubble formation are not considered. Overall, large (≫100 nm) educt particles or dielectric materials (i.e., semiconductors and ceramics) can be fragmented effectively by both ns and ultrashort (ps/fs) lasers, particularly when using high pulse energies (at intensities below the colloid’s optical breakdown) and repetition rates adjusted to the colloid mass concentration (see section 4). Ionic crystal powders (ceramics, phosphors, and semiconductors) are available in larger sizes and are more brittle than the well-studied metal colloids. Laser excitation of dispersions of such educt materials could contribute to elucidating the full suite of LFL mechanisms. Nakamura et al. hypothesized two different fragmentation mechanisms during LFL of 100 μm phosphor powder that contributed to bimodal (2 μm and 100 nm) product formation via shockwave/thermal stress and heating−evaporation, respectively.266 Lau et al. analyzed the decreasing hydrodynamic diameter of ps-LFL-processed, submicrometer ZnO educt particles fragmented from ∼450 to ∼180 nm with crystal diameters from 250 to 100 nm218 and hypothesized that LFL could be regarded as a comminution process with the nonspherical particle product originating from educt disaggregation, breaking sintering bonds by LFL. Although a small fraction of spherical 60 nm particle products was observed, the attrition mechanism dominated the mass yield.218 Interestingly, under the same conditions, during LFL of boron carbide, the evaporation/ablation pathway dominates because the microsized solid educt is mostly converted into spherical NPs with diameters of ∼100 nm.218 Similarly, Schaumberg et al. found that the dominant LFL mechanism of micropowder dispersions changes from photomechanical (CuI educt) to ablation/evaporation (CuO, Cu3N educt).248,259 Given that most theoretical work and fundamental experiments have focused on Au NPs, extending the derived model to other materials, especially Pt, Pd (relevant for catalysis

applications), and semiconductors (longer electron−phonon coupling times), is anticipated. 2.3. Laser Melting in Liquids (LML)

Mild isochoric photothermal NP processing by pulsed lasers in liquids was reported more than two decades ago269,270 and has recently been advanced for the synthesis of submicron spheres (SMSs) by LML (Figure 21). Synthesizing SMSs by LML is

Figure 21. Scheme of LML. Initially nanometer-sized (not necessarily spherical) primary particles are aggregated/agglomerated, and the secondary particles are melted by a laser beam into submicrometer particles that typically exhibit increased sizes as compared to the educt primary NPs. Note that irregularly shaped particles can also be isochorically reshaped through this method.

remarkable because the transformation process does not require any chemical reagents or surfactants in the process chain271,272 and because it is a comparably simple method to increase the particle size in a controlled, product-selective manner. Koshizaki’s group performed pioneering work in this field and expanded the applicability of LML toward metals,272−274 semiconductors,275,276 oxides,277,278 and carbides.279,280 In general, LML can be divided into two classes: (1) fusion-LML, which increases the size of a particle crystal (Figure 22a−g), and (2) reshaping-LML, which is an isochoric process (Figure 22k− n). A representative example of fusion-LML is the transformation of raw CuO NPs into single-crystalline SMSs in acetone,272 as shown in Figure 22a−h. The raw materials with a primary size of 34 nm (Figure 22c) are transformed into SMSs with an average diameter of 300 nm (Figure 22d), as confirmed by scanning electron microscopy (SEM) (Figure 22b) and TEM (Figure 22f). The selected-area electron diffraction (SAED) pattern indicates that these SMSs are crystalline (Figure 22h). Note that SAED tells that crystalline domains are present in the SMS, but cannot exclude the coexistence of amorphous domains, as deduced from LSPC-synthesized magnetite281 or AuFe alloys.282 More importantly, increasing the laser fluence tunes the SMS size from 260 to 370 nm, demonstrating the possibility of SMS size control. Note that the SMSs are not always single crystalline but are sometimes a mixture of single-crystal and polycrystalline spheres, as revealed by electron back-scattering diffraction (EBSD) analysis.283 Inspired by their work, Okamoto et al. extended the application of LML to transform anisotropic materials (e.g., CeO2) into single-crystalline SMSs with high sphericity.284 Rehbock et al. demonstrated surface roughness alteration of Au SMSs dependent on the laser fluence and the particle size distribution using salinity-adjusted aggregation where the yield of particles with wrinkled surface increased 4007


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Figure 22. Fusion and reshaping LML. (a−h) FESEM image, particle size distribution histogram (number frequency), TEM image, and SAED pattern of raw CuO NPs and submicrometer spheres obtained by pulsed laser irradiation of CuO NPs in acetone (355 nm, 66 mJ pulse−1 cm−2, 30 min). Adapted with permission from ref 272. Copyright 2010 Wiley-VCH. (i,j) Fusion LML of NPs into nanowires. Reprinted with permission from ref 49. Copyright 2003 American Chemical Society. (k,l, and m,n) TEM images of gold nanooctahedra [Reprinted with permission from ref 285. Copyright 2015 Nature Publishing Group] and nanorods [Reprinted with permission from ref 286. Copyright 2000 American Chemical Society] before reshaping LML and the obtained Au NPs.

Figure 23. Fundamentals and variants of pulsed LML: (a) fusion-LML and (b) isochoric LML. Adapted with permission from ref 295. Copyright 2013 Japan Laser Processing Society. (c) Schematic illustration of the ns laser-irradiation-induced shape transformation of octahedral Au NPs in water. Adapted with permission from ref 285. Copyright 2015 Nature Publishing Group. (d) Schematic illustration of the evolution from Au NPs adsorbed on ZnO (supported particles) via an intermediate state consisting of size-increased Au NPs on reshaped ZnO to the embedment of Au NPs into the spherical crystalline ZnO matrix (product). Adapted with permission from ref 267. Copyright 2015 Royal Society of Chemistry.

with laser fluence and monomodality of the educt particles.271 This phenomenon was hypothesized to be based on a disaggregation−melting−deposition mechanism.271 Subsequently, Lau et al. advanced the LML ability to invert supported particles (Au on ZnO) into SMSs resulting in Au NPs embedded inside of ZnO SMSs.267 Clearly, simple LML not only allows size control of the synthesized SMSs but also enables the adjustment of their surface roughness and composition; thus, many other types of SMSs will likely be reported in the near future. LML can also be used to fuse nanorods287,288 or NPs289,290 (Figure 22i,j) into wires or nanochains with higher aspect ratios.102 This phenomenon is also observed after long-term LAL, in which postirradiation leads to anisotropic coalescence of the gold particles.291 Serkov et al. proposed that the ionization of Au NPs by either thermionic emission of electrons or twophoton absorption of laser radiation would lead to the formation of highly charged particles that attract neutral particles to form multipole structures, which eventually evolve into the nanowires after further laser irradiation.292 Poletti et al. also observed the fusion-LML behavior of gold NPs forming nanocorals and attributed the formation of this structure to the particles’

assembly resulting from variation of the colloidal electrostatic balance and attractive dipolar interactions.249 Using LML for isochoric reshaping can transform not only gold nanooctahedra285 (Figure 22k,l) but also nanorods269,286 (Figure 22m,n) and partially nanospheres. This isochoric LML educt−product relationship can be described as a nanoscaleequivalent chemical isomer in which the solid educt and product have identical volumes. Link et al. reported that the LMLinduced rod-to-sphere shape transformation is initiated by defect (twin and stacking) generation and proceeds through surface diffusion and crystal growth.293 Typical laser requirements for LML are wavelengths from UV267 to NIR,273 a “mild” fluence regime of approximately 10− 350 mJ/cm2,271,294,295 and pulse durations in the order of ns.275 Because the primary aim is mild selective absorption and laser heating of the dispersed particles, UV lasers providing interband absorption and longer pulse durations (ns regime) are often advantageous. At a relatively low laser pulse fluence of ≤150 mJ/ cm2, the laser melting of agglomerated NPs causes the merging of agglomerates into SMSs.295 Because this low fluence is required, laser beam focusing is unnecessary, minimizing the problems of 4008


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Figure 24. (a) Au SMS formation via growth processes during LML in citrate and NaCl solutions with a citrate induction period. Reprinted with permission from ref 274. Copyright 2015 Elsevier. (b) Influence of fluence on the SMS particle size: calculated phase diagrams for silver NPs during 532 nm LML. J1, the start of melting; J2, melting complete; J3, the start of evaporation. Reprinted with permission from ref 91. Copyright 2015 Wiley-VCH.

LFL associated with fluence deviation. If the laser fluence must be increased at a constant pulse energy, using a parallel unfocused beam with a smaller radius is a good method and can be easily managed with a lens system. However, even in this case, a fluence gradient will arise because of the absorption and scattering of the beam as it penetrates the liquid; thus, thinner liquid layers (see section 5.3.4) are advantageous for fluence gradient minimization, which is important for selective LML in which the product’s size is strongly influenced by the applied fluence. Aggregated or agglomerated NPs are currently the most common dispersed particle educts used to produce SMSs via LML, which involves fusion (volume increase) and subsequent isochoric reshaping of the educt. Wang et al. hypothesized two LML mechanisms for the shape transformation into SMSs (Figure 23a,b).296 Dispersed, less-aggregated educt particles undergo conversion to final SMSs via three primary LMLinduced steps: sphere formation in a low laser fluence irradiation volume, fusion of the (reshaped) primary particles, and, finally, SMS formation at higher LML fluence (Figure 23a). In comparison, hydrodynamically larger educt aggregates follow the second mechanism and directly undergo fusion and isochoric reshaping into final SMS morphologies (Figure 23b). In both routes, incomplete fusion of the particles leads to the generation of anisotropic particles, which are an intermediate state of the SMSs. Similar intermediates have been observed for dimers of LML-molten particles.285 Detailed material characterization of the intermediates of LML in a liquid jet passage reactor has revealed that AuNP fusion by LML may also occur on the surface of the support particles (ZnO crystals), which then undergo isochoric structural inversion to become ZnO SMSs with interior AuNPs (Figure 23d).267 Thus, fusion-LML and isochoric LML may be material selective. To this end, defined educt particle agglomeration in LML has been achieved by the following strategies: stabilizer removal273 or salt destabilization.271 Thus, the key to achieving controlled fusion-LML is controlling the source particle’s agglomeration state, which is, in most cases, represented by the hydrodynamic diameter difference relative to the primary particle size. This agglomeration brings nearby educt particles together, which is essential for aggregation, fusion, and volume increase. The isochoric reshaping step is less critical because it is driven by surface energy. In general, colloidal agglomerates are metastable and difficult to synthesize without steric stabilizers. In water, electrostatic destabilization and aggregation may be readily triggered by charge screening (salinity and pH), thermophoresis (heat), or surfactant removal. Recently, Tsuji et al. observed an

essential induction period during LML with no occurrence of agglomeration or fusion of the educt particles and only the removal of stabilizers (Figure 24a).274 They compared electrostatic (chloride anions) and electrosteric (citrate) stabilizers and found that a holding period was required to remove the citrate stabilizers during the initial stage of LML. Subsequently, the size of the SMSs suddenly increased; in contrast, in sodium chloride solution, because no induction period was required, the SMS size increased gradually.274 The Meunier group irradiated gold NPs in methanol with 532 nm ns pulses of up to 20 mJ/pulse with a beam diameter of 8 mm.219 Despite the increasing bimodality of the resulting particles (successful LML), they emphasized that Ostwald ripening can be excluded as a reason for the changes in particle diameter. At higher laser powers, the growth rate decreased, which cannot be explained by the redeposition of dissolved species on the surfaces of larger particles. In this case, increasing the NP temperature would lead to a faster exchange of atoms and, therefore, more rapid growth of the large particles. Their “diffusion coalescence” model explains the slow increase of the solvent temperature caused by the NPs, which in turn slowly increases the NP collisions. During reshaping by LML (Figure 23c),285 the energy could be much lower than that required for the sequential melting of agglomerates into spheres. For instance, Inasawa et al. calculated that a surface temperature ∼110 K lower than the melting point of Au (i.e., 1337 K) was sufficient to transform ellipsoidal Au NPs into spheres using a heat balance and surface melting model.297 The pulse energy fraction absorbed by a single NP is related to its absorption cross-section. To determine the laser fluence that facilitates particle melting, Wang et al. estimated the absorption cross sections of particles of different dimensions using the Mie theory and calculated the energy absorbed by a particle irradiated by a single laser pulse.91,277 As expected, the results indicated that smaller particles require higher energy because of their low optical cross sections and higher specific surface areas (Figure 24b).91 Shim et al. demonstrated that larger particles require more energy to melt or evaporate because of their larger heat capacity.298 However, for the example of gold NPs in water, Baffou et al. reported that the temperature increase was maximized when the particle size was 40 nm and was damped because of inefficient absorption by larger NPs, whereas for smaller NPs, the temperature increase was restricted by their faster heat release.299 For agglomerated NPs, in the ns pulse regime, Feldmann’s group demonstrated a further increase of the local heating effect.300 Agglomerated gold NPs were shown to 4009


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Figure 25. Size range of the NMs accessible by LSPC and methodologies for obtaining monomodal and monodisperse NPs through LAL via in situ size control. (a) Size ranges accessible via LSPC shown by representative images (from left to right): 1 nm to 10 μm (LAL): carbon dots (C-dots) [Adapted with permission from ref 24. Copyright 2015 Wiley-VCH], Au NPs [Adapted with permission from ref 313. Copyright 2010 American Chemical Society], Si@Au core−shell submicrospheres [Adapted with permission from ref 213. Copyright 2015 American Chemical Society], and AgGe microspheres [Adapted with permission from ref 23. Copyright 2015 Nature Publishing Group]; 1 nm to 10 μm (LFL) PbS quantum dots (QDs) [Adapted with permission from ref 250. Copyright 2013 Nature Publishing Group], Cd NPs [Adapted with permission from ref 311. Copyright 2015 Wiley-VCH], Al NPs [Adapted with permission from ref 253. Copyright 2014 Elsevier], and K2SiF6:Mn4+ phosphor particles [Adapted with permission from ref 266. Copyright 2014 Elsevier]; 10 nm to 10 μm (LML): Au NPs [Adapted with permission from ref 285. Copyright 2015 Nature Publishing Group], ZnO SMSs [Adapted with permission from ref 296. Copyright 2014 American Chemical Society], and ZnO microspheres [Adapted with permission from ref 284. Copyright 2014 Nature Publishing Group]. (b−e) Monodisperse Cd colloids after LFL of the LAL-synthesized Cd NPs (LAL +LFL). Adapted with permission from ref 311. Copyright 2015 Wiley-VCH. (f) Polydispersity control during LAL synthesis of Y2O3. Reprinted with permission from ref 306. Copyright 2011 American Chemical Society.

yielding up to 40% elemental Cu particles.272 This example strongly indicates that RLML is possible, thereby extending the applications of LML from simple SMS fabrication and NP shaping toward the chemical conversion of solids. Moreover, reactions with the liquid during LML have been reported. For example, Nakamura et al. used dissolved salts (ferric301 and ferrous302 ions) as laser sensitizers for the liquid and were able to embed magnetite into calcium phosphate SMSs. Kawasoe et al. reported that during LML of TiN colloidal NPs in water, both TiN SMSs and some byproducts (e.g., TiO2 and TiOxNy) were produced because of reactions between oxygen species and the TiN compound and decomposed Ti species.294 Therefore, chemical reactions may be triggered to accompany LML. Although reactive LSPC has been primarily investigated via LFL and LAL, it also occurs during comparable “mild” LML. More research is needed to understand how chemical conversion can be controlled independently from the product’s particle size.

accumulate heat between the individual NPs, which created a “nanostove” in the agglomerate center. When a NP absorbs enough laser energy to enable local heating of the particle surface beyond the melting temperature, particles in close contact merge to reduce their surface energy, thus leading to the formation of larger particles with improved SMS properties (size homogeneity and stability) and even initiating particle alloying. For example, Hodak et al. provided evidence for laser-induced interparticle diffusion in AuAg core−shell NPs.270 Large particles have been reported to take up small particles during LML,277 and as this process continues, smaller particles are steadily consumed, resulting in larger SMSs. Consequently, the hydrodynamic diameter of the educt agglomerates can be far smaller than that of the SMS product, indicating that LML of SMSs is a multipulse process (and that LML itself triggers aggregation). This process is also quite robust, leading to comparable SMS diameters for educt colloids with different hydrodynamic particle size distributions. Generally, increasing the laser fluence leads to a gradual increase in the particle size.91 For instance, the average diameter of CuO SMSs increased from 260 to 370 nm when the laser fluence was increased from 50 to 200 mJ/cm2.272 Notably, further increasing the laser fluence resulted in a dramatic decrease of the particle diameters to below 10 nm because the LFL fluence threshold was exceeded. Similar to LFL, the size and the chemical composition of LMLgenerated SMSs depend on the laser fluence. X-ray diffraction (XRD) measurements revealed that the LML process was accompanied by the chemical reduction of CuO and Cu2O,

3. SIZE CONTROL Although significant advancements in size control during LSPC have been achieved for metal NPs in recent years,11 especially with LFL and LML, monomodal and monodisperse colloids are challenging to produce by LAL without compromising colloidal purity. Even LAL at very low fluence (resulting in low throughput) produces a secondary size mode of particles,194 as is clearly evident in the volume-weighted particle diameter histograms.11 The origin of this second size mode remains under 4010


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Figure 26. Statistics of LAL, LFL, and LML product particle diameters reported in the literature on the basis of 150 articles cited in this Review.

Figure 27. Size control of gold NPs by four different methods: (I) LML, (II) delayed conjugation, (III) size quenching by salts during LAL, and (IV) LFL. Reprinted with permission from ref 11. Copyright 2014 Beilstein-Institut. Delayed conjugation is a method to downstream-conjugate NPs shortly after the NP formation.

reported for LAL are unclear. Regarding the primary particle diameter, precise size control in the range of 1−20 nm (Figure 26) has been realized by LAL with the assistance of surfactant termination,305 complexing (Figure 25f),306 polymer capsulation,307 bioconjugation,308 organic molecule polarity control,219 support loading,309 and salt quenching.310 As a downsizing method, LFL naturally covers the size range of available educt particles, reducing their sizes down to between ∼2 nm (educt: 20 nm)25 and 2 μm (educt: 110 μm),266 depending on the evaporation/attrition mechanism (Figure 25a). As compared to LAL, LFL exhibits better size control in the range of 1−20 nm, especially for 1−5 nm (Figure 26). One particular feature of LFL is its compatibility with LAL, which enables the transformation of LAL-synthesized NPs with a polydisperse distribution (Figure 25c) into highly monodisperse NPs (Figure 25d,e).311 Furthermore, LML allows the generation of SMSs with typical diameters of 100−1000 nm (Figure 26)272,294,312 or even up to several micrometers.284 LML can also be used to fuse NPs into nanowires/nanorods291 and reshape

debate, and reported methods capable of achieving monodispersity often require particle growth quenching by surface adsorbates or conventional downstream processing, such as centrifugation.303,304 Moreover, no theoretical model has yet been constructed to correlate the material and liquid properties with the primary particle diameter resulting from LAL. Despite the current inability to quantitatively predict the NP size solely on the basis of the physicochemical properties of the solid and liquid, multiple pathways can be effectively used to tailor the size and narrow the size distribution of laser-synthesized NMs (Figures 25−30). This section first provides an overview of the size ranges accessible by LSPC, and then size control strategies are explained in detail based on laser processing with liquid (surface chemistry) parameters. In general, among the three LSPC methods, the primary NP diameters fabricated by LAL to date cover a broad size range from ∼1 nm24 to 10 μm23 (Figure 25a). However, on the basis of the experimental design, the extents to which LFL (size decrease) and supersaturation-driven ripening (size increase) contribute to the particle diameters 4011


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Figure 28. LAL: (a) Average size of Pd NPs fabricated at different LAL wavelengths (355, 532, and 1064 nm) and fluence values (9, 13, and 20 J/cm2). Adapted with permission from ref 314. Copyright 2014 Elsevier. (b) Mean size of the Au NP two modes as a function of laser fluence using an fs laser. Reprinted with permission from ref 194. Copyright 2003 American Institute of Physics. LFL: (c) Correlation between the average radius of Au NPs and fluence during ns LFL in H2O. Reprinted with permission from ref 232. Copyright 2007 Royal Society of Chemistry. (d) Mean size of Au NPs after fs LFL as a function of input laser energy. Adapted with permission from ref 315. Copyright 2015 Institute of Physics. LML: (e) Laser fluence-dependent size changes of CuO NPs obtained via pulsed laser irradiation (532 nm). Adapted with permission from ref 272. Copyright 2010 Wiley-VCH. (f) Laser fluence dependence of the mean size and yield of B4C particles prepared by laser irradiation for 300 min in ethyl acetate. Reprinted with permission from ref 279. Copyright 2010 Springer.

results in smaller particles.322 These contradictory findings originate from the fact that it is difficult to exclude in situ LFL of the NPs resulting from the energy absorption by NPs during long-term LAL.21 Also, the pulse-number-induced incubation and screening effects323 and the possible modulation of the laser fluence by the focus position324 may also affect the size fluctuation under different conditions; consequently, clearly identifying the impact of laser fluence on NP size control during LAL remains difficult. Elucidating the influence of laser fluence on LAL requires excluding LFL effects; thus, SP-LAL fluence studies are needed. As compared to LAL, the laser fluencedependent size control is relatively straightforward for LPC. Two laser fluence thresholds exist for NP size tuning. When the energy is well above the fragmentation threshold, the particles evaporate or explode, producing smaller particles and dramatically reducing the NP size; in contrast, when the laser energy is above the melting energy threshold but below the fragmentation threshold, LML occurs, and the NP size increases. This trend has been thoroughly confirmed by ns lasers, in which the heating−melting mechanism is responsible for the size variation.325 Clear fluence dependence is also observed in LML, in which the SMSs increase in size as the laser fluence increases279 (Figure 28f). In contrary, the NP size decreases as the laser fluence increases232 via LFL until a certain size limit (Figure 28c). However, the laser fluence must be constrained to the LFL window; otherwise, secondary nucleation of the fragmented NPs into larger ones may occur at high fluence (Figure 28d).315 At a certain point, however, fragmentation of the melted particles occurs (Figure 28e).272 The ripening and growth of LFL-synthesized NPs may also be triggered by the aging of supersaturated naked atom clusters.164

nanorods269,286 or octahedral particles285 into spherical NPs with sizes in the range of 10−100 nm. Overall, LSPC is an emerging technique with the capability for wide-range size manipulation from several micrometers down to 1 nm. In particular, Au NPs with diameters from 400 to 2 nm can be obtained (Figure 27).11,25 Realizing size control in LSPC is often achieved by modulating the laser or processing parameters and the liquid medium, as explained below. 3.1. Laser Modulation

Most laser systems have multiple tunable laser parameters (e.g., laser fluence, pulse duration, repetition rate, and laser wavelength). However, one parameter often affects the others. For instance, increasing the repetition rate always leads to higher LFL effects, whereas decreasing the laser wavelength often decreases the laser pulse energy and leads to sharper focusing and stronger LFL (absorbance of colloids at wavelength of λ). In the case of LAL, high-energy laser pulses have often been reported to favor the formation of larger NPs,84,316 at least in terms of the diameter of the modes in the number-weighted size histogram (Figure 28a,b).194,314 This may be attributable to the enhanced aggregation and coalescence of the particles resulting from explosive boiling and higher NP yield.317 Reduced laser energy has been reported to decrease both the size and the size distribution of LAL-synthesized particles, as observed independent of the laser pulse duration (e.g., fs,84,318 ns,306 and CW140) and for various materials (e.g., Si,318 C,319 Au,194 and Ag305). Although low pulse energy may reduce the size of NPs, it leads to a drawback of lower productivity.320 In contrast, the opposite effect has been reported as well, where increasing the laser pulse fluence narrows the size distribution of the generated NPs321 and 4012


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parameters, the existence of mutual cross-effects between these parameters makes it difficult to develop a general concept. Indeed, it is straightforward to narrow the size distribution and minimize particle diameter sufficiently to avoid NP excitation with LFL, resulting in monodisperse colloids. However, stabilizer-free colloids undergo ripening and may be supersaturated with atom clusters,164 and thus should be immediately postprocessed into their final matrix or supports to stabilize their size. In contrast, intermediate sizes are much more difficult to synthesize by LFL, in which educt−product mixtures often exhibit a bimodal size distribution, requiring one to perform further fractionation, for example, by centrifugation. As a general rule, IR ns lasers have been shown to be beneficial for producing monomodal products via LAL and LML, whereas ultrashort (and shorter wavelength) pulses are beneficial for LFL. During LAL, lower fluences seem to favor smaller sizes, but higher fluences may benefit from combined LAL−LFL (in particular at lower wavelength), narrowing size distribution. Further investigation is recommended to elucidate the impact of each parameter on NP nucleation and growth while keeping all other parameters constant.

Note that this ripening (LFL of noble metals in water) is generally more rapid than TEM sampling and can extend over multiple days, severely compromising the colloidal stability.164 This finding implies that rechecking the NP size (or carefully setting the sampling interval following LFL) is indispensable after LFL, particularly during the early storage of the colloid. Hamad et al. reported that increasing the laser pulse duration resulted in smaller LAL-synthesized Ag, Au, and TiO2 NPs in deionized water.326 Notably, not only the average (numberweighted) particle diameter but also the fraction of large particles (>40 nm) and polydispersity significantly decreased with increasing pulse duration for the investigated fs, ps, and ns laser systems. Unfortunately, the laser wavelength, fluence, and repetition rate also differed in those experiments (fs, 800 nm, 28 J/cm2, 1 kHz; ps, 1064 nm, 0.4 J/cm2, 200 kHz; and ns, 532 nm, 12 J/cm2, 30 kHz); thus, drawing robust conclusions is difficult. As for the fs lasers, thermal-free ablation and plasma-induced heating/ablation of a target dominate at lower and higher fluences (i.e., so-called soft and hard ablation), respectively.194 Tsuji et al. defined two types of laser pulse absorption by particles:327 “inter-pulse” absorption of the following pulses’ energy by colloidal particles generated by the former pulses and “intra-pulse” absorption of a later part of a pulse. As a result, intrapulse absorption is negligible for fs lasers but should be considered for ns-LFL. For ps pulse durations, the LFL threshold depends on the electron−phonon coupling time (τep) of the NPs.220 If the ps pulse duration exceeds τep, heat delivery from the electrons to lattice occurs during the laser pulse. In contrast, if it is shorter than τep, the heat delivery is negligible during the excitation. Therefore, the energy increase is negligible in the particle lattice in the case of fs pulse irradiation328 because the photon−phonon relaxation occurs in the dark; this circumstance does not occur when ns lasers are used because photons continuously irradiate the particle during photon−phonon relaxation (providing inefficiently larger energy flux duration than minimally required for LFL). Therefore, the intra-pulse effect is negligible for ps and fs laser fragmentation, making fsLFL more efficient than ns-LFL for the size reduction of Au NPs.329 Also, the second-harmonic generation during nonlinear fs laser ablation can shift the initial laser wavelength close to the SPR band of plasmonic NPs, providing the possibility of inducing resonant NP fragmentation.330 Note that LFL efficiency does not always correlate with the quality of the resulting NPs’ size distribution. However, because the LFL fluence threshold (Fth) drastically increases for small particles if ns pulses are applied (whereas Fth is less size dependent for ultrashort pulses),220 much higher pulse energies must be applied to minimize the NP diameter with ns lasers. Overall, ultrashort-pulsed LFL is less sensitive to particulate educt diameter variation, rendering fs and ps LFL robust. In addition to the laser fluence and pulse duration, another important laser parameter is the laser wavelength. Under the same pulse duration, the use of shorter-wavelength (e.g., 532 and 355 nm) lasers favors the generation of smaller NPs326,331 because of self-absorption-induced fragmentation332 or laser wavelength located around the NP extinction maximum, which has been verified for many materials, such as Au,333,334 Pd,314 ZnO,321 and Ag NPs.327,335 This behavior implies that postirradiation using the second or third harmonic wavelength is often much more effective for LFL-induced size reduction than using the fundamental wavelength. Taking together all of the effects of laser parameters on NP size control, it can be concluded that, although one laser system offers multiple size-affecting

3.2. In Situ Size Control by Surface Chemistry

In addition to laser parameters, surface chemistry can effectively be used to control the size of synthesized NPs during both LPC and LAL. Surfactants, electrolytes, polymers, biomolecules, organic solvents, and support materials have been applied to tune the NP size (Figure 29), and their effects are straightforward to control in LAL experiments. Sometimes, this basic concept also works for LFL and LML. Because LAL often suffers from a wider size distribution and bimodality, most efforts relating to LSPC in situ quenching research have focused on LAL. The general principle of in situ NP surface deactivation by physisorption or chemisorption is the same for all cases of inorganic NP seeds, which are released into the liquid during LSPC (Figure 30). First, the growth termination of embryonic particles by surfactant molecules in increasing concentrations generally shifts the NP size toward smaller values.336,337 This phenomenon has been widely investigated using CTAB338 and SDS305,336,337 (Figure 29a,b). The surface coverage increases with the SDS concentration, and a double layer coating forms on the surface, thus increasing the yield of small NPs.339 However, the surfactant concentration should not be too low or too high.339 Initially, increasing the SDS concentration screens the repulsive electrostatic force between the NPs because of the cumulative coating in which the amphiphilic surfactant points its hydrophobic tail outward at submonolayer and monolayer concentrations.339 At higher concentrations, a double layer forms, and the hydrophobic, charged surface groups contribute to electrosteric stabilization. Finally, when the surfactant concentration exceeds the level of double layer formation, unbound free ligands contribute to the solvent’s bulk salinity and screen the electrostatic charge repulsion of the electrosterically stabilized AuNPs−SDS− conjugate. Mafuné et al. convincingly demonstrated that steric contributions are beneficial for stabilization by comparing surfactants with different carbon chain lengths and showed that n ≥ 12 (CnH2n+1SO4Na) resulted in superior particle stabilization.339 Note that in this LAL study, as in a series of studies by Mafuné’s group in that decade, the long focal length (∼250 mm) and large beam diameter (1−3 mm) may have caused progressive LFL in the 10-mL metal NP sample, particularly with the second harmonic wavelength. Conse4013


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Figure 29. In situ size control during LAL and LFL by surfactants, solvents, biomolecules, and anions. (a) Surfactant effects during LAL: average radius of the NPs (●) and average dispersion of the radius distribution (○) as a function of the SDS concentration. Reprinted with permission from ref 305. Copyright 2000 American Chemical Society. (b) Surfactant effects during LFL: size-quenching effect depending on SDS surfactant concentration. Note that the critical micelle concentration (CMC) is 0.008 M. Reprinted with permission from ref 243. Copyright 2006 American Chemical Society. (c) Solvent effects on LAL/LFL: average Co NP diameter as a function of solvent polarity with ablation for 4 min (◇) and 60 min (○). Reprinted with permission from ref 219. Copyright 2012 American Chemical Society. (d) Solvent chemistry during LAL: size-quenching effect of acetone via its enolate tautomer on LAL-synthesized Au NPs. Reprinted with permission from ref 334. Copyright 2012 Springer. (e) Biomolecule chemisorption during LAL: size distribution determined for AuNP-oligonucleotide bioconjugates as compared to unconjugated gold NPs and a TEM micrograph of the monodisperse bioconjugates. Reprinted with permission from ref 308. Copyright 2009 WILEY-VCH. (f) Charge delivery by anions during LAL: Au NP diameters resulting from size quenching with sodium phosphate buffer (NaPP) of different ionic strengths observed by SEM and ADC. Reprinted with permission from ref 310. Copyright 2013 Royal Society of Chemistry.

