Plasmonic Heating of Nanostructures | Chemical Reviews

Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

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Plasmonic Heating of Nanostructures Liselotte Jauffred,† Akbar Samadi,† Henrik Klingberg, Poul Martin Bendix,* and Lene B. Oddershede*

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Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark ABSTRACT: The absorption of light by plasmonic nanostructures and their associated temperature increase are exquisitely sensitive to the shape and composition of the structure and to the wavelength of light. Therefore, much effort is put into synthesizing novel nanostructures for optimized interaction with the incident light. The successful synthesis and characterization of high quality and biocompatible plasmonic colloidal nanoparticles has fostered numerous and expanding applications, especially in biomedical contexts, where such particles are highly promising for general drug delivery and for tomorrow’s cancer treatment. We review the thermoplasmonic properties of the most commonly used plasmonic nanoparticles, including solid or composite metallic nanoparticles of various dimensions and geometries. Common methods for synthesizing plasmonic particles are presented with the overall goal of providing the reader with a guide for designing or choosing nanostructures with optimal thermoplasmonic properties for a given application. Finally, the biocompatibility and biological tolerance of structures are critically discussed along with novel applications of plasmonic nanoparticles in the life sciences.

CONTENTS 1. Introduction 2. Synthesis 2.1. Spherical Gold Nanoparticles and Nanostars 2.2. Nanorods 2.3. Composite Nanoparticles 2.4. Nanocages 3. Thermoplasmonics 3.1. Interaction of Light with a Small Spherical Nanoparticle 3.2. Absorption Cross Section and Temperature Profile 3.3. Nanostructure Geometry Determines Optical Properties 3.3.1. Spherical Nanoparticles 3.3.2. Gold Nanoshells 3.3.3. Gold Nanomatryoshkas 3.3.4. Gold Nanocages 3.3.5. Gold Nanorods 3.3.6. Nanostars 3.3.7. Gold Nanodimers 3.3.8. Possible Gaseous Layer Formation during Plasmonic Heating 3.3.9. Thermal Stability of Nanostructures 3.3.10. Collective Heating from Dispersed Nanoheaters 4. Nanothermometry 4.1. Fluorescent Molecular Probes 4.1.1. Fluorescence Polarization 4.1.2. Fluorescence Intensity 4.2. Nanoprobes 4.2.1. Quantum Dots 4.2.2. Lanthanide Upconverting Nanoparticles

© XXXX American Chemical Society

4.2.3. Microwave Spectroscopy of Nanodiamonds 4.3. Changes in the Surroundings 4.3.1. Refractive Index Distortion 4.3.2. Viscosity Changes 4.3.3. Phase Transitions in Lipid Bilayers 4.4. Raman Spectroscopy 4.5. Thermal Radiation Spectroscopy 4.6. Comparison and Applicability of Nanothermometry Methods 5. Biological Distribution and Tolerance 5.1. Circulation 5.2. Biodistribution 5.3. Toxicity 5.3.1. Gold Nanospheres 5.3.2. Gold Nanorods 5.3.3. Gold Nanoshells 5.3.4. Gold Nanocages 5.3.5. Gold Nanostars 5.3.6. Final Remarks on the Toxicity of Plasmonic Nanostructures 6. Selected Applications in the Life Sciences 6.1. Plasmonic Photothermal Therapy 6.2. Molecular Delivery 6.3. Plasmonic-Nanoparticle-Mediated Membrane Heating 6.4. Thermophoresis 6.4.1. Fusion of Cells and Vesicles 6.5. Photonic Polymerase Chain Reaction 6.6. Plasmonic Heating for Controlling Thermoresponsive Materials

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Received: December 1, 2018

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DOI: 10.1021/acs.chemrev.8b00738 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 7. Summary and Outlook Author Information Corresponding Authors ORCID Author Contributions Notes Biographies Acknowledgments Abbreviations References

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1. INTRODUCTION Medieval alchemists created homogeneous solutions of metallic oxides that survived mixing with glass. The reflection and refraction of sunlight from such solutions depend on the size of the particles, which the alchemists controlled so that each solution would each have their own color. In this manner, spectacular windows which we still admire, for instance, in Notre-Dame de Paris, were created (Figure 1A). In spite of our modern understanding of light−matter interactions, there remain many unknowns, especially if the matter is in the form of particles or structures whose linear dimensions are comparable to or smaller than the wavelength of light. In this case, the optical properties are significantly different from that of the similar bulk material. When irradiated by an electromagnetic field, the oscillations of the conduction electrons within metallic nanoparticles (NPs) will be in phase with the incident field (Figure 1B), and at certain frequencies, the electromagnetic field may be at resonance with the oscillations of the conduction electrons. This will lead to a significantly increased absorption of the incoming light, and the phenomenon is called localized surface plasmon resonances (LSPRs). The resonance frequency depends on the size, shape, and material composition of the nanostructure; an example of absorption as a function of wavelength for gold nanoparticles (AuNPs) with different diameters is shown in Figure 1C and clearly demonstrates the effect of size on absorption. Since the absorbed energy will be released as heat to the environment, plasmonic NPs can serve as light-to-heat converters. One important advantage of a plasmonic nanostructure is that heating can be triggered and controlled by external irradiation, for instance, by a laser. Hence, plasmonic nanostructures have the unique property that their heating can be fine-tuned, turned on or off, and used for a large number of diverse applications. One important reason for the huge interest in plasmonic nanoparticles is their potential for thermoplasmonic cancer therapy (Figure 2B). In this type of therapy, plasmonic NPs located at the tumor site are irradiated by a laser and the heating kills the tumor cells. Although, photothermal therapy has been shown effective in living mice,1,2 important challenges still remain before this therapy can be translated into humans. Most importantly, efficient strategies for delivery of NPs to tumor sites need to be developed.3 Because of the potential medical applications, much effort has been invested in development of methods for particle synthesis and characterization of plasmonic NPs, with a resonance in the near-infrared (NIR) region (700−1300 nm).4−7 This spectral window is termed the biological transparency window. It permits deep penetration of light into biological material (see Figure 2A), making it the most relevant wavelength regime for biological applications such as photothermal cancer treatment. Lasers operating in the NIR regime can penetrate centimeters

Figure 1. Plasmonic properties of nanoparticles. (A) Window from the Notre-Dame Cathedral in Paris, where each glass section contains a homogeneous suspension of metallic oxides of a certain size which determines the scattering and absorption and thus the color of the glass. (B) Schematic illustration of the interaction between conduction electrons in a metallic nanoparticle with the incoming electromagnetic field. (C) Absorption of gold nanoparticles as a function of wavelength for particles with diameters of 50, 100, and 150 nm calculated by Mie theory.

into tissue; hence, utilizing fiber optics and endoscopes, most sites in the human body can be reached. In this review, we describe the most essential properties of NPs relevant for applications based on photothermal heating with a special focus on biological applications. Therefore, we focus on NPs with significant absorption in the NIR regime. In spite of the efforts in producing NPs with resonance or near-resonance in NIR wavelengths, standardized criteria for evaluating thermoplasmonic properties remain to be formulated. To aid the reader in choosing an appropriate NP for their B


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Figure 2. Applications of plasmonic nanoparticles in the life sciences. (A) Illustration of the biological transparency window (NIR-I and NIR-II). Reproduced with permission from ref 8. Copyright 2017 Royal Society of Chemistry. (B) Illustration of how plasmonic nanoparticles (green) in human blood vessels penetrate the surrounding tissue. Reproduced with permission. Copyright 2019 Chano Birkelind.

application, this review attempts to systematically evaluate the plasmonic properties of a variety of NPs. With this goal in mind, this review will describe plasmonic NP types represented by the categories shown in Figure 3 where we

properties of particles have only been experimentally quantified for a small fraction of these particles, we endeavor to expand our knowledge on these particle types. To this end, we present illustrative examples, calculated by finite element modeling (FEM), on how absorption and associated temperature change with systematic variation of size, composition, and aspect ratio of selected plasmonic nanoparticles. On the basis of this quantitative information, this review can help estimate the NP properties that need to be taken into account for a specific application. If the exact intensity distribution of the light on the nanometer scale as well as all relevant thermal conductivities and geometries are known, the temperature rise of an irradiated nanostructure can be accurately calculated by FEM implementing Mie theory.9 However, these parameters are often not known. Hence, experimental assessment of temperatures at the nanometer scale during irradiation of plasmonic nanostructures is important. Therefore, we review the most widely used methodologies for experimental assessment of nanoscale temperature elevation associated with irradiation of plasmonic nanostructures. The final part of the review focuses on novel and emerging applications of plasmonic nanostructures, including photothermal cancer therapy, fusion of cells and vesicles, and drug delivery.

Figure 3. Sketch of the categories of nanoparticles which are in focus of the current review: spherical and massive metallic nanoparticles made of gold (AuNP), silver (AgNP), platinum (PtNP), and titanium nitride (TiNNP); spherical nanoparticles with a layered structure and composed of metallic and semiconductor material such as gold nanoshells (AuNS) and gold nanomatryoshkas (AuNM); metallic nanoparticles with sharp edges such as gold nanoboxes, nanocages (AuNC), and nanostars (AuNSt); elongated plasmonic structures as gold nanorods (AuNR), silica-coated gold nanorods and dimers. The abbreviations used throughout the review are shown in the figure.

2. SYNTHESIS The explosion we have witnessed in the field of nanoparticle research during the past decade can be ascribed to the wealth of new methods for synthesizing nanostructures of almost any design and composition. Plasmonic nanoparticles have become commercially available in a high and biocompatible quality, and every day, new particles emerge on the market. In parallel, there are large exploratory efforts in producing tomorrow’s plasmonic NP and recent advances in synthesis methods offer possibilities to create a tremendous variety of plasmonic NPs with sizes, shapes, and compositions tailored for specific chemical, biological, or medical applications. We here summarize a few approaches for synthesizing certain important classes of NPs before reviewing thermoplasmonics in section 3. The conventional fabrication techniques of the plasmonic particles depicted in Figure 3 and further described in section 3 are categorized as bottom-up approaches. In bottom-up methods, material is assembled by stacking of atoms, which

also introduce the abbreviations used for each particle type. This review covers optical properties and photothermal heating of nanostructures and selected applications involving photothermal heating. It does not cover the large general field of plasmonics, for instance, of bulk materials. For each of the particle types outlined in Figure 3, their synthesis, toxicology, and biodistribution (if systemically delivered) is critically reviewed. As the thermoplasmonic C


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Figure 4. Methods for nanoparticle synthesis. (A) Schematic of AuNS synthesis where small AuNPs are seeded on a 120 nm silica dielectric particle. Reduction causes a gradual growth and coalescence of the gold colloid on the silica nanoparticle surface until there is a completed layer. Reprinted with permission from ref 28. Copyright 2017 Elsevier Science B.V. (B) TEM images of AuNS synthesis. Reprinted with permission from ref 25. Copyright 1998 Elsevier Science B.V.

gives rise to a structure where crystal planes build on crystal planes. Generally, these approaches are based on molecular selfassembly and colloid reduction chemistry including a two-step precipitation process. In two-step processes, the first step is a homogeneous nucleation of very small particles known as seeds followed by a secondary nucleation, using the seeds as catalysts, where the NP grows to its final size.

lattice faces. However, the same surfactant that is responsible for the anisotropic shape of the AuNR also hinders its biological compatibility, as CTAB exhibits high levels of cytotoxicity; see section 5.3.2. Chemical functionalization strategies of AuNRs are now available that limit the cytotoxicity of CTAB in vitro, but often they require multiple washing steps and are sensitive to physiological conditions.22,23

2.1. Spherical Gold Nanoparticles and Nanostars

2.3. Composite Nanoparticles

In 1951, Turkevich and co-workers proposed to chemically reduce chloroauric acid, HAuCl4, by aqueous citrate solutions for the fabrication of AuNPs in solution.10 This chemical reduction of a gold salt produces spherical NPs; however, more control is obtained with a two-phase redox reaction:11,12 The first step in such a reaction is the precipitation of gold ions into seeds, which then act as nucleation sites as the remaining gold ions undergo reduction, and the NPs grow until the gold ions or reducing agent are depleted. By optimizing the reaction kinetics of gold ions and other reagents, AuNPs with narrow size distributions can be produced in ranges from 5 to several hundreds of nanometers. Even though most commercialized methods follow this scheme, many other different methodologies have been proposed for the synthesis of AuNPs, including photochemical,13 polymer-mediated,14 or microbial-mediated synthesis.15 For a thorough review on the synthesis of NPs, see ref 16. Gold nanostars (AuNSts) in the form of multipod colloidal NPs were first reported synthesized in 2007.17 The synthesis was based on protocols for producing gold decahedral and octahedral nanoparticles18 employing the reducing ability of N,N-dimethylformamide (DMF) in combination with poly(vinylpyrrolidone) (PVP), as a stabilizer, and using ultrasound as an energy source and preformed NP seeds as catalysts.

