Stabilization of Silver and Gold Nanoparticles: Preservation and

Review pubs.acs.org/CR

Cite This: Chem. Rev. 2019, 119, 664−699

Stabilization of Silver and Gold Nanoparticles: Preservation and Improvement of Plasmonic Functionalities Hyunho Kang, Joseph T. Buchman, Rebeca S. Rodriguez, Hattie L. Ring, Jiayi He, Kyle C. Bantz, and Christy L. Haynes*

Chem. Rev. 2019.119:664-699. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/09/19. For personal use only.

Department of Chemistry, University of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455, United States ABSTRACT: Noble metal nanoparticles have been extensively studied to understand and apply their plasmonic responses, upon coupling with electromagnetic radiation, to research areas such as sensing, photocatalysis, electronics, and biomedicine. The plasmonic properties of metal nanoparticles can change significantly with changes in particle size, shape, composition, and arrangement. Thus, stabilization of the fabricated nanoparticles is crucial for preservation of the desired plasmonic behavior. Because plasmonic nanoparticles find application in diverse fields, a variety of different stabilization strategies have been developed. Often, stabilizers also function to enhance or improve the plasmonic properties of the nanoparticles. This review provides a representative overview of how gold and silver nanoparticles, the most frequently used materials in current plasmonic applications, are stabilized in different application platforms and how the stabilizing agents improve their plasmonic properties at the same time. Specifically, this review focuses on the roles and effects of stabilizing agents such as surfactants, silica, biomolecules, polymers, and metal shells in colloidal nanoparticle suspensions. Stability strategies for other types of plasmonic nanomaterials, lithographic plasmonic nanoparticle arrays, are discussed as well.

CONTENTS 1. Introduction 2. Synthesis of Ag and AuNPs and Stabilization with Adsorbed/Covalently Attached Ligands in Solution Phase 2.1. Theoretical Background of Colloidal Stability of the Plasmonic Nanoparticles 2.2. Conventional Surfactants for in-Solution Synthesis and Stabilization 2.3. PEG Ligands Stabilized Plasmonic Nanoparticles in Complex Matrix 2.4. Biomolecular Ligands Stabilized Plasmonic Nanoparticles 2.5. DNA and Other Highly Functional Ligands: Stabilization and Assembly 3. Plasmonic Nanoparticles with Shell Coating 3.1. Silica Shell-Stabilized Plasmonic Nanoparticles 3.2. Plasmonic Nanoparticles with Organic-Shell Coating 3.3. Plasmonic Nanoparticles Coated by Metal Shells 4. Two-Dimensional Plasmonic Nanoparticle Arrays 4.1. Plasmonic Nanoparticle Arrays via Lithography Techniques and Related Stabilization Strategies 4.2. Fixation of Plasmonic Nanoparticles on Solid Substrates via a Nonlithographic Technique © 2018 American Chemical Society

5. Conclusion and Prospective Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References

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1. INTRODUCTION The phenomenon of the surface plasmon resonance (SPR) was first reported by Wood in 1902.1 More than 100 years ago, he observed a form of abnormal incident, angle-dependent bands on a metal-supported diffraction grating shed by polarized light. Since this introduction, the SPR phenomenon has been explained more explicitly based on the work of many researchers including Kretschemann and Otto.2,3 Numerous scientific fields have taken advantage of the SPR either directly or indirectly from simple optical sensing techniques to solar energy conversion technology.4−7 Along with extensive and intensive work on the development of nanotechnologies,8−11 a subarea of SPR research has attracted a lot of attention: the localized surface plasmon resonance (LSPR).

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682 Special Issue: Chemical Sensors Received: May 28, 2018 Published: October 22, 2018

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DOI: 10.1021/acs.chemrev.8b00341 Chem. Rev. 2019, 119, 664−699

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Figure 1. Description of different stabilizing agents in colloidal plasmonic nanoparticle preparations and related functions/characteristics. Sizes of the nanoparticles and ligands/shells are not drawn to scale.

produce different shapes and sizes of AuNPs and AgNPs have been explored, resulting in the ability to tune their plasmonic functionalities17 and their application in the fields of energy, catalysis, sensing, and biotherapy.18−22 The life of nanoparticles, including plasmonic nanomaterials, can be divided into three stages: preparation, storage, and application. Especially for plasmonic nanoparticles, whose size, morphology, and chemical stability determine the overall level of plasmonic and application performance, conservation of particles’ physical and chemical characteristics is critical and must be carefully controlled. In most cases, how the nanoparticles are prepared is deeply associated with how the nanoparticles are stabilized. The methods for generating Ag and AuNPs can be categorized into two major classes: wetchemical synthesis and lithographic fabrication. In the wetchemical synthesis method, nanoparticle size and morphology can be tuned by various reaction parameters, such as chemical precursor choice, temperature, pH, or reaction time. Stabilizing agents must be present during and after nucleation and growth to imbue the nanoparticles with colloidal stability. Without suitable stabilizers, neither Ag nor AuNPs can maintain their structures and will aggregate or dissolve, resulting in loss of plasmonic functionality.23−27 These initial stabilizing agents can be replaced by other more robust stabilizers specific to an application’s needs. Choosing appropriate protecting materials is especially important for in vivo applications, where the nanoparticles must maintain their plasmonic properties until the nanoparticle arrives at a site of action and performs the desired function within a complex biological matrix. In current research, many studies exploit more than one stabilizing approach to set up a platform that can maximize the nanoparticles’ plasmonic abilities and perform other critical functions (Figure 1). In lithographic fabrication, metal nanoparticles are usually deposited using various patterning methods, under vacuum,26 and immobilized on supporting substrates to form nanoparticle arrays. Different from colloidally synthesized Ag or AuNPs, these plasmonic nanoparticle arrays do not always require stabilizing agents during the preparation step, but their stability during long-term storage and use is still important.

The SPR is a coherent oscillation of surface conduction electrons upon excitation by electromagnetic radiation at the interfaces between, for example, metal and dielectric media. The LSPR occurs when this surface plasmon is restricted to smaller volumes, that is, to nanoparticles, which are comparable in size to the wavelength of incident light. The dimensions of the nanostructures allow the plasmon to oscillate locally, within the near metal surface. The LSPR and plasmonic nanoparticles can provide a couple of advantages over traditional SPR. First, the LSPR measurement platform needs no prism, and the angle of incident light is not as important as in the SPR platform; this means that the design of a plasmonic device can be much more affordable and flexible, and it is not susceptible to vibration or mechanical noise. The LSPR shows relatively less sensitivity to bulk refractive index changes than SPR due to the short range of the enhanced electromagnetic field, so more focused studies on reactions or sites of interest are available without much interference from bulk solvent. The LSPR is dependent on the size, shape, and composition of the nanoparticles as well as other external factors; therefore, different types of plasmonic nanoparticles can be designed based on the needs of specific studies or applications. Both theoretical and empirical research have demonstrated various kinds of plasmonic nanoparticles over the last few decades.12,13 The most frequently and widely used metals are silver and gold nanoparticles (AgNPs and AuNPs), though metal plasmonic nanoparticles can also be fabricated from aluminum, copper, palladium, and platinum.14 On the basis of the dielectric properties, copper should also have good plasmonic performance, but its propensity to oxidize limits the use of plasmonic CuNPs. Typically, the surface plasmonic qualities of transition metals, such as titanium, cobalt, and nickel, are less compelling than those of the coinage metals.15 There are multiple reasons for the dominance of AgNPs and AuNPs in plasmonic nanoparticle research. AgNPs and AuNPs can be tuned to absorb and scatter light throughout the visible and nearinfrared regions (i.e., Ag LSPRs can range from 300 to 1200 nm). AuNPs are chemically inert, oxidation-free, and show high biocompatibility, which is critical for biomedical applications.16 Moreover, various synthetic strategies to 665


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The main focus of this review is how Ag and AuNPs, the most commonly used plasmonic nanoparticles, are stably prepared and applied in specific plasmonic applications without significant damage to their original chemical and physical properties. Three different types of nanoparticles are discussed based on the types of protecting materials and the synthetic strategies, since these two factors are deeply related to the origins of the particles’ stability, the media to which they are exposed, and the involved applications. Where relevant, this review also discusses the role of stabilizers that enhance plasmonic nanoparticles to achieve specific morphologies/ arrangements or to incorporate other functional chemicals to achieve more complex plasmonic designs.

Anisotropic Au nanorods have been extensively synthesized and developed since the late 1990s due to their distinct optical properties compared to common spherical nanoparticles, including multiple plasmon modes in the visible region of the spectrum, LSPR tunability into the infrared, and concentration of excited electromagnetic fields at the nanorod tips.42 Starting from electrochemical reduction preparation methods in earlier years, now the most widely used method is a silver-assisted seed-mediated growth where preformed small AuNPs act as a seed for further reduction of Au ions to generate anisotropic Au shapes in the presence of silver nitrate and surfactants.43 In nearly all Au nanorod syntheses, cetyltrimethylammonium bromide (CTAB) has been predominately used as a shape controller as well as a stabilizer; this method was first introduced in 2001 by the Murphy group and has been widely used for the production of anisotropic Au nanorods.44 In colloidal synthesis, also commonly called chemical synthesis, of AgNPs, the basic synthetic approach is similar to that of AuNPs. The synthesis of AgNPs generally requires three chemical functional compounds: a silver precursor, solvent, and a reducing/stabilizing agent. Like the synthesis of AuNPs, the reduction of AgNO3 with citrate in water was first reported in 1982.45 However, relying on citrate-stabilized AgNP synthesis usually produces nanoparticles with poor control of size and shape.14 Rather than this citrate reduction method, the reduction of the silver precursor in multivalent alcoholsso-called polyolsis a more popular chemical approach to synthesize various shapes of monodisperse silver nanoparticles.46 In a typical synthetic process, ethylene glycol (EG), AgNO3, and poly(vinylpyrrolidone) (PVP) serve as the solvent/reducing agent, silver precursor, and stabilizing/ capping agent, respectively. This polyol method can achieve a high degree of control over the morphology of the final products by controlling the types and amounts of capping agents and oxidative etchants, the availability of Ag+ ions, or reaction kinetics with temperature.47 Other methods, such as seed-mediated growth or light-mediated growth, have received great attention as well.48−51 The general synthetic strategies described above for both AuNPs and AgNPs mainly focus on solution-based synthesis. These are “bottom-up” methods whereby particles are produced by chemical reductions. Other fabrication methods for producing plasmonic gold and silver nanoparticles include mechanical grinding of bulk metals, thermal decomposition, and evaporation; the latter two of these methods will be discussed in later sections. The solution-based chemical approaches are advantageous due to their low cost, high yield, and ease of production. As described, under these chemical approaches, the metal precursors or seeds are treated with surfactants or other molecules as stabilizing agents during growth, such as citrate, CTAB, and PVP in previous examples. Those loosely bound molecules are somewhat limited in their ability to maintain the colloidal stability of plasmonic nanoparticles, so in the final products, these stabilizing agents can still be present in their original role or they can be replaced with other functional groups via substitution. However, the roles of the aforementioned stabilizing surfactants should not be undervalued as they have a critical impact in determining the basic plasmonic properties of the final products by controlling the size and morphology of the nanoparticles. In this section, the roles of those stabilizing surfactants and other replacement stabilizing molecules and ligands will be discussed.

2. SYNTHESIS OF Ag AND AuNPs AND STABILIZATION WITH ADSORBED/COVALENTLY ATTACHED LIGANDS IN SOLUTION PHASE The importance of controlled synthesis and fabrication of plasmonic nanostructures has grown along with development of increased applications for plasmonic materials. It is now well-appreciated that the plasmonic properties and, thus, the performance in various applications are largely determined by the size, shape, and composition of nanoparticles.28 Solutionphase synthesis of Ag and AuNPs is the most common way to generate monodisperse particles with intentionally varied size and shape.14,29 In this variety of solution-phase synthesis methods, the stabilizer (also known as the capping agent) must be present to control the size and morphology, prevent aggregation, and facilitate long-term storage. Both Ag and AuNPs have shared some common and popular stabilizers for synthesis, and each particle’s synthetic history is discussed herein. AuNPs were first introduced into the research field in 1857 when Michael Faraday reported the preparation of colloidal AuNPs by the reduction of chloroauric acid by phosphorus.30 Since that discovery, 20th century scientists have made large efforts to control nanoparticle size and shape with tailored synthetic designs. Among various experimental explorations, the work done by Turkevitch and Frens has offered one of the most important breakthroughs in AuNP synthesis by pioneering and further improving the citrate reduction of HAuCl4.31,32 This method is very often used for colloidal gold nanomaterial synthesis, where citrate plays the role of both the reducing and the stabilizing agents. In 1993, Mulvaney and Giersing reported the stabilization of AuNPs with alkenethiols of various chain lengths.33 This two-phase, thiolate-stabilized method was more clearly illustrated by the Schiffrin group in 1994,34 and it has enabled researchers to synthesize AuNPs at lower temperatures with relatively high stability and facile size control. In the same year, Reetz et al. also reported an electrochemical synthetic strategy for metal nanoparticles.35 This electrochemical technique involves nonaqueous media where the dissolved metals from the anode and the intermediate metal salts are reduced at the cathode. The stabilizer, usually a tetraalkylammonium salt, is required to avoid indiscriminate aggregation in solution as well as to prevent all particles from plating at the surface of the cathode.36 Due to the advantages of low cost, modest equipment, and ease of controlling the yield and size of the nanoparticles by adjusting the current density,37,38 solution-phase electrochemical synthesis of plasmonic metal nanoparticles is an important route to keep in mind.39−41 666


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combined analysis of measured distance between particles from TEM and calculated pairwise interaction forces suggested that the hydration forces become effective only when the nanoparticles are separated by a few water molecules, and in other situations, electrostatic repulsions and van der Waals attractions dominate the pairwise interaction. The metastable transient nanoparticle pairs occur when the distance between the particles is around 0.5 nm, and the particles’ proximity induces vanishing hydration forces and resulting attachment.