PVP to water slightly quenched the (number-weighted) particle size but did not narrow the size distribution.144 Although encapsulated NPs are small, and the colloid is typically very stable, their hydrodynamic diameters are enlarged (and their zeta potential, which is indicative of the particle mobility, is screened) by the polymer capsule.307 In addition to chemical polymers, interest in studying the size-quenching effect of biopolymers, such as albumin,310,344,346 starch,347,348 gelatin,349,350 and chitosan,351−353 is increasing because of their excellent biocompatibility and biodegradability. The dispersions and sizes of AgNPs and AuNPs prepared by laser ablation in such biopolymer-containing solutions have been found to be narrower and smaller than those prepared in neat water.349,350 These results constitute the basis for the single-step synthesis of NP− biopolymer composites. However, because of the high molecular weight of these polymers, high volume fractions of additives are required for LSPC size quenching. Organic solvents play different roles in size quenching and particle stabilization because (i) organic solvents may shift bonds or decompose under laser irradiation, and the newly formed molecules may quench the NPs growth354 or react with the ablated matter;355 (ii) the solvent molecules bear functional groups with high surface affinity, such as thiols, ketones (e.g., acetone260,334 and cyclopentanone356), or esters (e.g., ethyl acetate248); and (iii) the polarity of the solvent contributes to colloid stabilization. First, solvent decomposition may be regarded as pyrolysis,198 and only a few reports are available on the compositional change of the liquid.357 One example is chloroform, which can induce reactions during LAL of copper,

quently, their follow-up studies addressed surfactant effects during LFL, demonstrating the importance of reaching the surfactant’s CMC (Figure 29b).243,336 One general drawback of using surfactants is the risk of pyrolysis during high-fluence LAL, which may contaminate NP purity, as shown by infrared (IR) spectroscopy.169 Even in LML with comparably low fluence, the ionic surfactant citrate decomposes.273 In contrast to electrostatic or electrosteric stabilization, which is sensitive to stability screening by salting-out effects (including overdoses of the ionic surfactant itself during LAL339), the steric stabilization of particles with biomacromolecules or polymers is even compatible at physiological salinity35,310 or in nonpolar solvents.307 By physisorbing onto the synthesized NP surfaces through van der Waals interactions, polymers can easily exert size-quenching effects. Additionally, functionalized polymer endgroups, especially thioether and thiols, allow chemisorption on growing particles.227,340−342 Singh and Soni demonstrated that poly(vinylpyrrolidone) [PVP] (keto) and poly(vinyl alcohol) (PVA) (alcohol) were much more efficient at aborting NP growth during laser synthesis and producing smaller NPs than polyethylene glycol (PEG) (ether).343 When conjugated with mPEG-SH, LAL-generated gold colloids could be frozen and thawed several times, and even freeze-dried (complete water removal) and redispersed without impairing colloidal quality or stability.344 Tsuji et al. investigated the effect of PVP on LSPC in detail,144,345 as summarized in their review.92 In their studies on Ag NP synthesis, PVP was more efficient for LFL than LAL, and laser-fragmented particles underwent shape conversion and crystallization in PVP but not in PVA.345 During LAL, adding 4014


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resulting in Cu2OCl2 NPs with diameters as small as 3.7 nm.358 The volatile byproducts and the reactions of the solvent with NPs359 will be discussed in section 5. Second, ketones as a functional group and LAL-induced enolates have been related to the size-quenching and stabilization effects of acetone (Figure 29d).334 Third, regarding the effect of the liquid’s polarity on NP size, opposing trends have been reported. Studying free ripening kinetics after short (5 min) LAL revealed that less-polar liquids resulted in larger NPs (with smaller particles forming in water and acetone as compared to those forming in ethanol and ethyl acetate).360 In contrast, a long LAL irradiation period (60 min), during which LFL side effects are known to play a dominant role, yielded smaller particles in nonpolar solvents (hexane and diethyl ether) and larger NPs in 2-propanol and methanol, as confirmed using solvent mixtures (Figure 29c).219 Thus, liquid polarity effect may vary as the irradiation time increases and leads to NP heating. Indeed, Meunier’s group reported that kinetic energy allows NPs to overcome repulsion more easily in polar solvents during growth.219 Fundamentally, one would expect that relatively more polar solvents (higher relative permittivity) would better stabilize charged NPs because the Debye length (which reflects the electrostatic stabilization) is proportional to the solvent’s dielectric constant. Thus, water should be superior to, for example, methanol or hexane. Most studies on size control have utilized metal NPs; however, the surface adsorbates used for noble metal NPs usually differ from those used for the growth quenching of oxides. Naturally, optimal surface adsorbates for noble metal NPs are soft Lewis bases (thiols, polarizable charges, chaotropic anions), whereas oxides require hard Lewis bases or complexing agents. Carboxylic acids are a class of organic molecules that enable size control through different modes of coordinating their carboxylate groups to LAL-generated oxide NPs. Amans et al. demonstrated that the polyether chain of 2-[2(2-methoxyethoxy) ethoxy]acetic acid (MEEAA) stabilized Y2O3 NPs in aqueous media, whereas the complexing group limited the NP size and drastically narrowed the size distribution by bidentate binding to the oxide NP surface (Figure 25f).306 Size tailoring of LAL-synthesized NPs is also achievable by in situ bioconjugation, such as by thiolated oligonucleotides dissolved in water (Figure 29e).308 Petersen et al. found that there is a critical concentration of biomolecules to achieve maximal growth quenching.308 Lower concentrations led to NP aggregation because of partial coverage, whereas higher concentrations produced no further change in the size distribution because the NP surface was completely covered by molecules. This concept of in situ bioconjugation can be used to achieve monomodal NPs with both narrow size distributions and the grafting of biologically functional groups in one step, as validated for a set of biomolecules, including nucleic acids,308 aptamers,361 antibodies,362 and peptides.363 However, in all of the in situ size-quenching methods summarized above, the addition of molecules and their possible degradation (even if their byproducts are often volatile or biocompatible and despite the fact that they are employed to a lesser extent than in chemical synthesis) cannot be totally avoided. Salt quenching is convenient for ligand-free size manipulation and has been widely implemented.135,268,310 The first study on in situ size quenching was reported by Procházka et al., who showed that the presence of chloride anions enabled size reduction of LAL-generated Ag NPs.364 In general, salts with higher ionic strengths produce smaller NPs at micromolar (μM) concentrations.268 For example, the quenching effects of NaCl, NaBr, and NaPP have been investigated, and increasing the ionic

strength from 1 to 50 millimolar (mM) was found to reduce the AuNP size from ∼20 to ∼6 nm (Figure 29f).310 Although electrolytes can quench the size of synthesized NPs and sharpen their size distribution, the production of large metallic NPs cannot be totally inhibited, as observed by Marzun et al. in the LAL of palladium targets in phosphate buffer, carbonate buffer, and sodium hydroxide.135 It is important to note that electrolytes with different affinities toward the synthesized NP surface and ions are present and may strongly affect NP stability.365−367 Investigating the anion-specific size-quenching and colloid stabilization effects during LAL of gold revealed a clear correlation between the anion polarizability and hydration number with its effectivity (stabilization per anion dose), and the strength of this correlation could be ranked as follows: SO42− ≈ F− < Cl− ≈ NO3− < I− ≈ SCN− ≈ Br−. Clearly, chaotropic anions most effectively deliver charge to the noble metal NP surface.268 Hence, not only the size is quenched effectively, but also colloidal stability increased during LAL in μM, chaotropic salt solution. Quantitatively comparing the colloid charge effects with X-ray photoelectron spectroscopy (XPS) data revealed that particle stabilization and size quenching were dominated by anion adsorption rather than surface oxidation.268 The surface charge density-driven growth-quenching mechanism was verified by identifying the linear correlation of the anion concentration with the surface area of the NPs fabricated using LAL in μM saline water.310 The constant of proportionality (1.5−1.9 nanomolar of anion per cm2 NP surface) enables adjusting the total NP surface in a colloid. The absolute NP surface in a given volume is a useful parameter in ex situ conjugation studies to adjust the intended final molecule grafting density independent of the NP size; indeed, this value can be easily set on the basis of the anion concentration if the LAL ablation rate is known. Another elegant strategy for NP size manipulation using inorganic additives is to directly load the charged NPs onto oppositely charged supports (e.g., silica, titania, or ceria), thereby controlling the size and fabricating supported particles in one step with high potential for catalysis applications. Hashimoto et al. synthesized Au NP-loaded zeolite crystals by ns LAL of Au flakes in a gel containing zeolite L crystals and successfully visualized the embedded Au NPs with dark field microscopy because of their strong light-scattering effect.254 Mafuné reported that the presence of silica during LAL of a Ni plate immediately caused size quenching of Ni NPs and resulted in the anchoring of 1−3 nm Ni NPs on silica supports, whereas the absence of silica caused aggregation of Ni NPs into 5−30 nm particles that precipitated after a few hours.368 Similar to the surfactant- or anion-based growth quenching mechanism, the surface footprint of the quenching agent that must be applied to the surface of the laser-synthesized NPs is the most important parameter. Because these supports’ volumes are larger as compared to those of the alternative quenching agents, the concentration of quenched NPs is limited by the maximum mass of supports that can be dispersed in the liquid without attenuating the laser beam, and thus transparent support materials are desirable. Note that the support concentration should be optimized to maximize the collision rate between the support and the synthesized NPs. Too many supports may result in the low loading mass of NPs, whereas too few supports may not quench the NP growth efficiently. Santagata et al. confined Ag NPs in nanoporous silica by laser ablation of a silver target in an aqueous suspension of ordered mesoporous SBA-15 and MCM-41 silica.64,309 The choice of the mesoporous silica material significantly affected the Ag NP size distribution. The size of the Ag/SiO2 composites 4015


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Figure 30. Size-quenching agent types and related effects during LSPC. In general, agents can be classified as organic molecules (solvents, ligands, surfactants, and polymers) and inorganic anions or supporting particles. The growth-quenching effects are essentially the same during LAL and LFL and are driven by purely electrostatic (anions), electrosteric (surfactants, biomolecules), or steric (polymer, supports) mechanisms, kinetically blocking and thermodynamically deactivating the NP surface against coalescence and aggregation.

synthesized in SBA-15 was smaller (15.4 ± 5.7 nm) than that of those obtained in MCM-41 (22.6 ± 2.6 nm) and pure Ag NPs (21 ± 1.3 nm) generated in water.64 Kim and co-workers synthesized Pd/CeO2 nanocomposites by inhibiting Pd NP growth in CeO2 colloids and found that the growth of Pd NPs terminated at 5−20 nm.369 Figure 30 summarizes the agent types (organic and inorganic) for in situ size quenching during LSPC. Organic agents with different molecular weights provide many options for size control and include organic molecules, surfactants, biomolecules, and polymers, which act via physisorption- and chemisorption-based mechanisms and result in electrosteric and steric stabilization. Inorganic agents offer two methods for size control: salt quenching and support loading, which are achieved by adjusting the surface charge density and steric adsorption, respectively. Because size-quenching effects address the NP surface, the surface adsorbate amount must be adjusted so that the total NP surface area is proportional to its mass at a given intended size. Note that one of the major problems in surface-quenching approaches during LSPC in batch processing is that the NP concentration changes with time. Thus, excess adsorbates are required at the beginning to reach the optimal grafting density (critical quenching concentration) at the end of the synthesis. If the emission of nanoparticles during LAL or LFL is considered as a point source emission (as ablation/cavitation space is negligible as compared to ablation chamber dimension), one can assume on the basis of volumetric considerations that the local concentration gradients from this point source scale with the cube of the distance z.370 As a result, the ligand-to-NPs surface ratios required experimentally that lead to effective size quenching are higher than those expected on the basis of monolayer calculations, particularly for high-molecular weight (i.e., small diffusion coefficient) surface adsorbates. Liquid flow techniques may help to minimize these problems, allowing the amount of adsorbate employed for size control to be adjusted more

precisely,310 as will be reviewed in section 5.3.5. Because the whole set of size-quenching methods works quite well during LSPC, particularly for metal NPs, the effective agents may be selected primarily on the basis of the intended application of the NPs, for example, biomolecules for cellular uptake studies, anions, which provide bare surfaces, for analytical chemistry studies, or support particles for heterogeneous catalysis applications (see section 6.6).

4. UPSCALING As a promising cross-disciplinary research field that integrates chemistry, physics, and engineering, the upscaling of NP synthesis via LSPC to multigram per hour22 is essential for preseries testing and many applications. Quantitatively, the multiple-gram range is important for two reasons. First, preseries testing requires large amounts of material with minimal batch-tobatch variation. Examples include biomedical applications, in which the in vitro testing of NP−polymer composites often requires more than 1000 samples371,372 for statistical reasons, with the overall weight of the composite quickly reaching the kg scale. Additionally, in systematic catalysis research, 100-g samples of heterogeneous catalyst powders may be required, for example, during fuel cell function preseries testing. For the bioactivation of 1 kg of polymer with laser-generated metal NPs,373 loading of 0.5−2 wt % is often required, and for oxidative gold catalysis, several wt % of NPs on supports is required.374 In turn, approximately 1−20 g of NPs enables nanointegration on the kg scale for functionalized materials testing. This regime of upscaling would allow the LSPC community to test whether the claimed advantages of LSPC-specific synthesis reported since more than a decade ago are truly applicable at all and at a relevant scale. Second, on the basis of simple economic considerations, LAL is expected to cross the break-even point in terms of wet chemical synthesis once it reaches a certain scale (Figure 3a). To increase the throughput, many researchers have investigated 4016


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Figure 31. NP productivity (ablation rate) as a function of laser and beam guidance parameters. (a) Laser fluence: Silicon ablation efficiency of ps LAL in water and the result of a logarithmic fit of the ablation efficiency model. Reprinted with permission from ref 21. Copyright 2014 Optical Society of America. (b) Fluence limit: Trends of ablation yield per surface unit irradiated, estimated by analyzing the Pd colloid absorption spectra, as a function of laser fluence. Reprinted with permission from ref 382. Copyright 2010 American Chemical Society. (c) Scan speed: Gravimetric Al2O3 ablation-rate productivity as a function of lateral interpulse distance using a laser pulse energy of 4.6 mJ with a 4-kHz repetition rate and a 4 mm liquid layer. Inset figures are schemes of the processes that occur during LAL, demonstrating the impact of the cavitation bubble and preheated zone on the subsequent laser pulse. Reprinted with permission from ref 20. Copyright 2010 American Chemical Society. (d) Repetition rate: Zn ablation per pulse in tetrahydrofuran (THF) with 125-μJ ps pulses as the repetition rate varies at lateral interpulse distances of 175 μm (○) and 125 μm (●), and NP productivity (mg h−1) at interpulse distances of 175 μm (△) and 125 μm (▲). Below 5 kHz, the ablation per pulse is constant (regime A), and, consequently, the productivity is linearly correlated with the repetition rate. At higher repetition rates (>5 kHz), the ablation per pulse decays monoexponentially (regime B), and the productivity gradually plateaus. Reprinted with permission from ref 381. Copyright 2010 American Chemical Society. (e) Focusing: NP productivity depending on the focal plane position. The thickness of the water layer above the target was 6 mm. Reprinted with permission from ref 380. Copyright 2013 Springer (f) Pulse duration: Intensity of the absorption spectra of colloidal gold as a function of pulse duration for a pulse energy of 5 mJ. Reprinted with permission from ref 386. Copyright 2012 Institute of Physics.

of this assumption, they predicted that the maximum yield per second is equal to η × Pavg, where Pavg = E0 × RR is the average laser power, and RR is the repetition rate. They concluded that the maximal ablation productivity could be as high as ∼6 g/h for a typical ps laser system with Pavg = 100 W (e.g., E0 = 1 mJ and RR = 100 kHz) and a wavelength of 1064 nm. Note that the experimental throughput the authors achieved was only 0.4 mg/ h21 because of the low laser power used, and therefore this model relies on extrapolation over 4 orders of magnitude. The productivity reaches saturation in this model (Figure 31a), as observed experimentally,381 because many other factors are involved in LAL, such as water breakdown,382 the hydrolysis/ decomposition of liquid molecules,383 cavitation bubble shielding,20 the screening effect of colloids,323 and the materialdependent ablation threshold and limited optical penetration depth.384 These factors should be considered during the extension and experimental validation of the LAL productivity model described above. There is no LAL productivity model until now that takes parameters of the liquid into account, such as specific heat, permittivity, or compressibility. Thus, systematic studies and the development of an advanced “LAL law” are required. Note that for the highest NP productivity, a fluence just below the water breakdown threshold (not the highest fluence) is optimal (Figure 31b).382 Similarly, during high-power ultrashortpulse laser machining in air, the laser fluence corresponding to the highest ablation efficiency was calculated to be the product of the ablation threshold fluence and e2 (where e is the Euler constant).385 This productivity-predicting ablation law was recently confirmed for high-power LAL.22

methods of increasing the yield, whereas other researchers have explored accurate particle-yield monitoring techniques (e.g., pulsed photoacoustics)375 to augment conventional methods, including comparing UV−vis colloidal absorption intensity,376 weighing the target before and after LSPC (gravimetrical measurement),320 and using volumetric measurements. After a decade of such investigations, substantial progress has been made, enabling us to envisage the great potential of this technique for pilot-scale colloid production. In the following sections, key factors for increasing NP throughput are summarized. 4.1. Laser Parameters for LAL Upscaling

Among all of the available laser parameters, laser fluence might be the most important factor for high LAL throughput (defined as ablated NP mass per time),20,84 irrespective of the pulse duration (ns,20 ps,21 fs,377 and CW laser378). In 2008, Abdolvand et al. used a high-power (250 W) CW fiber laser to ablate titanium in liquids and achieved a removal rate of ∼0.4 mg in 1 s.378 However, because of the heat accumulation resulting from the use of CW lasers, the liquid medium boils readily, and the throughput is severely diminished thereafter. Therefore, pulsed laser systems prevail for upscaling purposes. For many materials (e.g., silicon,320 alumina,178,379 and gold380), the laser fluence dominates NP throughput. Recently, Intartaglia et al. developed a model to predict the productivity based on ablation and in situ photofragmentation of Si NPs (Figure 31a).21 They assumed that the ablation efficiency was proportional to the laser energy and, thus, that the maximum ablation efficiency (Ma) was dependent on the fluence without energy loss during propagation (E0) and the ablation efficiency (η). On the basis 4017


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Figure 32. Cavitation bubbles in batch chamber induced at different pulse numbers: (a) irradiation geometry, (b) at the first pulse (fresh water), (c) at the 100th pulse, and (d) at the 3000th pulse. The ablated material was Zn, the pulse energy was 100 mJ at 1064 nm with a duration of 10 ns, and the spot size was 0.65 mm. Adapted with permission from ref 388. Copyright 2016 Angel Conference.

repetition rate regime, is desired. The repetition rate limit is normally determined by the time scale of the cavitation bubbles; however, temporal effects are limited by other transient phenomena, such as material accumulation or beam attenuation by sticky bubbles. Generally, successive laser pulses hit the target with a pulse interval that is controlled by the repetition rate. Recently, Momeni et al. reported that double-pulse ablation enabled a higher material removal rate than single-pulse ablation when the interpulse delay time was less than the pulse duration. In this case, the second laser pulse exhibited a higher coupling efficiency with the melted target induced by the former pulse.389 De Giacomo’s group also demonstrated enhanced LAL productivity and argued that the interpulse delay should approximate the time period required for the first pulse-induced bubble to expand maximally; in other cases (early bubble expansion and after bubble collapse), the second laser pulses only fragmented the ejected particles along their pathway.390 These investigations provide useful insights and may support future systematic studies on the repetitive application of so-called pulse trains or bursts, which have been shown to be advantageous in laser machining and structuring.391 By using MHz pulse bursts, Kerse et al. recently showed that it is possible to ablate a target material in air before the residual heat deposited by previous pulses diffuses away from the processing region leading to a huge increase in efficiency for the removal process.392 The available maximal pulse energy above the Kerr intensity threshold that leads to unintended excitation of the liquid is normally higher for longer wavelengths than for shorter wavelengths in the same laser system. Additionally, more LAL power is available for longer wavelengths, and thus a higher NP productivity is often achieved using larger wavelengths. For example, the maximal ablation rate at λ = 1030 nm is nearly twice that using the second harmonic at 515 nm because of the higher laser fluence (0.35 J/cm2) at λ = 1030 nm as compared to that at λ = 515 nm (0.22 J/cm2) because frequency conversion decreases the pulse energy.393 At different pulse durations, different degrees of self-focusing (see section 5.4) increase the complexity of evaluating LAL productivity. Self-focusing is a nonlinear optical phenomenon that tends to narrow the beam diameter as the laser pulse moves through the aqueous medium, causing the beam to focus in front of the geometric focal plane; as a result, the target position must be moved slightly toward the lens. Menén dez-Manjón theoretically simulated the laser beam caustic at different pulse durations and concluded that ns laser beams are minimally affected by nonlinear propagation but are refracted by the liquid, leading to a focus position below the original one (and requiring a longer target−lens distance as compared to the geometrical focus in air); in contrast, ultrashort pulsed laser beams, especially fs lasers, are strongly influenced by self-focusing (see section

The average laser power is correlated with the number of pulses, and each pulse will generate a cavitation bubble during LAL that may laterally and temporally shield the subsequent pulse.381 Therefore, optimizing the scanning speed and repetition rate to reduce the bubble-shielding effect should be considered when evaluating the ablation efficiency.323,381 In 2010, Sajti et al. achieved the gram-scale LAL by temporally and spatially bypassing the previously formed cavitation bubbles to minimize the shielding effect.20 Through adjusting the interpulse distance by changing scanning speed at an optimal repetition rate of 4 kHz with a thin liquid layer, a maximum corundum ablation rate of 1.3 g/h was extrapolated on the basis of 5−30 min of processing time, as shown in Figure 31c.20 It should be noted that uncertainty exists in this experiment: NP production was not confirmed by colloid analysis but rather by the ablated target mass. Ceramics made of pressed powders can release mechanically ablated microparticles, thereby contributing to the target mass difference but not to the NP yield.818 When a laser scanner is set to a constant speed, and the repetition rate is varied, the productivity typically follows the curve shown in Figure 31d, limiting the applicability of high repetition rates for high-pulse energy LAL; that is, a linear increase is observed until the temporal bubble-shielding effect arises.381 The lifetime of the cavitation bubble correlated with the ablated mass is proportional to the laser fluence387 (intensity) and the pulse energy (duration), as discussed in section 2.1.1. A shorter bubble lifetime may increase the applicable repetition rate limit and lead to a higher ablation efficiency per pulse. However, in commercial laser systems operated at maximal (fixed) laser power, the pulse energy is inversely proportional to the repetition rate, which leads to the assumption that an optimal valley may exist at a low pulse energy (less ablation per pulse with lower bubble shielding) and high repetition rate or at a high pulse energy and moderate repetition rate. Finding this optimal value theoretically is not straightforward because the bubble diameter scales nonlinearly with the pulse energy, whereas the (unshielded) ablation rate scales linearly with Ep (F); the shielding effect is also not linear because of the complex interactions between the existing bubble and the incoming laser pulse. Thus, systematic experimental studies on LAL with a process duration of ≥1 h are necessary,22 and the relevant NP concentration must also be determined. Regarding the latter, Ito et al. have reported that the bubble size and lifetime decrease sharply after several pulses (Figure 32) attributed to beam attenuation by the produced colloid.388 Shadowgraphy experiments are often used to measure bubble lifetime and size at negligible NP concentrations, and thus are less relevant for determining the productivity-related (multiple pulse) bubble shielding effects. Thus, correlating the cavitation dynamics with productivity-relevant NP concentrations in liquids at higher repetition rates, especially in the threshold 4018


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Figure 33. Realization of g/h-scale NP productivity. (a) Experimental configuration for MHz LAL using a high-power, high-repetition rate ps laser at a scan speed of 500 m/s. (b) NP productivity for different materials at 1 h experiments, each. Reprinted with permission from ref 22. Copyright 2016 Optical Society of America.

5.4).394 Bärsch et al. experimentally demonstrated the selffocusing phenomenon of ps lasers.395 They found that the ablation rates were different at different positions along the focusing beam, and the beam radius changed by only 2%, causing NP productivity to decrease by 90%.395 Maciulevičius et al. reported that the maximal Au NP productivity of ns LAL in water was obtained with a focus 1 mm above the geometrical focus in air (Figure 31e).380 This finding suggests that self-focusing may also occur in ns lasers because of the higher power (5 W) used in these experiments, which exceeds the critical power threshold (3.1 W for 1064 nm lasers, as calculated using Pcr = 3.77λ2/ 8πn0n2, where n0 is the refraction index of the medium and n2 is the coefficient of the nonlinear Kerr index)223 at which the selffocusing phenomenon is triggered in water. As a compromise between high intensity (peak power) and moderate nonlinear liquid side effects, ps lasers are desirable for the industrial-scale LAL synthesis of NPs. This was demonstrated by van’t Zand et al. in terms of the ablation efficiency of LAL of Zn in THF357 and Riabinina et al. according to the maximum ablation rate of LAL of Au in aqueous sodium citrate solution.386 At 40−150 fs, the LAL efficiency was constant, but it steeply increased by a factor of 100 (maximum) at 2 ps; simultaneously, the photoionization of the liquid (at a pulse energy of 5 mJ) decreased (Figure 31f). For even longer pulses (2−200 ps), the laser energy was absorbed by the expanding plasma in front of the target.386 In summary, ps lasers are the most efficient lasers for synthesizing NPs via LAL, but these lasers often have limited pulse energies; consequently, increasing the laser power requires a high pulse repetition. In turn, the high repetition rate implies higher shielding effects, whereas higher pulse energies lead to the excitation of the liquid. Thus, unless the cavitation bubble is spatially bypassed, ns lasers at moderate repetition rates constitute an uncomplicated and affordable substitute. Recent studies by Gökce and co-workers successfully demonstrated g/hscale NP productivities for many materials (maximum >4 g/h for Pt NPs, Figure 33b) by spatially bypassing the cavitation bubbles (Figure 33a) using a high scanning speed of 500 m/s and a laser with a repetition rate of 10 MHz, a laser power of 500 W, and a pulse duration of 3 ps.22,31 These are the first studies to report that when the cavitation bubble is completely spatially bypassed, LAL converges to laser machining in air in terms of its scaling factors (ablation law). Thus, pilot-scale productivities are well within the range of the current LAL method.