In 1997, a novel type of hybrid NP consisting of a dielectric core surrounded by a thin noble metal shell was reported.24 This socalled nanoshell was unique because its resonance frequency could be shifted from 650 to 900 nm depending on the geometry of the layers.24 This laid the ground for the versatile and widely used gold nanoshells (AuNSs) which are based on a silica core coated by a thin layer of gold.25 In contrast to metallic colloidal NPs, the plasmon resonance of a nanoshell can be tuned to specific wavelengths across the visible and even into the NIR range of the electromagnetic spectrum by adjusting the size of the dielectric core and the thickness of the gold shell, as characterized in section 3 of this review. The strategy for AuNS synthesis is first to prepare silica NPs with the Stöber method.26 Then, these silica cores are treated with an amine-terminated surface silanizing agent, e.g., 3-aminopropyltrimethoxysilane (APTMS), whereafter they are mixed with small, negatively charged gold nanoparticles (∼2 nm). As the negatively charged AuNPs are attracted to the amine groups, which are positively charged at the pH used for the attachment process, this strategy leads to an ∼25% coverage of the silica surface by AuNPs. Finally, the gold NPs are used to nucleate the growth of a complete gold shell; see Figure 4. The AuNS can be thiolfunctionalized like the ordinary AuNP,27 thus making surface modifications relatively straightforward. A related multilayered silica−gold NP (Au−SiO2−Au), known as a gold nanomatryoshka (AuNM), was reported in 2014.7 The AuNM offers the possibility to achieve NIR plasmon resonances for a particle less than 100 nm in diameter, as detailed in section 3. Nanomatryoshkas consist of a AuNP core, coated with a thin silica layer, and a final thin gold shell. Due to strong coupling between the plasmons supported by the gold core and the gold shell, known as plasmon hybridization,29 the plasmon resonance can be tuned to the NIR region in such particles with smaller overall dimensions than the standard NIR resonant AuNS, whQich typically have an overall diameter of 150 nm.

2.2. Nanorods

Chemical reduction of gold salts can produce spherical AuNPs. However, even though chemical reduction methods also have been proposed for nanorod synthesis, it is more common to use templates for making AuNR and nanowires. Porous alumina structures19 or carbon nanotubes20 are examples of such templates. However, the most conventional template for AuNR synthethis is hexadecyltrimethylammonium bromide (CTAB). CTAB forms a lipid bilayer with a positive surface charge that strongly adsorbs to the gold nanorod surface.21 This bilayer is critical in forming the desired rod-shaped morphology due to it restricting growth normal to the 110 and 100 crystal D


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2.4. Nanocages

The LSPR for a small spherical nanoparticle of diameter, d, in a quasi-static approximation can be expressed in terms of the particle’s polarizability. A spherical NP with d ≪ λ and volume, V, which is immersed in an external field can be considered as a dipole with a polarizability given by the Clausius−Mossotti relation

Porous gold nanocages with a hollow core were first described in 2007.30 These particles are prepared using a galvanic replacement reaction between solutions containing gold precursor salts and silver nanostructures prepared through polyol reduction.30 The electrochemical potential difference between the two species drives the reaction, with the reduced gold depositing on the surface of the silver NP and adopting their underlying form which can be cubic. Concurrent with this deposition, the interior Ag is oxidized and removed, together with alloying and dealloying, thereby producing porous gold structures.31 This approach is versatile, with a wide range of morphologies, e.g., nanorings, prism-shaped nanoboxes, nanotubes, or multiple-walled AuNS, available by changing the shape of the initial Ag template. In addition to gold-based structures, switching the metal salt precursors to sodium allows for the preparation of hollow platinum nanostructures.32

α(ω) = 3V

ε(ω) − ε0 ε(ω) + 2ε0

(1)

where ε0 and ε(ω) are the frequency-dependent permittivities of the medium and nanoparticle, respectively. For a uniformly polarized spherical NP, the extinction cross section, Cext, can be conveniently expressed in terms of the polarizability, α,36 Cext =

k4 2 |α| + k Im(α) 6π

(2)

where k = 2π/λ is the wavenumber and λ is the wavelength in the given material. We note that Cext = Cscat + Cabs, where Cscat denotes the scattering cross section and Cabs the absorption cross section.

3. THERMOPLASMONICS Metallic nanostructures produce heat when collective oscillations in the electron density at the surface of nanostructures, called localized surface plasmons (LSPs), arise via coupling to electromagnetic waves. The amplified movement of the conduction electrons increases the frequency of collisions with the lattice atoms and results in Joule heating, which is the mechanism behind thermoplasmonics. The coupling between incoming light and electronic oscillations is therefore a determining factor for the thermoplasmonic efficiency. Of special interest is the condition when resonance is achieved between the incoming light and the LSP frequency of the nanostructure. At the localized surface plasmon resonance (LSPR), a maximum of electromagnetic energy is converted into heat. The absorption of light is characterized by the absorption cross section, Cabs, which is a function of wavelength and can be directly applied to find the heat power produced by a nanostructure when irradiated at a given wavelength. Different parameters influence the interaction between a plasmonic nanostructure; for instance, the shape of the nanostructures is pivotal for the resonance conditions for the absorbed light. In this section, the absorption of light by a range of different nanostructures with different morphologies and compositions is calculated. In section 4, experimental methods to quantify thermoplasmonic heating are reviewed. To compare the photothermal efficiency of different materials, the dimensionless Joule number has been introduced. The Joule number scales with the absorption cross section divided by the nanoparticle volume.33 While this number is very useful for bulk materials, it is, however, not optimal for comparing different nanoparticles with complex shapes. Instead, to characterize the photothermal efficiency of nanoparticles, the absorption cross section, Cabs, is widely used. In the current review, we mainly focus on a few conventional materials, gold and silver, but also describe the plasmonic properties of some alternative materials, like titanium nitride34 and platinum,35 which are gaining popularity in thermoplasmonics due to their high absorption cross sections, also in the NIR. Gold is the most widely used plasmonic material for biological applications due to its exceptional biocompatibility, as detailed in section 5.

3.2. Absorption Cross Section and Temperature Profile

The temperature field caused by irradiating a nanostructure can be expressed via the general heat transfer equation37 ρCp

∂T (r) = ∇·[κ ∇T (r)] + Q ∂t

(3)

where ρ and Cp are the density and specific heat capacity at constant pressure, respectively. T(r) is the absolute temperature, and κ is the thermal conductivity of the surrounding medium. Q is the external source of heat in the medium (here, the heating generated by the nanostructure), and it represents the amount of heat produced per unit time and volume in the nanostructure, which originates mostly from Joule heating in the nanostructure. According to Poynting’s theorem, this dissipated energy can be written38,39 as

Q=

∭ q dV

(4)

1

where q = 2 Re(J·E*) is the electromagnetic power loss density within the nanostructure integrated over the NP volume and J = σE is the current density as a function of the conductivity σ. To calculate the heat production given by eq 4, the electric field, E, inside a nanostructure must be known. To find this, one can use finite element modeling (FEM) to numerically solve Maxwell’s equation: σ zyz ji ∇ × μr −1(∇ × E) − ko 2jjjεr − i zΕ = 0 j ωεo zz{ k

(5)

The electric field is the sum of the incident and scatter fields, E = Einc + Escat. μr is the relative magnetic permeability, ω is the angular frequency, ko 2 =

ω2 , c2

and c is the speed of light. The

frequency-dependent relative permittivities, εr =

ε , εo

for differ-

ent materials can be deduced from a refractive index database (https://refractiveindex.info). In practice, the electric field can be found by solving eq 5 numerically in a discretized space with tetrahedral meshes chosen much smaller than the wavelength. Together with appropriate boundary conditions, continuity of the electric field components inside and outside of the nanostructure, it is

3.1. Interaction of Light with a Small Spherical Nanoparticle

We first present the simple case of a spherical nanoparticle with diameter much smaller than the wavelength, λ; hence, the particle can be assumed to be uniformly polarized by the field. E


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Figure 5. Absorption by spherical solid NPs made from gold, silver, platinum, and titanium nitride. (A−D) The absorption cross section of 100 nm NPs versus wavelength. (E−H) The absorption cross section versus wavelength for NPs with diameters ranging from 10 to 250 nm. The dotted lines represent the profiles plotted in parts A−D. (I−L) The absorption efficiency versus wavelength for NPs with diameters in the range from 10 to 250 nm.

possible to obtain the electric field inside the nanostructure. Using adaptive refinements of the mesh sizes, the local fields can be accurately solved, even in highly curved regions where the fields exhibit strong gradients. This property allows FEM to be applied to complex structures with high accuracy and provides one of the strengths of FEM compared to other methods. FEM has been quantitatively compared to other methods and can accurately reproduce the results for Mie’s calculations on spherical objects.40,41 Dividing the rate of dissipated or absorbed energy in the nanostructure, Q, by the intensity of incident light, cε I0 = 20 |Einc|2 , gives an expression for the optical absorption cross section:38,39 Cabs =

Q 1 = I0 cε0 |Einc|2

∭ Re(J·E*) dV

constant phase within the entire NP, thus leading to a uniformly polarized NP. This approximation gives a closed form expression for the absorption cross section. The quasistatic approximation becomes increasingly inaccurate for larger NPs and the exact absorption cross section for any spherical nanostructure can be calculated using Mie’s equations.36 Mie’s equations are restricted to spherical shapes; however, one can use FEM to calculate optical cross sections of nanostructures of arbitrary shape using eq 6. The temperature distribution surrounding an irradiated nanostructure is governed by eq 3, which reaches a steady state after tens of nanoseconds, thus satisfying Laplace’s equation. The solution for the temperature gradient becomes a simple function of distance, r, to the surface of a nanostructure with radius R

(6)

ΔT (r ) =

Cabs is the quantity of interest for determining thermoplasmonic heating of irradiated nanostructures. The usefulness of this quantity becomes evident when using a known incident intensity of light, since the absorption cross section then gives the immediate production of heat by the nanostructure when multiplied by the intensity. Both numerical and analytical methods can be used to obtain optical cross sections for plasmonic nanostructures. For spherical NPs much smaller than the wavelength, the quasistatic approximation can be used while assuming the electric field has a

C I Q = abs 0 , 4πκr 4πκr

r≥R

(7)

where κ is the thermal conductivity. Often, experiments are performed at an interface, such as a glass−water interface, altering the thermal conduction significantly, especially for flat nanostructures with a large contact area with the substrate. In such cases, the thermal conductivity of the surroundings can be replaced by an effective conductivity which is the average of the conductivity of the two media κeff = (κ1 + κ2)/2.42,43 From eq 7, one can calculate the particle’s surface temperature as T(R) for spherical NPs. However, eq 7 can also be used to extract surface F


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Figure 6. Absorption by AuNSs at different wavelengths and with different core−shell ratios. (A) Absorption cross section versus wavelength for AuNSs with a constant silica core diameter of dSiO2 = 120 nm and with variable thickness of the gold layer. (B) Absorption cross section versus wavelength for a constant thickness of the gold layer, tAu = 15 nm, and variable diameter of the silica core. (C) Absorption cross section at λ = 800 nm as a function of the two variables, tAu and dSiO2. (D) Absorption efficiency at λ = 800 nm as a function of the two variables, tAu and dSiO2.