In this review, to have a clear distinct definition between ligands and shells (described in section 3), the term ligand will be limited to general, small molecules or compounds which form a coordination complex, mainly via metal−sulfur bonds, with the metal core but do not have strong intermolecular interactions between ligands. 2.1. Theoretical Background of Colloidal Stability of the Plasmonic Nanoparticles

Before exploration of various stabilizing agents in colloidal states, it is valuable to discuss the theoretical background which has supported the colloidal nanoparticles’ behaviors. Derjaquin−Landau−Verway−Overbeek (DLVO) theory has been widely used to study the behavior of colloidal particles and is therefore very applicable to this review. According to DLVO theory, the interparticle behavior in colloidal science is dominated by the interplay of attractive van der Waals forces and repulsive Coulombic forces.52 The classical DLVO theory calculates the total interaction potentials between two particles as the sum of the van der Waals attraction and the electrostatic repulsion. The size and the electrical double layer of the two particles are used to express the electrostatic repulsion potential along with other parameters. For the attraction force, the Hamaker constant plays a crucial role in the description of attraction energy between the particles. The Hamaker constant, which can be calculated based on the Lifshitz theory, refers to the relative strength of the attractive forces between the two surfaces.53 For the colloidal particles, the Hamaker constant can be used to estimate the forces between two particles of the same material separated by a continuous medium.54 More details and the full set of related equations can be found in previous studies.53 DLVO theory has often been used to interpret experimental results. In 2005, Kim et al. manipulated the interparticle interaction of the AuNPs to control the size of AuNP aggregates.55 The citrate-capped AuNPs underwent ligand exchange with the addition of benzylmercaptan ions, resulting in increased particle size due to aggregation. With the experimental results and the calculated interaction potentials based on DLVO theory, the authors suggested that the addition of benzylmercaptan ions lowered the energy barrier and reduced the colloidal stability of AuNPs via decreased surface potential and increased ionic strength, resulting in destabilization of the nanoparticles and aggregation. In another study, aggregation kinetics of the citrated-capped AgNPs were investigated in aqueous sodium chloride solutions.56 As the concentration of the sodium chloride increased, the attachment efficiency of aggregating particles also increased. The attachment efficiency, which can describe the aggregation kinetics of AgNPs, was both experimentally determined and theoretically calculated using a Hamaker constant for citratecoated aqueous AgNPs in varied ionic strengths. The results from experimentally obtained attachment efficiencies showed remarkable agreement with the values from DLVO predictions. Furthermore, this study compared the aggregation kinetics of PVP-coated AgNPs with citrated-coated AgNPs, and a significantly higher stability was found for PVP-coated AgNPs, probably due to steric repulsion imparted by the adsorbed PVP molecules. Recently, Anand et al. examined the solvation force between two adjacent CTAB-coated AuNPs by using in situ transmission electron microscopy and pairwise interaction forces derived from a fit function of repulsive forces, hydration forces, and van der Waals forces.57 The

2.2. Conventional Surfactants for in-Solution Synthesis and Stabilization

In the citrate reduction method, citrate anions reduce metal ions to atoms and stabilize clustered atoms, resulting in colloidal nanoparticles. Citrate-stabilized metal nanoparticles have played a crucial role as a fundamental material in a number of gold nanoparticle-based plasmonic applications. These days, as the applications of synthesized nanoparticles require more robust and versatile platforms, citrate-stabilized AuNPs are usually an intermediate product before further treatments. However, the citrate reduction method remains the most popular strategy to produce noble metal colloids with easily exchanged surface species. Until recently, many studies have relied on this facile synthesis method despite the fact that the exact structure/ orientation of citrate anions on gold and other metal surfaces were not well known. Recent studies have more closely analyzed the interaction between metal nanoparticles and citrate ions.58−60 AuNPs are positively charged during the gold ion reduction reaction. Charges on the gold nanoparticle surface are then neutralized, and the AuNPs are negatively charged due to the adsorbed citrate ions.61 The adsorbed citrate layers stabilize the AuNPs via electrostatic repulsions. There are a small number of published manuscripts that focus deeply on the citrate−metal interaction. In one example, Park et al. investigated the structure of citrate layers on gold nanoparticles via attenuated total reflectance infrared spectroscopy and X-ray photoelectron spectroscopy and concluded that, on a Au surface, η2-COO− coordination of the central carboxylate group of the dihydrogen citrate anions is dominant.62 The adsorbed citrate anions interact with adjacent citrate molecules through hydrogen bonds and van der Waals interactions, thus forming a self-assembled layer of 8−10 Å in thickness; steric repulsions between citrate anions provide dispersion stability of the particles in solution. To use the citrate during the nanoparticle synthesis more efficiently, many studies have introduced a secondary reducing agent along with the citrate and have widened the range of nanoparticles from the citrate-reduction solution synthesis. For example, in 2014, Bastus et al. synthesized highly monodisperse sodium citratecoated AgNPs with varying diameters by kinetically controlling the seed-growth method with sodium citrate and tannic acid as reducing agents.63 The researchers suggest that the functions of citrate as both a stabilizing agent and a weak reducing agent disturb the efficient and fast nucleation and growth of AgNPs wherein a monomer of silver ions (Ag42+) and oxidized citrates led to rather slow and heterogeneous nucleation and finally to a polydisperse AgNP product. Tannic acid was added to enhance the reduction reaction performance and achieve improved size control. The amount of tannic acid was carefully controlled to induce fast reduction and to avoid the formation of intermediate complexes so that homogeneous growth was possible. The synthesized particles showed improvement in 667


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Figure 2. (a) Transmission electron microscopy (TEM) images of regrown AuNPs from bipyramidal seeds in different conditions (1−5, singular surfactant; 2−10, binary surfactants. Scale bars, 200 nm for low magnification, 50 nm for high magnification). Different colors of arrows indicate the detailed condition for regrowth. Reproduced with permission from ref 69. Reproduced with permission under Creative Commons Attribution 4.0 International License http://creativecommons.org/licenses/by/4.0/. (b) TEM images of citrate-stabilized gold dimers. Particle diameter is 80 ± 2 nm. Distances in the figures indicate the gap between the particles. Scale bars are 100 nm. Reprinted with permission from ref 70. Copyright 2015 American Chemical Society.

sodium borohydride and L-ascorbic acid as reducing agents and trisodium citrate as a capping agent.64 As mentioned above, the role of CTAB cannot be neglected when reviewing synthesis methods for AuNPs. The Murphy group showed that anisotropic AuNPs could be obtained when the surfactant CTAB is coordinated with another mild reducing agent.65 The exact role of CTAB in the synthesis is still under debate,65,66 but it is obvious that in the final product, CTAB plays a role as a stabilizer in colloidal dispersion by protecting the gold from aggregation or dissolution. It is generally accepted that CTAB is present as a bilayer on the gold surface via electrostatic interactions, where the ammonium headgroups in each layer are facing the Au surface and bulk solvent media, respectively, and long hydrocarbon chains in both layers are located between the two sets of headgroups.42 One impactful aspect of this CTAB adsorption is that the packing density of bilayers on the side

size control (with a range from 16 to 118 nm diameter) and narrow size distributions. By changing the ratio of the two reducing agents and using the seed-mediated growth method, these nanoparticles also showed long-term colloidal stability, similar to that achieved for AgNPs stabilized using PEG, PVP, or bovine serum albumin (BSA). Interestingly, despite their similar colloidal stabilities, citrate/tannic acid-coated AgNPs exhibited improved ability as a catalyst in the electron transfer reaction between Rhodamine B and borohydride ions compared to PVP-coated AgNPs, likely due to the less dense AgNP surface coating with layers of citrate/tannic acid. The same synthetic strategy has been applied to Au ion reductions to obtain AuNPs smaller than 10 nm.26 The enhancement in monodisperse AgNP synthesis by focusing citrate as a stabilizing agent can be found in other studies such as the work of Haber et al., where the production of Ag nanoprisms with high stability and reproducibility was achieved with 668


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Figure 3. (a) (i) Schematic illustration of the process of fabricating Ag@Au cuboctahedra and Ag@Au concave cuboctahedra from a Ag cuboctahedron template. (ii) SERS spectra of 1,4-benzenedithiol adsorbed on each of three different nanoparticles at an excitation of 785 nm: (blue) Ag@Au concave cuboctahedra; (red) Ag@Au cuboctahedra; (black) Ag cuboctahedra. Intensity from Ag cuboctahedra substrate was 20 times multiplied. (bottom). Reprinted with permission from ref 82. Copyright 2016 American Chemical Society. (b) Scanning electron microscopy (SEM) images of Au hexagonal nanoprisms (left) and Au triangular nanoprisms fabricated by the PVP-induced photochemical irradiation− reduction method. Insets in each image show high-magnification SEM images (left insets from each) and the elemental distributions of nanoscale secondary-ion mass spectrometric images showing the 12C14N signals from PVP (green) and 127I signals (blue), respectively (right insets from each). Iodide ions were included to facilitate the production of a sharp triangular shape. Scale bars in all insets are 200 nm. Reprinted by permission from Springer Customer Service Centre GmbH: Springer Nature, ref 83. Copyright 2016.

and the tips of the Au nanorods are different due to the curvature at the tips, allowing site-selective further shape modification or surface functionalization.67,68 For instance, to take advantage of how nanoparticle morphology impacts the LSPR, bipyramidal AuNPs were synthesized with CTAB surfactant as a stabilizer and shape-guiding agent.69 Furthermore, with variation of the ratio of binary surfactants including two of the following, CTAB, cetyltrimethylammonium chloride, and benzyldimethylhexadecylammonium chloride, the colloidal stability of bipyramidal AuNPs was finely controlled, resulting in AuNPs with multiple novel morphologies via site-selective regrowth and etching (Figure 2a). When only one surfactant was used in the growth step, only size augmentation of the bipyramidal AuNP was observed. However, with the combination of two different surfactants, tip regions of the AuNPs overgrew, likely because of the exposure of less protected crystalline features based on the different binding affinity of the surfactants. This study reinforced the vital role of CTAB in stabilizing colloidal AuNPs and the application of surfactants to induce desirable AuNP morphology, which is critical for potential applications in optics or surface-enhanced Raman spectroscopy (SERS). In another study, citrate-capped AuNPs were investigated for

adjacent particle interactions. Yang et al. measured emission polarization from close AuNP dimers with varied internanoparticle gap widths.70 The samples were prepared by dropping AuNP dispersions onto a sample grid; since AuNPs were citrate-stabilized, varying gap distances were obtained randomly during solvent evaporation (Figure 2b). The authors argue that gap sizes less than a nanometer were feasible because particles were citrate coated, where van der Waals squeezing and capillary forces reduced the interparticle distances; thus, gap distances this small would be hard to achieve with more robust stabilizers, such as oxide shells. These varying gap distances between AuNP dimers enabled the investigation of polarization states of scattering and even the quantum effects from dimers forming quantum range gap distances. PVP is a nonionic polymer widely used in nanoparticle synthesis, especially for AgNPs. Possessing both a highly polar amide group in the pyrrolidone ring and a nonpolar alkyl backbone makes PVP highly soluble both in water and in nonaqueous solvents.71 Generally, PVP can act as a stabilizing agent in colloidal metal nanoparticle dispersion via the repulsive forces from its hydrophobic carbon chains and benefits from inert physicochemical properties over a broad 669


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Figure 4. (a) (i) Raman and SERS spectra of DOX molecules in four different conditions. Normal Raman spectrum of DOX molecules (1); SERS spectrum of DOX when bound to AuNPs at pH 7.4 (2)/pH 5.4 (3). SERS spectra of AuNPs without close DOX (4). (ii) Schematic diagram of DOX chemical structure (1) and DOX-conjugated, PEG-functionalized AuNPs at pH 7.4 (2)/pH 5.0 (3). (iii) Fluorescence spectra of Free DOX without AuNPs (1), bound to AuNPs (2), when DOX molecules are released from AuNPs (3), and AuNPs without DOX (4). Reprinted with permission from ref 89. Copyright 2013 American Chemical Society. (b) Schematic diagram describing the process of fabricating PEGfunctionalized AuNPs. One type of the particles is conjugated with malachite green isothiocyanate (MGITC) and f-PSA antibody, and the other type is conjugated with X-rhodamine-5-(and-6)-isothiocyanate (XRITC) and c-PSA antibody. Reprinted with permission from ref 90. Copyright 2017 American Chemical Society.

pH range.72 Even though a nanoparticle-adsorbed PVP layer is often categorized as a shell, it is included in this section due to its common use as a stabilizer during the synthesis step and relatively weak adsorption on the metal surface, which is similar to other popular molecular stabilizers, such as citrate and CTAB. The polyol synthesis, which is the most popular method to produce AgNPs in solution, was introduced by Xia et al. in 2002 as a system for the preparation of Ag polyhedra, where a diol solvent reduces the Ag salt at high temperature with PVP as a capping agent.73 Significant studies over the past decade have studied the interaction between PVP and Ag nanocrystals during and after synthesis, and it is clear that PVP plays a critical role in the Ag polyol method via stabilization of lowest energy crystal {100} facets.71 This surface-selective adsorption of PVP on AgNPs has been examined in many ways, such as Raman, IR, and X-ray photoelectron spectros-

copy.39,74,75 Among the various approaches, Saidi et al. investigated the PVP−AgNP interaction through density functional theory.76 In the study, the authors found that the interaction between PVP and {100} and {111} Ag crystal facets occurs via direct binding and van der Waals forces. The study clearly demonstrated that the PVP molecules bind to Ag in a flat conformation, and the binding energy of oxygen atoms in the carbonyl group is stronger with Ag{100} than Ag{111}. The surface-selective stabilization of PVP on AgNPs can be used to alter the morphology of the nanoparticles during synthesis. Xia and co-workers showed that with different concentration and molecular weight PVP, they achieved different AgNP shapes, including cubes, truncated cubes, and octahedra.77 The authors suggest that the concentration of PVP changes the surface free energy of Ag facets and that the molecular weight can affect the effective coverage of PVP on 670


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the Ag surface, both resulting in altered final morphologies of the particles. Other morphologies, such as pentagonal wires, bipyramids, and decahedral AgNPs, have been demonstrated with the polyol synthesis.14,78,79 The PVP-stabilized Ag nanocube synthesis method has been investigated and further refined in many ways, such as in seed-mediated growth.46 More recently, PVP has also been applied to AuNP synthesis, where PVP adsorption enables the formation of triangular plates, octahedra, and other morphologies of AuNPs.80 Interestingly, for AuNPs, PVP binds primarily to {111} facets. A recent computational study by Liu et al. reasoned that in the presence of PVP, {111}-faceted Au nanostructures are thermodynamically more favorable.81 In addition, researchers suggest that different from Ag{100}, which does not reconstruct, Au{100} shows surface reconstruction, resulting in PVP binding preference to Au {111} facets. As shown, due to its excellent ability in shape control and colloidal stabilization, PVP-coated Ag and AuNPs with novel morphologies are commonly observed. For example, in 2016, Zhang et al. synthesized Au-coated Ag concave cuboctahedra as a SERS monitoring platform.82 In this study, Au was first deposited on the colloidal PVP-stabilized Ag cuboctahedra; during the initial deposition, Au atoms covered the entire surface of the Ag nanoparticles. In the continuing and subsequent Au deposition steps, Au was preferably deposited onto the {100} Au facets due to the selective passivation of the {111} Au facets by PVP, leading to a concave cuboctahedral structure. This alloyed nanoparticle proved to be a more efficient SERS probe than the original Ag cuboctahedral nanoparticles with a 70-fold higher SERS intensity for the same analyte (Figure 3a). The particle also exhibited fine performance for in situ SERS monitoring of 4-nitrothiophenol reduction, with colloidal stability maintained throughout the reaction. The particles showed high stability against hydrogen peroxide etching as Ag was fully coated by Au. In another example, Zhai et al. observed the effect of PVP in a plasmondriven synthesis of gold nanoprisms.83 The authors revealed a unique function of PVP, that of preferentially adsorbing on the perimeter of Au nanoprisms, inducing the anisotropic growth of Au nanoprisms (Figure 3b). Under photochemical irradiation, it was observed that the PVP adsorbed onto the AuNPs prolonged the hot-electron lifetime to expedite the reduction of Au ionsnot a usual capability of PVP in normal conditions. The authors suggested that adlayered PVP molecules were capable of stabilizing electrons generated through plasmon excitation. As described here, PVP can act not only as a stabilizing agent but also as a morphologyinducing agent like other weakly binding molecules such as CTAB or citrate.