4.2. Material Geometry for LAL Upscaling

In addition to the laser parameters, upscaling LAL throughput by material-based methods has also been discussed in the literature. Generally, 3D bulk targets with thicknesses of several millimeters are adopted rather than 2D thin films, which suffer from rapid consumption and difficulties in material feed during LAL. Interestingly, a quasi one-dimensional (1D), continuously fed wire target has been shown to be a good substitute for bulk targets to enhance productivity, despite its more complex experimental setup (Figure 34a).396 The maximal ablation efficiency of 1D wire ablation was far higher than that of the bulk material (Figure 34b).396 By comparing the difference in cavitation bubble dynamics between wire ablation and bulk ablation, De Giacomo et al. concluded that the unique cavitation

Figure 34. (a) Schematic diagram of the experimental configuration for wire ablation. Reprinted with permission from ref 268. Copyright 2014 American Chemical Society. (b) Ablation efficiency of silver targets at 1.5 J cm−2 as a function of wire diameter. The efficiency value for a bulk target is also reported for comparison. Reprinted with permission from ref 396. Copyright 2013 Royal Society of Chemistry. (c,d) Timeresolved shadowgraph (top) and laser-scattering images (bottom) of laser-induced bubbles on a Cu wire target and a Ti bulk target in water. Adapted with permission from ref 129. Copyright 2015 Elsevier. 4019


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Table 1. Comparison of Absolute and Power-Specific NP Productivities of Bulk Target Ablation Reported in the Literature material

laser system

Au

1064 nm, 5 kHz, 32 W, 10 ns 1064 nm, 100 Hz, 25 W, 10 ns 1030 nm, 10 MHz, 500 W, 3 ps 1030 nm, 10 MHz, 500 W, 3 ps 1064 nm, 5 kHz, 32 W, 10 ns 1064 nm, 5 kHz, 32 W, 10 ns 1064 nm, 50 kHz, 5.5 W, 10 ps 1064 nm, 5 kHz, 32 W, 10 ns 1030 nm, 10 MHz, 500 W, 3 ps 1030 nm, 10 MHz, 500 W, 3 ps 1030 nm, 10 MHz, 500 W, 3 ps 1030 nm, 10 MHz, 500 W, 3 ps 1030 nm, 200 kHz, 50 W, 10 ps 800 nm, l kHz, 0.4 W, 120 fs

Ag Pt Au Ag Ni Ag Cu Ag Al Cu Ti Ag Ag

chamber

target geometry

water

static

1D

water

flow

water

process duration [min]

quantification method

(extrapolated) productivity (mg/h)

laser power-specific productivity (mg/hW)

ref

15

extrapolation

550

16.9

398,399

1D

not specified

extrapolation

317

12.7

396

flow

3D

60

l h experiment

4050

8.1

22,31

water

flow

3D

60

l h experiment

3970

7.9

22,31

water

static

1D

15

extrapolation

220

6.8

398

water

static

1D

15

extrapolation

215

6.6

398

water

flow

3D

0.5

extrapolation

31

5.6

400

water

static

1D

15

extrapolation

175

5.4

398

water

flow

3D

60

l h experiment

1900

3.8

22,31

water

flow

3D

60

l h experiment

1500

3.0

22,31

water

flow

3D

60

l h experiment

1400

2.8

22,31

water

flow

3D

60

l h experiment

800

1.6

22,31

acetone

static

3D

0.5

extrapolation

300

6.0

395

water

static

3D

not specified

extrapolation

0.3

377

liquid

0.1

Figure 35. Effects of liquid flow and height on productivity. (a−c) Ablated line geometries on bulk silver samples produced by ps laser pulses (5.5 W, 50 kHz, 20 repetitions per line) in stationary (a and b) and flowing liquids (c). In all cases, the liquid vessel was moved linearly relative to the laser beam at a constant speed of 0.5 mm/s. Reprinted with permission from ref 400. Copyright 2007 American Institute of Physics. (d) Local NP concentration gradients during batch processing and flowing liquid LAL. Adapted with permission from ref 310. Copyright 2013 Royal Society of Chemistry. (e) Laser ablation rate of Si depending on water layer thickness at a laser fluence of 3.1 J/cm2. Reprinted with permission from ref 403. Copyright 2001 American Institute of Physics. (f) Au NP productivity in water as a function of the liquid flow velocity using a focused laser pulse energy of 100 μJ. Reprinted with permission from ref 402. Copyright 2010 Springer.

and then collapses back toward the target (Figure 34d).129 When the wire diameter is too large, the bubble dynamics are similar to those of bulk target LAL, and thus there are no differences in the productivity. Integrating this experimental design with highrepetition rate laser pulses deserves further investigation because the first bubble trajectory and the cloud of daughter bubbles

bubble dynamic behaviors were responsible for the NP productivity increase for wire-LAL.397 First, the bubble grows spherically during the initial bubble stages in wire-LAL, then encompasses the wire, and is finally expelled from the wire, thereby releasing ablated matter into the liquid (Figure 34c). In contrast, during bulk ablation, the bubble grows hemispherically 4020


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steadily increased by a factor of 7 as the liquid flow rate was increased to 450 mL/min (Figure 35f).402 Note that the laser beam absorbance of the NPs can be calculated from their extinction coefficient; for example, using a 10 mm cuvette, Au NPs (7 nm) at a concentration of 100 mg/L would attenuate approximately 0.01%, 0.16%, and 0.13% of the intensity of 1064 nm, 532 nm, and 355 nm beams, respectively. Additionally, intensifying the irradiation provokes bubble formation adjacent to the NPs beyond the threshold fluence (1 mJ/cm2 for 200 fs and 10 mJ/cm2 for 1 ns), which is below typical LAL fluence values,220 additionally attenuating the beam by bubble scattering before it reaches the target. Hence, the liquid thickness must be considered because the laser power is increasingly attenuated as the liquid thickness82 (UV lasers) and NP concentration increase.376 The optimal water thickness to achieve the maximum NP productivity is in the range of 1−5 mm, depending on the ablation efficiency or pulse energy. The lower limit is set by the onset of bubbles breaking through the liquid and splashing; additionally, optical windows (used in closed chambers) require a minimum distance from the target surface to avoid damage. For higher-energy pulsed ns lasers, this minimum safe distance is approximately 4−6 mm. An optimal thickness of 1−1.5 mm was reported by Jiang et al. during the laser ablation of a Ge target with a Nd:yttrium aluminum garnet (YAG) laser (wavelength of 532 nm, pulse width of 10 ns)376 and Zhu et al. via the laser ablation of silicon wafers using a KrF excimer laser (wavelength of 248 nm, pulse width of 30 ns).403 Similar results have been reported by other researchers. For example, Sajti et al. noted that the minimum liquid layer required for stable ns laser (4 kHz, 18.5 W) processing of corundum was 2.5 mm, which resulted in a NP productivity that was 350% of that obtained at a higher liquid thickness (8 mm).20 The reason that the ablation productivity changes as the liquid thickness varies will be further discussed in section 5.3.4. Size-quenching additives also affect NP productivity. Dissolved micromolar salts and organic solutions should positively affect NP throughput because the laser light-scattering intensity decreases with the sixth power of the particle diameter.404 As size quenching measures also contribute to colloidal stability reducing the abundance of aggregates, size-quenching additives increase the productivity by reducing both types of scatterers, large spheres, and secondary aggregates of primary particles. However, the degradation of additives (e.g., molecular adsorbents) can contaminate the colloids.405 Moreover, bulky additives may also alter the liquid viscosity, which affects not only the bubble dynamics but also the NP throughput. For example, an increase of approximately 50% of LAL productivity in aqueous solution was obtained by adjusting the PVP additive concentration and was maintained even when it exceeded the threshold concentration at which no further size-quenching effect was observed.144 The temporal cavitation bubble size and lifetime measured by shadowgraphy were significantly shorter when adding PVP. These differences were attributed to the tighter plasma confinement in PVP solution induced by its higher liquid density and viscosity, which supports the secondary ablation effect by plasma confinement (and verified by higher acoustic signal).166 Moreover, the liquid viscosity also influences the NP stability against aggregation and can be used to improve the ablation rate. Wagener et al. reported that 0.3 wt % thermoplastic polyurethane (TPU) is optimal for the fabrication of welldispersed Ag NPs; below this loading, no stabilization of NPs was observed, whereas above it, the highly viscous liquid reduced the NP production rate.307

emerging after the first bubble collapse are directed toward the incoming beam, thereby shielding subsequent pulses. In general, LAL of a wire offers two essential advantages over 3D and 2D target geometries, which are independent of productivity but highly relevant for continuous production. First, achieving a continuous feed rate of cubic centimeters per hour (g/h) using a 2D target is difficult, and 3D target replacement interrupts the process and disturbs the steady state of the drained colloid concentration. Second, colloidal application often requires a certain concentration or total NP mass, which can be directly controlled by simply setting the length of the wire fed into the ablation chamber. While the NP concentration can be monitored for 2D targets as well (e.g., gravimetrically or spectroscopically), the control is not as convenient as for a wire. Table 1 summarizes the NP laser power-specific productivity for different geometrical targets. As can be seen, experimental ablation setup using 1D wires allows continuous output with laser power-specific productivities comparable to those studies using 3D targets. However, only a few metals can be wired, but almost all materials can be obtained in 2D films and 3D bulk targets. On the one hand, substitution of 2D films or 3D bulk targets takes a few minutes, whereas wire ablation does not require process interruption for target feeding. On the other hand, beam focusing on a microscale wire is difficult and requires timeconsuming adjustment at start. After elucidating the productivity and processing strategy effects with target geometries confined from 3D to 1D, the scale-up effects on 0D targets (particles) will be investigated in section 5.1. Because NP productivity is related to the liquid as well, LSPC scaling figures will be addressed in the following section. 4.3. Liquid Properties and Process Parameters for LAL, LFL, and LML Upscaling

Liquid handling is crucial during NP production because of its impact on the local concentration of the colloid and the cavitation bubble size and mobility. In a stationary liquid (Figure 35a,b), bubbles adhering to a hydrophobic metal surface will scatter and shield the laser, reducing the incident energy on the target and, therefore, decreasing the ablation rate.400 Furthermore, it will cause the beam path to deviate, making the ablation process uncontrollable and irreproducible. In comparison, transporting the bubbles from the pathway of the laser beam and draining the newly generated NPs over time via flowing liquid310 (Figure 35d) could significantly reduce both the bubble and the NP shielding effects and ensure a uniformly successive ablation pattern (Figure 35c).400 In addition, liquid-flow LSPC would establish steady-state synthesis conditions with regard to NP concentration. In batch processes, strong spatial and temporal concentration gradients arise, but these gradients can be minimized by liquid flow (Figure 35d),310 which is beneficial for steady-state particle formation kinetics. Hence, flowing liquid also ensures a constant molar ratio of the size-quenching agent relative to the NP surface. A 20% increase in LAL productivity has been obtained using flowing liquid with the flow rate of 190 mL/min.379 A further support is provided by Resano-Garcia et al., who reported that the width of the laser-cut kerf (proportional to productivity) obtained in the stationary liquid was nonrepeatable and approximately 30−35% narrower than that achieved in flowing liquid.401 In contrast to a conventional liquid flow apparatus, here, the liquid flow was achieved by rotating the target. The productivity of fs-LAL-synthesized Au NPs strongly benefits from liquid flow: the NP productivity 4021


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Figure 36. (a) Depiction of a passage reactor design with a free liquid jet and magnification of the laser irradiation area with the parameters required for process characterization. (b) Laser fragmentation efficiency plotted versus laser fluence. (c) Particle size according to laser fragmentation of ZnO in flowing liquid with a controlled energy dose. Reprinted with permission from ref 218. Copyright 2015 Elsevier. (d) Relative jet illumination plotted versus the fluence for different laser energies in a geometry with a water jet diameter of 1.3 mm, a raw laser beam diameter of 3.5 mm, a focal length of 100 mm, a pulse length of 10 ps, a wavelength of 532 nm, a repetition rate of 100 kHz, and a particle mass concentration of 0.1 wt %. Reprinted with permission from ref 406. Copyright 2016 Springer.

after a defined number of passages together with the laser pulse energy enables the key parameters of LFL scaling, particularly the cumulative PPP and the energy-specific mass throughput converted to particles, to be characterized (Figure 36c).218 This upscaling parameter allows the LFL productivity to be extrapolated to higher laser powers (at fixed fluence). The crucial factors to increase LFL productivity are the particle mass concentration and liquid flow speed (or educt mass flow) with respect to the laser fluence and repetition rate; the limiting factor is the optical breakdown of the liquid. A typical plot illustrating the fragmentation efficiency for this geometry is shown in Figure 36b. After passing the fragmentation threshold, the LFL efficiency increases linearly until the laser fluence is close to the optical breakdown point, at which the LFL efficiency starts to decrease. Figure 36d shows the calculation of the relative jet illumination as a function of the applied laser fluence at different pulse energies. This calculation is based on a liquid jet diameter of 1.3 mm, a laser pulse duration of 10 ps, a wavelength of 532 nm, and a repetition rate of 100 kHz. A “relative jet illumination” of 1 is defined as the case in which every passing particle is illuminated by the laser beam; thus, for upscaling purposes, this value should be as close to 1 as possible. Clearly, to achieve a higher fragmentation efficiency and a higher throughput, either the illuminated volume at constant fluence or the laser fluence at constant jet illumination (ideally 1) should be increased. In both cases, higher laser power is needed. For the example given in Figure 36d, 100% educt LFL illumination is achieved at a power of 75−150 W. Specifically, 100% illumination efficiency during LFL of 0.1 wt % ZnO (430 nm in diameter) in water enables a productivity of 1.8 g/h with a size reduction (Δd) of 50 nm and a

Although substantial research efforts have been directed toward the ablation rate of LAL, investigations of LML and LFL productivity are still rare. One reason is that quantifying LPC productivity requires quantitative fractionation of the educt and product, which are both in the colloidal state. Also, the partial irradiation of the liquid volume and the beam propagation through both the educt and the product makes defining normative parameters difficult. As will be demonstrated in the discussion of liquid flow in section 5.3.5, in general, two different experimental configurations, a vessel (cuvette) and a flow arrangement (e.g., a free liquid jet), are used for both LFL and LML. The laser processing of particles in a vessel265 is a lesscontrolled process than that in a liquid jet218,253 because the geometry of a vessel does not allow full illumination of the particles inside. Thus, the educt particles are not uniformly affected by the laser pulses, and the irradiation time must be prolonged (with exponentially decreasing efficiency) to increase the chance of processing all of the particles. For LFL, strong focusing conditions are generally beneficial because productivity scales with fluence. However, in the case of a thicker liquid layer, the deviation of laser fluence within the irradiated volume is large. Relative to the vessel geometry, a free liquid jet with a passage reactor, as shown in Figure 36a, appears to be a good alternative for LFL and LML productivity studies because it facilitates a relatively controlled illumination process of the particle stream.218 Because the educt mass and particle number concentration are fixed at the beginning of the process during sample preparation, controlling the laser repetition rate allows the precise tuning of the number of nominal pulses per particle (PPP). Comparing the particulate educt and product properties 4022


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bandgap variation (ΔE) of 0.2 eV.218,406 As compared to LFL, low levels of fluence deviation during LML will not strongly affect NP productivity because an unfocused laser beam is used.279 The LFL and LML productivity parameters can be summarized as follows. Liquid flow should be used and the LFL fluence should be as high as possible but below the optical breakdown point of the working liquid to avoid significantly decreasing the LFL efficiency and productivity. Compared to LFL, LML is a more straightforward process, but its product size is sensitive to fluence deviation.91 Thus, both LFL and LML benefit from a short beam path and full illumination of the liquid volume. Fundamental research indicates that single pulses may be sufficient for LFL. However, multiple pulses are often required (LFL of larger NPs at lower laser energy and, in most cases, LML). Thus, demand exists for the experimental quantification of the “minimal required PPP” values for well-defined nanoproducts achieved by LFL or LML processing. Identifying key parameters for obtaining minimal PPP would enable one to determine the optimal educt concentration that allows 100% educt-to-product conversion, thereby avoiding educt−product mixing downstream. For LML-synthesized SMSs, the productivity is already moderate, with values such as 100 mg/h of CuO,272 but further systematic investigations are required for upscaling. Because SMS productivity depends on the material’s laser absorbance,407 studying the relationship between the material absorbance and the SMS productivity is necessary. The absorption efficiency (Qabs) is correlated with the material’s refractive index and extinction coefficient and the laser wavelength;91 thus, these are the governing parameters determining LML productivity. Generally, UV lasers (e.g., λ = 355 nm) and VIS lasers (e.g., λ = 532 nm) enable higher absorption efficiency than IR lasers (e.g., λ = 1064 nm).91 Pulse duration is another critical factor for LML productivity. For SMS formation, the laser fluence threshold increases with the pulse duration.408 Indeed, the productivity achieved using longer pulse durations is inefficient as compared to that at shorter ns-scale pulse durations (e.g., 50 ns vs 7 ns), because heat tends to dissipate from the SMSs to the surrounding liquid within several tens of ns.409 Overall, LSPC-upscaling comprises a systematic optimization process that involves every aspect of the laser−liquid−target system. At the lab scale, noncost-effective measures to increase productivity include using a 3−6 mm liquid layer (for LAL) and flowing liquid (for LAL, LFL, and LML). For LFL and LML, a simple funnel or, even better, a Marriott flask with a 1 mm capillary outlet irradiated by the perpendicular laser beam works well. With advances in laser technology, increasing power will become more accessible for all laser systems, especially pulsed laser systems (Figures 1 and 3). Consequently, even higher NP productivities are anticipated in the future. The successful decoupling of the cavitation processes has already enabled a NP productivity of 4 g/h for high-power, high-repetition rate LAL,22 demonstrating that the liquid medium is not a barrier blocking LSPC from utilization in series production with industrial-scale productivity. Furthermore, new possibilities, such as beam and pulse shaping with spatial light modulators or multibeam processing by diffractive optical elements, also offer straightforward strategies to increase the productivity of LSPC further.

5. MATERIAL, PROCESS, LIQUID, AND LASER PARAMETERS The decisive factors shown in Figure 37 impose synergistic effects during LSPC: material properties, liquid parameters, laser

Figure 37. Influencing factors responsible for determining the purity, productivity, shape, size, charge, crystal phase, stability, and dispersion of NMs synthesized by LSPC.

parameters, and process adjustment, with each determinant including several branching factors. 5.1. Material Properties

5.1.1. Composition. Precisely determining the time scales for NP nucleation and their chemical reactions on the atomic/ cluster scale, including the rough time frame32 based on spectroscopic studies147,180,187 and simulations,163,410,411 is an ongoing task in LSPC. Additionally, the extent to which NP formation is kinetically or thermodynamically controlled remains unclear. In this regard, LAL of a target containing two or three miscible elements to produce bimetallic or trimetallic alloyed NPs could contribute to our understanding of the formation mechanism, particularly the interplay between the target composition and NP stoichiometry. To date, a large variety of alloys have become available for processing by LSPC, including AuAg,125 AgCu,412 PtAu,413,414 PtPb,415 and FeMn.416 The appealing features of these alloys include stoichiometric compositions nearly identical to those of the target125,417 and high composition uniformity within the synthesized alloy NPs.108,125,412,413 However, because of the possible oxidization and leaching of alloy atoms, in some instances, nonstoichiometric alloying and segregation occur.356,418 The probability of stoichiometric alloying depends on the materials’ resistance to oxidization and, specifically, on the redox potentials of the ablated elements. No deviation has been observed for noble metals that have no miscibility gap and produce perfectly monophasic crystals (e.g., AgAu125), as confirmed by UV−vis spectroscopy (Figure 38a) and TEM-energy-dispersive X-ray spectroscopy (EDX) line scans (Figure 38b,c). The SPR peak position of colloidal AgAu alloy NPs linearly increases with the gold molar fraction (GMF); however, at a given molar ratio, it also depends on the synthesis method used to generate the NPs. Unlike alloy NPs derived from LAL, red-shifts of the SPR peak occur in alloy NPs synthesized by chemical and biological methods (Figure 38d).125 These differences are attributed to the presence of ligands during chemical and biological syntheses, which increase the refractive index in the nanovicinity of the particle surface and cause a bathochromic shift of the SPR peak maximum.419−421 Thus, laser-generated plasmonic AgAu alloy NPs have the lowest SPR peak wavelengths. In addition to these 4023


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Figure 38. LSPC synthesis of alloy and doped NPs. (a−c) Comparison of UV−vis spectra of Ag−Au colloids and the elemental distribution (TEM-EDX line scans) and composition of single Ag−Au NPs generated during LAL of bulk AgAu alloys with a GMF of 0.4. (d) Comparison of plasmon wavelength of laser-generated NPs with chemically and biologically synthesized AgAu NPs. Reprinted with permission from ref 125. Copyright 2014 Royal Society of Chemistry. (e) PL properties of Cu-doped ZnO NPs for different Cu concentrations (inset). Adapted with permission from ref 428. Copyright 2011 Elsevier.

Figure 39. AgCu alloy NP phase structure after LAL synthesis. (a−d) TEM images showing the phase segregation dependence on the Cu mole fraction for 20 nm. (e,f) Calculated free energy of the biphase (twin) and core−shell structures. (g) Calculated composition and size effects on the phase boundary surface energy. Reprinted with permission from ref 412. Copyright 2014 American Chemical Society.

bathochromic effects, the generation of monophasic AgAu NPs by chemical precursor reduction synthesis is challenging because of the inherent differences between the reduction potentials of Ag and Au precursors.422 The coreduction of Ag and Au precursors leads to the formation of NPs with a gradient in which the more noble metal (Au) is reduced first and is thus more abundant in the core of the resulting NP.423 Overall, LAL not only allows the synthesis of colloids of plasmonic AgAu alloys without any initial ligand-attributed SPR shift, facilitating the precise observation of the effects on the refractive index in the particle’s vicinity, but also provides access to a monophasic intraparticle structure inaccessible by aqueous chemical reduction methods without time-consuming tempering steps, which could be used to obtain ideal alloy model materials for catalysis and biomedicine.11,346,413,424 Beyond using a binary material target for alloy NP synthesis by LAL, laser reirradiation of mixed LAL-synthesized elemental colloids (e.g., Au and Ag NPs425 or Au and Ni NPs426) and successive LAL of two elemental (e.g., Ag and Au) targets427 are two additional routes to synthesize alloy NPs.

However, large deviations occur when alloying elemental Fe or Cu with Pt,126 Ag,412 or Au.174 To increase the stoichiometric alloying of non-noble elements, corrosive liquids or liquid molecules that decompose in the laser plasma into reactive oxygen species, especially water, are unfavorable, and organic solvents are preferable,356,418,429 unless galvanic replacement via RLAL is intended,160,430 as we will discuss in the “liquids” section (section 5.3.1). When designing materials for laser alloying, the interface energies (at the intraparticle phase boundaries) and relative volume fractions should be considered because these parameters determine whether the synthesized NMs will exhibit a two-phase microstructure or a core−shell structure.412 Recently, Malviya and Chattopadhyay synthesized AgCu alloy NPs by LAL of the bulk alloy in an aqueous medium.412 They reported a Cu concentration-dependent morphological transition within the NPs, from a defined two-phase intraparticle structure to a structure with random segregation, and, finally, a core−shell structure at a high molar fraction of Cu (Figure 39a− d). Their formation mechanism hypothesis was rationalized through the thermodynamic modeling of the free energy of phase mixing and the wettability of the alloying phases. The surface 4024


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enhanced by increasing the laser fluence.272 Attention should be paid to the LFL yield and product composition difference originating from the source materials’ ablation threshold. By investigating the formation of Cu colloids by LFL of CuO, Cu3N, Cu(N3 ) 2, Cu 2C 2, and CuI powders in organic liquids, Schaumberg et al. determined that LFL of suspensions is an educt powder composition-dependent process.259,449 The LFL efficiency of copper nitride (29.7 μg/J) is several times higher than that of copper oxide (2.5 μg/J) (Figure 40) under the same

energy as a function of both size and composition is shown in Figure 39g. At a given particle size, the surface energy increases as the Cu composition in the alloyed particles increases (Figure 39g). Additionally, the free energy increases as the particle size decreases (Figure 39e,f), and thus smaller or Cu-rich particles tend to form core−shell structures (minimizing their Gibb’s energy) rather than asymmetric segregation phases. Overall, this study indicates that the final phase structure of NP synthesized by LAL is determined by equilibrium solid-state thermodynamics. It seems that, although the net composition of a particle is set in a fast nucleation process, its final alloy phase structure builds up on a far longer time scale.418 In addition to metal alloys, the laser synthesis of doped NMs with variable dopant concentrations has also been investigated. LAL of a solid Zn/Cu composite caused the corresponding PL properties of the synthesized colloid to depend on the Cu concentration in the ablated target (Figure 38e).428 Ledoux et al. confirmed the feasibility of preserving the phase (with some lattice disorders) and stoichiometry of the targets by transforming them into Y2O3:Eu3+, Lu2O2S:Eu3+, Gd2SiO5:Ce3+, Lu3TaO7:Gd3+, and Tb3+ NPs with sizes as low as 5−10 nm using LAL.431 Other doped NMs have also been successfully produced,432−434 and researchers’ interest in doped NMs with upconversion properties is rapidly increasing.435−438 Overall, obtaining doped ternary or multinary oxides by LAL of doped solids is straightforward, and the colloids often exhibit related dopant functionalities (section 6.2.3). However, more studies that focus on the selectivity of the synthesis related to a defined crystal phase, amorphous byproducts, or lattice defects are required. Oxygen vacancies and interstitial cations can be regarded as dopants also (i.e., defect dopants) and have relevance in (e.g., titania-based) photocatalysis.439 Depending on the target, LSPC reduces or oxidizes the solid during its conversion into a colloid. For example, LFL of rutile TiO2 resulted in anatase, brookite, and amorphous species with Ti3+ defects identified in the TiOx colloids.111 LAL of elemental Ti resulted in particle oxidation and the formation of TiO and Ti2O3 in addition to TiO2 with a significantly reduced bandgap.440 Such LAL-synthesized defect-rich TiOx crystals show the roomtemperature ferromagnetism of TiO2 NPs.441 Although precise adjustment of the oxidation degree during LAL remains difficult, the oxygen affinities of many metallic materials (e.g., Cu, Zn, and Sn) provide opportunities to conveniently obtain (sub)oxides (e.g., CuOx,442,443 ZnOx,444,445 and SnOx446,447) by LAL of pure metal targets in oxygen-containing solutions. Furthermore, LAL of a bulk target consisting of binary elements (e.g., Ag/Ti) with different resistances against oxidation may allow the direct, onestep synthesis of doped oxides (e.g., Ag−TiO2448). When ablating oxidation-sensitive metal targets in water, synthesized NMs with mixed chemical compositions are often obtained, as shown for Ti-LAL.440 Lam et al. started with AlxOy as a simulation prototype and found that the stoichiometry of Al2O3 formation is coupled to the temperature in the early phase of LAL.155 High temperatures in the first submicrosecond time period determine the oxidation rates of Al3+ to Al2O or AlO NPs, and later during cooling, based on the molecular simulations of the nucleation phase, Al2O3 is formed.143 This result indicates the occurrence of chemical reactions at the ns time scale, during which ablated species react with the liquid in the vapor phase in the early phase of cavitation. In contrast to oxidative LAL, reductive laser ablation and excitation has also been observed during LAL,449 LFL,260 and LML272 of CuO, producing either Cu or Cu2O NPs. During LML, the reduction ratio can be

Figure 40. Yields of copper NPs after 1200 laser pulses with different LFL precursors. Reprinted with permission from ref 248. Copyright 2014 American Chemical Society.