the size of the NP. However, platinum and titanium nitride NPs provide exceptionally high absorption in the NIR and must be regarded as good candidates compared to other spherical solid NPs for photothermal applications. Experimental measurements of heating by spherical metallic nanoparticles have primarily been carried out with AuNPs.2,45−49 Quantitative studies of single particle heating were conducted with a NIR continuous wave (CW) laser to optically trap a 100 nm gold nanoparticle in water, and the measured heating did not deviate much from values calculated by the dipole approximation.46 Other studies have measured heating of single AuNPs embedded within ice48 or adjacent to temperature sensitive membranes where the thermal response of the material is known.2,45,47 Overall, there is good agreement between the theoretically predicted and measured temperatures. Discrepancies may originate from uncertainties in substrate effects and from lack of knowledge of the exact laser intensity distribution at the location of the particle. Bulk absorption or heating measurements have been conducted on gold NPs,2 titanium nitride nanoparticles,34,50 and platinum NPs;51 these are all consistent with theoretical predictions. To improve the absorption in the NIR region beyond the values given in Figure 5, novel types of layered spherical structures with silica and gold have been designed with LSPR shifted into the NIR. The plasmonic properties of such layered nanostructures are reviewed below. 3.3.2. Gold Nanoshells. As detailed in section 2, gold nanoshells (AuNSs) are silica cores coated with a thin layer of

temperatures of nonspherical particles by including a correction factor β in the denominator: ΔTnonspherical = CabsI0/4πκReffβ, where Reff is the radius of a spherical shape with a volume identical to that of the nanoparticle. β has been calculated for several shapes,44 and for spherical nanoparticles, β = 1. 3.3. Nanostructure Geometry Determines Optical Properties

In this section, we present calculations of the absorption cross section characterizing the particles’ thermoplasmonic properties for the different classes of metallic NPs described in Figure 3. For the different NP types, their geometries are systematically varied and their optical characteristics are determined by FEM. 3.3.1. Spherical Nanoparticles. The absorption cross sections of spherical gold, silver, platinum, and titanium nitride NPs calculated by FEM are shown in Figure 5. Spherical solid metallic NPs have plasmonic resonances in the UV and visible region, as seen for the four materials in Figure 5A−D. With increasing sizes, the absorption spectra exhibit a slight red-shift and a significant broadening (Figure 5E−H). To see the efficiency of a NP in converting light into heat, we define the absorption efficiency as the absorption cross section normalized with the projected area of the NP, Qabs = Cabs/πR2, as plotted in Figure 5I−L. When normalized with the projected area, it becomes evident that small NPs are more efficient absorbers than larger NPs at their resonance wavelength. From Figure 5, it is clear that absorption peaks of spherical solid metallic NPs lie in the UV or visible region, regardless of G


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diameter limits their applicability for systemic delivery and hence smaller structures with similar absorption efficiency are desirable. 3.3.3. Gold Nanomatryoshkas. Gold nanomatryoshkas (AuNMs), as introduced in section 2, are concentric core−shell NPs with a gold core and an outer gold shell separated by a silica layer. AuNMs exhibit similar tunable properties in absorption as gold nanoshells but with the distinct advantage that high NIR absorption can be achieved with diameters below 100 nm, a major advantage for systemic delivery. The resonances of AuNMs can be tuned across the optical spectrum into the NIR region by varying the core radius (rAu), the silica thickness (tSiO2), or the thickness of the gold shell (tAu), yielding an overall particle radius of rtot = rAu + tSiO2 + tAu. The effect of varying each of these parameters on the absorption spectrum is systematically explored in Figure 8 where the nanomatryoshka radius is systematically increased from 35 to 55 nm. These values are chosen because the synthesized and characterized nanomatryoshkas reported in ref 7 have an overall radius of 44 nm, (rAu, tSiO2, tAu) = (21 nm, 10 nm, 13 nm). Increasing rAu from 13 to 29 nm, with tSiO2 = 10 nm and tAu = 13 nm, results in a gradual increase in absorption for longer wavelengths; see Figure 8A. The emergence of a second red-shifted resonance is caused by hybridization between the inner core plasmonic modes and the plasmonic modes of the gold shell.29,52,53 The red-shifted peak is tuned into the NIR region for rtot < 50 nm. Similarly, the thicknesses of the silica and gold layers affect the absorption bands; see Figure 8B,C. The silica thickness controls the strength of the coupling between the gold core and gold shell, and its effect on absorption is calculated in Figure 8B. Lastly, the thickness of the gold shell affects the position of the absorption peak with thinner layers leading to increasing red-shift; see Figure 8C. AuNMs have been suggested as ideal candidates for biomedical applications due to their LSPR in the NIR region for particle diameters below 100 nm.7 Comparing their absolute absorption cross section at 800 nm (Figure 8A) with the absorption cross section of AuNSs at the same wavelength (Figure 6A), it is clear that r ∼ 50 nm AuNMs can exhibit higher absorption cross sections in the NIR than r ∼ 75 nm AuNSs. The absorption cross section and absorption efficiency versus nanomatryoshka size, at λ = 800 nm, are plotted in Figure 8D,E. Nanomatryoshkas with a total radius of rtot = 44 nm ((rAu, tSiO2 , tAu) = (21 nm, 10 nm, 13 nm)), shown with green arrows in Figure 8D,E, are considered as a reference, and the parameters rAu, tSiO2, and tAu are each changed individually while the other two parameters are kept constant in the simulations (the color codes show the parameter changing in each graph). These simulations show that the highest absorption can be achieved for overall radii between 40 and 50 nm. Interestingly, changes in the dimensions of individual layers of a few nm lead to a dramatic change of AuNM absorption. The normalized electric field amplitudes within each of the sizes of AuNMs corresponding to the peaks of the black, blue, and red curves, respectively, in Figure 8D,E, are visualized in Figure 8F at λ = 800 nm. In all cases, the field is high in the silica layer separating the metallic core and metallic shell. The plasmon mode gives rise to polarization of the charge distribution in the metallic layers, resulting in opposite charges at the two gold/silica interfaces and a strong electric field within the silica layer. The different plasmon modes depicted in Figures

gold. Their resonance depends on the relative size of the silica core, the thickness of the gold layer, and the overall size of the particle.2,29,40,52 The absorption by AuNSs with variable shell thickness and core diameter is calculated in Figure 6. The systematic variation of the gold layer thickness shows that the absorption is substantially red-shifted for thinner gold layers (Figure 6A) or by increasing the diameter of the silica core (Figure 6B). Notably, the absorption splits into a high energy and a low energy band which arises from the hybridization between the plasmonic modes of the cavity (inner shell surface) and of the sphere (outer shell surface). Coupling between the cavity and sphere modes can be symmetric or antisymmetric corresponding to the low and high energy peaks, respectively.29,53 The red-shift in the absorption is desired for a number of biomedical applications, in particular if the overall particle size can still be kept small. This is only realizable with an ultrathin gold layer (∼10 nm) and a small silica core; however, such thin gold layers are difficult to synthesize in a reproducible manner and most NIR resonant AuNSs studied so far have core sizes of ∼120 nm and a shell thickness of ∼15 nm,2,54 which gives a LSPR at around 800 nm; see Figure 6A. The photothermal generation by AuNSs was recently quantified versus similarly sized AuNPs in both single particle experiments and in vivo photothermal therapy application using NIR lasers.2 The heating from near-infrared resonant single 150 nm AuNSs and 150 nm solid AuNPs was measured on a temperature sensitive lipid bilayer, as shown in Figure 7; see also

Figure 7. Experimental assessment of heating of two sizes of AuNPs and one 150 nm AuNS under identical conditions. (A) Heating from irradiated AuNPs and AuNSs detected by a local lipid phase transition (elevated intensity area); see also section 4.3.3. Scale bars in all images: 2 μm. (B) Quantification of heating from the particles based on the size of the melted footprints in part A. The figure is reproduced with permission from ref 2. Copyright 2016 Nature Publishing Group.

section 4.1.2. The off-resonant AuNPs heated substantially less than the NIR resonant AuNSs, as predicted by the FEM calculations in Figure 5 and verified experimentally; see Figure 6. These results were translated into in vivo settings where irradiated AuNSs were found to be much more efficient in ablating tumors in mice than their AuNP counterparts. Despite their higher photothermal efficiency, the fact that NIR resonant AuNSs typically need to have an overall size of 150 nm in H


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Figure 8. Effect of varying the dimensions of AuNM (rAu, tSiO2, tAu) on absorption. (A) Absorption cross section versus wavelength for AuNMs with variable core radius, constant silica shell thickness (10 nm), and constant gold shell thickness (13 nm). (B) Absorption cross section versus wavelength with variable silica shell thickness, constant gold core radius (21 nm), and constant gold shell thickness (13 nm). (C) Absorption cross section versus wavelength with variable gold shell thickness, constant core radius (21 nm), and constant silica shell thickness (10 nm). (D, E) Absorption cross section (D) and efficiency (E) versus particle radius obtained by systematically varying the gold core radius (black), the silica shell thickness (red), or the gold shell thickness (blue), λ = 800 nm; the value for a AuNM with a size of rtot = 44 nm (21 nm, 10 nm, 13 nm) is shown at the green arrow as a reference. (F) Normalized magnitude of the electric field amplitude within the AuNM structure at a wavelength of λ = 800 nm. The three representations of the electric field distribution correspond to the black, blue, and red curve peaks in parts D and E, respectively.

Figure 9. Hybridizations between dipolar plasmons in core−shell nanostructures. (A) The dipolar plasmon modes in a AuNS can be modeled as a hybridization of a nanosphere plasmon with a cavity plasmon. The charge distributions can couple symmetrically in a lower energy mode (red-shifted) or antisymmetrically in a higher energy mode (blue-shifted), leading to splitting of the plasmonic spectra. The degree of splitting is determined by the spatial separation of the modes (shell thickness). The figure is reproduced with permission from ref 29. Copyright 2003 Science. (B) For AuNMs, the three plasmon modes arise from hybridization between plasmons from the outer shell surface, the inner shell surface, and the core nanosphere. (C) Colormap of the charge distribution corresponding to the three plasmon modes. Blue and red colors represent opposite charges. Parts B and C were reproduced with permission from ref 53. Copyright 2010 American Chemical Society.

I


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Figure 10. Absorption by gold nanocages with different geometries. (A) Effect of edge length (l) on the absorption cross section. (B) Effect of hole diameter (d) on the absorption cross section. (C) Effect of wall thickness (t) on the absorption cross section.

Figure 11. Optical properties of bare and silica-coated AuNRs. (A) Absorption cross section of AuNRs with width 20 nm and length 60 nm (gray curve) and length 100 nm (magenta curve). (B, C) Absorption cross section (B) and efficiency (C) versus AR for a 20 nm diameter AuNR with variable length. (D) Normalized electric field magnitude of AuNRs with a constant AR = 4 excited at λ = 800 nm. The smallest rod has dimensions 10 nm × 40 nm and width and length are multiplied in cardinal numbers for each following rod, thus increasing volume while keeping AR constant. The electric field vector is aligned with the long axis of the rod. (E, F) Absorption cross section (E) and efficiency (F) for the rods in part D. (G) Electric field distribution inside a 20 nm × 80 nm AuNR coated with a SiO2 layer of thickness tSiO2 = 20 nm irradiated by 800 nm laser light. (H, I) Effect of silica coating thickness, tSiO2, on the absorption cross section (H) or efficiency (I) for AuNRs with AR = 3, 4, and 5 (AuNR thickness 20 nm).