properties of Ag and AuNPs more effectively.88 In 2013, Kang et al. used PEG stabilization when performing plasmon-tunable Raman/fluorescence imaging spectroscopy with anticancer drug-loaded AuNPs.89 When doxorubicin was attached to peptide-functionalized PEGylated AuNPs via peptide−drug conjugation, the SERS spectrum of the doxorubicin could be detected while its fluorescence was quenched, indicating the short distance between the drug and AuNPs. However, upon release, a reduction in Raman enhancement was observed and the fluorescence signal became apparent (Figure 4a). This selective “on”/“off” behavior took place inside the lysosomes of a malignant epithelial cell, where high colloidal stability is required. In another example, Cheng et al. developed a SERSbased immunoassay for prostate cancer;90 two types of prostate-specific antigens (PSAs) were simultaneously detected, since the ratio of the two antigens is crucial for accurate analysis and diagnosis. Two different SERS nanotag molecules were adsorbed onto a single AuNP, which were further functionalized with thiolate-PEG-COOH ligands. The carboxyl groups of these ligands were conjugated with antibodies for the two antigens (Figure 4b). By measuring the SERS spectra of the two SERS tags, the quantification of each antigen was achieved in the SERS-based assay, opening a strong potential for more accurate diagnosis of prostate cancer. AuNPs played a crucial role in this SERS-based assay, and PEG on the surface enabled the detection of proteins in clinical samples by maintaining their plasmonic properties and supporting antibody conjugation. 2.4. Biomolecular Ligands Stabilized Plasmonic Nanoparticles

The physiological fate of plasmonic nanoparticles is of great interest to many scientists since the behavior of nanoparticles is related not only to their functionalities but also to their toxicity and induction of inflammation.91−93 Because the usual access for many intended biomedical applications, such as disease diagnosis and drug delivery are intravenous, the interaction and the stabilization of the nanoparticle with blood plasma are very important. Achieving nanoparticles that will be colloidally stable, biocompatible, and functional in blood is difficult when using stabilizing agents such as citrate, CTAB, or PVP, which have insufficient stability in high ionic strength and can be cytotoxic.94,95 Thus, proteins hold great promise as coating agents for biomedical applications.96 Their high molecular weights, charges, complex but well-defined structures, retained multifunctional chemical groups, and high affinity for metal surfaces have attracted many researchers who seek stable metal nanoparticle coating materials for use in biological matrices. The most widely used protein in this context is bovine serum albumin (BSA). However, due to the structural complexities and difficulties in controlling BSA during functionalization, this protein has been used mainly as a secondary stabilizing agent to enhance the functionalities of nanoparticles, and a number of researchers debate the stabilizing quality of albumin on colloidal AuNPs.97 However, many reports have shown use of BSA as a coating material for plasmonic nanoparticles and the enhanced stability of BSAcoated AuNPs. One such study in 2012 by Khullar et al. demonstrated the synthesis of BSA-conjugated AuNPs for biomedical applications.98 The BSA was unfolded for efficient coating of AuNPs via interactions with anionic, cationic, and zwitterionic surfactants and temperature control during synthesis. The unfolded state enabled the reduction of gold

2.3. PEG Ligands Stabilized Plasmonic Nanoparticles in Complex Matrix

Another commonly used molecular functionalization of plasmonic nanoparticles is the use of polyethylene glycol (PEG)-stabilized nanoparticles for a variety of applications.84−86 PEG-based stabilization offers two major advantages, especially in vivo where steric repulsions inhibit colloidal aggregation and imbue resistance to protein adsorption and uptake by the mononuclear phagocytic system.87 In many biomedical or therapeutic applications, where nanoparticles need to be dispersed in highly complex media, Ag and AuNPs are surface functionalized with thiolated-PEG ligands through strong metal−S bonds to stabilize and exploit the plasmonic 671


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Figure 5. Protein-stabilized plasmonic nanoparticles. (a) Surfactant-free, protein-coated colloidally stable Au nanorods. BSA-coated AuNPs with three different aspect ratios dispersed in DMEM + 10% newborn bovine calf serum were able to be dispersed in the same media after being freezedried and were stable in a high concentration of Au nanorods of 20 mg/mL. Reprinted with permission from ref 99. Copyright 2015 American Chemical Society. (b) Citrate-stabilized small AuNPs are first coated with BSA and then adsorbed onto large citrate-stabilized AuNPs. Reprinted with permission from ref 100. Copyright 2016 American Chemical Society. (c) Schematic illustration of the later flow assay for phospholipase A2 (PLA2). Liposomes containing biotyinlated PEG linkers were incubated with phospholipase A2 and cleaved, and the PEG linkers were released. The polystreptavidin-coated AuNPs were added to the mixture, and the solution was transferred to a lateral flow strip. Green line preprinted with streptavidin on the strip turned to red due to the biotin−streptavidin affinity and formed multivalent AuNP networks. Reprinted with permission from ref 101. Copyright 2015 American Chemical Society.

inducing colloidal stability as well as satellite-to-satellite and core-to-satellite plasmon coupling.100 This was accomplished when Höller et al. synthesized citrate- or tannic acid-stabilized AuNPs/AgNPs (5−21 nm) and then coated those particles with BSA via ligand exchange. The BSA-coated particles were adsorbed on the surface of larger, citrate-coated AuNPs (84 nm) randomly, leading to a disordered distribution of small particles on the larger cores (Figure 5b). The authors reasoned that the high colloidal stability of the protein-coated satellite nanoparticles enabled the highly concentrated particle suspensions, which is essential in core−satellite cluster formation. The weakly bound initial citrate ligands provided sufficient colloidal stability, and they could be easily removed during the adsorption of BSA-coated satellite nanoparticles. Thus, BSA played an important role in small/large particle stabilization in colloidal states and as a soft spacer to achieve plasmon coupling between small/large plasmonic particles. Of course, BSA is not the only protein that acts as a ligand for plasmonic nanoparticle surface stabilization. In 2015, Chapman et al. designed a lateral flow bioassay for phospholipase A2, a potential biomarker for diagnosing diseases such as pancreatitis and prostate cancer.101 For the assay, polystreptavidin-coated AuNPs were synthesized, forming an interparticle aggregation via PEG−biotin linkers, which are released upon the enzymatic activity of phospholipase A2 to liposome. These

ions due to the presence of several reducing amino acids such as cysteine. Additionally, BSA can act as an excellent capping/ stabilization agent for growing Au nucleation centers, which is rather hard to achieve with folded BSA structures. The BSAcoated AuNPs showed colloidal stability in various pHs and low cytotoxicity with no hemolytic response, a stark contrast to traditional surfactant-capped AuNPs. The decreased hemolytic response indicates the nearly complete passivation of crystal planes of AuNPs, which is also hard to achieve with a common surfactant coating. As mentioned above, anisotropic AuNPs and their resultant plasmonic properties are of great interest and the cationic surfactant, CTAB, usually plays a crucial role in synthesis and stabilization. However, CTAB displays significant cytotoxicity and must be removed before use in biomedical applications. In one example, Tebbe et al. nearly completely removed CTAB from Au nanorods and replaced it with BSA (Figure 5a).99 Fast and efficient ligand exchange for BSA showed no LSPR shift, and the protein-coated Au nanorods showed higher colloidal stability in high ionic strength conditions such as phosphatebuffered saline or cell culture medium. The particles were also able to be lyophilized to powder and redispersed in media with the same optical and colloidal properties as before lyophilization due to the robust protein coating. In another study, BSAcoated small AuNPs/AgNPs formed clusters on larger AuNPs, 672


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Figure 6. DNA-functionalized AuNPs. (a) Schematic representation of a AuNP dimer linked by long double-stranded DNA featuring a single ATTO647N fluorescent molecule (left). Single-emitter lifetime spectra of the 80 nm AuNP dimer featuring the single dye. Experimental data (red) and estimated value from the instrument response function (IRF, black dots). Lifetime was estimated below 10 ps (center). Rotational averaged fluorescence enhancement factors for three different dimer antennas in solution, extracted from the measurement and Mie theory calculations. (right). Reprinted with permission from ref 121. Copyright 2016 American Chemical Society. (b) Schematic illustration of nanoclusters of AuNPs linked with CdSe/ZnS quantum dots via DNA strands and spectra of photoluminescence when nanoclusters were excited by a 530 nm laser (circle dots) and 440 nm laser (square dots). Reprinted with permission from ref 122. Copyright 2015 American Chemical Society. (c) Schematic illustration of DNA-based Au nanorod dimers and chlorin e6-attached upconversion NP assemblies for multifunctional biotherapy applications and TEM images of the nanostructures. Scale bars represent 50 nm. Reprinted with permission from ref 125. Copyright 2015 John Wiley and Sons. (d) Illustration of the molecular dynamics simulation of a colloid model predicted a rhombic dodecahedron equilibrium crystal structure for the DNA− gold nanoparticle system (left). Experimental result of SEM image of microcrystal with well-defined facets. Small AuNPs are 20 nm (right). Scale bar is 1 μm. Reprinted by permission from Springer Customer Service Centre GmbH: Springer Nature, ref 131. Copyright 2014.

plasmonic nanoparticles, the utility of nanoparticle-bound DNA hybridization/assembly presents an exciting range of potential applications for these functionalized nanoparticles, taking advantage of programmable DNA base-pairing interactions.108,109 In 1996, Mirkin et al. first demonstrated the use of DNA-functionalized AuNPs for the detection of target DNAs via programmable assembly of two complementary DNA strands.110 Since this report, DNA−nanoparticle studies in various fields have resulted in a number of interesting applications in SERS, optics, energy transfer, etc.108 Before discussing how DNA-coated AuNPs assemble, the basic synthetic scheme and related properties will be reviewed here. For DNA conjugation on the nanoparticle surface, singlestranded DNA oligonucleotides are most frequently used. Since presynthesized AuNPs and DNA strands are both negatively charged, the synthetic solution requires a certain ionic strength to overcome the electrostatic repulsion forces between the DNA and the AuNP surfaces. At this step, a colloidal stability problem arises due to the aggregation of AuNPs because of charge-screening high ionic strength media. Thus, fine control of salt concentration is necessary for successful DNA coating of AuNPs without aggregation. To bypass this synthetic difficulty, other methods have been proposed,111,112 but this salt-mediated DNA functionalization method is still the most widely used.113 Typically, once 5′thiolated DNA strands have been layered on the AuNPs, particles can be made colloidally stable via steric stabilization

polystreptavidin-coated AuNPs, aggregated by streptavidin− biotin affinity, can induce interparticle surface plasmon coupling, which can be read with the naked eyes when loaded on a nitrocellulose membrane lateral flow strip (Figure 5c). 2.5. DNA and Other Highly Functional Ligands: Stabilization and Assembly

As shown in the previous section, biopolymers possess high potential as stabilizing agents for colloidal plasmonic nanoparticles based on their biocompatibility that will allow various applications in biological diagnostics or sensing. DNA is another biomolecule which has been widely used as a ligand to stabilize nanoparticles in different colloidal conditions. The strong negative charge on the phosphate backbone in DNA contributes high electrostatic repulsion forces to the nanoparticles, and the DNA layer also provides steric stabilization. In most cases, DNA or oligonucleotides are functionalized with alkylthiols at the ends to covalently bind to the metal surfaces.102 In previous research, DNA-stabilized AuNPs showed noticeable colloidal stability in high ionic strength103 or in complex media such as seawater.104 Beyond thiol-driven modification, adsorption-driven stabilization has also been reported using DNA, especially for the synthesis and stabilization of fluorescent DNA-Ag nanoclusters.105−107 Thus, DNA surface-modified nanoparticles have attracted much research attention due to their great potential in analytical, materials, and medical applications.108 While DNA itself has great stabilizing potential when covalently attached to 673


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via the DNA assembly method.122 The quantum dots and AuNPs were functionalized with aminated and thiolated singlestranded DNA, respectively. Heteronanoclusters of AuNPs− DNA−quantum dots with satellite-like structures were fabricated (Figure 6b). All spectroscopic experiments were performed in high ionic strength buffer, and no aggregation or loss of plasmonic properties of AuNPs was reported, indicating the high colloidal stability of AuNPs/quantum dot nanoclusters. In this study, the length of DNA ligands played an important role in controlling and tuning the plasmon−exciton interaction and the optical behavior, including photoluminescence quenching and enhancement. These colloidal nanoclusters represent plasmon-assisted light-harvesting systems to transfer light collected by quantum dots to the plasmonic reaction center of AuNP cores. DNA-driven dimer AuNP formation can be found in other types of applications as well. For example, in phototherapy, the engineering of multifunctional nanoparticle platforms for simultaneous imaging and therapeutic treatment holds great promising tools for enhancing current imaging techniques such as MRI, X-ray, or CT.123,124 In 2016, Sun et al. synthesized nanorod dimers further functionalized by the chlorin e6-attached upconversion nanoparticles, NaGdF4.125 In this research, a nanorod dimer functioned as a photothermal therapeutic agent, and NaGdF4 acted as a photodynamic therapeutic agent. DNA played a crucial role in forming the gold nanorod dimer. With careful conjugation design, two complementary oligonucleotides were selectively bound to the sides of two nanorods, respectively, and the nanorod dimers formed via DNA hybridization (Figure 6c). Even though the entire cluster platform was stabilized by a layer of dense polymer, the DNA-driven dimer was able to maintain its geometric conformation throughout the experiment due to the precise gap formed by DNA hybridization. The Au nanorod dimer nanoclusters showed higher photothermal conversion efficiency and photostability than the Au nanorod monomer clusters due to plasmonic coupling and formation of an electromagnetic “hot spot”, and they also showed fine performance for in vivo tumor therapy and as imaging agents. Nanoparticle assembly using DNA has expanded to threedimensional arrangements, known as crystallization of nanoparticles, whereby a single nanoparticle itself can be viewed as analogous to an atom.126,127 DNA-mediated assembly can be an ideal tool for the systematic crystallization of nanoparticles, since different DNA designs and interactions among DNAfunctionalized nanoparticles can be tuned to produce a number of unique crystalline structures.128,129 The challenge in this work is to control and fabricate a structure with a desired crystal symmetry and lattice spacing similar to that of atomic crystallizations. Among the different types of nanoparticles, plasmonic nanoparticles are surely promising candidates since the optical properties of the plasmonic nanoparticle can evolve via plasmonic coupling within periodic layers of nanoparticle structures. For three-dimensional DNA assembly, energyrelated stability is key in synthetically programmable colloidal crystallization. After appropriate single-stranded DNA ligands have been functionalized on AuNPs, sequence-programmed linker oligonucleotides are introduced into the colloidal system. From the energy minimization standpoint, each AuNP adopts the conformation that will maximize the number of neighboring particles via DNA ligand hybridization. A welldefined and close-packed crystal can be formed by slowcooling methods whereby the crystal formation is driven by