LFL conditions (532 nm, 45 mJ/pulse, 6 ns, 1200 laser pulses) because of the lower ablation threshold of copper nitride as compared to that of copper oxide.248 Interestingly, LFL of CuO and Cu3N and Cu(N3)2 follows a reductive mechanism, whereas that of CuI and Cu2C2 follows a fragmentation mechanism without the formation of elemental Cu NPs. In addition to investigations into the chemical composition of synthesized NMs, intensive effort has been focused on the synthesis of high-pressure crystal-phase NMs. For instance, after performing LFL in water, monoclinic H-type Nb2O5 powders were transformed into either orthorhombic T- or pseudohexagonal TT-type materials, with the amorphous phase acting as a precursor for the high-pressure phases.450 Irradiating rutile-type SnO2 powders led to the formation of a high-pressure-stabilized α-PbO2-type structure.451 Other high-pressure-phase NMs that have been successfully developed by LSPC include doped γ- and θ-Al2O3 nanocrystals,201 defective calcite II and hydrates,452 wurtzite-type (W)-ZnO and ε-Zn(OH)2 composite nanocondensates,453 baddeleyite-type TiO2,454 and the high-pressure phase of tetragonal Ge.455 The creation of high-pressure phase NMs has been hypothesized to result from the fast cooling of the plasma plume, which causes densification during nucleation.456 Similarly, defects and surface adsorbates resulting from solvent molecules may also stabilize the high-pressure crystal phases of the NPs, making it difficult to attribute the obtained crystal phase to the initial “high-pressure and high-temperature” characteristics of LSPC alone. Low-pressure phases, such as Al2O3457 and ZrO2,458 have also been reported to be produced by LAL. However, because the thermodynamically stable crystal phase at a given composition also depends on the particle size, differentiating the determining parameters would require a thorough analysis, that is, size-dependent crystal structure histograms. Both oxides and carbon can undergo a phase transformation into allotropes, with the carbon sp2 to sp3 bond, graphite to diamond, transformation being the most intensively studied.459−463 Substantial effort has been devoted to the synthesis of diamond. For example, Qiu and his co-workers 4025


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Figure 41. (a) Advantages (green) and drawbacks (red) of using 0D/1D/2D/3D materials for LFL and LAL. (b) Au NP mass in dependence of film thickness (red dots) and theoretical yield (black line). (c) Size distribution obtained from LAL of 1 mm (black), 226 nm (red), 154 nm (green), and 50 nm (blue) films (as shown on the right). Inset shows NP size in dependence of the target film thickness. Reprinted with permission from ref 484. Copyright 2016 American Chemical Society.

see section 6.4.4) is used to improve water solubility and bioavailability.480,481 Thus, in contrast to inorganic or carbon targets, in which phase changes and the introduction of defects are often envisaged, LFL of organic crystals aims to preserve the NM composition, which is often difficult to achieve quantitatively. Overall, LSPC enables the synthesis of a wide variety of NMs, including alloys, doped NMs, and crystalline NPs. Therefore, the design of the target composition is important for controlling the composition of the synthesized NMs. If the synthesis of an alloy or doped NPs is intended, consolidated powder targets are a good option. Thereby mixing the LAL target on the microscale presents an elegant approach to result in atomically “mixed” alloys on the nanoscale. It is possible to synthesize solid-solution AuPt and AuAg NPs via this method, as demonstrated by Guay, who mixed Au and Pt as well as Au and Mn powders,413,482 and Neumeister et al.,125 who mixed Au and Ag, as well as Marzun et al., who mixed Ni with Mo.483 The synthesis of a doped NP oxide by mixing a powder of the host oxide with a powder of the dopant could represent a flexible and rapid method of tuning the doping level of the NPs, similar to the metal alloy series. 5.1.2. Shape. If the characteristic target length or diameter (optical cross-section) is smaller than the beam diameter (typical for LFL), it can be regarded as quasi-0D matter that is excited in the liquid, whereas micrometer-thick wires are considered 1D targets. According to a recent work, ablation dynamics for LAL of a bulk Au target changes if the target thickness is lower than tens of micrometers (Figure 41b,c).484 Hence, we consider targets 10 μm as 3D. Of course, intermediate geometries exist, such as large micropowders, which often exhibit characteristics of both LFL and LAL. Also, freestanding 2D film ablation will have similarities to 1D wires, which display an elastic effect during ablation.398 The target geometries can be extensively varied during LSPC to achieve different goals.

synthesized diamond QDs with tunable particle size and adjustable PL properties.461 As for the diamond formation mechanism, Tian et al. theoretically analyzed the growth dynamics of nanodiamonds during LAL and reported that the radius and density of the plasma plume determined the cooling velocity, which in turn influenced the nanodiamond size.464 Yang et al. used time-averaged optical emission spectroscopy to evaluate the plume produced by 532 nm laser ablation of graphite in water and concluded that atomic H is necessary for diamond growth,465 which may explain why most diamonds have been synthesized in H-containing solutions (e.g., water,460 acetone,461 and ethanol461). However, negative control experiments would be required to validate this formation model. More recently, Xiao et al. discovered a reversible phase transformation between nanodiamond and carbon onion via an intermediary bucky diamond phase462 and a phase transformation from nanodiamond to n-diamond via an intermediate carbon onion466 by laser irradiation. Although much of this work on nanodiamond synthesis was focused on hypothetical formation mechanisms and the laser irradiation synthesis of carbon allotropes, nanodiamonds are often extracted in the supernatant after laser synthesis, and their yields relative to the total ablated mass are seldom reported. In this regard, the formation of sp bonds during LAL of carbon, resulting in polyynes467 and multiwall graphene species,468 has also been reported by Compagnini and coworkers. Thus, more research is required to assess the yields and formation mechanisms of different carbon species during LSPC. Organic particles with defined molecular structures, such as quinacridone,469,470 vanadyl phthalocyanine,471,472 paclitaxel,473 buckminsterfullerene (C60),474,475 3,4,9,10-perylenetetracarboxylic dianhydride,476 melamincyanurate,477 dendronized perylenediimide,478 and melamine cyanurate,479 have also been produced by laser treatment. Here, LFL of semipolar or poorly water-soluble educt particles (e.g., aiming at drug micronization, 4026


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For example, 0D powders,260,469,485 NPs,486 and nanoflakes487,488 are processed by LFL during investigations of the interaction mechanisms between materials and liquid molecules248 or for size reduction489,490 and phase transformation111,491 or by LML for SMS synthesis.273 Both chemically cosynthesized NPs492,493 and almost all LAL-synthesized NPs494 are appropriate LFL particulate educts. Also, both the physical state and the size of the educt powders affect the LFL yield. When adopting microparticles for fragmentation, a simple mechanical pretreatment (e.g., wet-grinding) enhances both the laser fragmentation efficiency and the NP productivity.258 The laser processing of powders at the millimeter scale enables the generation of nonspherical submicron particles with sharp edges.495 Consequently, the LFL efficiency is insufficient to completely fragment large educt powder, leading to the following drawback: the difficulty in separating the transient mixtures consisting of the educt and products.218 Downsizing from micropowders to NPs can cause a band-edge shift of materials (e.g., CoO), enabling the activation of water-splitting capacity under VIS light irradiation, which cannot be achieved with micropowders.485 When the educt is on the nanoscale (or at least smaller than the excitation beam diameter and well dispersed in the liquid to minimize light scattering), they undergo the transformation to the tens-of-nanometers scale much more readily and may even reach sizes of a few nanometers.25 NP suspensions in the agglomeration state are employed in LML to obtain SMSs.279 Particular attention has been paid to preparing alloys by the supercontinuum irradiation of 0D material mixtures.226 LSPC can be used to fuse two miscible NP elements (e.g., Ag and Au) into a single nanosphere. By mixing different ratios of LAL-generated AgNPs and AuNPs, alloyed NPs with different Ag:Au ratios can be prepared.496 After alloying, the surface of the LSPC-synthesized NPs can be further passivated with hydrophobic molecules to improve the NP stability by irradiating the NPs in a liquid mixture of oleic acid and oleylamine with sonication.497 For submicrometer-sized products, LML also enables the alloying of immiscible metals into new metastable Au1−xNix nanoalloys.498 Agglomeration of the NPs facilitates energy transfer from Au NPs to NiO NPs to decompose NiO NPs into Ni and enable subsequent fusion with Au NPs when the temperature during laser irradiation is above their melting points.498 Therefore, alloying different colloids using LML may pave a new pathway for the preparation of various metastable alloys using both miscible and immiscible materials. The main disadvantage of using 0D targets is the requirement for postfractionating (section 5.2.2, downstream processing) of the educt and product mixture (Figure 41a). As illustrated in the upscaling section (section 4), because of the unique bubble dynamics during LAL of a wire-shaped target,397 1D materials exhibit a higher ablation efficiency than bulk targets.396 However, the enhancement ratio depends to some extent on the thickness of the wires, with the optimum diameter being approximately 500 μm. Indeed, when the wire is too thin, the absolute removal mass is limited, whereas for very thick wires (quasi-2D), the cavitation bubble does not propagate into the liquid while releasing the matter trapped inside. The latter case is similar to the laser ablation of a bulk target, in which the bubble collapses toward the 2D target.397 Moreover, because the bubble propagation can be varied by using 1D wires rather than a bulk target, bypassing the cavitation bubble by adjusting the repetition rates should be reevaluated.396 Using an automated target feed allows full continuous operation, and this 1D target omits any waste, yielding highest conversion rates

of solid into colloid (Figure 41). In the ideal case, the wire is completely ablated after one run of laser ablation; in that case, we have 100% conversion because nothing is left; even if there is debris (redeposits) it will be ablated by subsequent pulses. Yet imprecise focus adjustment or immoderate wire feed may cause wire splitting off, reducing the effective conversion rate. Another disadvantage is evident in ref 183 where a hundreds of μm thick gold belt was continuously fed like a tape recorder. This configuration led to a limited target consumption because a belt with certain residual thickness is required for automated feeding. Previously, only a few researchers have used 1D material to synthesize NMs by LAL, and considerable work is still necessary to uncover the potential advantages and to understand the effects of the wire’s physical and mechanical properties on the ablation process. Recent findings further show that the polydispersity of NPs obtained from LAL of 1D wires decreases for thinner wires if the ablation is performed without cavitation intereference (i.e., low repetition rates).484 3D bulk targets,186 foils,171 and 2D films484,499 are the most frequently used targets for LAL because they are immobilized by their mass density, thereby avoiding the creation of an educt− product mixture; however, continuous NP output is limited because target replacement is required. When flowing liquid is employed or the targets are insufficiently heavy to stabilize themselves in the liquid, anchoring bulk targets with clamps or using special customized chambers is necessary to ensure the stability of laser ablation process. As compared to those of 1D wires and 0D suspensions, the features of 3D bulk targets are compatible with using a galvanometric scanner for high-scanning speed beam guidance (even up to the m/s range) during LAL;500 in contrast, 1D wires are affected by the delivery speed of the wire, and 0D suspensions suffer from precise excitation of the educt without product reirradiation during laser synthesis in a batch chamber. The 3D target surface may also be porous, such as porous silicon in liquid.501,502 Moreover, the microstructures503 and nanostructures504 generated from previous laser scanning will cause fluctuations in the energy absorption by the substrate surface.505 The particles that deposit on the target surface may also act as absorption centers, changing the effective laser fluence on the underlying substrate surface and affecting LAL process stability. Therefore, it is possible that subsequent laser ablation will generate different NPs from these modified areas. Determining how the ablated target surface geometry influences the formation of NPs requires further investigation. 5.2. Process Parameters

5.2.1. Processing Time. The processing time affects the processes and products of LAL, LFL, and LML differently. During LAL, increasing the processing time leads to a higher concentration of NMs (Figure 42c),393,506,507 as characterized by productivity studies with the aid of UV−vis absorbance spectroscopy.376,393 However, the concentration of the LALsynthesized NMs should not be too high. Otherwise, they will gradually aggregate over time and attenuate the beam. The effect of minute amounts of NPs in the liquid, for example, volume fraction as low as 0.5 × 10−6, will alter the optical properties of the colloid, as shown by Karimzadeh and Mansour,508 who reported that the absorbance increased proportionally to the Ag NP concentration and that the thermal nonlinear refractive index (−0.1 × 10−8 cm2/W) of the same colloid also changed. In general, moderate particle concentrations (well below 1 g/ L) are preferred if no stabilizers are used. Subjected to the light absorption and scattering by the synthesized NMs over time, the 4027


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Figure 42. Effect of processing time on morphology and productivity in LAL, size change in LFL, and morphology evolution in LML. (a,b) TEM images of C3N4 nanostructures synthesized at a laser fluence of 75 mJ/pulse after 3-h and 5-h ablation. Reprinted with permission from ref 200. Copyright 2006 American Chemical Society. (c) Effect of wavelength and material on NP productivity; cumulative ablated mass as a function of the ps LAL processing time of silver. Reprinted with permission from ref 393. Copyright 2011 Springer. (d) Evolution of the mean diameter of PbS QDs with the laser fragmentation duration; the inset shows the evolution of the size dispersions of QDs with the ablation duration. Reprinted with permission from ref 250. Copyright 2013 Nature Publishing Group. (e) Evolution of the average size of ZnO QDs with increasing irradiation time during LFL. Reprinted with permission from ref 265. Copyright 2011 American Chemical Society. (f−h) SEM images of products obtained by LML of Ag NPs with irradiation times of (f) 0 min, (g) 30 min, and (h) 60 min (355 nm, third harmonic, in acetone, 0.5 mg mL−1, 30 mJ per pulse per cm2). Reprinted with permission from ref 515. Copyright 2013 Royal Society of Chemistry.

Recently, the successive laser ablation of two materials was suggested to have the potential to enrich nanoproduct diversity because of extreme sensitivity to the ablation period. Singh and Soni found that increasing the irradiation period of Ag and Al NPs in water caused the particles first to evolve into a networktype structure, then into core−shell structures, and finally into rattle-type Ag@Al2O3 because of the Kirkendall effect.511 Varying the ablation period of gold or zinc targets in LALsynthesized Ag colloids allowed the modulation of the Au or ZnO shell thickness of the resultant Ag@Au512,513 and Ag@ ZnO514 core−shell structures, respectively. Nevertheless, most studies on the processing time during LAL either focus on the concentration or supersaturation effects in batch reactors or trigger LFL processes that are naturally more pronounced at elevated concentrations. Although it may be convenient to have multiple effects triggered in one step and one batch, reproducibility may be an issue if the LAL processing time is mechanistically coupled with concentration-dependent LFL. The processing time interval is a key factor governing LFL efficiency258 and has been extensively studied in terms of its correlation with particle size variation253,516 and corresponding (Au, Ag) SPR position shift107,487 or colloid color change.516 Yang et al. quantitatively investigated the effect of laser fragmentation duration on the evolution of the average size of PbS QDs.250 Raw PbS nanocrystals with sizes of tens of nanometers were converted to fairly small PbS QDs (3.5 ± 0.3 nm) after approximately 20 min (Figure 42d). Moreover, the

ablation efficiency generally decreases as the ablation time increases,61,375,506 resulting in the fragmentation of existing particles305 and decreased particle size.393 This saturation and shielding phenomenon has prompted investigations into using flowing liquid to drain the NPs from the incident beam path, which will be discussed in section 5.3.5. Note that longer ablation times may also increase the liquid temperature until saturation at a value dependent on the laser fluence.134 Additionally, even ultrashort pulses deliver residual heat to the target, which accumulates during the processing time,509 and the surface may transfer this heat to the liquid. Continuous heating will compromise the colloidal stability according to the Stokes− Einstein equation (stronger diffusion causes particle aggregation). Increased fluence and colloid concentration during a 1-h laser ablation of Pd in 2-propanol caused photofragmentation and an increased ratio of Pd(0) to Pd(II), as measured by XPS of the resulting colloids.510 In contrast, when the concentration was kept constant during the 1-h ablation in a flowing liquid system, the composition and the oxidation state of the synthesized NPs did not change, as shown by the Gökce group for the ablation of Pt and Ni in water by collecting the XRD and XPS spectra of the products over a 1-h period.31 Furthermore, dispersed particles were shown to evolve into different 3D structures under continued laser processing.38,200 After ∼30 min of laser ablation, C3N4 particles were found to exhibit leaf-like features (Figure 42a), and they transformed into nanorods after 5 h of ablation (Figure 42b),200 possibly triggered by concentration effects. 4028


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Table 2. Downstream Techniques Combined with LSPC To Produce Novel Functional Materials for Various Applications technique

effect

material

potential applications

aging

phase change, defect control

annealing centrifugation membrane filtration crystal inclusion drop casting

defect control size classification ligand removal/size classification host−guest composite production film deposition

EPD

film deposition

ex situ conjugation

conjugation

Au−ssO, immunoglobulin conjugates

mixing at right pH hydrothermal

supported particles solidification, doped-hematite nanocrystal synthesis powder coating 3D parts of polymer composites

Pt, Au, Ag... on TiO2, BaSO4, graphene Ag−TiO2; Zn2SnO4, ZnSnO3; (Ge, Si, Mn, Sn, Ti)-doped α-Fe2O3 Au-prepolymer blend UVE3003/P3307 TPU-barium sulfate/Ag

LAL-NP grafting into electrospun macroscopic polymer fiber pads core−shell transformation into hollow NPs film deposition NP−polymer composites

ZnO NP-polycaprolactone nanocomposites hollow ZnO shell

polymerization polymerization and 3D prototyping polymerization and electrospinning selective etching spin coating sol−gel encapsulation solvent drying solvent drying and 3D laser lithography ultrasonic waves

Ge submicrospheres/leaf-like WO3/treelike Zn/ZnO nanostructures GaAs crystals Si, Ag, AuNPs bioconjugated Au NPs Au NP-doped zeolite L crystals Co3O4-highly ordered pyrolytic graphite (HOPG) electrode Cu(In,Ga)Se2 (CIGS) thin film/Au film/ Si-TiO2 heterojunction/Pt electrodes

catalysis, PL, water cleaning PL PL, plasmon resonance biomedicine catalysis electrocatalysis solar cells, sensing, photovoltaics, medical implants cellular uptake, reproduction biology catalysis photocatalysis, photoelectrochemistry optical filters medical devices (e.g., catheters) biocompatibility

ref 40,521,522 520 304,526 528 254 536 263,303,529,530,532−535,563 558,559,564 135,565,566 544,545,551,553 567 372 357

PL, photocatalysis

568

InN film Au NPs into a DETA cross-linked GPTMS:TMOS matrix dilm patterning; NP decoration on Tb3+-Cu-PVA film; ZnO/fabric fabric, NP strand formation composite, FeNi-PMMA film increased photosensitivity 3D polymer structures doped with Au NPs

solar cells thermosets

537 543

conductive films, antibacterial products biomedicine

538,539,569 570

size reduction/redispersion

PL

527

silicon nanocrystals

have no effect on the products, as demonstrated by Sun and Tsuji.359 When using LML, the irradiation time is often used to control the size,279 chemical composition301 and morphology of the SMS products. Because the educt is aggregated (e.g., Ag, as shown in Figure 42f), time is needed to melt the particles and form homogeneous SMSs (Figure 42g,h).515 Adjusting the irradiation time also helps to clarify the temporal formation dynamics of SMSs in different solutions.273,274 SMSs were shown to begin to form in NaCl solution immediately after laser irradiation and subsequently grew slowly with irradiation time. In contrast, in citrate solution, SMS growth was first inhibited (defined as the induction period) and then increased rapidly because of citrate decomposition during the initial phase of laser irradiation.274 Clarifying the SMS formation mechanism in different ligand solutions is beneficial to control the LML process and ensure the desired characteristics. Furthermore, if the educt added as dissolved light absorbers is sensitive to laser irradiation, the reaction could be switched on and off by the irradiation time to form novel SMSs, such as calcium iron phosphate spheres originating from CaP LML with ferric ions (as sensitizers) under laser irradiation.301 Thus, it can be concluded that the processing time primarily influences the NP concentration productivity in batch LAL and the NP size in LFL and LML and that it sometimes imposes additional effects on NP assembly and chemical reactions with the particulate educt, which often reach either the steady state or the supersaturation-coalescence after tens of minutes (at typical milliliter batch sizes and moderate laser power). 5.2.2. Downstream Processing. Currently, downstream colloid processing methods are routinely applied to the products

polydispersity of the QDs decreased from 16.0% to 8.6% after prolonged irradiation (inset of Figure 42d). In addition to semiconductor NPs, the applicability of size and dispersity control to LFL-synthesized noncoinage-metal NPs was also recently demonstrated by changing the irradiation period.311 Because the selenium NP size is associated with the band gap energy,231 LFL irradiation time can alter the optical properties of the synthesized NPs. Additionally, the sizes of organic drug particles monotonically decrease with increasing irradiation time.517 However, growth and ripening stages may occur after certain fragmentation periods (Figure 42e), leading to size increase of the products.265 This behavior can be ascribed to the nucleation and growth of fragmented species and the subsequent Ostwald ripening of small nanocrystals into large ones.164 By recording their extinction spectra over time, Jendrzej et al. confirmed that the growth of LFL-synthesized Pt atom clusters/ NPs follows two kinetic models: a rapid barrierless model for the early stage ( n-pentane) in alkanes lead to a lower thermalization speed during LAL, and thus enable the assembly of Au particles with higher aspect ratios.696 In addition to the chain length, the functional groups of the solvent govern size quenching and stabilization. The adsorption of organic molecules via the carbonyl groups on LAL-synthesized NPs could improve the NP dispersion.260 Furthermore, enolate or alcoholate groups could form upon the laser irradiation of acetone or ethanol, and, as a result, adsorption onto the gold NPs could endow them with long-term stability.334 However, it should be noted that LAL sometimes triggers solvent pyrolysis357 or organic molecule decomposition,697,698 thereby raising the risk of organic contamination. The amounts of this byproduct are expected to be proportional to the (initially hot) NP surface area, and the resulting carbonaceous shell may also contribute to the protection of the inorganic core, as shown by Amendola et al. for LAL of Fe in toluene.198 Note that the reaction between the target materials and the solvent (reactive LSPC) may lead to the generation of novel carbides, oxides, and doped NMs, such as metastable phase Cr3C2−x;699 carbon shells and fullerene-like carbon spheres;700 (LiNbO3, Au, or Si)@C,697 (TaC, NbC, HfC, or MoC)@C,701 Pd@C,702 Ag@C,703 HfC,704 W2C@C core− shell NPs,62 and SiC nanorings;705 onion-like carbon-encapsulated cobalt carbide (Co3C) core/shell NPs;383 Si−H-doped graphene-based lamellae;609 and C−H-doped anatase nanocondensates.610 In addition, bonding of the dissociative byproducts to the surface of LAL-produced NPs706 may also occur, which is beneficial for PL emission enhancement707 or excitation wavelength-dependent multicolor emission of carbon-protected NPs.697 Reaction of liquid silica precursors and metal atoms during RLAL enables the formation of metal@silica core−shell particles.708 By comparing the phases of the nanostructures obtained after primary LAL and secondary LFL, Jung and Choi discovered that Ni/NiO phase mixtures were generated by LAL 4035


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monomer shell (ε-caprolactam),719 and monomers can be grafted and polymerized on ZnO NP during LAL of Zn,720 thus opening a novel pathway for straightforward nanocomposite production in aqueous monomer solutions. The applications of LSPC-synthesized polymer-capped NP composites have been systematically reviewed recently.105 Some efforts have focused on synthesizing NPs by LAL using vegetable oils, such as walnut oil,574 coconut oil,571 or castor oil.572 Lubricant oils are also attractive as the LAL liquid medium because the synthesized particles in the oil medium can be directly used for tribological tests.573 Attempts have also been made to synthesize nitrides in liquid N2, for example, TiN particles.577 The oxygen-free environment of liquid N2 is beneficial for obtaining pure Si particles with negligible oxidation.577 Interestingly, this technique has been extended to graphene via the exfoliation LAL of graphite in liquid N2,576,578 in which liquid nitrogen penetrates into the interlayer spacing of graphite and subsequent gasification upon laser heating causes the dissociation of graphene layers.578 5.3.2. Solid Additives. Solid additives in liquid media during LAL allow size quenching (see section 3) of NPs and one-step in situ synthesis of hybrid (adsorbate−absorbent) composites. For instance, LAL of metals in a dispersion of silica spheres directly produces supported particles (e.g., Cu/SiO2 and Al/SiO2, Au/ SiO2, Sn/SiO2, and Ag/SiO2). This process has been reported to be driven by the adhesion energy values of the respective material pairing and not by the additive size (Figure 48d,e).721

ripening/growth of some particles into large particles (up to 100 nm), resulting in nonhomogeneous particle size. When mesoporous silica materials are used as adsorbents, LALgenerated particles may block some pores of the silica materials, slightly decreasing the specific surface area and total pore volume as compared to those of the bare silica materials.309 One possible advantage of in situ LAL-synthesized supported composites (e.g., Ag−SiO2) is that their metallic adsorbates (e.g., Ag) are smaller and more uniform than those obtained via hydrothermal synthesis.309 Note that the risk of support damage should be considered. Seyedeh Zahra et al. showed that longer ablation of a Pd target in the suspension of multiwalled carbon nanotubes (MWCNT) would induce damage to MWCNT supports, resulting in reduced hydrogen storage capacity when using these Pd/MWCNTs as catalysts.405 Sai Siddhardha et al. reported that LAL of a gold target in a functionalized layered hydrogen exfoliated graphene (f-HEG) suspension led to the simultaneous deoxygenation of f-HEG and loading of AuNPs to form laser-converted, graphene-supported gold NPs.722 Partial structure destruction of the MCM-41 silica suspension has also been reported by Szegedi et al.309 Driven by the extensive applications of graphene and graphene oxide (GO), increasing attention has recently been focused on in situ LAL synthesis of layered supported composites. Ye et al. demonstrated that an oxidation−reduction reaction may occur between GO and LAL-synthesized suboxide (e.g., SnOx) colloids that are rich in electrons, surface hydroxyls, and defects.723 LAL of Mg in aqueous GO produces LAL-generated positive Mg(OH)2 nanosheets and negative GO sheets that form porous Mg(OH)2/GO nanosheet composites, which exhibit high adsorption efficiency toward water pollutants, such as methylene blue (MB) and heavy metal ions.663 During LML, codispersing carbon black has been employed by the Koshizaki group as a laser incoupling sensitizer that enables fabrication of ZrO2 SMSs.724 Guo et al. reported the possibility of using solid metallic particles (e.g., Au NPs) as the adsorbates for in situ conjugation with LAL-generated oxides to form hybrid composites (Figure 49e), such as Au−CoFe2O4 (Figure 49a−d) and Au−SrTiO3.725 The Bayberry tannin-stabilized size-tunable Au particles behave as heterogeneous nucleation sites for the LAL-generated oxides to form heterodimer hybrids with well-defined structures and compositions. Particles generated by LSPC, and especially LAL (elemental, alloy, and oxide particles), are also good candidates for the synthesis of hybrid composites. This method is termed two-step or consecutive LAL (laser ablation of another material inside the former LAL-synthesized colloidal solution) and has enabled the fabrication of doped/core−shell nanocomposites (e.g., Pt− TiO2,726 Ag−TiO2,675 Au−Si,727 Ag@ZnO,514 Al2O3@Ag,728 Al2O3@AgAu,728 and Ag@Au512). These two-step LALgenerated composites can be further used as colloidal additives for the in situ conjugation synthesis of trimetallic composites by a third LAL step. Subsequent laser irradiation can transform these trimetallic composites into multishell structures, such as Al@ Al2O3@Ag@Au and Al@[email protected] Hence, adding solids in situ to LAL or LFL may be a convenient step to create supported particles, in particular if size quenching of the NPs is intended, and modification of the dispersed solid by the laser is intended as well (or at least does not have to be totally avoided). Alternatively, solid supports may also be added downstream (section 5.2.2), not being excited by the laser. 5.3.3. Concentration. The concentration of the solvate affects LSPC sterically (macromolecules), electrosterically

Figure 48. Solid SiO2 spheres serve as adsorbates for LAL-generated NPs in hybrid composites: (a) Au−SiO2 and (b−f) Cu−SiO2 particles by LAL at different pulse energies (100 or 200 mJ), ablation periods (10, 20, or 30 min), and different sizes of SiO2 (100 or 1000 nm). Adapted with permission from ref 721. Copyright 2013 Institute of Physics.

Nonuniform Ag or Au spherical islands (Figure 48a) and irregular Sn and Al islands cover the SiO2 core, whereas uniform, smaller Cu particles (Figure 48b) cover the SiO2 core and form uniform shells after 30 min of LAL at a pulse energy of 100 mJ (Figure 48b).721 The pulse energy and ablation time, which affect the particle size and concentration, also affect the particle uniformity on the SiO2 adsorbents (Figure 48b,c and e,f).721 In general, longer ablation or irradiation periods during LAL or LFL result in higher colloid concentrations. Consequently, the collision rate between adsorbents and their adsorbates is increased, leading to more particle loadings on the adsorbents. When the ablation period is too long, for example, 1 h, the particles (e.g., Pd NPs) may exceed the maximal “occupancy” capacity of the absorbents (e.g., CeO2).369 This will cause the 4036


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trations will result in more stable colloidal solutions and smaller particle sizes,594 as discussed in section 3.2. For example, as the concentration of aqueous cyclodextrins is increased, the size of the LAL-synthesized AuNPs can decrease to 2−2.4 nm (numberweighted) and exhibit a size width of less than 1−1.5 nm.731,732 To obtain larger NPs with LML, a higher concentration of polymer is unfavorable, as demonstrated by increasing the dextran concentration from 0.01 to 1 g/L to decrease the size of Au NPs (from 80 to 3 nm) via fs laser irradiation (Figure 50a).227 In general, an optimal concentration threshold exists at which both stabilization and particle dispersion are achieved.308,336,618 However, more is not always better. First, at a certain threshold ligand concentration, the minimal size is achieved, and further increasing the concentration only decreases the conjugation efficiency without affecting the NP size.733 Furthermore, in the case of copper and silver NPs obtained by laser ablation in TPU polymer-containing THF solution, the optimal concentration is approximately 0.3 wt % TPU; above this value, the particle dispersion does not improve further, but the production rate of the NPs decreases.307 By comparing the emergence of AuIII or Au0 on the AuNP surface at different CTA+ concentrations, Fong et al. concluded that only when the surfactant CTAB fraction approximates the aqueous CMC (0.94 mM) the colloid stabilized.734 Because this threshold depends on the final concentration of NPs, such values are reported as the grafting density of ligands per NP surface area (pmol/cm2) to facilitate comparison.308 Interestingly, at lower CTAB concentrations (in anion solutions and pure water), no trace of oxidized Au could be observed by XPS.734 This finding is contradictory to the report of Merk et al., who found 5% oxidized surface atoms in Au NPs laser-synthesized in water values as high as 11% in halide anion solutions.268 Also, Mafune et al., who did work at CMC of CTA+ as well, found signs of oxidation by XPS and calculated the

Figure 49. In situ conjugation with solid additives: (a−d) Au−CoFe2O4 heterodimer hybrid composites with solid Au NP additives and LALgenerated CoFe2O4 NPs as adsorbates. (e) Schematic diagram showing the one-step in situ particle-conjugation process. Scale bars represent 5.0 nm. Adapted with permission from ref 725. Copyright 2013 Wiley.