J


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whereas the transverse resonance lies close to 500 nm and is not included in Figure 11. The maximal absorption is obtained at the resonance wavelength with the electric field vector oriented parallel to the long axis of the rod. As shown in Figure 11B,C, the peak absorption for a 20 nm diameter AuNR can be finely tuned from 600 to 1200 nm by varying its length and increasing AR from 2 to 7. If excited at resonance with the electric field aligned along the rod axis, then AuNRs are among the best plasmonic absorbers of light. By comparing the absorption efficiency for AuNRs (Figure 11C) with the absorption efficiency of, e.g., AuNMs (Figure 8E) or AuNSs (Figure 6), it is clear that AuNRs have approximately 2-fold higher absorption efficiency. As mentioned earlier, the resonance behavior of AuNRs is often discussed on the basis of their aspect ratio. However, as can be seen from the simulations in Figure 11D−F, this parameter cannot alone characterize the resonance behavior of nanorods. AuNRs with identical aspect ratio, AR, but having different volumes, display LSPRs at different wavelengths. Parts E and F of Figure 11 show the absorption cross section and absorption efficiency of AuNRs with a constant AR = 4 but different volumes. As can be seen from Figure 11F, increasing the volume of nanorods red-shifts the LSPR peaks, while the most efficient morphology of AuNRs for photothermal applications, at λ ∼ 800 nm, is a nanorod with AR = 4 (10 × 40 nm). The effect of the silica coating on a AuNR is a slight red-shift which increases with the silica thickness, as shown in Figure 11G−I. As a rule of thumb, the simulations show that the maximum achievable red-shift caused by the silica coating equals AR × 15 nm. The electric field distribution for a SiO2-coated AuNR is shown in Figure 11G, and the corresponding absorption cross sections and efficiencies are shown in Figure 11H and I, respectively. Conversely, thicker coatings with a lowabsorption material reduce absorption efficiencies, as shown in Figure 11I. Due to their extremely high absorption, AuNRs have been extensively used as thermoplasmonic heaters in biological applications.67 AuNRs have been investigated quantitatively with respect to their photothermal efficiency in bulk measurements and compared with AuNSs and other particles.68 Importantly, when comparing AuNRs with other nanostructures, it is necessary to account for the lower average absorption of rods which are oriented randomly in a bulk environment.68 Temperature measurements on single nanorods include both optically trapped AuNRs69 and AuNRs immobilized on a surface.64 In ref 69, a AuNR (25 × 60 nm) was optically trapped by a 1064 nm laser which induced heating.69 This off-resonant heating increases the temperature by 70 K at P = 80 mW, which was detected through the rod’s Brownian fluctuations. Optical trapping has been shown to align rods along the polarization vector of the electric field,69−71 thus causing maximal absorption by the AuNR. To detect the effect of orientation of the AuNR relative to the electric field vector, one can fix the rod on a substrate while altering the direction of the electric field. In ref 64, NIR resonant rods fixed on a coverslip were heated by a focused laser beam while the electric polarization vector was rotated using a λ-half plate. The temperature oscillations were found to be in phase with the oscillations in angle between the electric polarization and long rod axis with high and low values differing by ∼200 °C; the results are shown in Figure 12A, along with FEM simulations (Figure 12B). These results show the importance of the angle between the laser’s polarization and the orientation of the rod

6 and 8 and the associated charge distributions in core−shell NPs have been analyzed in refs 29 and 53. Splitting of the plasmonic spectra results from hybridization between plasmons in the core and shell, as shown in Figure 9. From Figure 9A, we see that for a AuNS two peaks are predicted by this model which is consistent with the calculations in Figure 6, but three different plasmon modes are predicted for a AuNM; see Figure 9B. However, the third mode, which has intermediate energy relative to the other modes, is considered to be very weak29 and is barely resolved in the spectra shown in Figure 8A. The temperature of single irradiated AuNMs have not been experimentally measured, but bulk measurements have shown that AuNMs with a diameter of 90 nm can be designed to produce more heat than AuNSs with a diameter of 150 nm when irradiated with NIR light.7 The small size of the AuNMs combined with their high absorption makes these particles interesting candidates for photothermal therapy. 3.3.4. Gold Nanocages. Gold nanocages (AuNCs), as introduced in section 2, offer a new type of highly absorptive nanostructure with a porous structure and a hollow interior. Such AuNCs, with controlled pore sizes,55 can be loaded with chemical drugs and triggered to release their contents by a photothermal mechanism.56−60 Nanocages are characterized by an edge length (l), the diameter of the holes at each corner (d), and their wall thickness (t); see Figure 10. Keeping the hole diameter and thickness constant, the absorption peak can be shifted into the NIR spectrum by increasing the edge length; see Figure 10A. The localized plasmons associated with the edges and corners, respectively, are responsible for the splitting of the absorption spectra into two peaks.61 Increasing the hole diameter while keeping the edge length and thickness constant allows for redshifting the peak absorption while maintaining a constant overall size of the nanocage; see Figure 10B. Finally, decreasing the thickness can red-shift the resonance peaks while keeping the hole diameter and thickness fixed; see Figure 10C. The gold nanocages, like the AuNMs, are NIR resonant with overall particle sizes ∼60 nm, and the absolute absorption cross sections attainable with gold nanocages are comparable to the absorption cross sections of similarly sized nanomatryoshkas, as shown in Figure 8. Theoretical investigations of the optical properties of AuNCs have been conducted,61 but no quantitative experimental measurements of the thermoplasmonic properties have been done at a single particle level. Bulk measurements have shown that AuNCs can be readily tuned to absorb optimally in the NIR region62 even with higher efficiency than NIR resonant AuNS.63 The AuNCs were also found to be more efficient than AuNSs in photothermal therapy of mice.63 Importantly, their strong NIR resonance for sizes around 60 nm, combined with their ability to retain and release chemical compounds,58 makes these nanostructures ideal candidates for a number of biomedical applications. 3.3.5. Gold Nanorods. The resonance behavior of gold nanorods (AuNRs) is characterized by their aspect ratio AR, i.e., the length to width ratio. Most AuNRs studied in relation to thermoplasmonics have been resonant with NIR wavelengths.64−66 Rods have two resonances, one transversal and one longitudinal. The latter is red-shifted relative to the former. The absorption cross sections for rods with AR = 3 and AR = 5 are plotted in Figure 11A and show that the LSPR shifts ∼250 nm when the AR is increased by 2. The absorption spectrum in Figure 11A is associated with the longitudinal resonance, K


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and have to be considered in bulk experiments where randomly oriented anisotropic nanostructures are illuminated by a plane wave. 3.3.6. Nanostars. A gold nanostar (AuNSt) is another complex plasmonic nanostructure with tunable optical properties.72 The possible variations in the structure’s morphology include spike length, number of spikes, and tip curvature of spikes.73 Absorption by AuNSts is exquisitely sensitive to the length of the spike, ls, with an increasing length causing a redshift, as shown in Figure 13A. Changing the tip’s curvature radius, rt (Figure 13B), or number of spikes, Ns (Figure 13C), does not significantly change the location of the absorption peaks while interacting with a linearly polarized electric field. Interestingly, the presence of two spikes along the electric field in Figure 13C is sufficient to generate a resonance at ∼800 nm, whereas additional spikes do not affect the absorption spectrum noticeably. In treatment of tumor bearing mice, AuNSts have been shown to efficiently reduce tumor size upon irradiation with NIR light.74 Finally, we note that AuNSts can have plasmonic hot spots in the spike and tip region, as shown in Figure 13C (top panel), and therefore have great potential in photochemistry. 3.3.7. Gold Nanodimers. Plasmonic coupling among closely positioned nanostructures is of considerable interest due to the strongly enhanced near-fields existing between irradiated nanoparticle assemblies. Coupling of plasmons across the gap leads to redistribution of the incoming fields and extreme local enhancements for gap sizes of ∼1 nm.75 Applications of this near field enhancement include nanotweezing,76 surface-enhanced Raman scattering,77,78 and development of plasmonic-based nanorulers.79,80 However, placing plasmonic nanostructures in close proximity can also lead to strong electric hot spots within the nanostructures at the regions facing the gap and the optical interactions are found to be highly sensitive to the separation between the nanostructures.

Figure 12. Orientation of a gold nanorod with respect to the electric field polarization, which has a strong effect on heating. (A) Irradiation of a surface immobilized AuNR with an electric field vector having a variable orientation relative to the major rod axis, resulting in a sinusoidal temperature variation (green squares). The corresponding variation in the absorption cross section is shown by the red dashed curve. The inset images show the detected heating using a lipid bilayer as a sensor; see also section 4.1.2. The orientation of the rod with respect to the field polarization is indicated in the figure. Reprinted with permission from ref 64. Copyright 2012 American Chemical Society. (B) FEM calculations of the surface temperature of the AuNR as a function of the relative orientation between the AuNR and the laser’s polarization calculated at two laser powers. The inset shows the magnitude of the electric field for three selected relative orientations: top, parallel orientation; middle, 45°; bottom, orthogonal. The rod dimension was 20 nm × 100 nm.

Figure 13. Optical absorption by gold nanostars as a function of geometry, while the overall volume of the star is kept constant. (A) Absorption cross section versus wavelength for AuNSts with increasing spike length (ls). The overall volume of the star is constant, which implies that the core particle diameter is decreasing with increasing spike length, while the radius of tip curvature rt = 5 nm and number of spikes Ns = 14 is kept constant. (B) Absorption cross section of AuNSts versus wavelength for different tip curvatures. The volume and length (ls = 25 nm) of the spike as well as their numbers (Ns = 14) are kept constant while changing rt. (C) Absorption cross section versus wavelength for AuNSts with increasing number of spikes. The spikes have the same volume with ls = 25 nm and rt = 5 nm, and the size of the core particle is decreased with increasing number of spikes to ensure a constant volume of the nanostar. All AuNSts have a volume corresponding to a spherical gold nanoparticle with a diameter of 80 nm (NP80 in figure legend). L


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Figure 14. Absorption by Au nanodimers for different nanoparticle sizes and gap distances (dgap). (A) Absorption cross section versus wavelength for a Au dimer consisting of two AuNPs (40 nm) separated by 2 or 50 nm, respectively. The inset shows the relatively enhanced electric field within the gap region when the electric field vector is oriented along the dimer’s long axis. (B) Absorption cross section versus wavelength for a range of gap distances. The dotted lines correspond to the spectra in part A. (C, D) Absorption cross section (C) and efficiency (D), at λ = 800 nm, versus particle radius and gap distance, respectively. (E, F) Absorption cross section (E) and efficiency (F), at λ = 1064 nm, versus particle radius and gap distance, respectively.

Figure 15. Optical printing of gold dimers on different substrates. Substrates with low thermal conductivity lead to thermal repulsion between the AuNPs which prevents close location of the two NPs. (A) Schematic illustration of the setup. (B, C) Thermal distribution of a single irradiated gold nanoparticle on glass (B) and sapphire (C) substrates.

nanoparticles have a radius of ∼45 nm (Figure 14D) and at ∼70 nm (Figure 14F), respectively. The optical properties of dimers of different particle types have been theoretically predicted.81−83 However, so far, no measurements have assessed the thermal generation of single plasmonic dimers. The heat released from pulsed laser irradiated gold dimers in bulk was found to be sufficient to dehybridize a dsDNA used to link the two AuNPs.84 However, the gap distance in this study was several nanometers and the heat could thus be ascribed to heat generation in the two NPs without coupling. In a recent study, the effect of optical absorption of single gold dimers was indirectly detected and the temperature was calculated in optically printed gold dimers on glass and sapphire substrates.85 Optical printing of AuNPs on glass with gap distances below a critical value was compromized by thermal repulsion (Figure 15). However, printing could be achieved with a sapphire substrate with higher thermal conductivity and by switching to nanostructures with a larger contact area with the substrate which provided a higher cooling effect. The thermal effects in the experiments shown in Figure 15 were minimized by using a wavelength of 532 nm (see

The effect of plasmonic coupling between two particles on their optical absorption is shown in Figure 14 for two AuNPs (40 nm radius) irradiated by a plane wave. In Figure 14A, the particles are placed 2 and 50 nm apart, respectively, and irradiated by an electric field polarized along the dimer’s axis. At dgap = 50 nm, the absorption spectrum resembles the absorption of spherical gold nanoparticles, due to the absence of plasmonic coupling, and the spectrum peaks at ∼550 nm. However, at dgap = 2 nm, plasmonic coupling results in appearance of a second absorption peak at ∼710 nm. The absorption cross section versus wavelength for a range of gap distances is shown in Figure 14B, which reveals how the second peak is gradually red-shifted with decreasing values of dgap and finally reaches into the NIR at dgap = 1 nm. The effect of particle size on the dimer absorption cross section is shown for two commonly used wavelengths in thermoplasmonics in Figure 14C (λ = 810 nm) and Figure 14E (λ = 1064 nm); the corresponding absorption efficiencies are shown in Figure 14D and Figure 14F. For all particle sizes, the absorption increases with decreasing dgap, whereas the efficiency of absorption, at the two wavelengths, peaks when the M


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Figure 16. Effect of the environment on the thermoplasmonic properties of an irradiated AuNP. (A) Temperature increase of an irradiated 200 nm AuNP as a function of distance from a glass−water interface, λ = 1064 nm and P = 100 mW. (B) Absorption cross section of a 200 nm AuNP surrounded by a vapor layer of varying thickness. The larger the bubble, the less the overall effective permittivity which is responsible for absorption. (C) Temperature increase of an irradiated 200 nm AuNP surrounded by a vapor layer of varying thickness, λ = 1064 nm and P = 100 mW. The reason the temperature increases with bubble size, despite Cabs decreasing, is that the bubble forms an insulating layer around the metallic center. The inset schematically shows the vapor layer surrounding the AuNP. The beam waist in parts A and C was 850 nm.