and electrostatic repulsions of the DNA. Due to these dual stabilization effects, DNA-coated AuNPs showed enhanced colloidal stability in high ionic strength solutions compared to citrate-coated AuNPs.103 The single-stranded DNA-functionalized plasmonic nanoparticles enable various application opportunities to utilize the plasmonic properties of the core metal with DNA-driven assembly. In the past decade, plasmonics have been adopting classical antennae concepts from the field of optics. Within optics, it is known that carefully fabricated metal nanoparticles with sophisticated assembly of two or more nanoparticles have great potential to decrease luminescence lifetimes and maintain a high emission quantum yield.114 In 2011, Busson et al. demonstrated AuNP dimers linked by a single DNA double strand, producing a substantial scattering cross-section and plasmon coupling.115 This research revealed the important relationship between colloidal stability during the DNA conjugation on the AuNPs and the resulting plasmon coupling effect from particular geometric conformations. Electrophoresis has been proven to be an efficient way to purify AuNPs conjugated with a known number of DNA single strands,116 but in typical electrophoretic purifications, the diameter of the AuNP generally has to be small enough (less than 20 nm) and the grafted DNA strands need to be between 90 and 100 bases long.117,118 To satisfy the need in the application of AuNP groupings where larger diameter AuNPs and smaller interparticle gaps are required,119,120 researchers have studied efficient purification and synthesis methods to generate AuNPs as large as 36 nm in diameter with grafted DNA strands as short as 19 nm. To maintain the colloidal stability of these AuNPs, they were first coated with bis(p-sulfonatophenyl)phenylphosphine. This labile stabilizing agent shows weaker affinity to the gold surface than DNA but stronger binding affinity than the citrate surfactant, so it can not only achieve colloidal stability but also be displaced by DNA strands in the presence of an ionic strength sufficient for charge screening (which would be too harsh an environment for citrate-coated AuNPs). The particles were further stabilized by passivation via thiolated-PEG oligomers before electrophoretic purification, which relies on surface charges and a different size of DNA−AuNP conjugates to induce separated bands. During the synthesis thiolated-DNA strands on the AuNPs were lengthened by hybridization to overcome midpurification aggregation and to achieve a good separation of the products. After removal of the lengthening DNA strands and further purification, 36 nm diameter AuNPs conjugated to a single DNA strand of 10 nm in length were obtained. Upon assembly, the interparticle gaps measured were in good agreement with the lengths of the linker DNAs between the particles. A few years later, Bidault et al. fabricated DNA-templated colloidal gold dimer nanostructures that behave as single-photon emitters with short lifetimes and maintained the quantum yield.121 The same synthetic scheme with slight modification was applied as in the work from Busson et al., and the final products were 80 nm diameter AuNP-based dimers linked by a single DNA double strand and one dye molecule in the center (Figure 6a). The interparticle gap was found to be 14 nm, in excellent agreement with the length of the DNA molecule. This dimer system achieved up to an estimated 70% quantum yield and average luminescence lifetimes on the order of 50 ps, proving the benefits of the programmed DNA assembly designed from stabilized nanoparticles. In another study, CdSe/ZnS quantum dots were assembled around the AuNPs 674


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Figure 7. Ligand-assisted assembly of various plasmonic NPs. (a) Schematic illustration of light-induced AuNP assembly functionalized with PEGdiazirine ligands. Reprinted with permission from ref 132. Copyright 2017 John Wiley and Sons. (b) Illustrations of a Au nanorod functionalized with PEG and PLGA ligands and an assembled small vesicle. Reprinted with permission from ref 133. Copyright 2015 John Wiley and Sons. (c) SEM images and 3D illustrations of bypiramidal AuNPs formed on Si waters with different orientational packing orders. Scale bars indicate 100 nm. Reprinted with permission from ref 134. Copyright 2016 American Chemical Society. (d) Schematic representation of light-controlled reversible assembly of AuNPs. Solution contains light-responsive molecule, protonated merocyanine. Particles were functionalized with ligands terminated with carboxyl groups. Blue light irradiation increases the acidity of the solution, triggering the disassembly of the AuNPs assembled by hydrogen bonds. In the solution in ambient conditions or the dark the acidity drops and particles reassemble. Reprinted by permission from Springer Customer Service Centre GmbH: Springer Nature, ref 137. Copyright 2015. 675


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superstructures. The fabricated superstructures showed collective plasmonic coupling and different Raman enhancement factors depending on the orientational packing orders (Figure 7c). Interestingly, the length of the polystyrene ligands proved crucial in determining the nanoparticle packing behavior. The van der Waals forces among AuNPs and the steric hindrance force of the ligands needed to be balanced correctly to achieve the desired assembly. For example, if the length of the ligand was too long, the dominant force becomes steric hindrance and ordered lattices fail. As described above, many research efforts have worked to exploit the plasmonic properties of Au/Ag nanoparticles via systematic assembly, but sometimes assembly-driven functionalization of the nanoparticles can be burdensome and induce adverse effects on the performance.135,136 A new methodology was envisioned by Kundu in 2015 to overcome these limitations, whereby a medium consisting of light-switchable molecules drives colloidal stability in AuNPs functionalized with a pH-sensitive ligand, resulting in reversible aggregation and dispersion.137 In this system, AuNPs were functionalized with 11-mercaptoundecanoic acid, suspended in methanolic solution, and stabilized with a small amount of HCl. A photoswitchable molecule was synthesized and dispersed in the same medium and competed for protons with the −COOH group of 11-mercaptoundecanoic acid (Figure 7d). The suspension medium was comprised of this photoresponsive molecule and was capable of releasing or capturing protons. Due to the pH-sensitive nature of the ligand on the AuNPs, their colloidal stability was directly affected by the behavior of the photoresponsive molecules in the medium. The AuNPs were stable only under continuous light exposure, and without light they aggregated and precipitated. However, the existing ligands on the aggregated AuNPs could be disassembled again, and the colloidal stability was restored. Depending on the state of the AuNPs, different LSPRs were achieved. This reversible assembly-disassembly system was conducted over 100 cycles with no appreciable change found from the particles, proving the solid stability and functionality of this novel approach to control nanoparticle assembly.

thermodynamic forces not kinetic energy. The balance between entropic and enthalpic energy must be properly managed by temperature, size of particles, and length and binding strength of DNA.130 Recently, Auyeung et al. explored the packing behaviors of DNA-AuNP superlattices and discovered that in the described system the rhombic dodecahedron structure is the most thermodynamically favorable form for a range of different particle sizes (Figure 6d).131 The conclusion was that similar to the formation of atomic crystallization, DNA assembly-driven AuNP crystallizations also have the structure of Wulff polyhedra in which the crystal form of minimizing surface energy can be favorably shaped. This work is an interesting example of how DNA functionalization of plasmonic nanoparticles drives assembly and gap control due to the strong steric hindrance and electrostatic repulsion from DNA ligands using control of the DNA length to determine the physical gaps between adjacent nanoparticles. Along with DNA, there are other ligands that are able to induce the assembly of plasmonic nanoparticles for a desirable research platform with appropriate particle stabilization. Cheng et al. focused on the weak points of ligand-dependent assembly in previous research where controllable aggregates of AuNPs in vivo are not very viable.132 In such environments, lighttriggered assembly has great potential since light application can be localized and assembly of nanoparticles at a specific site is feasible. In one example, AuNPs were synthesized and functionalized with a PEG ligand having diazirine terminal groups. The diazirine groups were triggered by 405 nm laser irradiation and transformed to carbenes, which eventually formed covalent bonds with ligands of nanoparticles nearby (Figure 7a). The covalently cross-linked AuNPs showed the surface plasmon resonance peak at a longer wavelength due to their strong coupling behavior. The controlled red shift in the LSPR was achieved because the colloidal stability of individual AuNPs was maintained upon aggregation due to the existing PEG ligands on the gold surfaces. This photoinitiated assembly was successful in in vivo experimentation, proving the potential for an effective photothermal treatment platform for malignant tumors. Amphiphilic ligands can also be utilized to induce assembled plasmonic nanostructures. In 2015, Song et al. fabricated biocompatible and degradable plasmonic vesicles of assembled Au nanorods coated with hydrophilic PEG ligands and hydrophobic poly(lactic-co-glycolic acid) (PLGA) ligands (Figure 7b).133 This work demonstrated that each Au nanorod can be stabilized by a PEG coating, and the entire assembly can also be stabilized by extended PEG on the outside and inside of the vesicles. The formation of the vesicles was derived from an oil-in-water emulsion. The evaporation of oil and the resulting assembly showed significant red shifts in the LSPR (from around 790 nm to longer than 1000 nm) due to strong plasmonic coupling. The photothermal conversion efficiency of the assembled vesicles reached 51%, which is 2-fold higher than the 23% of uncoupled Au nanorods. Not ligand-driven but a solvent-evaporation-driven distinct orientational packing assembly was also reported in 2016.134 Bipyramidal AuNPs were synthesized and functionalized with hydrophobic, thiolated polystyrene via a ligand exchange method. The polystyrene-coated bipyramidal AuNPs were dispersed in chloroform and drop cast onto the surface of a convex-shaped water droplet on a silicon wafer. The evaporation of chloroform and water droplets led the formation of two-dimensional nanoparticle liquid crystalline

3. PLASMONIC NANOPARTICLES WITH SHELL COATING Ligand- or adsorbate-stabilized nanoparticles can be formed when ligand molecules are chemically or physically bound to the surfaces of the nanoparticle core. Often the nomenclature of the ligand or shell is used without distinction to describe the coating materials around the nanoparticle core. In this review, nanoparticle shells are defined as chemically and physically robust structures homogeneously covering the nanoparticle core. Shells are not easily removed or exchangeable as ligands are, even though no covalent links, such as Au−S bonds, may exist between cores and shells. Furthermore, shells provide another chemical environment not only to stabilize the cores but also to add desired functionality on the designed nanoparticles. In most cases, the shell is formed by strong chemical interactions or bonds among the shell chemical compositional units. In many cases, ligand-stabilized nanoparticles are synthesized first and then shell structures are formed via the ligand exchange method. The thickness of the shell sometimes can determine the distance between the plasmonic cores and the outer environments, and this distance gap can be used to tune the interactions between plasmonic cores and other molecules, such as fluorescent dyes. This 676


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Figure 8. (a) Visualized 3D reconstructed TEM images of silica-coated Au nanorod (left) and branched AuNPs (right). Silica shells form the mesoporous radial channels. Reprinted with permission from ref 151. Copyright 2015 American Chemical Society. (b) Schematic illustration of formation of silica-coated, CXCR4-functionalized Au nanorods. Particles are loaded in human-induced pluripotent stem cells and injected in vivo for photothermal tumor target therapy. Reprinted with permission from ref 163. Copyright 2016 American Chemical Society. (c) TEM image of silica-coated, gadolinium-hybridized Au nanorods (left). High-angle annular dark-field scanning TEM elemental mapping (HAADF-STEM) images of silica, oxygen, gold, and gadolinium localization in the silica-coated, gadolinium-hybridized Au nanorods (right). Reprinted with permission from ref 164. Copyright 2016 John Wiley and Sons.

shell surface properties can easily be modified due to reactive silanol groups on the surface of the silica. These silanol groups also contribute to negative zeta potentials in aqueous solutions and, thus, good colloidal stability due to electrostatic repulsion. Silica-shelled nanoparticle stability can be achieved through a relatively large pH range via surface modifications with species such as PEG−silane to add steric hindrance.138 Coating plasmonic nanoparticles, such as AuNPs, with a nonporous silica coating improves their use in various applications such as for photothermal treatment,141 photoacoustic imaging,142−144 and DNA quantification.145 Besides imbuing colloidal stability with a silica shell, if interaction of solution-phase analytes with the plasmonic

section will discuss recent trends and applications in plasmonic nanoparticles, focusing on three major types of shells: silica, organic polymer, and metal. 3.1. Silica Shell-Stabilized Plasmonic Nanoparticles

Silica nanoparticles themselves have high colloidal stability and dispersibility, and these properties are conserved when silica is used for coating materials.138,139 Silica is optically transparent in the visible region of the spectrum, and due to its chemical inertness, it is able to coat the core particle’s surface without sacrificing the ability to perform reduction−oxidation reactions at the surface of the plasmonic cores.140 Silica is known to be biocompatible; therefore, the cytotoxicity of the core nanoparticle is often reduced with the addition of a silica shell. Silica 677