(surfactants), and electrostatically (pH, anions), influencing colloidal stability, surface charge density, and reaction or oxidation of the ablated matter. The adsorption of anions310 or molecular ligands, such as surfactants,305 biomolecules,730 and polymers,104 will not only prevent particle growth but also inhibit their aggregation. Solutions containing higher ligand concen-

Figure 50. Solvate concentration effect: (a) Size variation of gold NPs as a function of the dextran-to-gold molar ratio after white-light fs laser irradiation determined from TEM micrographs. Adapted with permission from ref 227. Copyright 2009 American Chemical Society. (b) ζ-potentials of LALsynthesized gold NPs in aqueous SDS (○) and CTAB (●) with different concentrations. Adapted with permission from ref 664. Copyright 2009 American Chemical Society. (c−e) Influence of surfactant concentration on ZnO formation. (c) Illustration of the formation process of ZnO/Zn composite NPs with increasing SDS concentration. (d) Fourier transform IR spectra of NPs fabricated at different SDS concentrations. (e) XRD patterns of the products prepared in pure water and surfactant solutions with different SDS concentrations: 0.0001, 0.001, 0.01, 0.05, and 0.1 M (from bottom to top). Adapted with permission from ref 597. Copyright 2005 American Chemical Society. 4037


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surface oxidation by the particle’s charge.664 Note that the laser parameters, liquid flow, and target geometry of both studies were different, indicating that controlling surface charge of lasersynthesized Au NPs is based on more than just the adsorbate concentration. The matching between the ligand charge and the NP charge is another important factor that influences the stability and size of the synthesized NPs, especially for RLAL.447 Taking the ablation of a tin metal plate, for example, the neutral solution becomes acidic (pH = 4.5−5.3) after laser ablation, which may result in a positive zeta potential of the synthesized semiconductor NPs (e.g., SnO2).735 In this case, the negatively charged C12H25SO4− (SD−) ions preferably attach to the particle surface by electrostatic forces and thereby stabilize the NPs. When the SDS concentration is above its CMC (8.6 mM), the LAL-produced NMs are well-dispersed with a very narrow size distribution; however, when its concentration is below the CMC, small particles coagulate and form large particles.447 In LAL synthesis of Au NPs, because their surfaces are negatively charged in neutral water, the addition of anionic (e.g., SDS) and cationic surfactants will result in different effects on the surface charge of Au NPs. Muto et al. demonstrated that increasing the CTAB concentration gradually increases Au NP surface charge because of the adsorption of CTA+ ions, rendering the Au NP surface positively charged, whereas those synthesized in SDS remained negatively charged (Figure 50b).664 In addition to stability control, increasing the anion concentration increases the number density of the nascent charge state of Au NPs by a factor of 10−710 at the CMC, which positively affects the Coulomb explosion-driven ns LFL of gold NPs.243 Competition between surface oxidization and capping protection occurs during LAL of Zn in SDS solution,597 as shown in Figure 50c−e. Laser ablation in water gives rise to the formation of ZnO and less Zn; however, the ablation products transform as the SDS concentration changes. When the SDS is below its CMC, only wurtzite ZnO NPs are formed. As the SDS concentration increases, Zn crystals reappear and spread, and the ZnO peaks decrease. When the SDS concentration reaches 0.1 M, the ZnO crystal completely evolves into Zn(OH)2, which coexists with abundant Zn crystals, as evidenced by the FTIR and XRD spectra shown in Figure 50d,e. The schematic of the UV ns laser ablation products of Zn at different SDS concentrations597 is presented in Figure 50c. Similarly, Liu et al. synthesized various core−shell structures by mediating the copper oxidization in SDS-containing solutions, which affected the position and intensity of the copper localized surface plasmon resonance (LSPR) peaks.337 Note that Zn(OH)2 may also generate layered structures in the presence of different concentrations of SDS (wavelength: 1064 nm).629 Jung et al. reported the production of NaOH-concentration-dependent Cu-based nanoproducts (Cu, Cu2O, and CuO) with Cu and Cu2O formation occurring at lower pH values and increased oxidation to Cu2O and CuO observed at higher NaOH concentrations or pH values,736 as expected on the basis of the Nernst equation. 5.3.4. Layer Thickness. Controlling the liquid layer height above the target is crucial for maximizing the yield of LALgenerated NPs,20,178,376 and the layer thickness can be optimized to achieve maximum productivity (Figure 51d; see also section 4.3). The liquid always attenuates the laser intensity, particularly if the light is absorbed or scattered by dispersed NPs and bubbles, both of which reduce the energy absorbed by the target substrate. When the liquid layer thickness is too thin, the plasma cannot be fully confined by the liquid,403 and, as a result, the plasma or cavitation bubble breaks through the liquid−air interface, leading

Figure 51. Effect of the liquid thickness on LAL productivity and NP size. (a) Liquid splashing and fume emission from the liquid when the liquid thickness is too thin (regions A). (b) Optimal liquid level for maximized LAL and minimized volume (V) of LFL (regions B). (c) Beam attenuation when using a high liquid thickness, resulting in lower productivity but intensified LFL and producing smaller NPs (regions C). (d) Al2O3 NP production in distilled water conducted under various water heights. A laser pulse energy of 3.8 mJ at a 5-kHz repetition rate and 120 mm/s scan speed were employed. Adapted with permission from ref 20. Copyright 2010 American Chemical Society. (e) Ge NP mean diameter versus water layer thickness at a pulse energy of 60 mJ/ pulse. Adapted with permission from ref 376. Copyright 2011 Springer. (f) Dependence of the sound amplitude (proportional to productivity) on the liquid layer thickness during excimer LAL of Si. Adapted with permission from ref 403. Copyright 2001 American Institute of Physics.

to splashing and the emission of fumes or mist into the workplace (Figure 51a). Yet the liquid layer does not only affect the productivity. A thick liquid layer during LAL will increase the 4038


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Figure 52. Liquid handling configuration for LAL and LFL. Laser ablation in (a) batch processing, (b) semibatch processing, and (c) flow chamber configurations and the corresponding NP concentrations as a function of time (t) and liquid depth (z). (d) Configuration of laser fragmentation and melting in flow jet and the corresponding NP concentrations as a function of time and liquid flow axis. (e) Laser fragmentation in batch processing. (f) Example experimental schemes. From left to right: Reprinted with permission from ref 739. Copyright 2015 American Chemical Society. Reprinted with permission from ref 740. Copyright 2015 Royal Society of Chemistry. Reprinted with permission from ref 363. Copyright 2014 American Chemical Society. Adapted with permission from ref 310. Copyright 2013 Royal Society of Chemistry. Adapted with permission from ref 1027. Copyright 2017 Nature Publishing Group.

Figure 51f shows that the ablation rate correlates with the sound amplitude. A specific sound frequency (10 kHz) far higher than the repetition rate of the laser (not specified in ref 403, although it is noted that the laser LPX 100 has a maximum frequency of 100 Hz) was found to be optimal for correlation of LAL productivity of Si at 248 nm, 30 ns, and 31 J/cm2.403 5.3.5. Flow. Liquid batch and liquid flow processing are two different strategies used for LSPC, especially LAL and LFL, and have several fundamental differences regarding process control, yield, and byproducts.106 In batch LAL (Figure 52a), the incident light is scattered by both the bubbles rising upward and those adhering to the surface.400 This effect can be minimized in batch reactors using horizontal LAL (e.g., cuvette LAL with the target on the side). However, in any batch case, the newly formed NPs will accumulate in the liquid and consume some of the laser energy arriving at the target surface, leading to NP saturation and decreased productivity.401 Additionally, the colloidal concentration c changes temporally and spatially according to the trends (∼t1/A, A ≥ 1; ∼1/z3, t is the ablation/processing time, A is a constant, z is the distance away from the laser ablation spot) marked in Figure 52a−c. In batch processing, the ablation reproducibility is weak because both the effective fluence and the ablation rate change with time. Therefore, NP and bubble removal by liquid draining are essential for controlling LSPC. Resano-Garcia et al. reported that the yield of LAL-synthesized AgNPs in stirred liquid was approximately 30% higher than that

portion of LFL effects that occurs within the excitation volume fraction above the target (V in Figure 51b,c). Thus, increasing the liquid thickness may indirectly decrease the particle diameter and size distribution.178,737 Accordingly, interpretations of the effects of the liquid level reported in the literature should consider this effect, particularly for high-repetition rate or ultrashort-pulsed lasers (which are efficient for LFL). Another side effect is triggered if the vessel diameter is constant, which causes the liquid volume to increase with the liquid height: For example, less concentrated seeds ripen into smaller germanium particles.376 Thus, an increased liquid level may lead to opposing effects on the particle size, depending on the sensitivity to the LFL (e.g., Au, Ag) and supersaturation ripening (e.g., Ge) conditions (Figure 51e). Overall, liquid layer minimization is a simple, cost-effective method to increase the NP yield.106 Upper and lower limits of the optimal liquid thickness for ablation productivity exist. Generally, a minimal layer is required to avoid instability from the cavitation splashing of ablated matter through the liquid−air boundary, and lower pulse energies cause smaller cavitation bubbles, allowing the use of even thinner liquid. If LFL parallel to LAL is desired (at the expense of productivity) to minimize particle size, a high liquid level may be a simple method. In this case, reproducibility will be an issue because this method is not stable, and decoupling LAL by second-step LFL may be an alternative providing better control. Adjusting the optimal liquid layer thickness for high ablation rates can be facilitated using an external microphone. 4039


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Figure 53. Impact of liquid flow on the NP size distribution, NP productivity, and biomolecule degradation. (a,b) Size distribution of Ag NPs obtained by LAL in flowing and stationary liquid, respectively, and the corresponding TEM images of the NP morphologies. Reprinted with permission from ref 401. Copyright 2015 American Institute of Physics. (c) Relationship of the volumetric flow rate with the productivity and outflow concentration of CuLAL at 500 W, 10.1 MHz, 3 ps, and a scanning speed of 484 m s−1. Reprinted with permission from ref 31. Copyright 2016 Institute of Physics. (d) Biomolecule degradation during fs LAL of Au in aqueous oligonucleotide solution as a function of the liquid flow rate with increasing laser pulse energies. Reprinted with permission from ref 402. Copyright 2010 Springer.

pumped liquid may induce fluctuations of the liquid surface during LAL, which may refract or reflect the laser light in different directions and change the beam profile. Thus, liquid flow chambers should include an entrance window for the laser beam on top of the liquid or comprise a vertical chamber when a horizontal beam is used. The same is true for LFL or LML, in which the cuvette or vial arrangement usually suffers from mixing of the particulate educt with the product (Figure 52e). Liquid flow diminishes these side effects, particularly if the whole flow channel diameter is irradiated (Figure 52d). In fully irradiated flowing liquid channels, unirradiated solid educt is avoidable, and the transmitted laser energy may be measured to determine the energy-specific process efficiency parameters. These parameters help to provide energy-specific mechanistic insights and to identify the scale-up parameters for LFL218 and LML (see section 4.3).267 Because of the constant downstream conditions, flowing liquid may also allow well-controlled ripening, conjugation, and size quenching, and thus the characterization of LFL and LML intermediates.218,267 In general, the batch processing of both LFL and LAL causes nonstationary laser excitation conditions, which may compromise the quality of the colloid. Semibatch processing (and stirring) may help to avoid particle sintering (by fluence fluctuation), as shown for LAL of Ag in water.401 Overall, flowing liquid is superior to stationary liquid for laser-based colloid generation96 for three reasons: (i) stationary synthesis conditions without particle melting or fragmentation occurring in the beam path, (ii) bubble removal, and (iii) stationary conditions of continuously drained colloidal particles for downstream processing. These factors perfectly match with the continuous particle emission source character of LSPC, which would improve both the processing reproducibility and the yield for all LSPC methods. 5.3.6. Viscosity, Temperature, Pressure, and Supercritical Fluids. The liquid viscosity determines the solvent mass

in unstirred liquid, that the ablation reproducibility was also significantly improved (over four repeated experiments), and that the NP size distribution was narrower (Figure 53a,b).401 Similar findings were reported by researchers using a semibatch apparatus equipped with a magnetic stirrer380,738 (Figure 52b) and single-pass flowing liquid experimental configurations310 (Figure 52c). The removal of NPs by flowing liquid is effective and inhibits bubble settlement in the vicinity of the ablation grooves,400 thereby improving the ablation reproducibility and dramatically increasing the NP productivity (by 380% for ns LAL of alumina and 700% for fs LAL of gold) relative to the stationary liquid by adjusting the flow rate.20,402 Note that the NP productivity increases linearly with the flow rate at the expense of the outflow concentration (Figure 53c), as was recently demonstrated by Streubel et al. using a single-pass flow configuration.31 Additionally, the degradation of biomolecules during LAL of gold NP in an oligonucleotide solution was significantly reduced when the liquid was drained from the processing zone, thus minimizing the unintended post irradiation of the Au NP bioconjugates (Figure 53d).402 In a semibatch chamber (Figure 52b), because the colloids remain within the constant volume even though they are locally dispersed just after their formation, the NP concentration will gradually increase, and the colloid shield effect will be further enhanced, diminishing the laser efficiency. That is, semibatch or stirred processing constitutes a compromise that exhibits temporal changes in the NP concentration, bubble removal, and a technically simple colloid concentration method. When flowing liquid is used, because the inflow is fresh, colloid saturation will not occur, and the NP concentration will remain constant at all times (t) and all positions z (stationary fluid condition), as shown in Figure 52c. However, the concentration may be lower, and either a highpower laser or subsequent evaporation of the colloidal solution is required to increase the outflow concentration. Note that the 4040


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Figure 54. Influence of the liquid viscosity on the particle morphology and cavitation bubbles: (a) Au NP chain aspect ratio and position of the highwavelength plasmon peak as a function of the hydrocarbon chain length (liquid viscosity) during LAL. Reprinted with permission from ref 696. Copyright 2003 American Institute of Physics. (b,c) TEM images of gold and platinum nanowires deposited on a carbon-coated copper grid after superfluid LAL. (d) Schematic of nanowire formation mechanism from the quantized vortices during ablation in superfluid helium. Reprinted with permission from ref 585. Copyright 2014 American Chemical Society. (e) Inner bubble and outer, partly transparent shell acquired during LAL CO2 near the critical point (T = 302.0 K, P = 7.30 MPa). Adapted with permission from ref 748. Copyright 2013 American Institute of Physics. (f,g) Particle populations generated during LAL in supercritical CHF3 with (f) dominant morphologies at reduced densities of 0.8 and (g) 0.2. Reprinted with permission from ref 34. Copyright 2012 American Chemical Society.

flow, the diffusion constant of the particle nuclei, and their collision rate to form larger particles and aggregates. According to the LSW theory,741 the final particle size is related to the viscosity through the reaction constant k = CT/η for diffusionlimited reactions: R3 = R03 + CTt/η, where T is the liquid temperature, η is the liquid viscosity, C is a constant, t is the growth time, and R0 is the initial radius. Thus, theoretically, a liquid with a high viscosity should generate smaller particles during LSPC or reduce the particles’ aggregation. LSW theory was applied to the ripening (growth and aggregation) kinetics of Cu, Ag, and Au NPs fabricated by fs LAL and revealed two-step diffusion-controlled ripening in ethanol (1.04 mPa s) and ethyl acetate (0.43 mPa s).360 For gold and silver, both kinetic steps were quicker in the less viscous liquid, whereas the opposite was observed for copper.360 During LFL of Pt and Au in water, the initial LSW kinetics were also observed, followed by Ostwald ripening, and the kinetics were accelerated by increasing the temperature, which decreased the viscosity and increased the diffusion constant.164 Several reports have addressed the influence of viscosity on LAL. For example, the Ag NPs generated in propanol (1.94 mPa s) are smaller than those obtained in H2O (0.89 mPa s).607 Liquid viscosity also affects cavitation bubble dynamics, and the Chrisey group attributed the formation of larger hollow spheres in ethanol−water mixtures than in water to the generation of larger cavitation bubbles with longer lifetimes and less damped bubble oscillation. 55

Compagnini et al. described the relationship between the hydrocarbon chain of alkanes and the asymmetry of plasmonic aggregates after LAL of Au696 and observed that the aspect ratio increased from 4 to 6.5 (extracted from the longitudinal plasmon band) as the hydrocarbons became lighter (C10 to C5). They hypothesized that either higher thermalization (at lower liquid mass) or lower viscosity favors NP chain formation (Figure 54a).696 Indeed, the viscosity changes by a factor of 4 from pentane to decane. Notably, increasing the liquid viscosity may decrease the ablation productivity by reducing the solution transport, thus hampering the removal of scattering bubbles or particles.720 The liquid viscosity is known to decrease as the temperature increases. Therefore, increasing the temperature may increase the hydrodynamic diameter of LAL−AuNPs by raising the collision probability of the particles.134 Similarly, increasing the liquid temperature to 70 °C promotes the formation of Au NPs and Ag NP nanochains and nanonetworks.742 For ZnO in water, dissolution effects may trigger crystalline growth as the liquid temperature is increased. For instance, ZnO particles grow into columnar structures743 or nanoflakes744 when the water temperature is above 60 °C. To date, motivated by “nonviscosity” and unusual mass transport properties, LAL has been conducted in various superfluids, including trifluoromethane,34 helium,583,585,587 xenon,745 and carbon dioxide,66,581,746 and some unique features of using superfluids as LAL liquid media have been identified. 4041


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fluid density and temperature. Gold nanonecklaces and large nanospheres are produced at lower and higher densities of supercritical CO2, respectively.66 Moreover, LAL in supercritical CO2 triggers the formation of sp3-hybridized materials because of the selective dissociation of C−H bonds, such as diamantane and higher-order diamondoids (M > 12).581 For pressurized LFL, the size reduction ability is very different between atmospheric and high pressures.87 At ambient pressure with a spinodal temperature of ∼573 K, explosive evaporation will create a layer of superheated water in the vicinity of Au NPs.87 Subsequent bubble formation reduces the stability of fragments and causes them to aggregate because of insulated heat transfer (Figure 55a,b). In contrast, at high pressures (e.g., p > 22 MPa), the

Time-resolved shadowgraphy revealed that superfluid helium could give rise to different cavitation and cooling processes that include two separate phase transitions (gas to normal liquid and normal liquid to superfluid) because of the high thermal conductivity of superfluids and their photoinduced breakdown.584 Approximately 20% of the laser pulse energy could be directly delivered to the superfluid, thereby leading to the formation of a large cavitation bubble of up to several millimeters in diameter depending on the laser pulse energy.584 Quasi-1D quantized vortices (Figure 54d) produced during LAL in superfluid He dominate particle growth, resulting in thin and long nanowires with regular internal structures585 (Figure 54b,c). These synthesized nanostructures, which exhibited improved conductivity, were determined to be suitable for use as superconducting materials583 because an electrical interconnection exists between the nanowires in the webs,585 and they exhibit good stability at room temperature.586 In liquid He, Bulena et al. recently concluded that most of the ejected nanometer- and micrometer-sized metal particles originate directly from the primary ablation event (and not the collapse) because the typically observed tightly focused cavity collapse events are absent in superfluid helium.747 LAL in He can help one to study the mechanism of the primary ablation process and the noncollapse ejection of nanoparticles into the liquid. Remarkably, Saitow et al. reported that two particle populations are formed during LAL in supercritical CO2 and CHF3.34,66 They discovered that the larger population, which has an average diameter of 400 nm (Figure 54g), is dominant at high pressures (stabilizing large Au droplets because of the higher liquid density) and acts as a precursor for the nanonetworks (20 nm primary particles assembled into micrometer-long networks) (Figure 54f). This fragmentation of the large fraction was found to be governed by the permittivity of the supercritical fluid rather than by its thermal properties, as found by comparing nonpolar CO2 and bipolar CHF3.34 Varying the laser ablation/irradiation process induced by external pressure is a novel area in which interesting mechanistic insights are emerging. Interestingly, together with temperature modulation, the ambient environment can be tailored continuously from gas-like to liquid-like by varying the ambient pressure, with the gas−liquid critical point acting as the supercritical fluid threshold.749 The maximum volume of a cavitation bubble produced during laser ablation can be fitted with the empirical formula Vmax ∝ Pext−1, where Pext is the applied pressure;182 thus, the dynamics of cavitation bubbles respond strongly to external pressure. For example, increasing the liquid pressure from atmospheric pressure to 3 MPa dramatically shortens the cavitation bubble lifetime from 185 to ∼11 μs.182 Additionally, near the critical point of liquid CO2 (T = 302 K, P = 7.30 MPa), cavitation bubbles are capped by partially transparent shells via solvent gasification748 (Figure 54e), and the bubble size is maximized at the pressure (8.8 MPa) associated with the maximal density fluctuation.750 Furthermore, high pressures (up to the supercritical state) also facilitate controlling the atomic emission density and the degree of hydrolysis.579 Moreover, because both the bubble lifetime and size and the physical properties of the environment can be varied by changing the pressure, the cooling rate of the NPs and their coalescence rate can also be altered, directly affecting the particle size, phase, and assembly, as revealed by time-resolved spectroscopy.751 For example, silicon clusters with different electronic structures749 and various near-UV, violet, blue, green, and red PL emissions752 have been selectively generated by changing the supercritical

Figure 55. Effect of the liquid pressure on LFL: (a) Schematic illustration of ns laser-induced size reduction of Au NPs in water at atmospheric pressure and above the critical pressure of 22.1 MPa and (b,c) the resultant AuNP morphology. Reprinted with permission from ref 87. Copyright 2012 American Chemical Society.

formation of a SCL layer around the gold particles allows precise control of particle evaporation. However, a higher laser fluence is necessary at higher ambient pressures to achieve the same LFL efficiency as at lower pressures.88 At the expense of slightly lower efficiency, using higher pressures provides a good opportunity to precisely control the particle size (Figure 55) and elucidate the LFL mechanisms.87 Overall, the SCL state plays an important role during LAL and LFL, not only during ablation in SCL medium, but also an SCLlayer is created in the vicinity of the ablation and fragmentation space in normal liquid, anyway. 5.4. Laser Parameters

5.4.1. Pulse Duration. The pulse duration, which is also known as the pulse width, determines both the degree of selffocusing during laser propagation in liquid394 (Figure 56a) and the efficiency of LAL, LFL, and LML.269,286,357,400 Ns lasers do not suffer from severe self-focusing when typical LSPC parameters are employed, and the laser focus is always located at longer focal distance than in air because of liquid refraction (Figure 56b).394 In contrast, ultrafast (ps and fs) lasers are subject to a stronger self-focusing effect (Figure 56c), and, as a result, the focus position is elevated above the ns-LAL focus 4042


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Figure 56. Self-focusing and refraction at different pulse durations. (a) Nonlinear effects: Calculated caustic of a laser beam with different pulse durations focused in water. (b) Linear effects: Variation of the focus position caused by ns laser beam refraction in a liquid. (c) Sketch of the focal displacement caused by the combined refraction and self-focusing effect during LAL. Reprinted with permission from ref 394. Copyright 2011 American Chemical Society. Adapted with permission from ref 395. Copyright 2009 Institute of Physics.

Figure 57. Effect of the pulse duration on the ablation threshold, ablation mechanism, NP productivity, and photoionization. (a) Threshold laser fluence for the ablation of gold targets versus the laser pulse duration. Reprinted with permission from ref 762. Copyright 2002 American Institute of Physics. (b) Thermodynamic trajectories under fs (dashed-dotted line), ps (dotted line), and ns (thick solid line) laser irradiation. Thin solid line, binodal; dashed line, spinodal; cross, critical point. L, liquid; G, gas. Reprinted with permission from ref 759. Copyright 2003 American Physical Society. (c) Intensity of the absorption spectra (productivity) of colloidal gold as a function of the pulse energy for pulse durations of 1 ps and 150 fs. Reprinted with permission from ref 386. Copyright 2012 Institute of Physics.

point, especially for fs lasers.395 Hammer et al. demonstrated that the energy threshold of laser-induced breakdown in water for ns lasers is higher (∼10−4 J for 5 ns) than that of ultrashort lasers (∼10−6 J for 3 ps to 300 fs),753 whereas Ganeev et al. reported that slow thermal-induced self-defocusing processes occur as well when the pulse duration is on the ns scale.754 Both studies indicated that different energy attenuation mechanisms arise for different pulse durations during LAL. The effects of the pulse duration are dependent on the electron cooling time (electron−phonon coupling constant) of the material. For fs lasers, the pulse duration is shorter than the electron cooling time; thus, the electron−lattice (phonon) coupling is negligible, and the ablation process can be considered as a solid−vapor transition.328 Fs laser−matter interactions are common when the intensity exceeds 1013 W/cm2, at which point any dielectric can be fully ionized in the interaction zone. Kabashin and Meunier experimentally confirmed that the thermal-free mechanism dominates the fs laser ablation of gold in water, leading to the formation of small (3−10 nm) gold colloids at low fluence.194 However, when the laser fluence is high (e.g., F > 400 J/cm2), plasma-induced heating and ablation are triggered and result in a bimodal particle size distribution. The ablation process associated with ns pulses is believed to be a thermal ablation process involving laser heating and laser melting. Ps lasers lie between fs lasers and ns lasers, and when

they are used, thermal effects or a solid−vapor transition may occur depending on the pulse duration. The threshold is closely related to the electron−phonon relaxation time, which is particlesize dependent and is typically on the order of several ps (e.g., Au, 3−4 ps;755 Cu, 1−4 ps;756 Si, 0.35 ps;757 ZnO, 0.5 ps,758 with values depending on experimental setup and mathematical function). Lorazo et al. identified the thermodynamic pathways of the ablated material for short and ultrashort pulsed laser ablation using a combined MD and Monte Carlo approach (Figure 57b).759 The same trend was also reported by Lewis and Perez using an MD model.760,761 According to their simulations, fs laser ablation induces isochoric heating followed by fluencedependent mechanisms, including spallation, phase explosion, fragmentation, and vaporization plasma ablation.100 Extending the pulse duration to the ns regime leads to a situation in which the materials are heated along the solid−vapor and liquid−vapor coexistence lines (binodal). The pulse duration also affects the ablation thresholds of metallic and dielectric materials. Preuss et al. demonstrated a 2 orders of magnitude reduction of the ablation threshold for nickel films by decreasing the pulse duration from 14 ns to 0.5 ps.763 Link et al. discovered that the threshold for the complete melting of gold nanorods into nanodots for 100 fs pulses was 100 times lower than that for 7 ns laser pulses.286 Gamaly et al. calculated ablation thresholds and ablation rates for metals and 4043


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Figure 58. Effect of the pulse duration on LPC-induced cavitation bubbles. (a,b) Schematic comparison of thermo-mediated and plasma-mediated nanocavitation mechanisms. Adapted with permission from ref 765. Copyright 2014 American Chemical Society. (c) Simulated temporal evolution of electron temperature Te (dashed red curve), lattice temperature TL (solid black curve), and maximum water temperature Tm at the NP−water interface (dashed−dotted blue curve) for a 55 nm-diameter gold sphere absorbing a 150 fs laser pulse (full-width at half-maximum [fwhm] of the Gaussian time profile) at 400 nm and a laser fluence of 10 mJ cm−2 (Pmax = 6.3 × 1010 W cm−2). Horizontal lines represent the temperatures corresponding to Tce, electron temperature; Tbp, boiling point of bulk gold (∼3100 K); Tmp, melting point of bulk gold (1336 K); Tcp, critical point of water (647 K); and Tcav, cavitation threshold (573 K). Adapted with permission from ref 221. Copyright 2012 Elsevier.