absorption at λ = 532 nm in Figure 14); however, the thermal interactions between the AuNPs would be greatly amplified by using NIR laser wavelengths which interact strongly with closely spaced AuNPs. 3.3.8. Possible Gaseous Layer Formation during Plasmonic Heating. The conduction of heat through the matrix surrounding the heated nanostructure strongly affects the equilibrium temperature distribution. Most experiments reviewed here are for nanostructures immersed in water or immobilized at a glass−water interface. Laser-induced heating of plasmonic nanostructures often leads to temperatures exceeding the atmospheric boiling point of water without any signs of boiling.45,47,86,87 In the absence of any cooling substrates, there are other effects related to bubble nucleation which elevate the boiling point. A direct consequence of Laplace law is that formation of nanoscale bubbles requires high pressure and this nucleation barrier can significantly increase the boiling temperature.88 Formation of a nanoscale bubble surrounding the nanostructure acts like an isolating layer due to the thermal conductivity, which is lower than that for water. However, nucleation of a bubble is first expected to occur at around T = 200 °C where the thermal energy is sufficient to trigger a transformation from water to gas state.86 To explore the effects of the environment on the achieved temperatures, we calculated the temperature of a NIR irradiated 200 nm AuNP at different locations relative to a glass−water interface or in bulk with a thin vapor layer surrounding the AuNP; see Figure 16. The cooling effect of the glass is evident from Figure 16A as the distance from the glass−water interface increases. The difference in heat conduction in water compared to glass affects not only the temperature of the AuNP but also the absorption cross section of the AuNP. The resulting increase in particle temperature, when the particle is at the glass−water interface compared to inside the water phase, is minor, since the decrease in absorption and decrease in heat conductivity have opposite effects on the temperature. Figure 16B demonstrates the decrease in absorption cross section with vapor layer thickness observed for a AuNP surrounded by a water layer. The heating of

a AuNP surrounded by a vapor layer of increasing thickness is shown in Figure 16C. Constant irradiation of a 200 nm AuNP surrounded by vapor layers of different thicknesses leads to rapidly increasing temperatures for thin layers up to ∼50 nm, due to an insulating effect of the vapor layer, but levels off for thicker vapor layers due to the lower optical absorption of the AuNP in a vapor phase versus water. Although the experiments shown in Figure 16 are thought examples, they illustrate the effect an interface between two dielectric media has on the thermoplasmonic properties of a nanoparticle. Quantitative experiments with laser irradiated NPs are often performed on a glass substrate in water, and results from such experiments should be carefully interpreted to include the possibility of both vapor generation and cooling effects from the substrate. The scenario of having a nanoparticle partially immersed in the glass substrate was observed in ref 89 where a AuNP optically heated to 1100 K was found to leave a nanoscopic imprint on the glass substrate as measured by an atomic force microscope, thus confirming a partial embedment into the substrate. The importance of the particle environment plays a major role for the actual heating of nanostructures and therefore also strongly affects the thermal stability of laser irradiated nanostructures. 3.3.9. Thermal Stability of Nanostructures. Metallic nanoparticles used in thermoplasmonics must tolerate high temperatures to avoid morphological changes, which could alter their thermoplasmonic properties. Since shape is tightly linked to optical properties of NPs, any deformation will lead to altered thermoplasmonic behavior. Most bulk metals have high melting temperatures, but recent evidence has revealed that nanostructures melt or reshape at much lower temperatures than their bulk material. Reshaping of nanostructures has been primarily reported for gold nanorods64,90,91 which drastically alter their plasmonic properties. This occurs since even slight changes in their aspect ratio, as shown in Figure 11, change their optical properties substantially. Shape changes in AuNRs have been reported after ultrafast laser exposure where the AuNRs were found to tolerate up to 700 °C. However, prolonged heating of AuNRs leads to extensive surface melting and reshaping, even at ∼200 °C, as shown in Figure 17A.90 In summary, optimal N


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Although their thermal stability has not been reported, on the basis of results from the extensively studied nanorods, it is likely that the thin spikes will not tolerate high temperatures. The spikes could easily reshape during heating which will change their optical properties accordingly, thus limiting their use in high temperature thermoplasmonics. The thin shell structures in AuNMs and AuNSs are only a few nanometers thick and may be susceptible to thermal damage. AuNSs were directly compared to PtNPs in terms of heating and thermal stability at a single particle level in ref 35. Scanning electron microscopy of laser irradiated AuNSs and PtNPs showed clear degradation of the AuNS, probably caused by a greater thermal expansion of the core than the shell. In contrast, the spherical PtNPs remained intact even up to 700 °C. The thermal stability of AuNMs remains to be examined. However, they do have a similar outer shell structure as AuNSs, and this, combined with their high photothermal efficiency, renders them potentially thermally unstable at high laser intensities. Spherical shapes are most stable with respect to morphological changes, and hence, spherical AuNPs retain their optical properties at higher temperatures than is the case for AuNRs. However, at very high temperatures (>1000 °C), spherical AuNPs were found to fragment; see Figure 17B. The diameters of the fragments were quantified and found to decrease with increasing laser power and with duration of laser exposure (Figure 17C).89 The temperature and, hence, the stability of plasmonic nanostructures depends critically on the thermal conduction of the underlying substrate and the surrounding medium. The laser heating conducted in Figure 17B,C was carried out in air, and the nanostructures were immobilized on a glass substrate. Air has a low thermal conductivity compared to water which is used in many laser heating experiments; however, the high conductivity of glass and sapphire substrates can facilitate cooling of the nanostructures and prevent thermal degradation of the plasmonic nanostructure.85 Finally, we note that the irradiation laser power used for applied thermoplasmonics can differ by 6 orders of magnitude. In photothermal medical applications, a laser power of ∼1 W/ cm2 is typically used2,93 in contrast to a laser power of ∼1 MW/ cm2 or higher, which is used for optical trapping or laser heating of single NPs.2,45,49 However, even under low intensity in bulk experiments, in which single particle heating is negligible, reshaping of AuNRs has been detected.91 This shows that high local temperaturessufficient to reshape nanostructurescan be generated with low laser intensity in a dense suspension of NPs. 3.3.10. Collective Heating from Dispersed Nanoheaters. For biological applications like photothermal therapy, it is desirable to know both the single particle nanoscale heating as well as the global temperature resulting from irradiating a known density of plasmonic NPs. To explore the transition between single particle and bulk heating, influenced by collective effects, a controlled study was performed with temperature measurements of arrays of plasmonic absorbers with different spacings fabricated on a substrate.94 On the basis of these measurements, the authors derived theoretical expressions that are useful to predict the temperature gradients in suspensions of different densities of NPs. This study closes the gap between single particle thermoplasmonics and collective thermal effects from an ensemble of NPs. In bulk heating experiments, two temperature regimes exist: (i) the temperature in the nanoscale region surrounding the NP

Figure 17. Thermal stability of AuNRs and AuNPs. (A) TEM images of AuNRs after incubation at different temperatures for 20 h. The figure is reproduced with permission from ref 90. Copyright 2006 Royal Society of Chemistry. (B) SEM images of AuNPs exposed to CW laser irradiation (λ = 488 nm) for different durations and at different laser powers. Scale bars: 100 nm. The AuNPs were deposited on a glass substrate in air. (C) Evolution of the measured particle diameter with increasing laser power. The particle temperature, Tp, as a function of laser peak power density (ascending dashed red line; scale: right) calculated using a 1D heat conduction equation and using four different exposure times, as denoted by the legend. Parts B and C were reproduced with permission from ref 89. Copyright 2014 Royal Society of Chemistry.

absorption by AuNRs leads to extreme heating but has been linked to thermally induced structural changes which also strongly influences the optical properties of the rods.64,90,91 Fortunately, the thermal stability of AuNRs can be markedly improved by applying a SiO2 outer layer.65,92 Nanostars are highly absorptive nanoparticles, and their thin spikes might similarly suffer from poor thermal stability. O


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Figure 18. Measurements of bulk heating to assess the photothermal efficiency of plasmonic NPs. (A) Time scales involved in single particle versus bulk heating. (B, C) Ensemble measurement of photothermal heating generated by AuNS and AuNM upon irradiation with 810 nm laser light at P = 2 W/cm2. Reproduced with permission from ref 95. Copyright 2006 American Chemical Society. (B) Heating versus time for AuNS (red curve) and AuNM (blue curve), respectively. (C) Photothermal efficiency of AuNS (red bar) and AuNM (blue bar), respectively. All measurements were performed at the same optical density to facilitate comparison. The dimension of the AuNS was dcore = 124 nm with an Au layer thickness of 14 nm, and the AuNM had a dimension of (rAu, tSiO2, tAu) = (21, 10, 13). Parts B and C were reproduced with permission from ref 7. Copyright 2014 American Chemical Society. (D) Pt50s (green), Pt70s (red), and AuNSs (blue) with fixed optical density were irradiated with λ = 1064 nm laser light until they reached equilibrium temperature. The gray curve shows the heating of water alone. Pt70s have higher average efficiency (56 ± 3%) than AuNSs (39 ± 2%), in agreement with previously calculated values for AuNSs.7 Hence, platinum particles are 1.2 times more efficient than AuNSs at a constant optical density of Aλ = 0.14. (E) Photothermal transduction efficiencies, η, obtained from experiments and eq 9; same color code as that in part D. Parts D and E were reproduced with permission from ref 35. Copyright 2018 Royal Society of Chemistry.

equilibrium within the cuvette is much smaller than the time it takes to reach thermal equilibrium with the surroundings. From this relation, the photothermal transduction efficiency η of nanoparticles to convert absorbed radiation into thermal energy can be found:7,62,68,96−99

and (ii) the global temperature of the sample which typically has a length scale of 10−3 to 10−2 m. With a thermal diffusivity of water of D = 10−7 m2/s, the characteristic time for equilibration of the temperature of a region of size L is τ = L2/D. Nanoscale heating equilibrates during 100 ns, whereas the characteristic heating time for a volume with a radius between 1 and 10 mm is several hundreds of seconds. These time scales are plotted in Figure 18A which shows the two plateaus originating from nanoscale heating and the effect from collective heating.95 The collective heating from NIR irradiated nanostructures is readily obtained from bulk measurements by using thermal imaging35 or a simple thermometer. Bulk measurements therefore provide a robust readout for the relative photothermal heat generation obtained from ensembles of different nanostructures. To compare the capability of nanoparticles to generate heat, a common strategy is to perform laser heating on ensemble solutions having identical optical density7,62,68,96−98 or having the same metal density.2,99 The energy balance in a system of irradiated nanoparticles can be expressed as96

∑ miCp,i i

dT = dt

η=

I(1 − 10 Aλ)

(9)

Here, Tamb is the ambient room temperature, Tmax is the equilibrium temperature, Q0 is the baseline energy input by the solvent and the sample cell without nanoparticles, I is the laser power, Aλ is the optical density of the nanoparticle solution at the laser wavelength, λ, h is a heat-transfer coefficient, and A is the surface area for radiative heat transfer. The laser power I, the ambient temperature Tamb, and the equilibrium temperature Tmax are experimental values. The constant hA can be found from a control experiment where a cuvette with water is exposed to the laser. The photothermal transduction, defined in eq 9, was compared for AuNMs and for AuNS irradiated under identical conditions; see Figure 18B.7 By matching their optical densities (Aλ = 1), the photothermal transduction efficiency could be directly compared between these nanostructures, which revealed a significantly higher efficiency for AuNMs than for AuNSs; see Figure 18C. Despite the large difference in the total diameter of the AuNMs (88 nm) and the AuNSs (152 nm), the AuNMs generate significantly more heat than the AuNSs, consistent with the absorption values calculated in Figures 6 and

∑ Qj j

hA(Tmax − Tamb) − Q 0

(8)

where the left side is the summation over products of masses, mi, and the corresponding heat capacities, Cp,i, of the different components, T is the absolute temperature, and t is time. The right-hand side is the sum of energy terms, Qj. Equation 8 is valid when the time it takes for the system to reach thermal P


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Figure 19. Nanothermometry based on fluorescent molecular probes. (A) Sketch of experimental setup including a heating laser, a spherical NP, and a laser to excite fluorescent molecular probes. D is the distance between the focal points of the two lasers. (B) Fluorescence anisotropy of fluorescent molecular probes versus D. Figure adapted with permission from ref 100. Copyright 2009 Optical Society of America. (C) Fluorescence intensity versus wavelength, λ, obtained at different distances, D, from the heating laser. Figure adapted with permission from ref 101. Copyright 2016 Optical Society of America.