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properties. Among the many different morphologies of AuNPs, anisotropic AuNPs, especially Au nanorods, have been heavily researched in recent years due to their distinct optical properties based on longitudinal plasmon resonances.42 Abadeer et al. used mesoporous silica-coated Au nanorods to elucidate the relationship between gold−fluorophore distance and plasmonic−fluorescence enhancement and quenching.152 Silica shells with a range of thicknesses from 11 to 26 nm were coated onto a series of Au nanorods that had a plasmonic extinction maximum ranging from 530 to 850 nm. An IR dye was covalently attached onto the silica shells, with the distances between the dyes and Au nanorod cores tuned by the range of shell thicknesses used. Steady-state fluorescence measurements were carried out with Au nanorods coated with dye-conjugated silica shells suspended in methanol. During the measurements, the plasmonic properties of Au nanorods and the distance between the dyes and the cores were maintained by the stable silica shell to reveal the fluorescence enhancement/quenching behaviors that resulted from the plasmon-generated electromagnetic fields. As was shown in this study, the thickness of inorganic shells can be a crucial factor for the application of plasmonic nanoparticles, and silica shell thickness can be tuned without detracting from the stability of the Au cores. Recently, the thickness of the silica shell was tuned down to around 2 nm with a large-scale synthesis, producing about 190 mg of the nanoparticles.153 The enhanced thermal stability of shell-coated AuNPs is critical in application spaces such as hyperthermic cancer treatment therapy, photoacoustic molecular imaging, and image-guided drug delivery.154−156 With appropriate excitation, plasmonic nanoparticles can produce heat following targeted delivery to particular in vivo destinations. Thus, the thermal stability of the plasmonic nanoparticles in a complex medium is critical. Several researchers have addressed this issue and shown that AuNPs have much lower melting temperatures than their bulk counterparts depending on NP size and structure.157,158 Silica coatings have been shown to increase the thermal stability of AuNP cores. Previous studies demonstrated that silica-coated AuNPs possess higher photothermal stability than CTAB or PEG-coated AuNPs under the influence of nanosecond higher energy laser pulses.159−162 In some studies, this silica shell-induced enhanced photothermal stability has been used to tune the LSPR and morphology of the AuNPs while they reside within the silica shell via oxidative etching or femtosecond laser pulse excitation; this method allows modification of the aspect ratio of AuNPs without irreversible dissolution or aggregation. For another in vivo application, Liu et al. designed and fabricated silica-coated AuNPs conjugated to an antibody for the chemokine receptor known as CXCR4.163 The prepared nanoparticles were loaded into human-induced pluripotent stem cells, which serve as the delivery agent, since they are capable of targeting tumor cells (Figure 8b). The performance of this platform was evaluated via photoacoustic tomography, two-photon luminescence for tracking the nanoparticle localization in the tumor tissue, and in vivo photothermal therapy toward gastric cancer cells in mice. Plasmon-related evaluations showed a prolonged retention time, good biodistribution characteristics, and therapeutic efficacy against the growth of tumors in mice, proving that the colloidal stability of the injected particles was maintained during the journey. In another recent biomedical application of silica-coated plasmonic nanomaterials, Wang et al. synthesized a single nanocomposite capable of photo-

nanoparticle surface is desired, silica shells can be synthesized to have sufficient porosity to allow analyte transfer to the plasmonic core. A porous silica coating still provides protective benefits and also allows access of the external media to the core structure. Since the early 1990s when micro/mesoporous structures of silicate (MCM-41) were observed with an electron microscope,146 mesoporous silica nanoparticles have been synthesized in various formats and applied in many different fields.147 Pore structures can play an important role in plasmonic catalysis or sensing, where the reactants or analytes must be placed within several nanometers of the plasmonic core.148,149 Effective roles for mesoporous silica have been frequently reported, especially for colloidal AuNPs. Zhang et al. studied the effects of porosity on the catalytic activity of silica-coated AuNPs.150 The permeation levels of the reactant, 4-nitrophenol, were tuned by the degrees of porosity within silica shells via etching time control. The fabricated silicacoated AuNPs showed more improved activity as catalysts for the reduction of 4-nitrophenol to 4-aminophenol as the porosity increased. The unetched silica shells were impenetrable for the reactant molecules to the Au cores, resulting in no catalytic conversion. Even though the changed shell morphologies increased the exposure level of the AuNPs to the outside media, the silica shell still maintained the stability of the AuNPs during the catalytic activity. Furthermore, the stability was proven to provide consistent catalytic behavior after 12 continuous cycles, with a nearly 100% conversion rate, while bare AuNPs underwent coalescence and aggregation after the first catalytic cycle. This porosity-dependent plasmonic performance of silica shell-stabilized AuNPs is also relevant for solution-phase SERS sensing. For example, Gao et al. investigated how the size of the pores and the size of various analyte molecules impacted SERS detection of those analytes.148 In 2015, Sanz-Oritz et al. synthesized novel mesoporous silica-coated branched AuNPs for colloidal SERS sensing.151 The authors optimized a synthesis method to obtain pore structures in the silica shells that are oriented radially from the core AuNPs to the outer bulk in such a way that analytes entering the pores will be adsorbed or placed very near to the surface of the AuNPs. The authors also improved the colloidal stability of the nanoparticles by heat treating dried nanoparticles; a lower temperature than is conventionally used was employed in this study so as to cause minimal deformation of the shape of the AuNP core. The silica shell in this study had three major roles: shells for AuNP cores, passageway for the analytes, and structural templates for branched gold growth. To improve SERS performance, the Au nanorods, preformed within radial silica shells, were overgrown and transformed to branched structures during the Au reduction catalyzed by Au nanorod cores through the silica shell pores (Figure 8a). The further reduction of the Au overgrowth and the formation of final structures occurred in the silica shell; thus, the aggregation was prevented, and controlled shape change was achieved. In application, the limit of detection for crystal violet in ethanol was lowered by 4 orders of magnitude when branched AuNPs were formed within the silica shells compared to the silica-coated spherical AuNPs. Due to the structural changes, where the sharp Au tip features render high electric field enhancement, mesoporous silica-coated branched AuNPs showed enhanced Raman signal intensity suitable for colloidal SERS measurements. In addition, the thickness of the silica shell can be well controlled and applied in the study of plasmonic nanoparticle 678


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Figure 9. (a) (i) Schematic illustration of PANI coating on Au nanorods via surfactant-assisted, oxidative polymerization. (ii) Oxidation−reduction reaction for PANI. (iii) Plasmon peak wavelengths during 200 switching cycles of PANI-coated Au nanorods at the oxidized and reduced states (bottom right). Reprinted with permission from ref 177. Copyright 2016 John Wiley and Sons. (b) Schematic illustration of aggregation and disassembly behavior pNIPAM-coated AuNPs at the critical temperature, Tc, induced by collapsing and swelling pNIPAM. Adapted figure and figure caption reproduced with permission from ref 179. (c) Description of inducing crosslike arrangement of Au nanodumbbell dimer via controlling steric hindrance and encapsulation of the dimer in PS403-b-PAA62 polymeric micelles. Reprinted with permission from ref 184. Copyright 2015 American Chemical Society.

thermal heating, in vivo tracing, and tumor-targeting drug delivery.164 In this nanocomposite, Au nanorods were first coated by mesoporous silica, and gadolinium was then loaded in and onto the pores/surfaces of the silica shells for highsensitivity bioimaging by enabling the simultaneous use of CT and MRI (Figure 8c). Further, the anticancer drug, doxorubicin, was loaded on the outside of the silica shell via electrostatic interactions, and finally, the composite was layered with hyaluronic acid for effective tumor targeting. The gadolinium doping onto the silica helped to avoid direct modification on Au cores, enhance the gadolinium loading capacity, and prevent undesirable toxicity from gadolinium ion leakage.165−167 Laser irradiation of the injected nanocomposite in vivo increased the penetrability of the nanocomposites into the inner regions of the tumors and induced disruption of the electrostatic interaction between the drug and the silica shell via the plasmonic-heating effect from the Au nanorod cores. Overall, the silica shells in this nanocomposite fulfilled multiple roles while protecting the colloidal stability of the inner Au cores.

line), poly(diallyl dimethylammonium) chloride, and poly(2hydroxy-3-methacryloxy-propyltrimethylammonium chloride) have all been commonly used in situ to produce Au or AgNPs coated by these charged polymers.169,170 Sulfurterminated polymers can also be applied during the metal salt reduction steps. McMormick and co-workers in 2002 mixed Au salts with various polymers bearing dithioester end groups and reduced the Au salts to AuNPs.171 The reduction of both the metal salt to metal nanoparticles and the dithioester groups of the polymers to thiol groups occurred simultaneously, leading to polymer-stabilized AuNPs. The high affinity of the thiol for Au surfaces promoted facile surface functionalization in situ. Thiol-functionalized polymers such as poly(N-isopropylacrylamide) (pNIPAM) and polystyrene (PS) can be prepared first and then mixed with Au precursors in a solution.172,173 The grafting-to method is sometimes used interchangeably with the in situ grafting synthesis method, but in this review the meaning is limited to methods where AuNPs or AgNPs are first prepared with loosely bound capping agents such as CTAB or citrate and then those surfactants are replaced with polymers via ligand exchange. During the exchange the polymers can bind to the metal surface covalently or via chemi/physisorption, depending on the properties of the polymers. In contrast, the grafting-from method first synthesizes metal nanoparticles functionalized with monomers, and polymerization reactions are done on the nanoparticle surface. Fabrication schemes can vary depending on the application intended for the nanoparticles. Many of the previously mentioned polymers, such as PS or pNIPAM, can coat the metal cores by either the grafting-to or the gratingfrom method.174,175 Particular polymers are chosen as the shell materials based on the chemical or physical characteristics that they will impart onto the plasmonic core. For example, creating a nanoparticle shell from conducting polymers, such as polyaniline (PANI), polypyrrole (PPy), or polythiophene (PTh), has great potential for electrical or electrochemical applications based on their conductivity as

3.2. Plasmonic Nanoparticles with Organic-Shell Coating

Due to the variety of chemical and physical characteristics of the organic polymers, they are applied to fabricate advanced and stable plasmonic nanocomposites for a variety of application areas.140 Grafting the metal nanoparticle surfaces with polymers can be categorized broadly into three groups, differentiated by how the polymers graft onto the metal surfaces during preparation: (1) in situ grafting methods, (2) “grafting-to” methods, and (3) “grafting-from” methods.168 For the in situ grafting method, either already-prepared polymers or monomers are introduced to the metal salt reduction solutions and serve as a shape-guiding and stabilizing agent. PVP, which has been discussed in previous sections, is one of the most widely used polymers for Ag and AuNP modification. Other polymers such as cationic polyelectrolytes can be introduced during synthesis to stabilize the final products via in situ grafting. Poly(2-(methacryloyloxy)ethyl phosphorylcho679


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well as high stability.176 In 2016, Lu et al. synthesized PANIcoated colloidal Au nanocrystals by seed-mediated growth wherein aniline monomers underwent oxidative polymerization to form a polymer shell around the Au cores with the help of the surfactant, SDS.177 It was expected that PANI would induce effective plasmonic switching with encapsulated Au cores and function to control the spaces between Au nanocrystals to avoid aggregation. The synthesized particles were placed in an electrochemical set up, and applied potential could drive PANI in the nanoparticle shell from half-oxidized to fully reduced states, resulting in a change in the refractive index and significant shifts in the longitudinal and transverse LSPR of the Au nanoparticle cores (Figure 9a). The plasmon shifts of the Au cores could be reversibly controlled via the PANI shells, and the particles showed remarkable stability over 200 cycles of reversible plasmonic shifts. Thermal-responsive behavior is another characteristic of some polymers that can be applied in plasmon-related applications. pNIPAM has a property of temperatureresponsive coil-to-globule transition in which the polymer above the lower critical solution temperature (LCST) undergoes dehydration and conformational change.178 In 2016, Ding et al. investigated the combined activity of the temperature-responsive pNIPAM and the plasmonic heating ability of AuNPs to design light-induced actuating nanotransducers.179 The citrate-stabilized AuNPs were functionalized with amine-terminated pNIPAM. Upon the irradiation with a resonant laser at 532 nm, the extinction peaks of colloidal AuNPs shifted from 535 to 645 nm, indicating that the conformational changes of the polymer shells induced particle aggregation (Figure 9b). The optical heating caused the AuNPs to aggregate within a microsecond. Interestingly, when the irradiation ceased and the temperature dropped below the LCST, the clusters disassembled within less than 1 s due to strong elastic forces from the hydrating and swelling polymers. These temperature-dependent LSPR changes did not occur when AuNPs were suspended with −COOHterminated pNIPAM which do not attach as strongly to the Au surface as amine-terminated pNIPAM, showing the importance of proper binding of the polymer shells on the AuNP surfaces. The laser-induced aggregation and isolation behaviors of the pNIPAM-coated AuNPs could be performed over many continuous cycles. This reversible aggregation−isolation behavior was achieved by balancing the forces between van der Waals attractions and strong elastic repulsions by controlling the thickness of the polymer shells which also colloidally stabilized AuNP cores. Polymer shells can also be used to drive plasmonic nanoparticle assembly. For example, amphiphilic polymers can drive nanoparticle assembly based on the segregation of hydrophobic and hydrophilic polymer components in polar or nonpolar solvents. The interactions between amphiphilic polymer shells and solvents can be programmed to design various assembled plasmonic nanoparticle platforms.168 One recent focus area in self-assembly is specifically anisotropic nanoparticle assembly, where the tailored arrangements of nonspherical plasmonic nanoparticles generate complex plasmonic phenomena such as chiral nanoparticle assemblies and plasmonic circular dichroism.180 The assembly of anisotropic nanoparticles requires a delicate approach since both the distances between and the orientation of the nanoparticles are important, and different parts of the nanoparticle may be subjected to different physical or chemical

forces. The poly(acrylic acid)-block-polystyrene (PAA-b-PS) diblock copolymer is a popular polymer that has been widely used for plasmonic nanoparticle assembly.181−183 In 2012, Grzelczak et al. manipulated the combination of attraction forces and steric hindrance to assemble Au “nanodumbbells” by using PAA-b-PS as a stabilizing and orientation-guiding agent.184 CTAB-coated Au nanorods were first synthesized, and then the tips were selectively overgrown to produce the dumbbell shape and enhance anisotropy. Then the tips were further functionalized by thiol-terminated polystyrene (PS). At this point, the amount of the polymer was optimized to cover the tip area only and to maintain colloidal stability in the solvent mixture of tetrahydrofuran/dimethylformamide. As the polarity of the solvent increased upon addition of water, the solubility of both CTAB at the sides of the particles and PS at the ends became poor, forming a side-to-side dimer that eventually aggregated to larger clusters. To impede further clustering after dimer formation and to orient the dimers into a cross-like formation, PAA-b-PS polymers were added to encapsulate the dimers by forming polymer shells with the PAA block at the outer parts. Furthermore, to overcome the steric hindrance between the Au nanodumbbells, the temperature, and the water amount were increased to force organic solvents away from the hydrophobic core. Then the mobilities of PS shell and CTAB surfactants lowered, putting more mechanical stress on the dimers; this resulted in the transformation from side-to-side dimers into cross-like structures (Figure 9c). The final products were stable in water due to the electrostatic repulsion forces of the hydrophilic PAA corona. In each step, PS and PAA-b-PS copolymers prevented the aggregation of the Au nanodumbbells and guided the dimers into specific formations while protecting the particle and interacting with solvent mixtures. In subsequent research, Smith et al. used a similar shell formation methodology to explore the effects of different dimer geometries on scattering properties by fabricating achiral and chiral Au nanodumbbell dimers.185 For biomedical fields, not only is colloidal stability important but also the biocompatibility of the nanoparticles must be considered for in vivo applications. Polydopamine (PDA) is an analogue to the pigment eumelanin, a type of melanin. PDA is known to be biocompatible, exhibiting low cytotoxicity, and is therefore capable of attenuating the adverse biological effects of materials when used as a coating.186 More importantly, PDA is best known for its ability to act as an adhesive with virtually any solid surface, so it has been widely used to coat a number of materials through covalent and noncovalent interactions.187−189 In 2015, the Duan group suspended AuNPs in a dopamine solution to deposit PDA onto the Au surfaces.190 The self-polymerized PDA on the Au surface gave the nanoparticles excellent colloidal stability and drove further nucleation and growth of heterogeneous metal−organic frameworks on the PDA surfaces. Another polymer, poly(sodium 4-styrenesulfonate) (PSS), also contributed to enhanced colloidal stability and low cytotoxicity as a coating for Au cores in work by Ye et al. The authors synthesized Au nanotubes capable of absorbing in the near-infrared (NIR) that therefore have potential for various photothermal therapy and photoacoustic imaging applications.191 Contrary to CTABcoated Au nanotubes, PSS-coated Au nanotubes showed negative zeta potentials and colloidal stability in a serumcontaining medium over 7 days without significant loss in NIR 680