Figure 59. Comparison of fs lasers and ns lasers for biomedical applications. (a) Maximum temperature increase of (*, Te) electrons, (●, Tl) lattice, and (■, Tm) water at the Au/water interface for different fluence levels. (b) Maximum temperature increases on the surface of Au nanoshells in water as a function of the fluence of a 5 ns laser pulse at 800 nm. Adapted with permission from ref 770. Copyright 2015 Elsevier. (c,d) Particle size distributions of laser-fragmented naproxen obtained by fs and ns laser irradiation. Adapted with permission from ref 480. Copyright 2014 Springer.

dielectrics that agreed with the experimental results,762 as shown in Figure 57a. Clearly, the laser ablation fluence thresholds of the target for ps and fs lasers are similar and are well below the value for ns laser ablation. Additionally, the ablation threshold increases as the pulse duration increases from the ps toward ns domains, with the tipping point at ∼100 ps. Thus, a ps pulse duration may be a good compromise between high ablation efficiency and moderate system investment costs. The ablation thresholds of metal and dielectric materials for ultrashort laser pulses are calculated according to eqs 1 and 2, and the ablation threshold for long pulses is described by eq 3

Fthm ≡

λn 3 (εb + εesc) e 8 2π

(1)

Fthd =

ln 3 (εb + Ji ) s e 4 A

(2)

Fth ≈

(κt p)1/2 εbna A

(3)

where Ji is the ionization potential, εe is the electron kinetic energy, εesc is the work function, tp is the pulse duration, ne is the number density of free electrons, na is the number density of 4044


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atoms in the target, ls is the skin-depth, and A is the absorption coefficient. A low ablation threshold and a higher laser fluence substantially increase (3 orders of magnitude larger762) the ablation efficiency of fs laser ablation relative to that of ns laser ablation.357 The quantitative pulse duration effect in the ablation efficiency observed experimentally is lower in liquid than in air but still pronounced.386,400 Riabinina et al. reported that LAL productivity using a 1 ps laser is 20−30 times higher than that obtained with a 150 fs laser (Figure 57c), as indicated by the absorption intensity of the colloid.386 By investigating the NP concentration (deduced from the absorption intensity) with pulse durations from 40 fs to 200 ps, they also found that a pulse duration of 2 ps is optimal for maximizing the Au NP productivity; below this pulse duration, photoionization attenuates the beam at 5 mJ (Figure 31f). Saraeva et al. observed a minimum Au NP productivity at approximately the same pulse duration value,764 which coincides with the electron−phonon coupling time of Au. However, they used a 500-kHz laser system causing LFL of the LAL-generated particles.764 These two opposite pulse-duration dependencies indicate that high LFL efficiency decreases LAL efficiency at high repetition rates. Because of the higher ablation efficiency of fs lasers as compared to ps and ns lasers, the efficiency of fs LSPC has also attracted substantial attention and was reviewed by Tan et al. in 2013.100 Recently, Meunier and co-workers reported that plasma-mediated nanobubbles (Figure 58b) could be generated from plasmonic Au NPs using a fs laser with a considerable discrepancy between the ps−ns laser thermo-mediated nanobubbles (Figure 58a).765,766 The dimensions of the plasmamediated nanobubbles depend on the pulse polarization, and linear polarized pulses caused larger bubbles than circular ones. Hashimoto et al. theoretically demonstrated the electron relaxation dynamics of a Au NP as a function of time after 150 fs laser pulse irradiation using the two-temperature model and concluded that photothermal bubble formation occurs over several tens to several hundreds of ps upon laser excitation and is associated with heat dissipation from the phonon of the Au NPs to the liquid medium (Figure 58c).221 Note that bubble formation will shield the pulse energy, leading to reduced laser transmittance; this phenomenon is termed optical limiting (see section 6.2.4).603,767 A laser fluence of 10 J/cm2 was found to induce the optical limiting effect for a 3 ns laser at wavelengths of 532 and 1064 nm during the laser irradiation of LAL-generated Au, Ag, and AuAg NPs.768 Also, optical limiting in lasergenerated colloids was observed for laser fluences 0, blue line) magnetic focusing. MG is a cationic molecule which can interact with negatively charged NPs. Reprinted with permission from ref 67. Copyright 2015 Springer. (c,d) Thermal imaging of an Au NP aqueous solution in a quartz cuvette irradiated with a CW-laser from the visible to NIR region. Thermal imaging of polymer−AuNP nanocomposites before (left) and after (right) 785 nm laser irradiation. Adapted with permission from ref 249. Copyright 2015 Royal Society of Chemistry.

Figure 76. NP cytotoxicity: (a) Oocyte maturation (embryo development) exposed to different LAL-synthesized NPs showing how alloying may render Ag NPs safe. Reprinted with permission from ref 424. Copyright 2014 Royal Society of Chemistry. TEM micrographs of spermatozoa after coincubation with (b) ligand-free NPs and (c) ligand-covered NPs, (d) sperm motility, and (e) embryo development with ligand-free AuNP (violet bars) and oligonucleotide-conjugated AuNP (blue bars). Adapted with permission from ref 915. Copyright 2014 Informa UK Ltd.

understand the protein−particle interactions902 and to detect trace amounts of organic pollutants.903 It is found that the SERS performance can be further enhanced with magnetic focusing using LSPC-synthesized magneto-plasmonic alloy AgFe NPs (Figure 75a).67 Similarly, magnetophoretic amplification of plasmonic properties of AgFe NPs can further enhance

photothermal heating (Figure 75b).67 Note that the thermal heating effect of LSPC-synthesized NPs is dependent on the surrounding medium. Embedment of plasmonic Au NPs in polymers (PVA, PS, PMMA) enables one to achieve higher temperature increments upon laser irradiation than directly using 4057


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processes often result in particle aggregation914 and are timeconsuming. Rath and co-workers found that LAL-synthesized Ag NP coincubation in BSA-containing media has no adverse effect on sperm cell motility, sperm cell membrane integrity, and sperm cell morphology, but shows strong toxicity toward oocyte maturation rates (Figure 76a) and causes a slight decrease of blastocyst development rates.346,424 Therefore, mammalian embryo development is vulnerable to Ag NP but not to Au NP exposure.346,424 Hence, one possible solution to reduce detrimental reprotoxicic effects is to alloy Ag NPs with Au, creating AuAg NPs whose toxicity is negligible as compared to pure Ag NPs when the silver molar fraction is 50% or less.346,424 The reduction of silver-related cytotoxic effect using AgAu nanoalloys is also accompanied by a decreased antibacterial activity (see section 6.4.3).716 For fundamental toxicity studies, in particular, those addressing mechanisms of nanobioresponse, LAL provides ideal reference materials.11,12 First, NP alloy series and mixtures are easily synthesized, unraveling stoichiometrically disproportional effects.11,424,716 Second, ligand effects can clearly be distinguished from effects of the inorganic core (Figure 76b− d),11,915−917 contributing to mechanistic understanding and providing testing material closer to realistic exposure scenarios. For example, using sperm cells as a very sensitive toxicity model, unable to undergo NP uptake, it has been shown that only ligandfree NPs compromise their biofunctionality, whereas sterically (BSA or oligonucleotide) stabilized NPs did not compromise.346,915 Mimicking unintended exposure like implant debris requires a synthesis method that delivers ligand-free colloids, ideally represented by LSPC. These mechanistic insights could serve to render NPs safer (nanosafety-by-design), for example, alloying Ag NPs popular for high plasmonic activity (e.g., in SERS) with gold, creating less toxic AgAu NPs made by LAL.346 Mechanistic studies on toxicological effects caused by NP’s ion release greatly benefit from the availability of ligand-free reference materials. Mimicking ion release from Nitinol (widely used for stent application) for implant debris toxicity studies showed that the ligands cysteine (an amino acid present in serum proteins) and citrate significantly affected the Ni ion release from Ni and NiTi alloy nanoparticles. Interestingly, although cysteine enhanced more Ni ion release from Ni, NiFe and NiTi alloy NPs than citrate did, the metabolic activity of endothelial and smooth muscle cells in cysteine solution is almost the same as in citrate solution which is attributed to the cysteine’s ability to complex heavy metal ions.918 6.4.2. Bioconjugates. Noble metal NPs with a low degree of surface oxidation (Table 4) present hydrophobic adsorbent surfaces with soft Lewis acid character, prone to facile conjugation with soft Lewis base functional groups. Hence, laser ablation of a noble metal in a liquid medium containing biomolecules allows one-step in situ conjugation of any biomolecule bearing an electron-donating function (e.g., thiols/cysteine, disulfides) with the synthesized NPs without any limitation in the biomolecule type.361,558,730,919 Until now, a large variety of bioconjugates have been synthesized by in situ LAL, including nucleic acids (e.g., oligonucleotides308,561,919), aptamers,361 proteins (e.g., immunoglobulin E,558 Staphylococcus aureus protein,848 S-ovalbumin730), biopolymers (e.g., chitosan351,352,715,920), and peptides (e.g., penetratin,733 nuclear localization sequence-NLS,363,917 sweet arrow peptide-SAP303). The conjugation process is very rapid; for example, synthesizing 20 μg of prostate cancer targeting aptamer oligonucleotidefunctionalized gold NPs only takes less than 1 min at optimal

colloidal plasmonic NPs (Figure 75c,d) due to a lesser heat dissipation rate.249 For a multimodal application, LAL-generated (negatively charged) plasmonic (e.g., Au) NPs can also be mixed with (positively charged) magnetic NPs to assemble aggregates with both superparamagnetic and SERS properties for imagingassisted cell sorting830 or can be melted into a semiconductor (e.g., ZnO) SMSs to tune the material’s bandgap.267 Lysyakova et al. designed and successfully synthesized a type of novel plasmonic conjugate with a light-controlled plasmonic response by neutralizing or overcompensating the AuNP surface charge with an azobenzene-modified cationic surfactant.906 Note that achieving positive charge always requires crossing the point of zero charge through an intermediate state because the original charge of naked NP is negative in the case of noble metals.363 There are several clear advantages of LSPC-synthesized plasmonic NPs. First, if a particle surface is covered by ligands, the refractive index of the particle surface increases and will cause a bathochromic shift of the SPR peak,421 hence LSPCsynthesized alloy NPs typically have lower SPR wavelengths than those of chemically and biologically synthesized NP counterparts (Figure 38d).125 Second, given that the SPR wavelength can be linearly tuned simply by adjusting the molar ratio of two materials in the alloy LAL-target and thereby the alloy NPs,125 it is expected that alloy NPs allow broadband optical control (e.g., OL) of the plasmonics-related behaviors. For example, Dengler et al. showed that the OL properties of Ag or Au NPs could be tuned by alloying them.768 Moreover, Dallaire et al. demonstrated a higher stability of LSPCsynthesized alloy AgAu NPs over both LSPC-synthesized and chemically synthesized elemental Au NPs after oligonucleotide hybridization, making them promising for nucleic acid sensing.907 Hence, LAL has manifested its strength to “conveniently” fabricate binary and ternary alloys. It can be concluded that LAL is particularly advantageous for producing plasmonically or catalytically active alloy NPs that are still a challenge to synthesize by conventional methods. Today, advanced LSPC gives access to a defined alloy NP crystal structure and allows the high-throughput synthesis of molar fraction series of nanoalloys. 6.4. Biology

6.4.1. Toxicity. Biological applications of laser-generated colloidal NMs and NPs are often motivated by their purity or facile nanointegration capability. Purity is of particular importance for fundamental studies of the nanobioresponse,346 in particular mimicking unintended exposure to engineered NPs. It is known that stabilizers such as citrate,908 which is generally believed to be biocompatible as well as cytotoxic CTAB,909 interfere with toxicological assays.910 Uboldi et al. have shown that initial citrate surface coverage after chemical reduction synthesis of NPs negatively affected cell viability and significantly reduced cell proliferation. In their study, they further showed that the biocompatibility could be partly improved by citrate removal.8 Accordingly, a ligand-covered surface makes the evaluation of the NP toxicity very complicated. With ligands on the surface, even those kind of ligands (like citrate) that are not toxic at equivalent bulk solution concentration, it is impossible to distinguish between “real” nanotox effects and cross-effects caused by the nanoparticle’s surface adsorbates.12 Surface ligands can be removed by centrifugation911 or diafiltration912 or tangential-flow filtration.913 However, these 4058


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Figure 77. High grafting densities: (a) Regimes of AuNP surface coverages obtained by in situ and ex situ conjugation during laser ablation as compared to literature values. Reprinted with permission from ref 561. Copyright 2009 American Chemical Society. (b) Biomolecule grafting density of lasergenerated Au NPs as compared to chemically prepared Au NPs. Reprinted with permission from ref 923. Copyright 2013 AT-Fachverlag.

Figure 78. In situ and ex situ conjugation effect: (a,b) Size quenching effect of Si NPs during in situ conjugation with SSO-biomolecules. Reprinted with permission from ref 847. Copyright 2012 Royal Society of Chemistry. (b) Cell imaging using in situ LAL-synthesized Staphylococcus aureus protein Aconjugated Si nanoparticles. Adapted with permission from ref 848. Copyright 2013 Institute of Physics. Ex situ conjugation effect: (c) Scheme of ex situ conjugation and (d) colloidal stability of laser-generated and chemically prepared gold nanoparticles during surface modification with thiolated PEG. Reprinted with permission from ref 341. Copyright 2011 American Chemical Society.

balancing the charges of the net-negative noble metal NP and the net-negative (e.g., oligonucleotides) or net-positive (e.g., cationic peptides) ligands is the key to controlling ligand grafting density and colloidal stability after LAL.344,363 As LAL delivers naked NPs and bioconjugation starts from scratch, the effective ligand dose is exactly known during ex situ conjugation, without any trade-off for kinetic or thermodynamic barriers that typically have to be overcome after chemical citrate-based synthesis based on ligand exchange. In detail, the uniqueness of having ligand-free Au NP or Pt NP building blocks allows one to freely “titrate” different types of biomolecules sequentially on the NP surface.344 In situ conjugation will yield dense monolayers, whereas ex situ conjugation allows, for example, one to add a submonolayer dose of biomolecule A and subsequently saturate the surface (by the way improving colloidal stability) with biomolecule B. Hence, these naked building blocks give convenient access to bivalent

laser and process parameters, sufficient to validate selectivity of prostate cancer tissue targeting.361 Despite some reports showing the degradation of biomolecules during LAL402 or LFL,921 in most cases, no significant degradation of the molecules is observed during in situ bioconjugation at an optimized condition,308,730 which is promising for their further functionality in biological applications. Degradation can be minimized by liquid flow during LAL402 or tuning the pH during LFL.921 In general, conjugation efficiency is molecule-dependent. Because of the coiling effect of the flexible strand of spacerprolonged single-stranded oligonucleotides (ssO) on the particle surface that increases deflection angle of oligonucleotides and due to the repulsion force induced by the highly negative charged ligand’s phosphate backbone, the number of biomolecules attached to the NPs is significantly reduced for longer nucleotide sequences.559 In both in situ and ex situ conjugation protocols, 4059


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and even charge-twisted conjugates,363,917 that is, a net-negative NP bioconjugate having negatively and positively charged ligands. Net negative NPs have improved biocompatibility,922 whereas the cationic submonolayer nanoenvironment may trigger cellular uptake.917 In terms of the higher conjugation efficiency toward DNA (>90%), in situ conjugation of NPs during laser-ablation is quite advantageous as it leads to far higher surface coverages than conjugation of chemically synthesized NPs (Figure 77).561,923 Note that grafting density (surface coverage) has been reported to be proportional to biofunctionality. For Au−oligonucleotide conjugates, this effect has been known for more than a decade and is based on cooperative binding theory.924,925 The phenomenon of binding avidity is generally believed to be a universal concept, which is also transferable to folate loaded dendrimers, where multivalency could increase binding constants up to 170 000-fold.926 Recent studies seem to indicate that a precise control of ligand coverage is a vital prerequisite to harvest the benefits of avidity.915,927 However, it should also be noted that the highest surface coverage is not necessarily the condition with the highest avidity as ligands at too close proximity may also hinder efficient binding. On the basis of these findings, it can be assumed that systematically altering the surface coverage of biomolecules on nanoparticles, easily adjustable if the particles are naked, could be a key factor in achieving optimal biofunctionality. Similarly, it has been shown that bare planar gold surfaces adsorb 2 times more protein ligands than a citrate-covered gold surface.928 Furthermore, in situ conjugation is a simple one-step approach with size quenching involved simultaneously, both for Au308 and for Si.847,848 For instance, Intartaglia et al. showed that the average size of silicon NPs synthesized by in situ bioconjugation with oligonucleotides is 5 nm (Figure 78a), whereas that synthesized without conjugation is 60 nm.847 Bagga et al. reported that in situ conjugation decreases the size of LAL-synthesized Si NPs from 15 nm (in water) to 8 nm.848 The conjugated Si NPs had PL property and could be used for cell imaging (Figure 78b).848 Ex situ conjugation requires anion-based size quenching310 and/or centrifugation before conjugation (for monodisperse NPs). A direct comparison showed that both methods yield similar results regarding grafting density of biomolecules and biomolecular functionality test with cytokine production.558 Advantages of ex situ conjugation include precise control of biomolecule number per NP starting from zero (e.g., 0−350 ligands per particle363), NP dispersity control by a combination of different biomolecule types,929 and molecule combinations on NP surfaces (Figure 78c),341 yielding bivalent bioconjugates (such as cationic peptide plus oligonucleotide).562 The ability to create such bivalent bioconjugates is the key to transfect biological cells that do not undergo endocytosis, such as sperm cells.930 Besides this advantage of LSPC-synthesized NP, the higher stability of LSPC-synthesized NPs after PEGylation than chemically synthesized analogous NPs (Figure 78d) is another property allowing steady application of the PEG-conjugates in the biomedical field (as PEG is known to cause “stealth particles” invisible to the immune system).341 In ex situ conjugation, the unique LSPC-property of “building a NP from scratch” is utilized. Simply speaking, in situ bioconjugation is often more convenient and yields size-quenching in parallel, whereas ex situ bioconjugation provides more precise control over biomolecule grafting density and charge manipulation but requires prior size control preferably in the absence of ligands by salinity quenching or LFL. The conjugation effi-

ciency308,561,919 and biological functionality558 yielded by both methods are similar for monovalent (single biomolecule type) nanoparticle bioconjugates. 6.4.3. Antibacterial and Ion Release. The development of novel antibacterial agents, especially in the form of nanomaterials, is of great importance for biomedical applications because microorganisms gradually become resistant toward traditional antibiotics or disinfecting agents. Because of the absence of ligands, LAL-synthesized NPs are better candidates in comparison to wet-chemical synthesized NMs, enabling to clarify the effect of surface ligands on the resultant antibacterial performance.916 Many kinds of antibacterial nanomaterials have been produced by LSPC, including Ag NPs,931−934 Cu NPs,935 TiO2,439 and Ag−TiO2,448 as shown in Table 6. It can be seen that the most frequently studied NPs are based on Ag. It is confirmed that Ag+ ions released from the Ag NPs contribute to the antibacterial effect rather than the Ag NPs themselves.936 As expected, the antibacterial efficacy of Ag NPs is Ag-concentration dependent with higher concentration leading to enhanced antibacterial performance.675 Yet at the same time, increasing the concentration of Ag NPs from 5 to 70 μg mL−1 gradually causes toxicity, reducing the viability of the human tissue cells gingival fibroblasts as well.937 Hence, often the intended antibacterial effect is accompanied by the unintended toxicity. The applicable therapeutic window may be quite narrow and depends on ligand effects. Accordingly, LAL-synthesized ligandfree Ag NPs are helpful to identify how the presence of a ligand or protein influences the antibacterial activity. Interestingly, silver ion release and antibacterial effect of LAL-synthesized Ag NPs can be slightly enhanced by white light illumination (1.8 mW/ cm2).933 The groups of Bubb and Klein could clearly attribute this effect to enhanced ion release, rather than creation of reactive oxygen species and evidenced by TEM that mild irradiation of the colloid causes narrowed and smaller Ag NP size.933 The authors concluded a new clinical pathway for treatment of open wounds where the antimicrobial agent may be stimulated locally by white LED illumination.933 Moreover, LAL-generated AgNPs have shown potent antibacterial activity against Gram-positive bacteria (e.g., B. subtilis, S. typhi, S. typhimurium, and S. aureus) and Gram-negative (e.g., K. pneumonia, P. aeruginosa, and E. coli) human enteropathogenic bacterial strains.932 Interestingly, addition of the steric stabilizer BSA to the ligand-free LALsynthesized Ag NPs significantly reduced effectiveness (antibacterial effect against bacteria and toxicity against fibroblasts) of Ag NPs, diminishing the therapeutic window.937 On account of the antibacterial property, LAL-generated AgNPs are discussed as candidates to be embedded in a biopolymer (e.g., Agar) for food packaging to suppress foodborne pathogenic bacteria.938 Also, Klein et al. found Gram-positive bacteria to be more resistant against Ag NPs attributable to their thicker peptidoglycan cell wall as compared to Gram-negative strains.933 Embedding Ag NP in a thermoplastic polymer (e.g., TPU) via LAL in THF solution of TPU and subsequent part fabrication by polymer injection molding and extrusion allows the development of antibacterial medical devices (e.g., catheter) for series testing toward clinical application.372 Alloying 30% of Au to Ag disproportionally reduced the adverse effects to both fibroblasts and bacteria, with the antibacterial effect reduced by 80%.716 Note that this mechanism can only be observed clearly for ligand-free NPs, because addition of citrate diminishes this disproportional effect, making LSPC powerful for mechanistic studies on antibacterial effects of NPs.716 To reduce the cytotoxic effect but simultaneously 4060


LAL in citrate and BSA solutions

LAL in PVP or SDScontaining solutions LAL in water and chloride LAL in NaCl solution LAL in TPU−THF solution LAL of Ti/Ag in water

LAL in water

Ag, AuAg

Ag

AgCl (Ag, Cu, Zn, Mg)−TPU Ag−TiO2

TiO2

E. coli

E. coli

2−222 nm with peak at 31 nm 34 ± 1 nm

LAL in water Ag

Ag

preserve moderate antibacterial effect of Ag NPs, embedment of Ag NPs inside polymer to form nanoparticle−polymer composites is a feasible approach; for example, TPU capping makes Ag NPs become less cytotoxic toward cells but still toxic to some bacteria strains.371 Mixing Ag NPs and Mg NPs into a polymer matrix even enhances the antibacterial effect against S. aureus due to the improved Ag+ release from the sacrificial volumetric effect of Mg, whereas mixing Ag NPs with either Zn or Cu NPs leads to a diminished antibacterial effect.371 As suggested by the authors, alloying Ag NPs with other functional metals or mixing Ag NPs with other metal NPs in a polymer matrix may pave a way to find a compromise between cytotoxic and antibacterial effects. 6.4.4. Micronization and Solubilization of Drugs. Micro-/nanonization of insoluble drugs (e.g., megestrol acetate, fenofibrate, and naproxen, etc.) by LFL to improve drug solubility for intravenous administration is a developing subject of great significance for preclinical drug evaluation.480,517,940,941 Figure 79a shows that LFL of the model drug megestrol acetate leads to a size reduction from >3 μm to 95% antibacterial efficiency against 105 CFU/mL of bacteria in 3 h improved antibacterial efficacy by mixing Ag−TPU and Mg−TPU composites due to improved Ag ion release Ag−TiO2 NPs have better antibacterial effect than laser-generated TiO2 nanoparticles and chemically synthesized Ag NPs 85% antibacterial efficiency at concentration of 100 μg/mL after 60′ exposure by UV irradiation, much better than 75% commercial TiO2 nanoparticles

934

716

LAL in water Ag

BSA- and citratestabilized Ag NPs, AgAu alloys 19 and 25 nm with and E. coli after without irradiation 0.9−2.2 nm E. coli and B. subtilis

B. subtilis, S. aureus, E. coli, E. aerogenes, K. pneumonia, P. aeruginosa, S. typhi, and S. typhimurium S. aureus

S. salivarius, S. aureus, E. coli, P. aeruginosa

a reduced fraction of silver in AgAu nanoalloys disproportionally decreased the antibacterial effect; BSA maintains antibacterial activity, whereas sodium citrate reduces antibacterial effects

932

937

935

a cell load equal to 6.0 Log CFU/mL lower than that measured in control polymeric matrix (7.4 Log CFU/mL) an increased antibacterial activity with ligand-free as compared to BSA addition in AgNP-embedded solid agar hydrogel matrix minimum inhibitory concentration (MIC) values (2−6 μg/mL) for both Gram-positive and Gramnegative bacteria; more sensitive to Gram-negative bacteria LAL in acetone

36 ± 9 nm, 20−41% surface oxidation 10−60 nm with peak at 21 nm 9−27 nm, polycrystalline

a mix of P. fluorescens and P. putida

Review

Cu

particle properties synthesis NPs

Table 6. Antibacterial Activity of LSPC-Synthesized NPs

bacterial Strain

key aspects

ref

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Figure 79. Drug micronization by LFL. (a) Size analysis of untreated, ns laser and fs laser micro-/nanonization of megestrol acetate (MA). (b,c) Dissolution and in vivo blood plasma concentration of untreated MA powder (□), media milled MA (×), and fs laser (250 mW, 30 min, 2 mL) fragmented MA (○), respectively. (d) Permeability of irradiated and nonirradiated clobetasone butyrate suspensions across Caro-2 cell monolayers. Reprinted with permission from ref 940. Copyright 2011 Elsevier. Reprinted with permission from ref 517. Copyright 2014 Elsevier.

Figure 80. Thermoresponsive nanoparticle−polymer composites starting from LAL-synthesized Au NPs (a), and temperature controlled biorecognition (b and c). (d,e) Confocal microscopy examination of KB cells treated with AuNPs functionalized with folic acid/pNIPAM-co-Am. Reprinted with permission from ref 945. Copyright 2011 Royal Society of Chemistry.

to realize triggered cell binding and internalization (Figure 80a).945,946 When the temperature of the particle surface is below the lower critical solution temperature (LCST), the targeting ligands are shielded by polymers, in which case no surface

binding and particle uptake by cells is observed (Figure 80b,e); whereas above LCST, the ligands stick out sterically and show affinity to the cell-surface receptors, allowing surface binding and cell uptake (Figure 80c,d).945,946 As was demonstrated in section 4062


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6.1, MNPs are extensively used in biomedical applications, partly because of their response to a magnetic field enabling “action at a distance” (e.g., cell targeting) or “remote control” (e.g., cell sorting) once they are taken up by cells. To this end, Amendola et al. investigated the magnetic labeling and manipulation of macrophage cells using LAL-synthesized FeOx NPs.815 Flow cytometry showed that 95% of the macrophages could be effectively sorted by an NdFeB magnet after only 10 min of exposure. The cell sorting efficiency was also maintained even when the FeOx NPs are combined with LAL-synthesized Au NPs to AuFeOx aggregates for rendering magneto-plasmonic SERS function. Multimodal use of LSPC synthesized MNPs for in vitro tests can be further enriched, for example, photoluminescent bioimaging by cation doping, which has been reported by the Li and Chen groups.648,829 Ligand-free Au and Pt particles also allow biomedical researchers to increase the binding affinity of Aβ-specific small molecules to inhibit Aβ peptide aggregation into fibrils in vitro,344 relevant for Alzheimer disease. Binding affinities of AuNP-immobilized D3 (Aβ-specific D3 dodecapeptide) can be enhanced by a factor up to 7 as compared to the free ligands. Surprisingly, cellular excretion of neurotoxic small Aβ oligomers while using AuNP-immobilized D3 conjugates is as low as ∼20%, much better than pure particles (51%) and pure ligands (40−85%).344 Hence, effective ligand conjugation to the NP surface amplified the ligand’s application potential. Overall, D3-conjugated nanoparticles convert Aβ into nonamyloidogenic, amorphous, nontoxic aggregates. This finding may stimulate further comparative studies on bioresponse of ligands and their biofunctional particle conjugates, based on ligand-free building blocks. To overcome the lack of cell adhesion property of alginate, Blaeser et al. deliberately introduced metal (Au or Fe) nanoparticles in alginate by laser ablation in alginate solution.947 In vitro tests showed that endothelial cells seeded on iron-loaded gels have the highest viability in comparison to the alginate control and gold-loaded alginates. This is attributed to the release of Fe ions over a prolonged period, which improves the adsorption of cell adhesion proteins, relevant for extrusion-based biofabrication.947 With molecular medicine delivery in mind, Krawinkel et al. reported the photodispersion effect of cell-internalized (cellpenetrating peptides-conjugated) NP agglomerates by ns laser irradiation (Figure 81a) for intracellular release of molecules carried by the agglomerates (Figure 81b,c).897 Key innovation of using aggregates of primary small particles over the widely used large spheres, nanoshells, or nanorods is that NPs with primary diameters of ≤5 nm are considered to be biodegradable (not biopersistent).897 At the same time, the aggregates benefit from plasmon coupling increasing their photosensitivity. Also, LAL creates negatively charged bare Au NPs, which can be precisely aggregated by oppositely charged, cationic peptides crossing the isoelectric point during conjugation and stabilized against further aggregation by BSA.897 In a different study on molecule delivery (plasmids), Durán showed that LAL-synthesized ligand-free AuNPs were superior to chemically synthesized Au NPs for nanoparticle-mediated transfection in terms of higher transfection efficiency and lower cytotoxicity,930 indicating that LSPC-synthesized NPs are good candidates for in vitro transfection assays.948 Another advantage of LSPC is its convenience for micronization or nanominzation of drugs (section 6.4.4), which will lead to the improved drug bioavailability in vivo and dissolution in vitro.940

Figure 81. Cell perforation and laser transfection. (a) Schematic illustration showing uptake and laser-based release of molecules using ex situ LAL-synthesized cell-penetrating peptides (CPP)-AuNPs into cells relevant for human mammalian adenoma. (b,c) Visualization of calcein uptake and release before and after laser irradiation. Adapted with permission from ref 897. Copyright 2016 BioMed Central.