4.1. Fluorescent Molecular Probes

8. Similar comparative experiments have been carried out using PtNPs and AuNSs, as shown in Figure 18D,E. These experiments show that PtNPs are highly efficient in generating heat upon NIR irradiation despite their peak absorption being in the UV range.35 Overall, our understanding of the mechanism behind optical heating and its dependence on the characteristics of the nanostructure has increased immensely thanks to the extensive efforts made with bulk and single particle experiments. These experiments have provided a wealth of quantitative data on plasmonic heating of nanostructures, and together with numerical methods, it is now possible to perform quantitative predictions of heat generation in nanostructures with complex shapes. To compare optical heating with theoretical predictions, it is critical that single particle heating can be accurately quantified. A number of novel methods have recently been developed for measurements of nanoscale heating, and a selection of these are reviewed in the following section.

The techniques reviewed in this section are based on the use of fluorescent reporter molecules dispersed in the medium around a heated NP; see Figure 19A. The fluorescence properties of the probing molecules, e.g., the emission intensity, emission spectrum, or polarization anisotropy, all depend on temperature through the temperature-dependent lifetime of the fluorescent state102−104 or on the effect of Brownian dynamics on the light emission.105 4.1.1. Fluorescence Polarization. The temperature distribution around plasmonic NPs can be measured using an optical technique based on molecular fluorescence polarization anisotropy measurements.100 For this approach, fluorescent molecules are dispersed in the fluid surrounding the NPs and illuminated by linearly polarized light. Because each dipole of the fluorescent molecule is randomly oriented, the emitted fluorescence of an ensemble of illuminated molecules is partially depolarized. When the kinetic energy of the fluorescent molecules at the vicinity of the heated NPs increases, the rotational motion of the molecules is accelerated. Hence, heating will further depolarize the emitted fluorescence as the molecules rotate more during their fluorescence lifetime.43,100 In this way, thermal information is translated into optical information through fluorescent molecules; see Figure 19B. Brownian motions have to be damped for this technique to be useful, either by increasing the viscosity of the medium or the size of the proteins, such that the time scale of molecular rotation corresponds to the fluorescence lifetime. As this measure is based on the ratio between the intensities of light with different polarization, it is not sensitive to photobleaching. Hence, this technique has been used for thermal measurements of plasmonic heating in vivo in C. elegans, as shown in Figure 20A,B.106,107 4.1.2. Fluorescence Intensity. In 2014, a new method using nucleic acid oligonucleotides with fluorophores coated onto a AuNP was proposed as a thermometer.108 The principle is that DNA oligos are labeled at one end with a specific fluorophore and attached to AuNPs by thiol chemistry at the other end. At room temperature, the structures are all in the closed form and the fluorescence is quenched by the AuNP. When the particle is heated, the nucleic acid structure unfolds and the quenching ceases. Hence, this rise in intensity is temperature-dependent; however, even though the authors calibrated the system, it still has to prove its value for accurate nanothermometry or in vivo thermometry. A drawback of this technique is the sensitivity to salt and pH, and as the cellular environment is very heterogeneous, it might be difficult to calibrate such a system.

4. NANOTHERMOMETRY Nanothermometry is the measurement of temperature at the nanoscale and is emerging as an important tool in science with applications to biology, soft-matter physics, and materials science. An ideal nanothermometer should be not only accurate but also applicable over a wide temperature range and reliable under diverse environmental conditions. Additionally, the readout time should be fast enough to allow rapid temperature variations to be detected. However, many of the existing techniques are limited by drawbacks such as low sensitivity and systematic errors due to fluctuations of fluorescence, local environment, or optical properties of the surrounding medium. Hence, a number of competing techniques have been developed, each with certain advantages and disadvantages. In this section, we focus on the published methods that have been presented or refined since 2013 where the field was thoroughly reviewed.42 We categorize the methods based on whether they involve florescent molecular probes, nanoprobes, or temperature-induced shifts in the surroundings. Furthermore, we review techniques based on Raman scattering and thermal radiation. In the future, this large variety of techniques may develop into a toolbox of a few well-calibrated methods of use for a large variety of nanometry applications. However, for the time being, an optimal approach should be chosen for each application and one purpose of this section is to guide the reader to the optimal nanothermometry method. Q


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study, methods based on temperature-dependent fluorescence are often performed by detecting intensity changes from fluorophores in a solution. This poses a limitation, since irradiation of NPs generates insufficient heat to achieve a sufficient fluorescence contrast to be detected in a confocal microscope; however, with the micrometer sized gold shells used in ref 101, it is possible to achieve intensity changes that span the entire focal volume. 4.2. Nanoprobes

Methods based on reporter particles like quantum dots, upconverting NPs, or nanodiamonds have the ability to quantify single NP heating. These approaches are all based on a two-laser system and are independent of fluorescence intensity, as sketched in Figure 21. Furthermore, they are based on the following two measures: (i) spectral shifts and (ii) distortion of the spectral line. 4.2.1. Quantum Dots. Quantum dots can also be used as temperature nanoprobes, this was first demonstrated in a solution containing laser irradiated gold nanorods.112 The fluorescence spectral line shape of CdSe quantum dots exhibits a red shift upon heating at a rate of 0.1 ± 0.05 nm/K. Since this method is based on spectral shifts, it is less sensitive to bleaching. With a flow system and a confocal microscope implementing two lasers that can be operated simultaneously, one for gold nanorod plasmonic excitation and one for quantum dot excitation, one can perform point measurements of temperature in a gold nanorod solution within a microfluidic channel. In ref 112, temperature increases of 1−40 K were observed. The authors also quantified the absorption efficiency (the ratio of absorption to extinction) of the gold nanorods. In 2014, this experimental approach was further developed by focusing both laser beams using separate microscope objectives located on both sides of the microfluidic channel; the principle is sketched in Figure 21A. This technique was used to investigate the photothermal properties of a larger variety of NPs, such as carbon nanotubes, nanocages, nanoshells, and nanostars.113,114 In this work, the primary aim was not to image a temperature field but rather to perform point measurements to investigate the photothermal properties of nano-objects. In principle, this could be combined with a scanning system. However, this illustrates a limitation of the approaches based on spectrum acquisition: acquiring an image requires measuring and

Figure 20. Intracellular temperature measurements in C. elegans induced by external heating. (A) Left: A bright field image of C. elegans. Right: Fluorescence intensity of fluorescent neurons, temperature image with no laser (OFF), and temperature image when a AuNR next to the worm is laser radiated (ON). (B) Zooms from the upper panel. Reprinted with permission from ref 106. Copyright 2013 American Chemical Society.

As shown in 2015, the temperature-dependent fluorescence of tryptophan can be used as an in situ fluorescent thermometer to measure the photothermal transduction efficiency in a bulk solution containing AuNPs with diameters ranging from ∼20 to 80 nm.109 Although this simple fluorescent method is inadequate for single particle measurements, it clearly has the ability to accurately quantify optical heating in bulk ensembles of NPs which have equal optical densities. Recently, a two-laser system was proposed for nanoscopic thermometry. One laser was used to trap and heat micron sized gold shells, using a NIR laser, and a second laser was used for excitation of two fluorescent dyes (at 445 nm);101 see Figure 19C. The fluorescence was excited by the second laser in a spot separated by a distance from the particle. The increased temperature changes the ratio of the fluorescence emission from the two dyes: a temperature sensitive dye (rhodamine B) and a nonsensitive dye (rhodamine 110). As the method is independent of bleaching, the ratio has a linear relation to the temperature and can be accessed through a calibration where the sample is heated in 1 K steps. The authors found a heating rate of 360 K/W for the investigated particle. As exemplified by this

Figure 21. Nanothermometry based on nanoprobes. (A) Technique based on quantum dots. Two-photon emission spectra generated from quantum dots are obtained at two different distances, D, from the heating laser. The vertical dashed lines indicate the signal from which the temperature increase, ΔT, is inferred. The inset is a sketch of the experimental setup including a heating laser, a spherical NP, and a second laser to excite the quantum dot (gray). D is the distance between the focal points of the heating (red) and excitation (blue) lasers. (B) Technique based on upconverting NPs. Twophoton emission spectra generated from an upconverting NP at two different distances, D, from the heating laser. The inset is a sketch of the experimental setup including a heating laser, a spherical NP, and a second laser to excite the upconverting NP (gray). D is the distance between the focal points of the two lasers. Parts A and B are adaptions with permission from ref 110. Copyright 2013 Elsevier Publishing. (C) Technique based on nanodiamonds. Optically detected spin resonance spectra corresponding to different distances, D, from the heating laser. The vertical dashed lines indicate the splitting parameter, from which the temperature increase, ΔT, is inferred. Figure adaption with permission from ref 111. Copyright 2010 American Chemical Society. The inset is a sketch of the experimental setup including a heating laser, a spherical NP, and a second laser to excite the nanodiamonds (gray). D is the distance between the focal points of the heating (red) and excitation (blue) lasers, and the wave pattern indicates the microwave. R


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processes between Yb3+ and Er3+ ions include a red emission band (654 nm) and two green emission bands (526 and 533 nm). The intensity ratio between the two green levels, S and H, is thus sensitive to temperature and can be used for nanothermometry following a technique known as luminescence intensity ratio (LIR).122 The intensities of the two transitions, IH and IS, are usually defined to be proportional to Boltzmann’s factor123

processing one spectrum per pixel, which can be timeconsuming. 4.2.2. Lanthanide Upconverting Nanoparticles. Semiconductor NPs or quantum dots have often been demonstrated to emit light of shorter wavelength than the excitation wavelength following a two-photon absorption mechanism. This process relies on high photon fluxes and simultaneous absorption of two photons. In contrast, lathanide-doped nanocrystals are used to upconvert infrared light to visible light through a multistep process (anti-Stokes). This is the opposite of regular Stokes shift luminescence, where the excitation wavelength with a shorter wavelength (higher energy) leads to the emission of a photon with a longer wavelength (lower energy). Photon upconversion relies on metastable states to facilitate sequential energy uptake. This new class of materials may broadly be referred to as upconverting NPs (UCNPs). One important property of UCNPs is their long lifetime which lies in the microsecond to millisecond range and allows for use of temperature-dependent spectroscopy; see Figure 21B. The fluorescence intensity, lifetime, and spectra of lanthanides are all dependent on temperature and the chemical environment such as pH.115 In 2011, it was suggested to embed Er3+ ionic centers in a thermal sensor, consisting of an Al0.94Ga0.06N layer, for temperature measurements of single AuNPs.118 Soon after, in 2013, a related approach was adopted by using lanthanide-based compounds.119 They used Nd3+-doped LaF3 (core) NPs shielded by nondoped LaF3 (shell), excited at 808 nm and emitting around 885 and 1060 nm. Their approach contained the modification that Nd3+:LaF3 NPs were suspended within the medium of interest, instead of having Er3+ ionic centers within the substrate. Interestingly, the context of this work was to use these NPs as a means to measure the temperature directly inside biological tissues. This was facilitated by the IR absorption and emission range of their compounds overlapping the biological transparency window (Figure 2). The thermal sensitivity of this sensor was ±2 K. The authors quantified the brightness of their Nd3+:LaF3 probes and found it to be more than 1 order of magnitude higher than that generated by a CdTe quantum dot colloidal solution under similar conditions, e.g., NP concentration and excitation intensity. This larger efficiency was attributed to the fact that Nd3+:LaF3 NPs are excited by means of one-photon excitation, whereas, for CdTe quantum dots, a twophoton excitation process through virtual states is needed.120 In parallel, the same group reported the use of other UCNPs consisting of Er3+ and Yb3+ codoped NaYF4 nanocrystals and compared them with CdTe quantum dots. The achieved temperature resolutions were 1 and 0.5 K for the UCNPs and quantum dots, respectively. Here again, the thermal sensitivity of the UCNPs was given by relative intensity changes between two emission spectral peaks, while the sensitivity of the quantum dots was related to a spectral shift.110 The lanthanide NP system is, hence, more sensitive to bleaching. In 2015, the method was improved by using a similar hybrid mixture of luminescent UCNPs (NaGdF4:Yb3+ 20%, Er3+ 2%) with gold nanorods (aspect ratio of 2:10) for nanothermometry.121 The localized plasmon resonance (980 nm) of the AuNP matched with the absorption wavelength of the first excited state of Yb3+, which led to a large plasmonic resonant enhancement. Hence, in this study, the role of the AuNR was twofold: collective optical heating and luminescence enhancement, the latter depending on the distance between the AuNR and the UCNPs. The emission bands resulting from the upconversion