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dynamically unstable, and the morphology of AgNPs changes easily when stored in aqueous solutions, even with PVP.207 To overcome these drawbacks, Wang and co-workers overgrew Ag on bipyramidal AuNPs, creating Ag-coated AuNPs.204 The bipyramidal AuNPs were synthesized first; then AgNO3 was reduced on the Au surface in the presence of cetyltrimethylammonium chloride. The study with the final product showed that Ag-coated AuNPs possessed narrow longitudinal plasmon peak line widths in ensemble and single-particle measurements due to high monodispersity. Moreover, the structural and optical colloidal stability of the particles were investigated by checking and comparing the extinction spectra of the Agcoated AuNPs and that of triangular Ag nanoplates as both particles aged in aqueous solutions (Figure 10). Even though

absorbance. In contrast, CTAB-coated Au nanotubes without the PSS aggregated within 30 min in the same medium. 3.3. Plasmonic Nanoparticles Coated by Metal Shells

While a large majority of plasmonics research has focused on nanostructures made of silver, gold, and copper,15 the skew toward these three materials is perhaps most pronounced when it comes to using plasmonic materials for SERS. The enhancement factors that can be achieved in the visible part of the spectrum where there are good detectors with other metals such as Pt, Pd, and Co are far lower than those that can be achieved with Au or Ag.15,192 However, these same lowenhancing metals are very important in application areas such as catalysis.193,194 From the various efforts to improve the plasmonic properties of transition metals, the “borrowing SERS” strategy was born.195−197 In this method, the Au or Ag is coated by transition metals, and the Raman signals of the target molecules or analytes adsorbed on the transition metal surfaces can still be enhanced without direct contact since the Ag or Au can produce enhancement in the electromagnetic fields a few nanometers away from the surfaces.198 To make this approach work, the catalytic metal shells must be ultrathin so that the electromagnetic fields extend through those layers. Additionally, the thin shell must be pinhole-free to avoid direct interactions of the target molecules with the plasmonic core surfaces. Thus, in the “borrowing SERS” strategy, unlike other stabilizing agents that coat the Au or Ag cores to protect the core, the main purpose of coating plasmonic nanoparticles with transition metals has been to take advantage of the outstanding plasmonic properties of core metals and to fuse the distinctive capabilities of transition metal shells into the overall plasmonic nanoplatforms. Generally, the stability of transition metal-coated plasmonic nanoparticles is lower than that of particles coated with other shells such as silica or polymers, and the synthesis requires more complex instrumentation to perform atomic layer depositions or galvanic replacement.199 Thus, much of the research into metal-coated plasmonic nanoparticles has employed the help of other stabilizing agents such as CTAB, citrate, or PVP during experimental preparation.184,200,201 Even though the aggregation behavior of the AuNPs were found to be somewhat attenuated when transition metal-coated particles were suspended in aqueous solutions without stabilizers, the metal-coated particles are often applied in applications which require the use of a substrate to immobilize the particles to avoid colloidal instability. Interestingly, transition metal-coated plasmonic nanoparticles demonstrate overall enhanced durability and stability during repeated use;202,203 however, the improved performance is not attributed to the shells but from structural interactions between the plasmonic cores and the metal shells. For example, Chen et al. demonstrated stability and enhanced catalytic activity of Pd-coated AuNPs for oxygen reduction and attributed this stable performance to the lattice tensile effect in the Pd shell induced by the Au core.203 A similar synergy of behaviors of bimetals can be found in the work of Wang and co-workers in 2015.204 The authors focused on improving the refractive index sensitivity and plasmon-enabled field enhancement of AgNPs by controlling nanoparticle size/morphology and tuning the bimetallic character of the nanostructures.205,206 Generally, it is more difficult to control of size and shape of AgNPs, a critical parameter for achieving strong and narrow plasmon bands, compared to AuNPs. Furthermore, bare AgNPs are thermo-

Figure 10. (a) Schematic illustration of Ag growing over bipyramidal AuNPs. (b) Extinction peak wavelength changes of triangular Ag nanoplates (green) and Ag overgrown on bipyramidal AuNPs (pink) incubated in aqueous solutions. (c) Extinction peak wavelength intensity changes of triangular Ag nanoplates (green) and Ag overgrown on bipyramidal AuNPs (pink) incubated in aqueous solutions. Reprinted with permission from ref 204. Copyright 2015 John Wiley and Sons.

Ag nanoplates have been known for narrower plasmon line widths than other Ag nanomaterials,159 the Ag-coated AuNPs demonstrated narrower extinction peaks and improvement in the maintenance of the structures and the LSPR peak wavelength during storage. The authors posited two reasons for this higher colloidal stability: (1) the reduction in the electron density of Ag due to the large electronegativity of Au cores or (2) charge redistribution in the Ag atom orbitals during synthesis, resulting in increased resistance to oxidation. This research is a fine example of the enhancement of plasmonic ability and chemical stability of outer shells that result from the synergy of combining different plasmonic inner cores. Improved chemical stability due to bimetallic formation can also be found in the work of Huang et al.208 The authors synthesized Au nanorods coated with a AuAg alloy and tested the chemical stabilities of the nanoparticles suspended in a strong oxidizing environment. For comparison, four different particles (Au nanorods coated with AuAg alloy, Au, Ag, and Ag/Au (not alloy)) were incubated in an aqueous solution containing H2O2. The particles coated with Ag and Ag/Au and incubated in 0.5 M H2O2 showed disappearance of the LSPR after 1 h, but the particles coated with Au only and AuAg alloy maintained LSPR peak wavelength positions and intensities 681


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4.1. Plasmonic Nanoparticle Arrays via Lithography Techniques and Related Stabilization Strategies

(Figure 11). The Ag in AuAg alloy showed higher resistance to oxidative etching due to electron redistribution upon alloy formation.

Lithographic techniques have been utilized to make plasmonic nanoparticle arrays for over 30 years, including use of electron beam lithography (EBL),210 focused ion beam lithography (FIB),211 and nanosphere lithography (NSL).212 The basic schematic description of each technique is described in Figure 12. In EBL, an electron-sensitive resist is deposited on a sample via spin coating and an electron beam induces a specific pattern on the resist. After exposure to the beam, the solubility of the exposed resist changes to selectively dissolve in a suitable solvent. A layer of plasmonic metal is deposited on the resist via vapor deposition, and the resist and the metal deposited on it are lifted off to reveal the metal nanoparticle arrays as defined during patterning (Figure 13). FIB is similar, but it is capable of locally depositing213 or milling away material.214 Both methods offer high-fidelity fabricated nanostructures but require expensive and time-consuming sample preparation. Nanosphere lithography (NSL) is a simpler, cheaper patterning method where nanospheres are first deposited in a single layer on a supporting substrate to act as a deposition mask. When plasmonic metals are deposited onto the nanosphere mask, a portion of the metal deposits through the voids in the assembled pattern. The nanospheres are then removed to leave a nanopatterned array. While NSL is an effective way to create a nanoparticle array, researchers are limited by the void pattern created as nanosphere assembly; arbitrary nanoparticle patterns can only be achieved by using EBL or FIB. Regardless of which lithographic method is used, the lithographically defined nanoparticles are different from nanoparticles in suspension because nanoparticles on a solid substrate are not likely to aggregate. However, unlike colloidal suspensions of nanoparticles where the liquid media helps dissipate thermal energy, two-dimensional nanoparticle arrays on substrates can be used in air or aqueous media. Arrays used with air as the surrounding media can suffer changes in their plasmons due to thermal degradation and adsorption of contaminants from the air.215,216 Also, since lithographically defined nanoparticles without a protective shell may be directly exposed to the media and laser irradiation, the plasmonic nanostructures and properties may degrade with use. In the fabrication and application of plasmonic nanoparticle arrays, where direct exposure of the arrays to air is common, the stability of the nanoscale metals in air must be considered. Among the various plasmonic metals, Ag has many advantages even over Au. Ag has the highest thermal and electrical conductivity over all metals. It is known to support a LSPR in the widest range of visible to near-infrared regions, from 300 to 1200 nm.47 Simulated calculations28 and experimental results220 show that AgNPs can possess sharper and more intense LSPR and stronger plasmon field intensity than AuNPs and thus are more suitable for plasmon sensing. However, Ag has not been studied in plasmonic applications as much as Au, mainly due to its chemical instability, especially in applications involving air exposure. Ag is likely to be oxidized upon exposure to the atmosphere,221 resulting in an attenuation of plasmonic properties.222 Moreover, sulfidation of the bare Ag surface occurs frequently at room temperature in air; this can occur in a single day and significantly influences conductivity and the extinction wavelength.223 Au is chemically much more stable at room temperature in air and does not suffer from oxidation.224 However, photo-

Figure 11. (a) HAADF-STEM image of AuAg alloy-coated Au nanorods. (b) Extinction wavelength spectra of AuAg alloy-coated Au nanorods in varying concentrations of aqueous solutions of H2O2. Reprinted with permission from ref 208. Copyright 2015 John Wiley and Sons.

4. TWO-DIMENSIONAL PLASMONIC NANOPARTICLE ARRAYS In the chemical synthesis for colloidal plasmonic nanoparticles discussed in previous sections, Au or Ag precursors are presented and reduced in solution with capping agents that chemically and/or physically block possible sources of destabilization such as contaminants, oxidants, and aggregation due to other nearby plasmonic nanoparticles. Thus, each isolated particle is formed in solution and coated by ligands or more rigid shells which protect the inner plasmonic cores. However, particles produced from chemical wet synthesis may have limitations. First, due to the thickness of coating materials, the plasmonic performance can be impacted as direct contact with the plasmonic metal nanostructure is limited. Second, since particles are in suspension and continually move in solution, precise control of plasmonic properties is difficult to achieve. In some application areas, such as solar energy conversion or electrochemical catalysis, it would be beneficial if plasmonic nanoparticles could be fixed in a small area with a particular formation. On the basis of this need, there has been significant focus on fabrication of plasmonic nanoparticle arrays using a variety of methods.209 682


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Figure 12. Schematic description of the preparation steps for plasmonic nanoparticle arrays in three different lithographic techniques.

Figure 13. SEM images of nanoarrays fabricated by lithographic techniques. (a) Array of the Au in the third-order Cayley trees structures fabricated by EBL. Scale bar for the array is 2 μm and 300 nm for the inset. Reprinted with permission from ref 217. Copyright 2015 American Chemical Society. (b) Array of the Au in “L”-shaped chiral structures fabricated by FIB. Scale bar represents 1 μm. Reprinted with permission from ref 218. Copyright 2016 American Chemical Society. (c) Array of the Au in nanotriangle structures fabricated by NSL. Inset shows the monolayer of polystyrene nanospheres. Reprinted with permission from ref 219. Copyright 2012 American Chemical Society.

deformation and LSPR shifts when the temperature exceeded 600 °C. The authors also measured the third harmonic generation (THG, i.e. third-order hyper-Rayleigh scattering) intensity, emitted light from nanostructures where the optical frequency is three times that of the irradiating laser beam; THG is an emerging technique in nonlinear optical imaging.229,230 The investigation of THG was chosen specifically as THG signal is known to be crucially dependent on the exact size and shape of the nanoparticles.229 During the stability investigation of Au nanorods exposed to 10 GW/cm2 of laser radiation (a pulse duration of 16 fs, 44 MHz repetition rate, and 180 mW average power), the nanorods showed a continuous decrease in THG intensity due to cumulative damage at the Au surface. The authors reasoned that the change of the linear spectra overlapped with the laser spectrum reduced the nonlinear signal resulting in THG intensity decrease. A similar thermal study on Ag nanorods was performed as well in 2018 by Albrecht et al.231 The 60 nm wide Ag nanorods showed significant loss of plasmonic response at 500 °C. The substrates on which plasmonic arrays are deposited also impact the overall optical properties, in part because the

thermal stability is another issue in two-dimensional plasmonic nanoparticle arrays, and Au is not free from this issue. Generally, structural changes of AuNPs are found to occur at lower temperatures than the melting temperature of the bulk.225,226 Exposure to laser irradiation can also induce shape deformation. Petrova et al. exposed Au nanorods to a pulsed 400 nm laser with energies between 0.1 and 20 μJ/pulse, and Au nanorods maintained their structures up to around 700 °C, the heat induced by the laser.157 It was found that this temperature was much higher than 250 °C, the temperature at which thermally heated Au nanorods showed rapid transformation. The authors reasoned that the thermal diffusion between pulses induced higher deformation temperatures for Au nanorods exposed to the laser. Similar work was done by El-Sayed and co-workers, where a femtosecond pulsed laser showed much more efficient photothermal reshaping of Au nanorods than nanosecond pulsed laser irradiation.227 In more recent research, Hentschel and co-workers examined the thermal stability of Au nanostructures deposited on a substrate through EBL in ambient atmosphere by imposing high temperatures and intense laser pulses.228 60 nm wide Au nanorods of various aspect ratios showed significant shape 683


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Figure 14. (a) (i) Long-term extreme-ultraviolet intensity measurements of bow-tie Au nanoantennas fabricated on sapphire (red) and on mica (black). Nanostructures on sapphire substrates show much more stable and consistent yields and structures than nominally identical nanostructures on mica substrates (insets). (ii) SEM images of the structures right after sample preparation (left), after several hours of exposure to incident intensities less than 0.15 TW cm−2 (middle), after a few minutes of exposure to incident intensities higher than 0.15 TW cm−2, and up to 0.3 TW cm−2 (right). Scale bars represent 200 nm. Reprinted by permission from Springer Customer Service Centre GmbH: Springer Nature, ref 244. Copyright 2013. (b) (i) Cross-sectional SEM images of PZT-sandwiched Au nanostructures on ITO substrates. (ii) Time-dependent short-circuit photocurrent for sample PZT-sandwiched Au nanostructures array in three different states: as deposited and after +10 and −10 V poling. Structures show high stability and reproducibility on photoelectrochemical performance. (c) Schematic designs of the electronic band structures when the injected hot electron transfers from PZT films to the electrolyte for the two poling configurations,: (i) +10 and (ii) −10 V. Reprinted with permission from ref 245. Reproduced with permission under Creative Commons Attribution 4.0 International License http://creativecommons. org/licenses/by/4.0/.

nanoparticles induced the splitting of plasmonic dipolar peaks, which are dependent on the dipoles parallel or perpendicular to the surfaces. This splitting was proven to be influenced by increasing substrate permittivity. Other studies have emphasized the importance of adhesion layers on deposited metal nanoparticles because these layers can impact the stability and plasmonic response of the nanomaterials on a substrate.242,243 Carson and co-workers showed that the adhesion layer composition and thickness between Au and a Pyrex substrate had effects on the optical resonance properties of Au film.242 The Au films on chromium or titanium adhesion layers showed larger optical resonance bandwidths than the films on etched adhesion layers, and the increased thickness of the adhesion layers induced a reduction in the magnitude of the optical resonance peaks.