Some attempts have been made to use LSPC-synthesized NPs (e.g., silica,949 TiO2,950 etc.) as “nanofertilizer” to increase the germination percentage and enhance the seed growth, but up to now it is still difficult to make any conclusion whether LSPCsynthesized NPs are advantageous over chemical-synthesized NPs for this type of bioapplication because of the limited number of studies. 6.4.6. In Vivo Application. Successful in vivo tests pave the way for nanoparticles toward clinical use. Increasing studies are now directed toward using LSPC-synthesized NPs for in vivo applications. For instance, Baati et al. systematically investigated the biodistribution, biodegradability, and toxicity of LALsynthesized Si/SiOx NPs.951 Their results showed that these particles were biocompatible and biodegradable with negligible toxicity or biopersistence. The small LSPC-synthesized Si/SiOx NPs were completely cleared within 1 week,951 while in comparison, electrochemically synthesized Si/SiOx NPs took longer (4 weeks) for clearance,952 maybe due to their larger size. These findings have aroused increasing interest to apply lasergenerated Si NPs for drug delivery and cancer theranostics. Tamarov et al. applied both LAL-synthesized and chemically synthesized Si NPs as sensitizers to inhibit Lewis lung carcinoma development.953 Figure 82a shows the mechanism that the authors proposed for radio frequency (RF) radiation-induced hyperthermia using RF-absorbing Si NP-based sensitizers: First, Si NPs will be accumulated in the tumor cells and then heated by RF radiation to increase local temperature leading to selective death of cancer cells without negative effects toward healthy tissues (Figure 82b,c). Figure 82d supports the finding that LALsynthesized Si NPs perform superior to chemically synthesized porous silicon (PSi) NPs against cancer growth, which can be attributed to a better in vivo delivery and uptake of LALsynthesized Si NPs as well as their ideal spherical shape, smaller mean size, and low size-dispersion. As introduced in section 6.1, LSPC-synthesized AuFe alloy particles are promising candidates for bimodal SERS-MRI-CT imaging; hence their in vivo biocompatibility becomes important for their practical applications in clinical trials. Figure 83 shows the in vivo biodistribution in different organs of a syngeneic mouse deduced from MR images at 4.7 T within 24 h. It is clearly seen that liver and tumor display different extents of NPs accumulation at different times. Figure 83a shows a ∼10% decrease of the T2 signal in the tumor and an increase of the T2 4063


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Figure 82. In vivo radio frequency (RF) radiation-induced hyperthermia using chemically synthesized and LAL-synthesized Si NPs as sensitizers. (a) Schematic representation of the treatment procedure. Parts (b) and (c) are histology images of a tumor area 1 h and 3 days after the PSi NP injection and RF-based treatment using PSi NPs as nanosensitizers, respectively. (d) Inhibition of the tumor growth under different conditions. Reprinted with permission from ref 953. Copyright 2014 Nature Publishing Group.

6.5. Analytical Chemistry (SERS, SEIRA, and LDI-MS)

signal in the liver, meaning an increase in NPs accumulation in the tumor and a decrease of NPs accumulation in the liver. Selective accumulation in tumor and their retention effect render AuFe agents interesting for tumor detection and treatment. Yet from the Raman spectra (Figure 83b) and the images of the pellets from organ lysates (Figure 83c), AuFe agents also accumulated in liver, spleen, and kidneys with only lungs and blood free from nanoparticle accumulation. NP accumulation in organs is a common drawback while using insoluble or larger NPs as contrast agents for bioimaging in vivo, pointing at further research needed addressing the long-term biocompatibility of the biopersistent (bioaccumulated) NP fraction. In a work on NPs immobilized on implant surfaces, Angelov et al. revealed that LSPC-synthesized NPs increased the impedance/stability of neural electrodes in vivo, which are good candidates for neuroprosthetic devices for recording neuronal activity and neural stimulation.532 This can be attributed to the tight bonding of ligand-free Pt NP to Pt electrode surface after EPD and controlled electrophoresis during EPD. The latter feature originates from the ligand-free NPs, which have reduced drag force and barrierless deposition during EPD, allowing linear control of deposition rate.303 Through evaluating the gliosis and the number of neurons at the electrode tip implanted in rat brains, no adverse effect was observed toward the neuronal cells,532 which indicates safe use and biocompatibility for practical biomedical treatment. Most importantly, long-term in vivo impedance of the neural electrode was stabilized,532 making neural stimulation more energyefficient and motivating further research on these LSPC-NP modified implants for treatment of Parkinson’s disease.

Laser spectroscopy using the surface plasmon resonance of metallic nanostructures has given rise to a highly sensitive and label-free spectroscopic analysis technique: SERS,954 which allows single molecule detection by measuring the enhanced Raman signal from the molecules adhering to the metallic NP surfaces for biomedical applications955 as well as quantification of SERS active NP uptake by specific cells (see also section 6.1 and section 6.3 for magneto-plasmonic SERS).898 Increasing efforts are currently underway to prepare various SERS agents by LSPC driven by the cleaner spectral background of the Raman spectra in comparison with chemically synthesized counterparts. This phenomenon was first reported by Neddersen et al. in 1993 (shortly after Fojtik and Henglein18 discovered laser synthesis of NPs) who showed that LAL-synthesized Ag, Au, Pt, Pd, and Cu colloids had SERS activity comparable to or superior than chemically prepared colloids,956 which was later supported by the studies of Kneipp et al.957 and Intartaglia et al., who used LALsynthesized AuNPs (Figure 84a).783 In comparison to the featureless Raman background signal of LAL-synthesized NPs, chemically synthesized NPs have an apparent fluorescence background and a feature peak band from 2800 to 3100 cm−1 due to the C−Hx stretching vibrational band of the contaminants (Figure 84a).783 The featureless Raman background using LSPCsynthesized NPs is assigned to the ligand-free surface, unlike chemically synthesized NPs with stabilizing ligands on their surfaces, which show a significant background noise and might hinder precise molecule labeling. Moreover, the reproducibility of surface-enhanced resonance Raman scattering (SERRS) spectral monitoring of the kinetics of a free base porphyrin 4064


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Another type of widely used analytical methods is LDI-MS, which is a technique useful for detecting various analytes. Recently, a comparative study of the LDI-MS performance of LAL-produced AuNPs and chemically synthesized citrateAuNPs revealed that laser-generated ligand-free AuNPs are better candidates for LDI-MS7 (Figure 84b,c) because of their remarkably cleaner background (Figure 84c) and almost no spectrum disturbance in the whole mass range during detection of different molecular structures (e.g., arginine, fructose, atrazine, and anthracene). The stabilizers’ presence on citrate-AuNPs causes a strong background spectrum of the LDI-MS spectra even after dialysis (Figure 84d),7 whereas only a small amount of analyte-sodium adducts on LAL-synthesized AuNPs lead to an almost negligible background peak in the LDI-MS spectra (Figure 84e). Hence, in studies on laser-generated nanoparticles as a matrix for matrix-assisted laser desorption/ionization mass spectroscopy (MALDI-MS), the low detection limit for low molecular weight analytes resulting from the ligand-free matrix7,960 and the good reproducibility resulting from electrophoretically deposited nanoparticles for surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) using laser ablation in liquids961 were highlighted. Accordingly, the ligand-free feature of LALsynthesized NPs has clearly proven to be beneficial for areas of analytical chemistry such as SERS in the visible and infrared regions as well as LDI-MS.

Figure 83. In vivo biodistribution of AuFe alloy NPs multimodal contrast agents (MCAs). (a) Plot of in vivo relaxation time in liver (■) and tumor (red ●), rationed to the value before injection of the CAs (T2/T2(0)), versus time after injection. The plot shows that MCAs in the liver decrease and accumulate in the tumor over a time scale of several hours, as sketched in the left side of the figure. (b) Plot of Raman intensity at 1505 cm−1 in different organ lysates, showing preferential accumulation in liver, spleen, kidneys, and tumor, while negligible accumulation was found in lungs and blood. (c) Image of pellets obtained from organ lysates, derived from MCAs treated or mocktreated mice sacrificed 24 h after the injection. Dark-brown staining depends on the loading of MCAs. Reprinted with permission from ref 822. Copyright 2014 Wiley-VCH.

6.6. Catalysis and Energy

Catalysts are the most important building blocks of devices for chemical energy storage and conversion. Nowadays, the most commonly used nanoscale catalysts are often synthesized by chemical bottom-up methods such as impregnation or salt reduction,962 where indispensably ligands or reactants are needed to aid the synthesis or stabilize the colloids. After the colloid synthesis, catalysts’ postcleaning procedures like calcination,963 centrifugation,964 or ozonolysis965 are necessary, which may cause particle growth and/or agglomeration, leading to the loss of catalytic activity.966 Although facile cleaning of chemically prepared Au−TiO2 catalysts by reflux significantly increases its performance, only 20% of residuals are removed by this method.9 In comparison, LSPC is an efficient technique, which allows one-step synthesis of the nanoparticle catalysts without any requirement of ligands or postsynthesis cleaning.967 Today, relevance of LSPC has been shown for hydroxides,601 semiconductors,439,968 as well as noble metals967,969 and alloys.970 Easy LSPC synthesis of metal and alloy nano-

metalation is low for chemically prepared Ag colloids but high for LAL-synthesized colloids.364 Thus, this analytical test proves the advantages of the LAL-generated colloids over the chemically prepared ones for SERS and SERRS analysis. LSPC-synthesized NPs have also been applied for surface-enhanced infrared absorption spectroscopy (SEIRAS),958 which is a complementary technique of SERS for biochemical applications due to its ability to probe almost all bands of the adsorbed species, resistance to fluorescence interference, high sensitivity, and good enhancement factor (10−100× enhancement of vibrational bands).959 Given that very promising pioneering works have covered SERS and SEIRAS, further investigations are expected in the future.

Figure 84. Featureless background of LSPC-synthesized NPs in analytical chemistry because of the absence of ligands on the surface. Raman spectra (a) using picosecond laser ablated Au NPs (psLA-NPs, ligand-free surface) and chemically synthesized NPs (chem-NPs, citrate-capped surface). Adapted with permission from ref 783. Copyright 2013 Royal Society of Chemistry. LDI-MS spectra (b,c) using citrate-AuNPs (green) and bare LAL-AuNPs (red) and (d,e) schematics of surface chemistry. Adapted with permission from ref 7. Copyright 2013 American Chemical Society. 4065


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Figure 85. Reference material for catalytic reaction modeling: (a) Schematic of modeling by Langmuir−Hinshelwood kinetics of reduction of 4nitrophenol (Nip) to 4-aminophenol (Amp) using (b) LAL-generated ligand-free Au NPs (Adapted with permission from ref 969. Copyright 2015 Springer) as compared to (c) chemically generated Au NPs. Reprinted with permission from ref 980. Copyright 2014 Springer. Process stability: (d) SPR shift after reaction as compared to initial SPR position before reaction, and (e) catalytic activity comparison between LAL-AuNPs/TiO2 and commercial AUROlite catalyst AuTiO2. Reprinted with permission from ref 967. Copyright 2015 Elsevier.

catalysts108 (e.g., AuPt,970 PtCo160,430), a good size control of both LSPC-synthesized (section 3) and chemically synthesized NPs (e.g., size reduction of encapsulated Ag NPs of Ag@ SiO2493) by LFL, as well as “smart” catalysts assembly/ construction (CO2-switchable poly(N,N-diethylaminoethylmethacrylate) (PDEAEMA)−AuNPs,714 AuNP decorated NaYF4:Yb3+,Er3+,Tm3+-core@porous-TiO2-shell microspheres971) starting from LAL-synthesized ligand-free NPs facilitate optimization of the catalytic performance. For example, the highest reported performance has been achieved for LSPCsynthesized heterogeneous catalysts, such as LAL-AuNP/CeO2NT catalysts972 for 4-nitrophenol reduction with 1.40 × 10−2 s−1 (μmol Au)−1, LAL-Co3O4 NPs536 for water oxidation with a high mass activity of 1.02 × 108 mA cm−2 g−1 and a high turnover frequency of 0.21 mol O2 (mol Cosurface)−1 s−1, and Pt/rGO catalysts973 for methanol oxidation with 333.3 mA mg−1. The purpose of synthesizing catalysts by LSPC and evaluating their performance is to utilize them for catalysis-based energy devices, such as water oxidation/splitting for fuel cells,974 glucose oxidation for biofuel cells, etc. In light of decreasing energy consumption, LSPC has addressed many routes to save energy by friction reduction573,975−977 or develop sustainable energy (e.g., solar cells,263,978 supercapacitor,601 Li-battery979) . In this section, we first demonstrate the advantage of using LSPCsynthesized NPs as reference materials for model reactions and for creating supported catalysts and photocatalysts. Subsequently, taking solar cells and fuel cells as typical examples, the advantages of high purity (ligand-free) and surface charge

features of LSCP-synthesized NPs for energy applications are clarified. 6.6.1. Reference Materials, Model Reactions. The most valuable feature of LSPC-synthesized NPs for catalysis application is their ligand-free, naked surface where ligands cannot block the catalytic activity like at chemical-synthesized catalysts, which indicates that LSPC-synthesized NPs are excellent catalytic reference materials for kinetic modeling of various catalytic reactions. For example, LSPC-synthesized Au NPs have been successfully used as reference materials for mechanistic studies of the reduction of 4-nitrophenol (Nip) by sodium borohydride using Langmuir−Hinshelwood kinetics (Figure 85a).969 As shown in Figure 85b, the experimental data fit well with the kinetic scheme based on the reduction mechanism of metallic NPs on the direct route; even after 70% Nip conversion, the experimental data still fit,969 much better than mere fitting by the chemically synthesized reference material (Figure 85c).980 Heterogeneous Au/TiO2 catalysts consisting of such ligand-free Au NPs have higher “process stability” (durability) than commercial AUROlite Au/TiO2 catalysts due to less NP aggregation and/or particle growth after reaction with less SPR shift (Figure 85d).967 At elevated temperature, the activity of laser-generated Au/TiO2 is 1.5 times higher than that of AUROlite catalysts even though their size (7.8 nm) is much larger than 3.0 nm of AUROlite catalysts (Figure 85e).967 These works indicate that LAL-synthesized ligand-free NPs are durable good reference materials for evaluating the ligand’s effects during various reactions. Because productivity of such reference materials (e.g., Au,374,972 etc.) generated by LSPC 4066


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Figure 86. Higher adsorption efficiency and turnover rates using LSPC-synthesized NPs due to the ligand-free surface. (a) Adsorption efficiency (of metal NP on catalyst support) as a function of citrate ligand concentration on LAL-synthesized metal NPs. Adapted with permission from ref 984. Copyright 2012 American Chemical Society. Reduced incubation time and higher activity due to surface charge: (b) “Free” AuNPs remaining in the solution as a function of incubation time for LAL-AuNPs and Chem-AuNPs after the addition of CeO2 NT supports, respectively. Catalytic activity: C/ C0 and −ln(C/C0) as a function of reaction time for the reduction of 4-nitrophenol using (c) LAL-AuNPs/CeO2-NTs and (d) Chem-AuNPs/CeO2NTs. (e) Superior performance of LAL-synthesized Au/CeO2 during heterogeneous CO oxidation reaction. Adapted with permission from ref 972. Copyright 2013 Elsevier.

has reached a high throughput (4 g/h),22,31 expansion of LSPCNPs as reference materials for catalysis can be anticipated. Moreover, for screening purposes of catalysts of different composition such as alloys or mixed metal oxides, LSPC is probably the only method to achieve a high throughput of different catalyst materials in a limited amount of time such as shown for a NiFe layered double hydroxide (LDH)981,982 series testing for water oxidation and PtAu alloy series for electrochemical reactions.413,414,983 6.6.2. Supported Catalysts. The synthesis of heterogeneous catalysts is a widely adopted strategy, often aiming at defined metal−support interactions (MSI), in which case the adsorption efficiency of adsorbates on adsorbents should be as high as possible because it determines the active site quantity available for the reaction. Because of the absence of shielding and pH effect by ligands, LSPC-synthesized colloidal NPs are more prone to reach the dispersed adsorbent surface efficiently. Wagener et al. showed that adsorption efficiency of LALsynthesized Ag NPs on BaSO4 microparticles is 20 times higher in the ligand-free state than those covered by citrate-stabilizers (Figure 86a).984 Those NPs with less than 50% ligand coverage show an adsorption efficiency of almost 100%, providing unparalleled high noble metal adsorption yield. Transferability has also been proven for metal (Ag, Au, Pt) adsorption onto various supports (BaSO4, TiO2, CaCO3).984 Hence, the cumulative material yield of precious metal in the process chain from precursor over the colloid to the catalyst is almost 100%,135 presenting an economic (and ecologic) advantage. Additionally, the intrinsic surface charge of LAL-synthesized NPs (e.g., Au) is also beneficial to increase the adsorption efficiency and reduce the induction period in combination with oppositely charged supports (e.g., CeO2-NTs) (Figure 86b).972 Because of the high adsorption efficiency (∼100%) and ligand-free surfaces,

LAL-AuNPs/CeO2-NTs are much more active than the ChemAuNPs/CeO2-NTs for the reduction of 4-nitrophenol into 4aminophenol (Figure 86b). The reaction almost ends within 1200 s for LAL-AuNPs/CeO2-NTs with a reaction rate constant (kapp) of 1.40 × 10−2 s−1 m−2 (Figure 86c), while in their chemically derived analogue (Chem-AuNPs/CeO2-NTs), the rate constant kapp = 2.25 × 10−3 s−1 m−2 (Figure 86d) is 1 order of magnitude lower, and after 1500 s only a yield of 0.5 is reached.972 The same catalyst was tested in heterogeneous CO oxidation, yielding high conversion rates at lower temperature for the laser-generated catalyst (Figure 86e). Other evidence to support the amenities of LSPC-synthesized NPs over chemically synthesized counterparts was shown by Li et al., who reported that LAL-Au NPs/BiFeO3 nanowires are ∼30% more active than the chemically synthesized Au NPs, evaluated by the water oxidation (water splitting) reaction.374 Furthermore, although the ligands on the NP surface are deleterious to the catalytic performance in many cases, recent reports showed that some agents (e.g., organic ligands, polymers) provide metal−organic interfaces that can significantly enhance strong metal−support interactions (SMSI).985 From this perspective, the ligand-free properties of LSPC-synthesized metallic NPs allow the surface and interface control from scratch for SMSI useful for catalytic and electrocatalytic applications. The Liang group reported three routes to synthesize supported catalysts using LAL-produced active nonstoichiometric MOx oxides (e.g., SnOx, TiOx, MnOx, etc.). One is to utilize MOx oxides (e.g., SnOx) as a sacrificial template to reduce metal ions (e.g., [PtCl6]2−) in the support (e.g., GO) suspension solution.973 Other routes include the development of metal− oxide (e.g., Ag−TiO2544) and M-doped (M: Ge, Sn, Mn, etc.) hematite (α-Fe2O3) nanostructures551,554 by hydrothermal treatment of MOx colloids and metal ions (e.g., AgNO3 and 4067


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Figure 87. Biofuel cells based on a catalytic reaction of glucose oxidation using LAL-synthesized Au NPs. (a) Schematic presentation of a hybrid biofuel cell for implantable biodevices. (b) Voltammograms of CTAB-Au NPs (red), Citr-Au NPs (blue), and LAL-Au NPs (black) electrodes in 0.1 mol L−1 NaOH recorded at 20 mV s−1 and 25 °C in the presence of 10 mmol L−1 glucose. Adapted with permission from ref 998. Copyright 2015 American Chemical Society.

presence of rich defects of the LSCP-synthesized oxides, some of them have already been proven to be superior to TiO2 (P25) for photodegradation of methyl orange, which is considered to be the best commercial photocatalyst.993 6.6.5. Hydrogen Fuel Cell. Hydrogen is considered as a very promising next-generation fuel (energy carrier) that could replace fossil fuels because of its high energy density (142 MJ kg−1) and abundance (e.g., water, organic solutions, biomass). Many researchers have shown the feasibility to generate hydrogen through water splitting (section 6.6.3) and hydrocarbon oxidization (methanol,994 ethanol,995 and formic acid983) via LSPC-generated reduced TiO2/GO,996 Au/MwCNTs,995 Au/graphene,994 CoO,485 and PtAu983 nanoalloys as well as the hydrogen storage by Mg@PMMA composites.588 In the literature, one apparent advantage of using LSPC-synthesized NPs as compared to chemical-synthesized catalysts is the bandgap decrease of the photocatalysts caused by nonstoichiometry oxides (e.g., TiO2−x996) and electronic reconstruction (e.g., α-Ag2WO4997), which can significantly increase the visible light absorption485 and result in the enhancement of photocatalytic performance. 6.6.6. Biofuel Cell and Glucose Oxidation. Biofuel cells are a kind of power suppliers that are different from hydrogen fuel cells in two aspects. First, the chemical energy is obtained from abundant biomass (typically glucose or other sugars), which makes them available to supply power for implanted biosensors or pacemakers (Figure 87a).998 Second, biofuel cells can be used at ambient temperatures. Enzymes serve as biocatalysts for glucose oxidation, but considering their vulnerability to the formed hydrogen peroxide, there is a demand for developing selective electrocatalysts for glucose. Hebié et al. recently showed the promising prospect of electrodes containing LAL-generated Au NPs for biofuel cell applications.998 They compared the glucose oxidation performances of LAL-synthesized Au NPs, CTAB-, and citrate-capped Au NPs and showed that the current densities (at 0.4 V) obtained from LAL-generated Au NPs are ∼5 times higher than those from ligand-coated Au NPs, and the ratio stays almost constant for the successive oxidation of lactone (Figure 87b), as indicated by the second peak in the range of 1.15−1.3 V. The mass activity of LAL-generated Au NPs was 65 A g−1 (6 mA cm−2), that is, much more active and more efficient than chemically synthesized Au NPs for glucose oxidation, which was ascribed to the high “purity” of LAL-generated NPs, thus demonstrating the great promise of LAL-synthesized Au NPs to be applied for biofuel cells.998

FeCl3) mixtures, respectively. Hence, LSPC-synthesized oxygendefect-rich nonstoichiometry MOx oxides are good reducing agents and sacrificial reagents to elegantly synthesize oxide or metal supported catalysts via RLAL or RLFL. 6.6.3. Water Oxidation (Splitting). Water oxidation (2H2O → O2 + 4H+ + 4e−) catalysts, generally characterized by oxygen evolution reaction (OER) or oxygen reduction reactions (ORR), are widely studied materials.974 This trend is driven by the fact that water oxidation is the first step of water splitting (2H2O + hν → 2H2 + O2) for sustainable energy resources and requires robust catalysts to extract 4H+/4e− from two water molecules and to form an oxygen−oxygen bond.974 Recently, Müller and co-workers have shown the potential to produce the most active electrocatalysts for water oxidation via LAL. 536,982 For instance, Ti 4+ and La 3+ ion doped [Ni1−xFex(OH)2](NO3)y(OH)x−y·nH2O yielded a very low overpotential (260 mV at 10 mA cm−2).982 Their work demonstrated the possibility to systematically screen mixedmetal catalysts’ composition series using aqueous solutions with metal ions (e.g., Ti4+ and La3+) during RLAL.981 Mukherjee and co-workers further confirmed superior ORR/OER activities of RLAL-synthesized PtCo nanoalloys and PtCo/CoOx nanocomposites.160,430 The mass-specific activities of LAL-synthesized PtCo NPs with a Co molar ratio of 22.1% are approximately 2.7 and 5.5 times higher than commercial Pt/C catalysts toward water oxidation, respectively.160 PtCo/CoOx composites at the optimal mixture (33.3 M%, 62.3 wt %) allowed a combined overpotential of 756 mV vs RHE (reversible hydrogen electrode) toward water oxidation in alkaline media taking into account both ORR and OER activities, which is the highest value reported so far using carbon black as supporting material.430 In a different study, Liao et al. achieved the highest (∼5%) solar-to-hydrogen efficiency of water splitting under visible-light irradiation taking advantage of LFL-CoO NPs.485 6.6.4. Photocatalyst. Because of the ability to mineralize various organic pollutants, such as dyes, herbicides, pesticides, aromatics, etc., and photocatalytic reduction of heavy metal ions (e.g., Cr(VI)),986,987 photocatalysts have occupied an important position in the field of pollutant removal.988 Although the investigation of LAL-synthesized NMs as photocatalysts just started several years ago,989 applying LSPC-synthesized NPs as photocatalysts for pollutant removal has advanced rapidly, as indicated by the synthesis of TiO2,176,968 α-Bi2O3,612 TaxO@ Ta2O5,615 TiO2−Ti5O9,990 Pt−TiO2,726 ZnO/TiO2,991 ZnS/ Zn,986 MWCNT/ZnO,992 Sn6O4(OH)4,993 etc. Because of the large surface-to-volume ratio of LFL-synthesized NPs and the 4068


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Figure 88. Effect of NP surface chemistry on bulk-heterojunction (BHJ) solar cells performance. (a) Schematic representation of the BHJ organic photovoltaic (OPV) cell architecture with an active layer embedded with either LAL-synthesized ligand-free bare or chemically synthesized TOABterminated or P3HT-terminated Au NPs, (b) I−V characteristics of reference and BHJ solar cells of the configuration ITO/PEDOT:PSS/ P3HT:PCBM/Al incorporating different types of Au NPs into the active layer, and (c) the incident photon-to-current conversion efficiency (IPCE) curves of reference and OPV devices incorporating different types of Au NPs into the P3HT:PCBM. Adapted with permission from ref 999. Copyright 2015 American Chemical Society.

6.6.7. Solar Cell. Conventional chemically synthesized NPs are always covered by insulating ligands, which inevitably exert barriers for electron transfer and arouse a side effect for tailoring the band gap of semiconductor NPs. In the preparation of solar cells, expensive equipment such as chemical vapor deposition (CVD) is often employed for material deposition. Ligand-free NPs and cheaper film deposition devices are desirable for a costsaving preparation of solar cells with high efficiency.109 Addressing these problems by LAL-synthesized semiconductor NPs are of great appeal because of their ligand-free and highly charged surface. For example, Guo and Liu have fabricated Cu(In,Ga)Se2 (CIGS) solar cells on Mo sheet substrates by combining LAL and EPD (a method for film deposition based on electrokinetic mobility of the charged colloids under an external electric field) techniques,530 which eliminates the risk of film contamination by impurities (e.g., adhesives or binders). The energy conversion efficiency was already quite well 7.37%, but further studies are required to elevate the efficiency above 10%, which is needed for practical solar cell applications. Bulk-heterojunction (BHJ) solar cells are another potential low-cost, lightweight photovoltaic technology because of their hybrid polymer blends’ layout (polymer/nanocrystal serves as donor/acceptor for hole/electron transport)1000 and the simple manufacturing process by roll-to-roll coating or printing. To address the conflict between efficient light absorption, which is higher in thicker devices, and efficient photogenerated carrier collection, which is higher in thinner devices, plasmonic metal NPs are designed to be embedded inside BHJ solar cells to trap incident light and increase the optical absorption.1001 Hence, evaluation of the embedded plasmonic NPs is important for the performance of BHJ solar cells. Kymakis et al. compared the effect of three types of Au NPs with different surface chemistry on the performance of poly(3-hexylthiophene-2,5-diyl): phenylC61-butyric acid methyl ester (P3HT:PCBM) BHJ solar cells

(Figure 88a), including LAL-synthesized Au NPs, TOABterminated Au NPs, and P3HT-terminated Au NPs.999 A 20.3% increase in the power conversion efficiencies (PCE) is observed for LAL-generated bare Au NPs (Figure 88b), much better than TOAB-terminated Au NPs, which is even inferior to the reference device. The same trend is also achieved regarding the enhancement of the incident photon-to-current conversion efficiency (IPCE) (Figure 88c). As compared to P3HTterminated Au NPs, better performance for IPCE enhancement is displayed by LAL-generated bare Au NPs. Through analysis of the active layer structures by atomic force microscopy (AFM) and time-resolved PL spectroscopy, the authors concluded that only when the plasmonic NPs are in direct contact with the active layer polymer donor can the solar cells’ performance be enhanced, while for an extra ligand that does not consist of the same materials the active layer will deteriorate the active layer morphology and result in exciton quenching, which is unfavorable for BHJ solar cells.999 Besides plasmon-enhanced absorption effects, embedment of LSPC-synthesized Al, Ag, and Au NPs in the photoactive layer can also enhance the structural stability of the active blend to decrease the device degradation rate upon long-period illumination due to NP-mediated mitigation of the photo-oxidation effect.1002−1004 Analogously, the PCE and the stability of ITO/GO/AuNPs/P3HT:PCBM/Al organic photovoltaic devices are both improved by anchoring LSCP-synthesized Au NPs between the interface of the active layer (P3HT:PCBM) and the hole transport layer (GO).1005 These works clearly demonstrate the positive effect of ligand-free plasmonic NPs on the performance of BHJ solar cells. Hence, the application of LSPC-synthesized NPs as plasmonic additives to optimize organic solar cells in various configurations (e.g., active layer, hole transport layer, and between their interfaces) may become fertile.109,1006 4069


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Figure 89. Heavy metal, food pollutant, and hazardous gas detection using LSPC-synthesized NPs assembled electrodes. Heavy metal detection: (a) TEM images of LAL-synthesized Au NPs. (b) Differential pulse anodic stripping voltammetry (DPASV) response of the AuNPs/GC electrode for the simultaneous detection of Cd2+, Pb2+, Cu2+, and Hg2+ over a concentration range of 0−1.4 μM for each metal ion. Reprinted with permission from ref 563. Copyright 2013 American Chemical Society. Food pollutant detection: (c) TEM image of LAL-synthesized Ni NPs. (d) Electrochemical response studies of BSA/a-AfB1/DMSO/RnNi-film/ITO bioelectrode with respect to AfB1 toxin concentration (5−100 ng dL−1). Reprinted with permission from ref 1009. Copyright 2012 American Institute of Physics. Hazardous gas detection: (e) TEM image of LAL-synthesized mesocrystal CuO NPs (M− CuO) and (f) real-time gas-sensing response of mesocrystalline (M−) (black), singlecrystalline (S−) (red), and polycrystalline (P−) (blue) CuO NPs toward ethanol at concentrations of 12.5−200 ppm. Reprinted with permission from ref 1010. Copyright 2016 Royal Society of Chemistry.