IH ji ΔE zyz ∝ expjjj− z j kBT zz IS k {

(10)

where ΔE is the energy difference between the two levels, kB is Boltzmann’s constant, and T is the temperature. This ratio was calculated for the measurements whose power dependence is shown in Figure 21B. The longitudinal surface plasmon resonance of the AuNR is tuned to be in resonance with the UCNP absorption wavelength, so that the two can be excited simultaneously. Thus, the AuNR both enhances the local field and heats up to a temperature found from eq 10. This provides a method for detecting the in situ temperature by optical means.121 However, the platform is restricted to usage with specially tuned nanorods and is far from a general method for nanothermometry. Another approach which emerged in 2016 relies on optically trapping temperature sensitive μm-sized particles in a fluid medium and using this as a probe to measure a temperature field in three dimensions.117,124 In one implementation, colloidal NaYF4:Er3+,Yb3+ particles were used as temperature microprobes in living cells heated by AuNPs.117 The UCNPs were trapped by a 980 nm laser, and the same laser was responsible for a bright anti-Stokes emission in the visible range, characterized by two narrow peaks at 525 and 550 nm. The intensity ratio of these peaks was highly temperature-dependent, as described in eq 10. The same authors also used this kind of particles in 2010 for temperature mapping (Figure 22B) in living cells.125 In a second implementation, optically trapped 150 nm erbium oxide NPs were used as temperature probes.124 In this study, an image was generated where each pixel quantifies the emission spectrum of the optically trapped UCNP. However, for these approaches, it may be problematic that the trapping and heating laser is the same in all of the investigations mentioned.126 4.2.3. Microwave Spectroscopy of Nanodiamonds. Nanodiamonds or hyperdiamonds are nanoscopic diamonds with impurities, or variation, in their face-centered cubic crystal structure. The fluorescence from nanodiamonds arises from these defects, or so-called color centers, in the crystalline structure. Of particular interest is the nitrogen-vacancy (NV) color center where two carbon atoms in the diamond lattice are interchanged with a nitrogen atom and an adjacent vacancy. This color center is responsible for a fluorescent color in the red or near-infrared regime (550−800 nm) and is remarkably photostable. Nanodiamonds are not as popular and widely used as quantum dots but are finding increasing applications, as they are single photon sources with a high efficiency. Interestingly, the spin states of a NV center can be optically polarized and coherently manipulated by microwave radiation, a technique known as optically detected magnetic resonance, even at the single molecule level.128 When exposed to a microwave, the electron spin of the negatively charged NV center is changed. It has a convenient optical transition in the visible between its triplet ground state, |0⟩ and its excited states |±1⟩. The fluorescence intensity, however, is directly related to the S


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find the detuning by measuring the probability to transfer |−1⟩ to |0⟩ states.130−132 In 2013, this approach was tested in nanoscale thermometry;116 see Figure 21A. The authors spin-coated a coverslip with AuNRs and single-crystalline nanodiamonds containing approximately 500 NV centers each. When the AuNR is heated with a resonant laser, the spin states can be coherently manipulated using microwave pulses and efficiently initialized and detected by means of laser illumination. In the absence of an external magnetic field, the precise value of the transition frequency between the |0⟩ and |±1⟩ states has a temperature dependence due to thermally induced lattice strains.129,133 Thus, the nanodiamonds are probing the temperature in different distances from the AuNR by electron spin resonance spectroscopy and the result is a temperature map of the surface. Furthermore, as nanodiamonds are inert and supposedly nontoxic, this method is applicable for nanothermometry inside living cells (Figure 22). However, it is time-consuming to acquire the entire spectrum for each pixel and subsequently calculate the peak position. Therefore, temperature profiles of a system far from equilibrium are still challenging. A three-point sampling method allowing for real-time temperature measurements of AuNPs freely diffusing in water was reported in 2015.134 This method is based on the fact that, for bulk diamonds, the widths of the optically detected magnetic resonance peaks are almost constant, whereas their peak positions are shifted by more than 20 MHz in the temperature range from 300 to 700 K.116,129,133 Assuming a constant width, one can determine the temperature shift by measuring the changes of the fluorescence dip at three fixed frequencies without scanning the whole spectrum. This enables real-time measurement of the temperature changes over ±100 K. With this method, the accuracy is lowered from 0.1 to 1 K, but the measurement time is shortened from 30 s to 10 μs, thus enabling temperature measurements on the fly. This method was further developed to combine the heat mapping with a topological scan of the sample.111 In order to obtain spatial information about a sample, the nanodiamond must be scanned relative to the sample. This is commonly achieved by use of an atomic force microscope (AFM) with a nanodiamond at the AFM tip. This is the only method that provided detailed topography of the sample in addition to quantum sensor information.111 The authors dispersed AuNPs on the sample surface that produce a fluorescence signal, which is maximized when the nanodiamond is directly above a particle. The fluorescent spot corresponds to a particle and indicates a

Figure 22. Microwave spectroscopy of nanodiamonds. (A) Confocal scan of a single cell under laser excitation (532 nm). The cross marks the position of the gold nanoparticle used for heating, and circles represent the location of the nanodiamonds (NV1 and NV2) used for thermometry. The dotted line outlines the cell, and the color bar indicates the fluorescence in counts per seconds. Reprinted with permission from ref 116. Copyright 2013 Nature Research. (B) Maximum temperature increment induced in the surrounding of a cell incubated with AuNR (circles) and without (squares) as a function of the laser power (800 nm). The dashed lines are linear fits. The insets are luminescence images of cancer cells incubated with (AuNR) and without AuNRs (No AuNR). Reprinted with permission from ref 117. Copyright 2016 WILEY-VCH Verlag.

microwave resonance such that an increase in triplet population corresponds to a decrease in the fluorescence intensity.128 At zero magnetic field, the |±1⟩ states are split from the |0⟩ state by a temperature-dependent zero field splitting.129 One way to accomplish precision spectroscopy is to apply an external oscillating electromagnetic field at a fixed frequency and then

Figure 23. Nanothermometry based on changes in the surroundings. (A) Changes in the refractive index as experimentally measured at two different laser powers (P). Figure adapted from ref 127. The inset is a sketch of the experimental setup including a heating laser, a spherical NP, and plane wave illumination. (B) Changes in viscosity measured by obtaining power spectra (PSD) of an optically trapped NP at two different laser powers (P > 0); the splitting parameter, from which the temperature change, ΔT, is inferred, is the roll-off frequency. Figure adapted from ref 46 with permission. Copyright 2006 Optical Society of America. The inset is a sketch of the experimental setup including a heating laser, a spherical NP, and changing viscosity of the surrounding. (C) Changes in lipid phase transitions can be detected via the intensity as a function of distance R from the heated NP for two different laser powers (P). The extent of the melted (fluorescent) region is the parameter from which ΔT is inferred. Figure adapted from ref 45 with permission. Copyright 2010 American Chemical Society. The inset is a sketch of the experimental setup including a heating laser, a spherical NP, and a fluorescent lipid bilayer. T


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significant absorption of the laser light, which is expected to turn into heat. To form a temperature map, a full spin resonance spectrum is recorded at each pixel of the scan. The spectrum is acquired under continuous laser excitation (250 mW) in order to maximize the heating. From the fits to the spectroscopy data, the temperature increase is inferred, as shown schematically in Figure 21C. 4.3. Changes in the Surroundings

When a NP is heated, heat will be dissipated into its surroundings, thus potentially changing their physical properties in a detectable manner. For instance, any liquid varies its refractive index upon heating. Thus, when locally heating a NP in a solvent, this variation provides a way to optically probe a local temperature variation. As detailed below, also changes in viscosity or temperature-induced phase transitions are detectable shifts in the surroundings from which the temperature of a plasmonic NP can be inferred. 4.3.1. Refractive Index Distortion. On the basis of the temperature-induced change of refractive index, an optical microscopy technique for nanothermometry was proposed on the basis of quadriwave shearing interferometry (TIQSI).135,136 The principle behind this technique is that, when the temperature increases, the liquid changes its refractive index, which creates a distortion of the planar wavefront of incident light, this being measurable by a wavefront analyzer (Figure 23A). The TIQSI method has a high acquisition speed (∼10 μs) and a sensitivity of ∼1 K.136 However, the main advantage is that the measurable temperature range is arbitrarily large compared to methods based on probes, e.g., fluorescent molecules and quantum dots, which will bleach for temperatures approaching 100 °C.137 The TIQSI method has been used to measure the local steady-state temperature increase, the heat source density, and the absorption cross section both of a AuNP array (Figure 24) and a complex nanowire system.137 The plane wave, which crossed the region of interest, was distorted due to the thermally induced variations of the refractive index of the liquid. This wavefront distortion was then processed using an inversion algorithm to retrieve the temperature increase. Interestingly, integration of the experimental maps of the heat power density allows measurement of the total power absorbed by the composite NP.137 Hence, the absorption cross section could be found by dividing the total absorbed energy by the incident irradiance, I, as in eq 6, and the steady-state temperature distribution is a simple function of distance from the NP surface; see eq 7. In 2015, the TIQSI technique was developed to measure the temperature of plasmonically heated metal nanowire systems of varying complexity.138 Furthermore, the technique was used to measure the temperature threshold prior to bubble formations around a heated plasmonic NP.86 The authors found that it is likely to reach the super heated liquid temperature of water of ∼220 °C. Recently, the TIQSI technique has even proven useful for in vivo heat measurements in cell culture.139 This effect of refractive index variation was also used to measure the temperatures of resonant AuNPs.127 The temperatures were determined by comparing the peak positions of experimentally obtained scattering spectra with temperaturedependent scattering spectra, calculated by Mie theory. The temperature increase was found to depend on both the surrounding medium and the supporting substrate.127 A more sensitive and faster thermal sensor, termed plasmonic thermal microscopy (PTM), was developed in 2015. This

Figure 24. Nanothermometry based on changes in the refractive index. (A) SEM image of a quasi-hexagonal array of gold NPs. (B) Extinction spectra of the sample in air with a plasmonic resonance at 520 nm. (C) Distribution of NP radii. (D) Temperature image measured using the TIQSI technique when illuminating the AuNP array with a laser beam radius (24 μm). (E) Expected temperature increase as a function of the laser power (solid line) and experimental measurements of the temperature increase at various laser powers and at three different locations in the sample (crosses, circles, and diamonds). Parts A−E reprinted with permission from ref 94. Copyright 2013 American Chemical Society.

method is based on the temperature dependence of the refractive index, which is detected using surface plasmon resonance with high sensitivity. The temperature is measured through a relation between the temperature and the change in the reflected angle of the plasmonic resonance of the heated NP. The strength of this technique lies in the ability to detect temperature variations as small as 6 mK with a temporal resolution of 10 μs. However, as this approach only works in the presence of a gold film, it can not be used more generally to measure the temperature of a single nanostructure on more commonly used glass substrates.79 4.3.2. Viscosity Changes. Most nanothermometry techniques reviewed above were only applicable to surface-attached nanostructures and only few for freely diffusing or optically trapped NPs. One of the first implementations of nanothermometry using optically trapped silica and polystyrene particles was based on the analysis of the power dependence of Brownian motions of trapped particles.140 Here, the temperature increase was attributed to the absorption of NIR light primarily by the solution and not by the dielectric particle141 and U