anisotropic environment can induce LSPR peak splitting, LSPR peak shifting, and other unusual effects.232−234 Among the many possible substrates available, transparent conducting oxides such as indium tin oxide (ITO) are commonly used due to their electrical conductivity, optical transparency, and ease of thin film formation.235 For this reason, the effects of ITO substrates on plasmonic nanoparticles have received much attention, and a number of studies of Au− or Ag−ITO hybrid plasmonic platforms have been reported.19,236−238 Glass substrates such as fused silica or borosilicate are also popular, and plasmonic nanoparticles deposited on different types of glass showed different LSPR peak wavelength positions and varying sensitivities to bulk environments.239,240 Halas and coworkers studied the influence of substrates with varied dielectric properties, including glass, sapphire, and ZnSe, on deposited plasmonic AuNPs.241 The substrates beneath the 684


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improve the catalytic activities of two-dimensional plasmonic catalytic platforms.249,250 A rather unique oxide layer was used by Wang et al. in 2015 for a plasmonic photoelectrochemical device. Instead of using a conventional photoelectrochemical semiconductor, such as TiO2, they used ferroelectric Pb(Zr,Ti)O3 (PZT) to make tuning the band bending at the ferroelectric and electrolyte interface possible. An array of square Au dots (270 × 270 nm2) was deposited between PZT layers on ITO substrates (Figure 14b).245 The steady-state external quantum efficiency spectra showed hot-electron injection from excited Au dots to PZT layers via higher and more distinctive external quantum efficiency spectra than that of PZT without Au dots. This result qualitatively matches with the Au dot−PZT layer absorbance spectrum. The ferroelectric domains in PZT films are known to be poled, resulting in the capability to switch the direction of depolarization of electric fields.251 This characteristic enables the manipulation of hotelectron injection and transfer (Figure 14c). However, then the Au dots were not sandwiched between two PZT layers, and a degradation of performance was observed when the electrodes were poled with different potentials in a propylene carbonate solution. The performance could be maintained only when the structure had double layers of PZT film, indicating the positive effects of coating layers on stability and reproducibility (Figure 14b). There are a few other methods also known to improve nanoparticle array stability. For example, Bosman et al. focused on the damping effects of grain boundaries or surface roughness underneath lithographically fabricated plasmonic nanomaterials, reducing plasmonic response.252 The researchers worked to reduce grain boundaries by applying a thermally stable, but removable, 30 nm thick layer of hydrogen silsesquioxane during the annealing on the EBL-fabricated Au nanoparticle array. Au nanostructures with this encapsulated annealing experienced fewer grain boundaries while preserving the designed shapes. Scuderi et al. used a hexanethiol monolayer for the passivation of EBL-fabricated Ag nanodisk surfaces.253 This monolayer overcomes drawbacks of metal oxide coatings, which can alter the achievable plasmonic shifts due to the larger refractive index and blocking of plasmonic interactions on the targets due to the thick oxide layers. To achieve a similar result, Losurdo et al. imposed a low-temperature hydrogen processing on the EBL-fabricated Ag surface, rather than coating them, to inhibit oxidization and maintain the optical properties under ambient conditions.254 The authors argue that treating Ag nanostructures with lowtemperature hydrogen processing not only can make the surface chemically passive but also can avoid the additional heating process that would change the morphology or grain sizes of the original Ag nanostructures. Thick layers of oxide or other coating layers can provide plasmonic nanoparticles with protection from shape deformation or chemical contamination, but this protection can also reduce or hinder the functionality of the plasmonic nanomaterials since many plasmonic applications require interactions within a few nanometers of the plasmonic surface.255 Graphene is an alternative protective layer that displays many advantages over other types of protective layers. A layer of graphene has an average thickness of 0.35 nm, which is much thinner than most applied oxide layers.256 Even though it is quite thin, graphene monolayers have a great degree of impermeability, making it possible to block gas molecules as small as helium.257,258 Thus, passivating the surface of a metal such as Ag with graphene can

Many researchers have fabricated different metal nanoparticle arrays and studied their corresponding plasmonic capabilities/related stabilities. The critical role of the substrate was thoroughly investigated in 2013 when Sivis et al. measured nanostructure-enhanced high-harmonic generation in plasmonic bow-tie antennas.244 To study the specifics of extreme-ultraviolet generated by high-harmonic conversions from plasmonic nanostructures irradiated by high-energy pulses, the authors fabricated arrays of Au bow-tie antennas via FIB. Each triangle had lengths of 200−240 nm, and the gap distance between two triangles was 20 nm. An 8 fs light pulse with 800 nm center wavelength illuminated two-dimensional plasmonic devices placed in a vacuum chamber, and extremeultraviolet fluorescence and third and fifth-harmonic generations were observed. Studies of the durability of the devices under high-energy irradiation and achievable maximum local intensities followed. The same Au bow-tie antennas were fabricated on both mica and sapphire substrates. The longterm extreme-ultraviolet yield was measured from both devices, and the results showed that the Au nanostructures on mica gradually lost ultraviolet generation capacity, but the structures on sapphire were maintained during the exposure (Figure 14a). It was more obvious from SEM images that unlike the Au on mica which clearly showed the effect of cumulative damage, the Au structures on sapphire remained intact. Figure 14a ii shows SEM images that depict rapid and significant degradation of Au structures due to high photon energy on the substrates, which could result in a loss of plasmonic enhancement and extreme-ultraviolet generation. This study shows the importance and effect of the substrate on twodimensional plasmonic devices fabricated by lithography techniques. To enhance the thermal and chemical stability of twodimensional plasmonic nanoarray platforms, deposition of thin protecting layers of dielectric oxide layers such as titania or alumina is the most popular strategy. The enhanced stability of AgNPs fabricated by NSL was observed when the Al2O3coated nanoparticles maintained their LSPR wavelength despite exposure to femtosecond laser pulses.246 Albrecht et al. studied the enhanced thermal stabilities of various Al2O3coated plasmonic nanoparticles, including Ag and Au fabricated by electron lithography on glass substrates.231 All metals except copper and magnesium showed increased thermal stability with 4 nm thick layers of Al2O3. Photostability was also investigated by looking at third-harmonic generation. Bare Au and Ag nanostructures clearly showed a steady decrease in third-harmonic generation intensity, indicating the weakening of plasmonic responses from local heating or structural deformation. The nanostructures with layers of Al2O3 generated steady third-harmonic generation signals upon exposure to incident lasers. This improved stability facilitated by Al2O3 can be explained by the suppressed deformation of the plasmonic nanostructures. The surface melting temperature is increased by decreasing the mean square displacement of Ag atoms on the surface upon the oxide layer coating.247 Adibi et al. fabricated a platform for in operando plasmonic nanospectroscopy, where Au nanodisks coated with mesoporous Al2O3 layers impregnated with Pt nanoparticles could sense sintering kinetics of the Pt nanoparticles.248 Al2O3 layers protected Au from possible thermal deformation and prevented direct contact between Au and catalyst materials. TiO2 layers have also been known to enhance the thermal and chemical stabilities in harsh oxidative conditions and to 685


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Figure 15. (a) SEM images of (i) graphene-coated Ag nanostructures and (ii) bare Ag nanostructures after 30 days; scale bars represent 200 nm. (b) Normalized LSPR peak extinction spectra of (i) graphene-coated Ag nanostructures and (ii) bare Ag nanostructures over 30 days. Reprinted with permission from ref 259. Copyright 2012 American Chemical Society. (c) XPS spectra of the Ag 3d photoelectron (i) for Ag deposited on glass and (ii) for the graphene-coated Ag on glass after 1 month of air exposure. Reprinted with permission from ref 260. Copyright 2013 John Wiley and Sons. (d) (i) Schematic illustration of AgCo NPs. Ag oxidation can be suppressed by hydroxide formation on the Co surface via electron injection from Co into Ag. (ii) Plasmonic degradation graphs measured by the normalized inverse full-width at half-maximum of the LSPR peak of NP arrays, AgNPs, and AgCO NPs. Reprinted with permission from ref 261. Copyright 2013 John Wiley and Sons.

known to be higher than that of graphene,264 allowing the reduction of silver oxide to silver initiated by injected electrons from graphene. To prove this deoxidization process on the Ag surface, the authors fabricated Ag nanostructures via EBL, and graphene layers were formed on Ag surfaces. X-ray photoelectron spectroscopy results were consistent with the authors’ arguments: an increase in Ag metal, a reduction in silver oxide, and a decrease in oxygen amount on the surface during exposure in air, indicating that the Ag deoxidization occurs after graphene layers are deposited onto the Ag nanostructures (Figure 15c).260 This study proves the stability induced by graphene layers on plasmonic Ag nanostructures, not only through passivation but also through induction of Ag deoxidization via electron transfer. The similar enhanced stability behavior can be found when Ag is in contact with cobalt, resulting in oxidation-resistant Ag nanostructures (Figure 15d).261

conserve the plasmonic ability of the metal and effectively protect it from external contaminant sources such as hydrogen sulfide or carbonyl sulfide in the air. Graphene-protected AgNPs were fabricated in 2012 by Reed et al. On glass substrates, EBL-defined Ag nanoantenna arrays were deposited followed by coating with chemical vapor deposition-grown graphene.259 Two nanoparticle array substrates, one with graphene and one without, were stored in an ambient laboratory environment for 1 month. As shown in Figure 15a, it was obvious that significant morphological deformation had occurred on the sample without the graphene layer, but the sample covered by graphene maintained its morphology, indicating no sign of sulfidation. This observation was supported with data from X-ray spectroscopy and optical measurements (Figure 15b). When the refractive index sensitivity of the two nanoparticle arrays was evaluated, the graphene-protected Ag nanoparticle arrays showed a higher sensitivity to refractive index changes. On the basis of this work, it is clear that there will be many applications where a graphene protective layer is superior to traditional oxide layers. In 2015, Losurdo et al. evaluated another benefit of the graphene layer on plasmonic nanomaterials. The authors argue that a graphene layer on AgNPs can act as an electron shuttle and deoxidize the Ag surface, making the plasmonic platform more chemically stable.260 Generally, when a graphene layer is deposited or transferred to another material, the occurrence of a defect is inevitable. The defect sites of graphene layers chemisorb more oxygen, become more chemically unstable, and increase the carbonaceous material reactivity.262 Since the work function of Ag is known to be slightly different from that of the graphene monolayer,263 electron transfer from graphene to Ag happens during the graphene layer deposition. Finally, the reduction potential difference between silver oxide is

4.2. Fixation of Plasmonic Nanoparticles on Solid Substrates via a Nonlithographic Technique

As shown, two-dimensional lithographic fabrication of AgNPs or AuNPs enables various plasmonic applications. Fixing plasmonic nanoparticles on solid substrates can also be accomplished by first chemically synthesizing the nanoparticles, followed by adsorption onto a substrate of interest.265−267 For example, Wang et al. designed a plasmonic-assembled AuNP thin film for efficient solar-enabled evaporation. The idea was similar to biological evaporation in plant leaves: plasmonic nanoparticles can absorb sunlight and induce heating, acting as a light-to-heat converter. With the help of plasmonic AuNPs, vapor bubbles do not lose heat to bulk water during travel to the air−water interface because there is more intense formation of bubbles near the interface upon plasmonic heating.268 While this system demonstrated 686


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Figure 16. (a) Schematic illustration of the structure of AuNP films on paper. (b) (i) Maximum, minimum, and average weight change of water over 800 s by using AuNP films on paper. (ii) Total weight changes after 15 min illumination in each cycle; average weight change is 1.25 g. Reprinted with permission from ref 269. Copyright 2015 John Wiley and Sons.

create a layer of positive charge on the fabric, which would interact with carboxylate groups of citrate-stabilized AgNPs via hydrogen-bonding and electrostatic attractions, leading to successful deposition of AgNPs on the fabric. Next, the fabric was transferred to a fluorinated-decyl polyhedral oligomeric silsesquioxane (F-POSS) solution to give it superhydrophobic characteristics. Each product showed distinctive colors with superhydrophobicity, and the deposition of F-POSS enhanced the AgNP adherence to the fabric despite continuous washing and rubbing (Figure 17a).272 As is clear from the examples given here, nanoparticle fabrication via lithography or deposition enables precise control of nanoparticle size, morphology, and gap distances within arrays; however, their structural qualities can suffer due to grain boundaries and defects during preparation. In the case of wet-chemical synthesis, particles can display excellent plasmonic properties, but sophisticated arrangements and architectures over a large area or volume are limited. To harness the advantages of each method effectively, Flauraud et al. proposed topographical control of plasmonic nanoparticles via capillary force (Figure 17b).273 By dividing the dynamics of the capillary assembly of nanoparticles into three stages, insertion, resistance, and drying, a colloidal suspension of Au nanorods could be placed onto solid substrates with specific topographic patterns with ∼1 nm resolution. This study is meaningful in that scalable and lithographically accurate control of colloidal nanoparticles was achieved that is independent of the surface patterns and types of nanoparticles.

efficient evaporation, the system was not reusable and there was thermal diffusion to the nonevaporative portion of the liquid. To address these limitations, the authors employed an air-laid porous paper as a substrate to provide mechanical stability and low thermal conductivity.269 AuNPs were synthesized via citrate reduction and they were allowed to self-assemble onto the paper by incubating the particles in water and under formic acid vapor. The acid vapor diffusion to water neutralized, destabilized, and trapped the AuNPs at the water−air interface, resulting in a thin film. This film was transferred to the air-laid paper and dried. This relatively simple AuNP thin film preparation generated plasmonic paper in which each AuNP was separately sitting on fibers, resulting in nonaggregation and a robust plasmonic platform for an efficient, sunlight-enabled evaporation system (Figure 16a).269 The surface temperature of the paper floating on liquid under sunlight illumination rose to 80 °C, and the hot zone showed a power density of 4.5 kW m−2 after 15 min of illumination. Furthermore, due to the robustness of the fibers, the evaporation experiment showed consistent rates for 30 continuous cycles with the same paper, proving reusability and sustained plasmonic function (Figure 16b). In 2015, Wu et al. took a similar nanoparticle-on-fiber approach with AgNPs, taking advantage of the antibacterial properties of AgNPs as they release Ag ions.270,271 To create antibacterial fabrics, AgNPs were used both as the coloring and as the antibacterial agents. To solve the chronic problem of poor color fastness of these natural AgNP dyes, the authors dipped cotton fabric into a poly(ethylenimine) solution to 687


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Figure 17. (a) Cotton fabrics dyed with different morphologies of AgNPs, resulting in different colors. Inset graph shows water contact angle and sliding angle changes of the fabricated AgNP cotton fabrics during 80 cycles of dry cleaning. Reprinted with permission from ref 272. Copyright 2015 John Wiley and Sons. (b) (i) Schematic illustration of of the capillary force-driven assembly of AuNPs onto topographical traps. Dimension not to scale. (ii) SEM image of the accumulated AuNPs and AuNPs trapped in the fabricated holes. Reprinted by permission from Springer Customer Service Centre GmbH: Springer Nature, ref 273. Copyright 2017.