Quantum dot-sensitized solar cells (QDSSC) are thin film devices benefiting from the efficient charge transfer of photogenerated carriers and diffusion of redox couples in the electrolyte. Many LSPC-synthesized semiconductor QDs (e.g., ZnSe and CdS,613 CdTe,1007 and CdSe1008) and TiO2 SMSs278 are promising for use as either film building blocks or absorbers for QDSSC application, respectively. Until now, due to limited investigations, whether they perform better in comparison with chemically synthesized counterparts still needs to be explored. 6.6.8. Friction Reduction. The addition of nanomaterial additives in lubricating oil/grease to reduce friction, improve antiwear ability, or retard the thermo-oxidation of lubricating oil/ grease may become increasingly attractive for industry, mainly motivated by energy saving and lowering the maintenance cost. The potential of LSPC-synthesized NMs for this applications lies in two aspects. One is directly producing lubricant nanofluids by LAL in lubricating oil/grease, and the other is the suitability of LML-synthesized SMSs for rolling friction reduction. FloresCastañeda et al. recently showed the feasibility to achieve lubricating oil with bismuth NPs as the additives by direct LAL of a bismuth metal target in both heavy and light viscous mineral base oils (BS6500 and BS900).573 The concentration and the size of bismuth NPs could be tuned by adjusting the processing parameters, which in turn affect the friction performance of the synthesized bismuth NP-containing lubricant fluids.573 Cao et al. tested the performance of LML-generated ZnO/TiO2/CuO/ Fe3O4 SMSs as lubricant oil additives.975,976 When such SMSs are introduced into the lubricating oil, the friction coefficient could be dramatically reduced dependent on the concentration of the SMSs. Such LML-generated SMSs are thought to act as spacers

for the rubbing surface, thus reducing the friction under tribologic pressure strength over time. Because ZnO has a small hardness of 4.5 on the Mohs scale, the LML-synthesized ZnO SMSs may not be resistant to friction over time. Harder materials (e.g., Al2O3) may show increased antiwear properties,977 enabling them to be used for long-lasting frictional applications. 6.7. Environmental Protection

Industrial wastewater discharge, motor vehicle exhaust, and extensive use of pesticides and chemical fertilizer in agriculture have produced many kinds of pollutants, including heavy metals, poisonous gases, and biopersistent molecules. Portable miniaturized electrochemical detectors are strongly desirable for pollutant detection. Driven by increased electron transfer rate from pure NP surfaces and large surface areas from ultrasmall dimensions, LSPC-synthesized NMs have been widely employed for trace detection of poisonous substances, such as heavy metal ions, food or water pollutants (e.g., xanthine,1011 pathogen,1012 phenyl hydrazine1013), as well as flammable and explosive gases (e.g., CO, ethanol, HCl, H2).53,68,1014−1017 Note that selectivity is very important for pollutant detection, for example, during HCl gas sensing using LAL-synthesized FeOCl nanosheets.68 In the following, one representative detector is introduced to show the potential of LSPC-synthesized NPs for various detection applications, covering heavy metal ion detection, biotoxin detection (immunosensing), and selective sensing of volatile compounds. Xu et al. assembled LAL-synthesized spherical Au crystalline grains with a size of 5 and 35 nm (Figure 89a) into a sensitive electrochemical detector for heavy metal ions via 4070


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Figure 90. Pollutant adsorption (a) Comparison of the equilibrium isotherms between the experimental and theoretical data of amorphous LFLsynthesized NiO nanostructures. (b) The effect of initial pH on MB adsorption using NiO nanostructures. Adapted with permission from ref 1018. Copyright 2015 Nature Publishing Group.

Figure 91. Advantages of the LSPC process and synthesized materials.

EPD.563 This sensor can simultaneously detect four heavy metals (Cd, Pb, Cu, and Hg) through four peak currents (Figure 89b) with potentials of −0.78, −0.56, 0.00, and 0.25 V corresponding to the oxidation potentials of Cd(0), Pb(0), Cu(0), and Hg(0), respectively.563 Besides the most frequently investigated Au NPs for colorimetric assays, other kinds of nanomaterials are also good candidates as electrochemical catalysts for detection applications. For example, Kalita et al. developed an immunosensor for aflatoxin (AfB1) detection with a broad detection range (5−100 ng/dL), high sensitivity (0.59 μA/(ng/ dL)), and low detection limit (32.7 ng/dL) using LAL-generated ring-like nickel NPs (Ni ≈ 10−20 nm, Figure 89c) as a good conductor for electrons to be transferred to the electrode via Ni2+/Ni3+ redox couple (Figure 89d).1009 This detector has been claimed to be fine for AfB1 toxin detection in grains, nuts, rice, and oil seeds to prevent the carcinogenic, mutagenic, teratogenic, and hepatotoxic effects in the case of an overdose. Furthermore, Li et al. recently reported an ethanol detector using LALsynthesized self-assembled CuO (M−-CuO) interconnected spindles (Figure 89e), which showed the highest sensitivity from 12.5 to 200 ppm (Figure 89f), fastest response, and the best selectivity ever reported (as claimed by the authors).1010 The superior performance as compared to chemically synthesized single/poly(S−/P−) crystalline CuO structures is ascribed to the unique properties (e.g., lower initial resistance in air/higher working resistance in ethanol, active sites, and larger specific

surface area) originating from the quasi-monocrystal lattice, higher density of oxygen vacancies, and branched structures of the M−CuO assembled spindles. Therefore, similar to the catalysis applications (section 6.6), the ligand-free surfaces feature of LSPC-synthesized NPs is very appealing for designing and constructing electrocatalyst-based detectors. Because of the high toxicity, once the pollutants enter the food chain or are directly exposed to living beings, they may cause fatal diseases. Many approaches have been proposed for pollutant removal and were generally classified into physical and chemical methods. Physical adsorption is a widely adopted physical method, not only effective for heavy metals but also for dyes and preservatives. Recently, LFL-synthesized NiO NPs have shown their promise as adsorbents for pollutant removal in light of the highest adsorption capacity (beyond 104 mg/g) toward MB dyes reported so far (Figure 90a).1018 Many factors lead to this substantial adsorption capacity as compared to the starting NiO NPs with only ∼255 mg/g. The first reason is related to the high surface area of synthesized amorphous NPs, which is about 10− 30 times larger than the raw materials. Second, a strong reaction between the dye and OH radicals on the NiO NP surface provides more adsorption sites for MB’s adsorption. Third, the formed positively charged center M(Fe, Co, and Ni) (OH+) at pH = 6.5 enables surface bonding toward the negatively charged functional groups of dye (−SO3−), allowing further adsorption (Figure 90b). All of these features can be easily achieved by LFL 4071


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Table 7. Advantages of LSPC-Generated NPs and Corresponding Exemplary Applications advantage of LSPC convenience

high purity

start from scratch

supersaturation charged surface

defect introduction

product

size control

model substance or reaction

PtAu Fe@Au

LAL LAL

formic acid electro-oxidation PMMA substrates

AuFe

LAL

mouse mammary tumor cells

AuAg

LAL

mixed metal (NiFe) hydroxides doped semiconductor NPs (e.g., Cudoped ZnO) ternary oxide (e.g., NiCo2O4)

RLAL RLAL

nonlinear scattering and nonlinear absorption water oxidation Cu doping-induced band-edge emission

Gd2O3

RLAL +hydrothermal RLAL

AuAg, Au Au

LAL LAL

Au Au/TiO2 Au Au, Pt Au

LAL LAL+adsorption LAL LAL LAL

Au Au Au Au Au

LAL LAL LAL LAL LAL

Pt AgGe Au nanocorals CuO spindles Mn3O4 Au/CeO2 Pt ZnO TiO2 Ag/TiO2

LAL RLAL LFL RLAL RLAL LAL+adsorption LAL+EPD LAL RLAL RLAL +hydrothermal RLAL +hydrothermal RLAL +hydrothermal RLEL mixed RLEL RLAL RLAL LML LAL LAL

M-doped α-Fe2O3 (M: Ge, Mn, Si, Sn, Ti, etc.) Pt/rGO

carbonization

employed technique

α-Ag2WO4 TiO2−x/GO Pd@C Co3O@C ZnO, TiO2, CuO SMSs Si QDs PtCo

Co2O4 + OH− + H2O ↔ NiOOH + 2CoOOH + e− nasopharyngeal carcinoma xenografted tumor cresyl violet and Rd6G arginine, fructose, atrazine, anthracene, and paclitaxel reduction of 4-nitrophenol ethanol oxidation glucose oxidation neurotoxic fibril inhibition germ cell function and embryo development plasmid DNA P3HT:PCBM BHJ system PEGylated Au pNIPAm-co-Am coated AuNPs CO2-switchable Au-PDEAEMA composite EXAFS, supported particles 4Ag+ + Ge → 4Ag + Ge4+ human fibroblasts and tumor cell lines ethanol gas pentachlorophenol nitrophenol reduction neural implant interstitial-zinc-related defect Escherichia coli and methylene-blue dye Ag/TiO2 pentachlorophenol (PCP) degradation reversible hydrogen electrode (RHE)

potential applications catalysis983 transparent and electrical conducting coatings418,540 multimodal imaging (SERS-MRICT)822 optical limiting768 catalysis981,982 photoluminescence428 pseudocapacitors1020 MRI828 SERS783 LDI-MS7 catalysis969 stable catalytic process967 biofuel cell998 biomedicine344 toxicity mechanism346 transfection930,1021 solar cell999 biomedicine341 cell-specific internalization945,946 catalysis714 catalysis566 photoelectronics23 photothermia249 gas detection1010 pollution remediation810 Co oxidation catalysis972 Parkinson’s disease device (in vivo)532 photoluminescence218,444 photocatalysis439 photocatalysis544 catalysis552,554

methanol oxidation

electrochemical catalysis973

MO degradation and H2 evolution water splitting nitrobenzene reduction magnetic and C-shell photoluminescence rolling friction photoluminescence oxygen reduction of HClO4 electrolyte

photocatalysis997 photocatalysis996 catalysis702 multiplexing383 friction reduction296,975 LED869 electrocatalysts160

in water where water breakdown is thought to provide the OH radicals for surface functionalization,263 and the educt particles are transferred into an ultrasmall size that provides a high surface area for pollutant adsorption. Meanwhile, surface charge is an intrinsic property of LFL-synthesized nanoproducts. Another often investigated series of adsorbents for pollutant removal are mesoporous nanomaterials, which can also be obtained by LSPC. For example, Wang et al. synthesized layered mesoporous Mg(OH)2/GO composites by RLAL of a Mg target in GO solution, which were shown to efficiently adsorb both dye model contaminants (MB) and heavy metal ions (Zn2+ and Pb2+).663

Therefore, in light of the capacities of LFL-size control and LSPC-surface chemistry, many other effective adsorbents’ fabrications may be anticipated for adsorption and removal of dyes as model pollutants from two routes of LFL and LAL.

7. SUMMARY OF ADVANTAGEOUS PROPERTIES CORRELATED TO APPLICATION PROSPECTS As demonstrated by the studies discussed in section 6, LSPC has proven to be a synthesis method that provides nanoparticles with numerous advantages. Figure 91 and Table 7 summarize these 4072


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their chemically synthesized counterparts.374,972 Elimination of the ligand effect also makes LSPC-synthesized “naked” nanocatalysts unparalleled reference materials for modeling chemical reactions with perfect fits to theory969 and results in significantly greater process stability (durability) during alcohol oxidation in heterogeneous catalysis at elevated temperatures than the chemically prepared “gold standard” reference catalysts.967 For the purpose of constructing stable supported heterogeneous catalysts using LSPC-synthesized NPs as adsorbates, greater adsorption efficiency984 and net charge transfer to the supports during the adsorption process135,565 are beneficial. From ecological and economic perspectives, LSPC synthesis of heterogeneous catalysts produces a cumulative yield of nearly 100% of the noble metal throughout the entire process chain.135,967 Additionally, because of their ligand-free nature, LSPC-synthesized colloidal NPs can contribute to our understanding of the principles of barrier-less charge-transfer-tosolvent (CTTS), allowing particles to chemisorb to the naked NP surface directly.164 Because no ligands or surfactants are present to interfere with, LSPC-synthesized ligand-free NPs offer a complete noise-free spectral/imaging background for analytical chemistry methods (section 6.5), such as LDI-MS7 and SERS.783 Regarding SERS applications, ligand-free particles are also helpful for adsorbing analyte molecules to achieve highly active SERS detection with very good stability and reproducibility.535 The absence of ligands’ blocking effect on MNPs is also advantageous for enhancing the longitudinal relaxation rate in T1-weighted MRI applications because of the shortened distance between ions of MNPs and water protons828 and because of the increased number of coordination sites for water hydration.811,826 For biomedical applications, the absence of ligands is also uniquely advantageous for distinguishing the toxicity of NP surface ligands from NPrelated toxicity in vivo and in vitro.346,917 In this way, unintended nanoparticle exposure scenarios may be mimicked more realistically, as for example implant abrasion will not involve any stabilizing ligand but biofluid’s protein corona, only. Hence, the nanotoxicities of ligand-coated and ligand-free NPs may be compared on the basis of LSPC-synthesized reference materials. Thereby, these materials essentially contribute to the rational design of NP toxicology assays as reviewed recently.12 Note that ligand-coated nanoparticles are not inferior per se in comparison with LSPC ligand-free nanoparticles for biomedical applications because ligands have the double function of increasing NPs stability in vivo (for instance, stealth effect provided by polymers affecting protein corona10 and capping the surface of toxic metals containing NPs to reduce the leakage of toxic ions).918 That is, ligand-free particles are good candidates as naked building blocks where functional ligand carriers can be precisely set and combined. For example, Streich et al. successfully used LALsynthesized gold and platinum nanoparticles to increase the binding affinity of Aβ-specific small molecules to inhibit Alzheimer Aβ peptide aggregation into fibrils in vitro.344 Increasing efforts have been devoted to embedding plasmonic NPs in bulk-heterojunction solar cells to promote photoconversion,1022 for which LAL-synthesized pure NPs are preferable because of the exciton quenching effect induced by ligands.999 To develop precise nanostructured coatings via NPEPD, two clear advantages result from ligand-free, charged NPs. First, no barrier build-up is observed during deposition. Second, fully linear electrophoretic mobility is a property of LSPCsynthesized ligand-free NPs, which allows for the precise scaling of the deposited NP mass with EPD time.303 Because no steep

advantageous properties that can be related to both LSPC itself as well as the resulting nanomaterials. LSPC offers a continuous, steady-state process, where seeds can be continuously supplied for downstream unit operations (e.g., ex situ conjugation or catalyst adsorption onto supports) to allow for direct integration of the synthesized NPs into stable hybrids that are easy to handle and store. The use of state-of-theart laser systems with MHz repetition rates and subkilowatt laser powers has demonstrated that NP productivities greater than 4 g/h are possible,22,31 and that the productivity is linearly scalable and goes along with the advances made in laser system technology. From the perspective of occupational safety, LSPC is a safe technique that can prevent health hazards caused by NPs’ inhalation because it is operated in a sealed environment (the liquid). Having a sealed vessel that can be sterilized along with the target and liquid provides advantages for sterile synthesis conditions, for example, for the synthesis of immunogold. These process-related advantages are all clearly important properties for any method intended to be used at an industrial scale. A very attractive feature of LSPC for use at laboratory or pilot scales is that it provides a quick and convenient platform for the rapid prototyping of various NMs at room temperature and under ambient conditions in a short period (from several seconds361 to 1 h22,31). Simple replacement of the solid target or the liquid enables the high throughput synthesis of systematic colloid variations (e.g., bioconjugates), which is particularly attractive for biological tests where normally only milligrams of NPs are needed. Consequently, the easy operation and quick synthesis of LSPC make it highly accessible to researchers from a wide range of fields. Another important convenience provided by LSPC is the easy synthesis of nanoalloy series by simply ablating an alloy125,174 (e.g., AuAg, AuFe) or irradiating a mixed colloids (e.g., Ag and Au).593,1019 Because alloy targets may not be available in various mole fractions, a very convenient approach is LAL of a powder mixture, consolidated by pressing. Here, LAL converts a “micromixture” into atomically mixed (solid−solution) NPs,125,413,483 which are much more convenient than complex chemical alloy-NP synthesis methods, in particular for alloy series with tunable composition. LSPC-synthesized alloy NPs have been successfully used for multimodal imaging822 (section 6.1, magnetism), optical limiting,768 magneto-amplified SERS detection67 (section 6.3, plasmonics), toxicity reduction11,346,424 (section 6.4, biology), and catalysis413,970,983 (section 6.6, catalysis and energy). LSPC also allows for facile and unparalleled synthesis of mixed metal (e.g., Ni−Fe) catalyst series for high throughput screening and the systematic variation of molar ratios981,982 as well as metal cation doped luminescent NPs with tunable dopant concentrations.428 Selectively mixing two series of RLAL-synthesized oxide/hydroxide colloids and subsequent hydrothermal treatment offers a convenient approach for the production of ternary oxide nanocrystals for optoelectronic and electrochemical applications, such as Zn2 GeO4 , NiCo 2O4, Zn 2SnO 4, ZnFe2O 4, ZnMnO3, and Fe2GeO4.1020 Therefore, LSPC has emerged as a NM synthesis technique with greater convenience than other synthesis methods and unparalleled flexibility. The extreme purity achieved by the ligand-free synthesis process is probably the most unique feature of LSPC-synthesized NPs. In the absence of the hindering effect of ligands or surfactants, more active sites of the LSPC-synthesized catalysts are available to be exposed to the reactants; as a result, LSPCsynthesized catalysts display better catalytic performance than 4073


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for recording neuronal activity in vivo with an improved longterm stability.532 Additionally, the surface charge of LSPCsynthesized NPs is beneficial for in situ308,717,730 and ex situ558 conjugation with proteins, DNA, or other biomolecules for medical and clinical applications. The introduction of high defect densities is normally conducted by LAL of ignoble metals in water (section 5.3.1).440 These defects either facilitate PL emission of the host nanoparticles444 or delay the electron−hole recombination and result in enhanced photocatalytic performance.439 Because charge transport is proportional to the defect quantity, small semiconductor particles with many defects will increase the charge transport and have been reported to enhance photovoltaic performance.1024 Sacrificial substoichiometric defect-rich oxides (e.g., TiOx, x < 2) derived from RLAL can reduce metal ions (e.g., Ag+) during hydrothermal treatment, which enables the direct synthesis of conjugated photocatalysts for pollutant degradation.544 Carbonization is another attractive characteristic of LSPCsynthesized NMs. The carbon shell capsule is produced in organic solvents (e.g., acetone,383 acetonitrile,702 etc.) where the carbon of the organic molecules serves as the resources during RLAL or RLFL. The carbon shell is particularly appealing for catalytic applications because it may increase the adsorption rate of the reagent and the electron concentration due to efficient interfacial electron transfer from the carbon shell to the palladium core, while also acting as a barrier to prevent the aggregation of catalysts during reactions.702 Additionally, a carbon shell can impart carbon-triggered PL properties to the synthesized NMs.383 As discussed in section 3, LSPC provides access to a wide range of sizes from 1 nm to 10 μm. With respect to LAL, multiple choices are selectable to realize in situ size quenching during NP formation. For example, using micromolar saline solution310 to inhibit NP growth enables the generation of monodisperse noble metal NPs. The use of solid additives725,903 or biomolecules11,848 enables direct fabrication of supported particles or bioconjugates for catalytic, environmental, and biological applications, respectively, thus providing an opportunity to realize NP size control and function introduction in a single step. Although LAL sometimes produces bimodal size distributions with large particles, LFL of these LAL-synthesized NPs can not only reduce the particle size substantially but also improve the NP monodispersity,25,90,1025 resulting in NPs with a coefficient of variation less than 10% under optimized conditions.250 In addition to the post-treatment of LAL-synthesized NPs, LFL also allows for fragmentation of micropowders or particles, extending the size control ability of LSPC to additional classes of materials, thus facilitating their applications in various fields, including catalysis,493 LED,865 organic/inorganic hybrid solar cells,674 and pollutant removal.1018 For specific materials, LFL enables the generation of atom clusters with size less than 2 nm in size, which not only help to clarify the particle nucleation mechanism164 but also enable anisotropic growth of plasmonic structures for photothermal applications.249 LML is a unique method to produce crystalline SMS particles294,296 that have also the potential for friction reduction application because of the sizedependent friction mechanism of additives.975 In summary, as compared to chemical-synthesis methods, LSPC is appealing for two reasons. First, a scalable, continuous, and convenient process is performed in a sealed environment, which enables high throughput of a large variety of NPs in a steady state with high occupational safety. Second, the

phase boundary originating from direct hard binding of the ligand-free NPs to the EPD-produced implant surface was observed via cross-sectional analysis,1023 EPD coating of ligandfree NPs is preferable to conventional “coating” methods that often require postannealing or ligand removal steps. As for plasmonic-related applications, without a bathochromic shift of the SPR peak maximum as a result of ligand-induced refractive index variation, LSPC-synthesized plasmonic NPs have lower SPR wavelengths as compared to their chemically or biologically synthesized counterparts,125 making them favorable for the design of optical applications. Hence, pure colloids not only provide application-oriented advantages but also bring the studied NP properties closer to textbook-like fundamental behaviors such as exhibiting ideal Stokes’ hard spheres (as an assumption for calculating charge-mobility relations) or ideal plasmonic particles (for experimental evaluation of the plasmon ruler equation or the Mie−Gans fit). Defined surface functionalization starting from scratch is an advantage rendered from the ligand-free surface of the LSPCsynthesized NMs. Different types of molecules can be precisely grafted onto the naked surfaces of LSPC-synthesized NMs to develop “smart” nanomaterials for catalytic and biological applications, such as CO2-switchable PDEAEMA-Au NPs714 and thermosensitive pNIPAm-co-Am functionalized Au NPs.945,946 Additionally, multiple types of molecules (e.g., PEG with different functional groups341 or biofunctional biomolecules with twisted charges363,917) are available to be functionalized in a sequence of specific ligand-free NPs, which enables them to be selectively used in surface-chemistry sensitive bioenvironments. A high local supersaturation concentration increases the collision rate between the atoms or the clusters (i.e., the seeds) and results in particle attachment and asymmetrical growth into high aspect ratio structures after LAL1010 or after laser irradiation (LPC);200,249 the resulting structures are suitable for use in gas detection1010 (section 6.7) and biotherapy249 (section 6.3). Surface charge is an additional useful feature provided by LAL and results in electrostatic stability of the synthesized NPs. Surface charge is also the reason why some LAL-synthesized NP colloids are stable when highly pure (at moderate concentrations), without the need for surfactants. Surface charge is beneficial for many applications. First, it facilitates the NPs to physically attract and adsorb the oppositely charged contaminants for pollution protection.1018 Surface charge can also substantially enhance the adsorption efficiency984 of LSPCsynthesized NPs on adsorbents via a purely electrostatically driven adsorption process. This simple process is realized by colloid mixing and is unparalleled for the direct assembly of highly active supported heterogeneous catalysts without the calcination process often required for chemically synthesized NPs.972 Additionally, catalyst loading can be set independent of particle diameter, unlike conventional wet-impregnation-derived NPs whose loading capacity is correlated with particle size. This independence is valuable for fundamental studies on catalysis where the effect of loading must be distinguished from size effects. Moreover, the electrokinetic mobility of charged NPs under an external electric field allows them to be directly assembled into thin films or electrodes through EPD (without the need for any surface functionalization or modification for charge manipulation) for electrochemical detection,563 solar cell,530 and biomedical applications.532 In particular, the electrode impedance of neural implant electrodes has been demonstrated to be lower after 3 weeks operating in vivo, created by EPD of LAL-synthesized Pt NPs, making the electrodes useful 4074


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the mechanism of both LAL and LFL could be supported by welldefined single-pulse experiments. Another goal of the fundamental research involved in LSPC is to accurately manipulate the NPs surface chemistry, size, and crystal phase of NPs with the aid of external stimuli or by adjusting target properties (e.g., geometry208) or the local liquid environment (e.g., bilayer liquids,1026 liquid mixtures,55,416 or reactive liquids445). Currently, LAL often leads to the formation of phase mixtures with yet-unknown determinants (laser, liquid) governing the crystal phase. Because of the possible chemical reactions (RLEL) originating from the laser-stimulated active species of materials and liquids, both the NP composition and the size quenching effect of NPs should be understood from the perspective of chemical reactions within LSPC time scales. Additionally, most research groups primarily study LSPCsynthesized colloids. Only a few correlate their findings to the structured surface or grooves after ablation,116,400,401 where new information can be obtained to better understand the properties of the synthesized NPs and to better understand the laser− liquid−target system and its productivity/reproducibility characteristics. The long-term prosperity of the LSPC technique depends on a clearer understanding of the aforementioned aspects, particularly in a wider field than the most-studied platinum-group-element colloids. Recently, facilitated by advances in LFL, ligand-free monodisperse noble metal NP colloids have become accessible.164 Their size “cartography” is wellestablished approximately as 1 nm to 10 μm (Figure 26); however, the size map still contains some nearly blank ranges. Medium-sized monodisperse NPs (which are relevant for many applications) such as 50 nm Au NPs are still difficult to obtain. Driven by their wide plasmonic-related applications, size control of metal NPs has reached an advanced stage; by comparison, studies on oxides are still limited and therefore require further investigation. In the case of LML, the synthesized SMSs are often polydisperse (sometimes approaching monodispersity) and educt size-dependent,1027 which raises the question of how to precisely narrow the size distribution of SMSs. Solutions to this problem require extensive studies on beam attenuation, beam shaping, and particle educt aggregation/dispersion control. The high purity of LSPC-synthesized NPs is well acknowledged to be their main advantage over chemically synthesized ligand-covered NPs. However, although LSPC-synthesized particles are referred to as ligand/surfactant-free, they are not totally naked and still have their own surface chemistry; that is, some of the NP surface atoms are oxidized or complexed. Therefore, differentiation of the surface chemistry for multiple applications (e.g., catalyst, energy conversion and storage, analytical chemistry, and biology) should be performed in the future (e.g., using in situ EXAFS566,831) to realize performanceoriented colloidal synthesis and processing. Although many other challenges remain and new questions will arise during the fast-paced accumulation of knowledge in this field, the progress made since the development of LSPC only a generation of scientists ago is encouraging for the following advances yet to come.

advantages of laser-generated nanomaterials, which include high purity, “starting from scratch”, defect introduction, carbonization, and size control, may not only help to clarify the fundamentals in nanoscience, but also facilitate the surface chemistry control/functionalization of LSPC-synthesized colloids.

8. OUTLOOK AND CHALLENGE As laser technology and nanotechnology have become vital parts of our daily lives, laser synthesis and processing of colloids has become an advanced discipline for addressing real-world problems. Only a few years ago, LSPC was described as a very promising synthesis method with unique properties, but lacked size control and high productivity. After recent advances in this field, these shortcomings have been remedied, resulting in the capability to synthesize monodisperse nanomaterials ranging from 1 nm to 10 μm with monodisperse size distributions at the lower end (

Laser Synthesis and Processing of Colloids: Fundamentals and

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