5. CONCLUSION AND PROSPECTIVE This review aims to survey the stabilization strategies for plasmonic Au and Ag nanoparticles utilized in various fields. As shown, the type of stabilizing approach applied to preserve plasmonic properties depends on the nanoparticle’s bulk environment, how nanoparticles are fabricated, and the types of plasmonic applications (Table 1). The most robust and thorough stabilization strategies are not always the most advantageous routes to take, due to their impacts on the refractive index and distance between the analyte and the core, which are vital for catalysis and SERS detection. Thus, researchers are always working to obtain a balance between achieving efficacious plasmonic properties and maintaining nanoparticle stability. In-solution nanoparticle preparations are often used because the nanoparticle crystallinity, and thus plasmonic behavior, is usually superior to that achieved using lithographic techniques. In colloidal synthesis and applications, initially AgNPs and AuNPs are mostly surrounded by stabilizing surfactants or ligands which also play roles in nanoparticle morphology. Even though their roles in determining morphologies are crucial, loosely bound stabilizers are often not sufficient to protect the nanoparticles in complex media or in vivo applications. Further functionalization of the colloidal nanoparticles with more strongly bound ligands, such as thiolated-PEG, can enhance colloidal stability, resulting in improved plasmonic performance.

Analysis of current studies reveals that near-future directions for plasmonic AgNP/AuNP research will be investigation of controlled nanoparticle self-assembly. During nanoparticle preparation it is desirable to separate each nanoparticle to avoid aggregation and reach the targeted morphologies. However, when nanoparticles are placed in close proximity to each other without aggregation, the gaps between the nanoparticles can be filled with coupled plasmonic electric fields. These short gaps enable unique plasmonic-related applications in a variety of fields such as biosensing, photovoltaics, photocatalysis, and photothermal therapeutics.274 This synergistic plasmonic amplification can be maximized when nanoparticles are very closely packed and arranged; thus, recent work has focused on decreasing the gap distance between the particles and designing unique plasmonic materials that are assembled using the nanoparticles as nanobuilding blocks. Among different assembly techniques, DNA-based nanoparticle assembly is particularly exciting as DNA enables very precise control over the distance between nanoparticles owing to Watson−Crick base pairing.108 As a ligand, DNA has excellent biocompatibility and feasible functionalization via nucleic acids, which make DNA a promising linker for plasmonic nanoparticle self-assembly. From a stability point of view, due to the predictable lengths of DNA strands and strong thiol bonds between modified DNA and the nanoparticle surface, aggregation can be prevented while the gap distances can be controlled to the nanometer 688


two-dimensional nanoparticle array

colloidal nanoparticle

689

nonconventional lithographical technique

conventional lithographic technique

shell

ligand

none or additional coating on array

coating

substrate or adhesion layer

metal

organic shell

silica

assembly inducing ligands

metal oxide (Al2O3,231,248 TiO2,249,250 PZT245, graphene,259,260 hydrogen silsesquioxane252 air-laid paper,269 fabric,272 specific topographic patterned substrate273

sapphire,244 titanium, and chromium242

mesoporous or nonporous shell148,151,152,163,164 poly(N-isopropylacrylamide),179 polyaniline,177 polydopamine,190 PAA-b-PS184 Pd,203 Ag,204 AgAu alloy208

bovine serum albumin (BSA),98−100 streptavidin101 DNA,121,122,125,131 diazirine132

thiol-PEG,89 thiol-PEG-COOH90

polyethylene glycol (PEG)

protein

specific examples citrate,63,70 PVP,82,83 CTAB69

stabilizing agents conventional surfactant or ligand

features

tuning the band bending at the electrolyte interface, increased external quantum efficiency, thin but great degree of impermeability preprepared nanoparticles are transferred to specific substrates

physical separation of nanoparticles on specific substrates

affecting optical resonance property

increased resistance to oxidation, enhanced catalytic activity

temperature-responsive conformational change, high conductivity, adhesive properties, amphiphilic

programmable assembly via DNA base pairing interactions (dimer, heteronanocluster, nanoparticle crystal), photoresponsive assembly tunable porosity, facile surface modification

biocompatible, stable at various pH levels

can be replaced with other stabilizing agents, controls nanoparticle morphologies in colloidal synthesis (spheres, prism, bipyramidal, etc.) resistant to protein adsorption

enhanced durability of the particles under high photon energy redistribution of the metal atoms at the surface layers, more facile electron transfer

core−shell chemical interaction

mainly via electrostatic repulsion and/or steric hindrance

stability methods

Table 1. Summary Describing the Types of Stabilizing Agents in This Review and Related Stability Methods, Features, and Related Research Fields main application area

photocatalysis, sensing, toxicity related research

photovoltaics, photoelectrochemistry, sensing

sensing, catalysis

quantum-level or single molecular interaction study, photonics, sensing biomedical field, sensing, catalysis electrochemical applications, sensing

biomedical sensing, imaging, or therapeutic applications

nanoparticle preparation steps in colloidal synthesis

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depositions of Pd on two different positions of the Au nanobipyramids and also protected coated regions during colloidal catalytic Suzuki coupling reactions under laser irradiation to examine the correlation between the plasmonic photocatalytic activity and the positions of the deposited Pd. Currently, there is a lot of research being performed on hybrid or heteronanostructures in the field of plasmonic photocatalysis to achieve both stability and high performance.293 With metal shells, the properties of a transition metal shell are affected by the inner plasmonic core. However, the fabrication of ultrathin (less than 1 nm thick) and pinhole-free shells that allow use of the plasmonic properties of the inner cores is not easy to achieve, and further functionalization is not straightforward, limiting more efficient plasmonic applications. To overcome these issues, nontraditional shells such as MnO2 have been applied to produce more stable and tunable plasmonic nanoparticles.294 Au−Ag bimetallic alloy plasmonic nanoparticles have been an exciting platform to maintain the excellent plasmonic properties of Ag while making use of the chemical inertness of Au.295,296 In lithographic plasmonic nanoparticle array fabrication, there are no issues with ligands or colloidal stability, but the nanoparticles are much more likely to be exposed to air or thermally harsh conditions. Silver is very susceptible to oxidation and sulfidation; therefore, coating it with oxide layers or graphene is necessary for protection and deoxidation via electron transfer, respectively. AuNPs do not suffer from oxidation, but exposure to high photothermal energy can reduce their stability. Plasmon damping is a major cause of losing their optical properties; this loss can be attributed to the presence of grain boundaries and surface roughness on the substrate or adhesion layers.252,297 For these reasons, alternative plasmonic materials such as aluminum or hybrid plasmonic nanoparticles have recently received significant attention.12,298,299 Whether future applications use new materials such as aluminum or traditional plasmonic materials such as silver and gold, researchers will have to continue to consider appropriate stabilization tactics to achieve performance stability without hindering the exciting and useful plasmonic properties. The overall rapid development of the plasmonic Ag/Au nanoplatforms has overcome many obstacles and is pushing the boundaries toward more sophisticated and enhanced plasmonic systems, but there is still the need for further improvements. From the stability perspective, most research has been performed in simplified or benign conditions which are far from a realistic environment. The stability as well as the preserved plasmonic properties must be tested in complex systems such as cell matrices. DNA and biomolecules show remarkable potential for enabling nanoparticle assemblies and chiral nanoparticles, but at elevated temperatures or in complex media, DNA can be denatured and biomolecules can be unfolded, resulting in loss of the plasmonic properties of the stabilized nanoparticles. For further practical applications, more proper tests of stability, reversibility, and reproducibility of various plasmonic nanoplatforms specific to each purpose must be performed and satisfied. Moreover, current plasmonic nanoplatforms mainly reside within the proof-of-concept stage, considering the high cost of complex templates such as DNA origami and the relatively low yield of lithographically defined plasmonic noble metal nanoparticles.300 Thus, future research will likely encompass the improvement and enhancement of the stabilities and viabilities of the developed AgNP/AuNP

level. DNA origami, a nanoscale folding of DNA to create a customized two- or three-dimensional structure, is of particular interest these days, as various designed templates enable a variety of plasmonic nanoparticle assemblies in controlled manners.275 From a basic structure as AuNP dimers,276 AuNP helices277 or toroidal AuNP superstructures278 have been fabricated based on different DNA origami structures. Considering the various morphologies of the AgNPs/AuNPs and available designs of the DNA origami structures, more diverse plasmonic platforms and related synergistic plasmonic properties will likely be studied. Another active and promising field enabled by DNA ligands is the fabrication of chiral plasmonic nanostructures. Chirality is a geometric feature where a structure cannot be superimposed with its mirror image.279 Nanomaterials with chirality have the capacity to rotate the polarization of light and interact differently with left circularly polarized light and right circularly polarized light.280 This phenomenon is recognized as circular dichroism (CD).281 Circular dichroism has a high potential to be used in many applications such as the detection of subtle conformational changes of biomolecules or proteins,282,283 measurement of circularly polarized light,284 and stimulating asymmetric catalysis.285 Plasmonic nanoparticles assembled with a chiral geometry can exhibit strong optical activity as well as enhanced chiroptical activity. DNA can be used to stably assemble achiral AuNPs/AgNPs into overall chiral plasmonic nanostructures of helices,277 spirals,286 rod dimers,287 pyramids,288 etc. Peptides and proteins are also promising stabilizing agents that can produce plasmonic chiral structures. The various ranges of functional groups on the peptides and proteins can be used for the controlled nucleation and stabilization of metal NPs during the association of the growing particles with their surfaces.279 Very recently, Lee et al. fabricated amino acid- and peptidedirected three-dimensional chiral nanoparticles in an aqueousbased synthesis.289 The presynthesized Au seed particles were mixed with chiral cysteine or cysteine-based peptides in Au growth solution; since chiral cysteine was used, the Au helicoid nanoparticles that are synthesized exhibit chirality. As introduced, DNA and biomolecules are expected to be employed actively in the near future as stabilizing and structure-designing agents to achieve a clear goal: the programmatic construction of highly effective plasmonic nanoplatforms. With DNA and biomolecule-assisted assembly/synthesis, AgNPs and AuNPs can go beyond general plasmonic performance to allow exploration of quantum-level phenomena and single molecular or structural interactions.290,291 Inorganic, organic, and metal shells can provide AgNPs/ AuNPs with stability against aggregation and dissolution in complex media. Further, shell components can act as a completely different intermediate environment from bulk media or the plasmonic cores, where further functionalization or pH/temperature-responsive behavior can be achieved for specific applications such as imaging or cancer therapy. Conventional silica shells are still being actively and widely utilized. However, in current studies, their use is pushing toward fabricating heterocomplex structures for improved plasmonic performance. For example, Wang and co-workers achieved selective deposition of Pd on Au nanobipyramids via predeposition of silica.292 Before the deposition of Pd, the surfaces of Au were site-selectively coated with silica, then the remaining exposed parts of the Au surface, either tips or sides, were covered with Pd. Silica assisted the site-selective 690


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systems and design of more simplified and market-friendly fabrication methods for the production of practical plasmonic metal nanoparticle platforms.

Kyle C. Bantz received her B.A. degree in Chemistry in 2006 from Cornell College and her Ph.D. degree in Chemistry in 2011 from the University of Minnesota under the supervision of Christy Haynes on the development of SERS sensors for detection in complex mixtures. She received postdoctoral training in SAMDI analysis of phosphatase enzymes at Northwestern University with Milan Mrksich. She is currently a term-assistant professor at the University of Minnesota.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

Christy L. Haynes received her B.A. degree in Chemistry in 1998 from Macalester College and her Ph.D. degree in Chemistry in 2003 from Northwestern University. As the Elmore H. Northey Professor of Chemistry, she leads the Haynes research group at the University of Minnesota. Her group focuses on exciting research questions at the intersection of analytical, biological, and materials chemistry. She is also the Associate Director of the NSF-funded Center for Sustainable Nanotechnology.

ORCID

Hyunho Kang: 0000-0001-9258-7168 Joseph T. Buchman: 0000-0001-5827-8513 Rebeca S. Rodriguez: 0000-0002-8994-554X Hattie L. Ring: 0000-0001-5779-2561 Jiayi He: 0000-0003-4361-3379 Kyle C. Bantz: 0000-0002-1732-2183 Christy L. Haynes: 0000-0002-5420-5867

ACKNOWLEDGMENTS This work was supported by National Science Foundation under the Center for Sustainable Nanotechnology (CSN), CHE-1503408. The CSN is part of the Centers for Chemical Innovation Program. J.T.B. acknowledges support by a National Science Foundation Graduate Research Fellowship (Grant no. 00039202). Portions of this work were conducted in the Minnesota Nano Center, which is supported by the National Science Foundation through the National Nano Coordinated Infrastructure Network (NNCI) under Award Number ECCS-1542202.

Notes

The authors declare no competing financial interest. Biographies Hyunho Kang received his B.S degree in Chemistry from the University of Illinois at Urbana−Champaign in 2014. He is currently a Ph.D. candidate in Chemistry at the University of Minnesota under the supervision of Christy L. Haynes. His current research is on the design and investigation of colloidal SERS substrates using silicacoated gold nanoparticles. He also does research as part of the Center for Sustainable Nanotechnology, where his research focus is on the synthesis of silica nanoparticles for the investigation of environmental impacts.

REFERENCES

Joseph T. Buchman received his B.S. degrees in Chemistry and Biology from Augsburg University in 2013. He is currently a Ph.D. candidate in the Department of Chemistry at the University of Minnesota, working under the supervision of Christy L. Haynes. He currently does research as part of the Center for Sustainable Nanotechnology, where he focuses on understanding the mechanisms of nanoparticle toxicity to environmentally relevant bacteria.

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Rebeca S. Rodriguez received her B.S. in Chemistry from American University in 2016. She is currently pursuing her Ph.D. degree in Chemistry at the University of Minnesota. Her research focuses on the design and fabrication of polymer affinity agents to detect small molecule toxins found in food. Using surface-enhanced Raman spectroscopy, this platform allows for molecular fingerprint identification as well as the possibility of multiplex detection. Her current work has focused on mycotoxin detection and will move to other classes of small molecules for food safety. Hattie L. Ring received her B.S. degrees in Physics and Chemistry at Iowa State University. She then earned her Ph.D. degree (2012) in Physical Chemistry at the University of California, Berkeley. Her postdoctoral training was at the University of Minnesota in the Department of Chemistry and the Center for Magnetic Resonance Research. Her research interests include biologically compatible nanoparticle coatings, iron−oxide nanoparticles, magnetic resonance imaging contrast agents, and magnetic fluid hyperthermia. She is currently a research associate at the University of Minnesota. Jiayi He received her B.S. degree (2016) in Chemistry with Honor from Wuhan University. Currently, she is a Ph.D. candidate under the supervision of Christy Haynes in the Department of Chemistry at the University of Minnesota. Her research interests focus on single-cell electrochemistry measurements and polymer-modified electrolytegated transistors for food safety applications. 691


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