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in this case the fluid temperature field T(D) decays radially in a distance, D, around the particle. Spectral analysis of the positional data of NPs can be used to extract the temperature of the NPs diffusing in their own temperature gradient. However, due to the inherent spherical aberration of the focus and the relatively large scattering cross section of the metallic NPs,35,51 the intensity distribution in the focal region is not trivial at the nanometer scale. In addition, stable trapping positions for optically trapped nanoparticles were identified outside of the most intense focal region.47,142,143 Therefore, the temperature generation depends nontrivially on laser intensity which complicates the quantification of the photothermal effect. Further complicating the picture is the fact that, if viscosity changes are used as a measure for temperature change, then one has two unknown parameters, both the viscosity and the temperature, and these two depend on each other in a nontrivial manner. The first report on using changes in viscosity to infer the temperature of optically trapped AuNPs was published in 2006.46 The authors determined the stiffness of the optical trap through three different estimations: (i) power-spectral analysis, (ii) the equipartition relation, and (iii) hydrodynamic drag calibration. They showed that the trap stiffness diverged for augmented laser powers, and the large disagreement between trap stiffness calculated with the equipartition and power spectral analysis suggested significant heating. To infer the temperature, the authors assumed a relation between temperature and viscosity and found temperature elevations up to 383 K at the surface of the particle with a heating rate of 266 K/W for 100 nm AuNPs.46 The analysis of the diffusive dynamics of a NP in a thermal gradient has been further explored and is termed hot Brownian motion.144−146 The concept of hot Brownian motion can be used to infer the temperature of NPs, as for instance in 2011 where the temperature of an optically trapped AuNR (aspect ratio 25/60) was determined.69 In this assay, the AuNR aligned along the polarization direction of the trapping laser, thus causing an angular dependence of the scattered intensity. The spectral intensity ratio is approximated by I I⊥

∝

κ kBT

on a lipid with a well-defined phase transition temperature and kept below the phase transition temperature. Then, the size of the melted region around the NP following irradiation should be measured. An illustration of the method is shown in Figure 23C, and experimental data is shown in the upper panel of Figure 12A. In the original assay from ref 45, the lipid bilayer was composed of saturated lipids with a phase transition temperature, Tm = 33.8 °C, a few degrees above ambient temperature. Upon irradiation of the NPs, the surrounding bilayer formed a circular fluid phase area which could be identified by using the fluorescent lipophilic tracer DiIC18 which prefers to partition in the fluid phase. The radius of the liquid region, Dm, equals the distance from the NP center to the rim of the melted region where T = Tm. To find the temperature increase at the surface (ΔT(D = R)) of the particle, eq 7 was used to obtain ΔT(R) = ΔT(Dm)·Dm/R. With this method, heating rates of 452 and 1640 K/W were found for AuNPs of 100 and 200 nm, respectively.45 These rates are somewhat higher than what was found earlier46 and could possibly arise from the use of the dipole approximation in ref 46 and the assumed relation between the two unknown parameters, T and viscosity. This lipid bilayer melting method was further developed to measure the dependence of AuNR heating on its orientation with respect to the laser polarization vector (Figure 19B) as well as the heating of complex composite surface fabricated nanostructures,64,150 where experimental results were in excellent agreement with FEM predictions. Recently, this methodology was used to quantify the extreme photothermal heating of platinum nanoparticles.35 Several assays have been employed to quantify the heating of NPs trapped or fluctuating in 3D environments.46,47,151 One such assay was based on the thermally induced leakage of a giant unilamellar lipid vesicle (GUV) carrying fluorophores47 and was used for nanothermometry of AuNPs (80−200 nm) optically trapped in 3D. AuNPs were trapped at varying distances to the GUV, and at a measured distance, Dm, the heat radiating from the trapped AuNP caused the GUV temperature to reach Tm, initiating a visible molecular efflux from the GUV. From Dm and Tm, the entire temperature profile could be deduced in the same manner as in ref 45. The accuracy of the methods based on a lipid bilayer phase transition is limited by how well-defined the phase transition temperature is and the width of the transition. The phase transition temperature and its width can be obtained by calorimetry and imaging of the phases with different lipophilic dyes.152 An advantage of a lipid-based method is that it is a direct experimental temperature assessment that does not rely on knowledge of, e.g., the viscosity of the medium, the thermal conductivities, or the exact laser intensity. Also, the measurements can be carried out at a distance from the irradiated NP in both a 2D and 3D environment. One drawback is that not all nanoscopic systems are compatible with lipid bilayers and that the chosen lipid bilayer may be sensitive to osmotic pressure and solution conditions.

(11)

where ⟨I∥⟩ is the intensity scattered from the parallel orientation and ⟨I⊥⟩ is the intensity scattered from the orthogonal orientation. κ is the rotational trap stiffness and T the effective temperature.69 From this dependence, the authors found a heating rate for the optically trapped nanorod of 900 K/W. This compares well to other reports of AuNR heating.64 An alternative method of inferring the heating of a AuNR is to analyze the translational and rotational movements of a spinning AuNR optically trapped in 2D; i.e., on a glass surface, the authors inferred the temperature by employing power spectral analysis and luminescence decay time measurements.71 However, the heating rates ranged from 1500 to 5000 K/W and therefore more work is needed before this method will become generally applicable. 4.3.3. Phase Transitions in Lipid Bilayers. Optical trapping of plasmonic nanoparticles51,147−149 raised the question of how much the particles would heat due to absorption of the trapping laser light. To answer this question, an assay based on a lipid bilayer undergoing a phase transition was developed.45 The idea behind this method is to place a NP

4.4. Raman Spectroscopy

Raman scattering is the in-elastic scattering of photons that interchange energy with their scatters. This energy change corresponds to the vibrational modes of the scattering molecules. In Raman scattering, the energy of the scattered photon is hν′ = hν − hνm, where ν is the frequency of the incoming light and νm is the vibrational frequency of the excited state. The opposite process also occurs, hν′ = hν + hνm, and this second mechanism is called anti-Stokes. Hence, in a Raman V


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scattering spectrum, the energy locations of corresponding Stokes and anti-Stokes peaks are symmetric around the excitation photon energy. The intensity ratio between the antiStokes, IaS, and Stokes, IS, peaks is related to the ratio between the occupation number of the ground state and the excited state which follow Fermi statistics:154 ij IaS −hνm yzz = AjjjB + exp z j IS kBT zz{ k

(12) Figure 25. Nanothermometry based on Raman spectroscopy. NP emission spectra using two different laser powers, P. The splitting parameter is the intensity ratio between the anti-Stokes, IaS (lower wavelength), and Stokes, IS (higher wavelength), from which the temperature increase, ΔT, is inferred. Figure is adapted from ref 153. Copyright 2018 American Chemical Society. The inset is a sketch of the experimental setup including a heating laser and a spherical NP in solution.

Here, A is an asymmetry parameter taking into account that the cross section of Stokes shifts is different from the cross section of anti-Stokes shifts and B is a term that describes vibrational pumping and depends on, e.g., the laser intensity and the vibrational excited state lifetime. As the second term is solely temperature-dependent, Raman scattering spectroscopy can be used for temperature measurements and has, e.g, been successfully applied to AuNPs,155 AuNRs,156 and gold nanodisks.157 Exciting a AuNR with a monochromatic beam at its resonance frequency generates a collective oscillation of the plasmons. The plasmons decay by forming electron−hole pairs with an energy that equals the laser photon energy hν. This hot electron−hole pair has a small probability of recombining while emitting photoluminescence either by a Stokes or an anti-Stokes process, but in both cases, the possible energies cannot exceed hν + kBT. If the excitation falls within the plasmon resonance, the spectrum is expected to follow the plasmon spectrum multiplied by a Bose−Einstein statistics factor arising from phonon population. One can assume that coupling to the phonons dominates the process153 instead of the carrier−carrier interactions, as described by eq 12, where the occupation number follows Fermi statistics. Under this assumption, the emission should be proportional to the phonon occupation number, n, for anti-Stokes and n + 1 for Stokes processes; hence, the anti-Stokes emission spectrum follows the form ij hν yz I(ν′) = ISPR (ν′)jjjexp m − 1zzz j kBT z k {

Figure 26. Nanothermometry based on Raman spectroscopy of a single AuNR. The green curve is the measured luminescence spectrum (532 nm excitation), and the red curve shows the extracted surface plasmon resonance spectrum, ISPR(ν′); see eq 13. The other curves show emission from the same AuNR (633 nm excitation) at different powers (see legends). Note the filtering of the notch filter to prevent laser damage of the detectors. The inset shows the ratio between the antiStokes intensity and the Stokes intensity as given in eq 12 versus laser power. The red line is a linear fit to the data, from which he temperature can be deduced. Reprinted with permission from ref 153. Copyright 2018 American Chemical Society.

−1

(13)

where I(ν′) is the emitted intensity and ISPR(ν′) is the surface plasmon resonance spectrum.153 The only remaining free parameters are the temperature T and a normalization constant which is not included in eq 13. A schematic of this nanothermometry method is shown in Figure 25. Since the only free parameter of the model is the absolute temperature of the NP, this technique does not require any calibration. In ref 153, the authors obtain an accuracy of the temperature determination of 6% by applying this technique with an integration time of 1 min. In practice, the Stokes and anti-Stokes spectra are recorded in a scanning confocal microscope with a coupled spectrometer, where the exciting laser is blocked by a notch filter, as shown in Figure 26. The potential of this technique is not only to access the temperature of a NP under excitation but to use the obtained data to deduce the temperature before excitation. Thus, it has potential to probe local temperatures in vivo. This technique has been further demonstrated to be useful for accessing the temperature of a gold bowtie nanoantenna (Figure 27). Nevertheless, the method still needs to be validated with less luminescent nanostructures and less invasive implementations of the method are needed before it can find practice for in vivo measurements.

4.5. Thermal Radiation Spectroscopy

Thermal (blackbody) radiation has a spectrum that depends entirely on the temperature of the particle. The absolute temperature of a particle can be determined by balancing the absorbed power with the emission power and the heat dissipation. The emission intensity for a specific wavelength can be calculated from Planck’s law. Unfortunately, standard thermal imaging, which is often used to measure heating of NPs in suspension,68,98,159 does not apply for nanothermometry. The reason is that the wavelength of the peak intensity (more than 10 μm) would lead to a very poor spatial resolution.160 As light is propagating, it can be detected far from the emitting object, but in contrast, heat spreads through diffusion. This means that the heat from a NP will only be measurable by near-field measurements. For these reasons, first attempts to probe a temperature field at small scales were based on the use of local small composite tips acting as a nanoscale thermo-coupler. This is the so-called scanning thermal microscopy (SThM) technique W


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4.6. Comparison and Applicability of Nanothermometry Methods

Table 1 provides an overview and comparison of the nanothermometry methods outlined in this review, the purpose of this table being to guide researchers in the search of an appropriate nanothermometry assay for a given application. An additional issue to notice is that the techniques based on optical detection, the majority of the reviewed techniques, have a spatial resolution that is limited by the diffraction limit of visible light (200−300 nm). An evaluation of the range of nanothermometry methods would require direct comparative tests of individual methods applied to identical nanostructures under identical conditions. Such tests, although not yet reported, hopefully will be conducted to help identify the most reliable nanothermometry for quantifying temperatures in vivo or in vitro. A number of methods for quantifying intracellular temperatures have reported increases of several Kelvins across the cytoplasm originating from endogenous thermogenesis,168,169 which significantly exceed theoretical estimates based on cellular metabolism.170,171 These reports on large temperature variations in living cells have led to serious doubts concerning the accuracy of the intracellular thermometry techniques used. This controversy emphasizes the requirement that nanothermometry methods are thoroughly calibrated and puts attention to the fact that complex environments, e.g., the cytosol of cells, probably influence the readout. The field of nanothermometry is still nascent and quickly evolving. Therefore, there is still ample space to develop suitable methods for determining temperatures around, or aided by, plasmonic nanostructures in vitro and in vivo. These methods should be calibrated and designed to quantify temperatures under specific conditions.

Figure 27. Nanothermometry based on Raman spectroscopy of bowtie nanostructures. (A) SEM images of gold bowtie nanoantennas of varying length (L) but with a width of 70 nm and a height of 30 nm. (B) Representative dark-field scattering spectra for various lengths; simulations are shown by thin lines and experiments by thick lines; same color code as part A. The polarization of the incident laser is parallel to the long axis. The dashed vertical line indicates the heating laser wavelength (1064 nm). (C) Simulated (FEM) internal heat dissipation density (L = 180 nm). (D) Measured scattering spectra for polarization of the incident laser along the short axis (color code as in part A). The dashed vertical line indicates the heating laser wavelength (633 nm). (E) Simulated (FEM) electric field enhancement in a bowtie (L = 180 nm) excitation polarized along the short axis. Reprinted with permission from ref 158. Copyright 2018 American Chemical Society.

5. BIOLOGICAL DISTRIBUTION AND TOLERANCE When introducing plasmonic nanostructures into biological systems, knowledge of the biological fate of the particles is vital for the success of the application. The stability and integrity of injected particles are challenged by the complex nature of changing physiological environments. These are in turn dependent on the selected delivery pathway, e.g., oral, inhalation, topical, local, or parenteral. The challenges posed by these interactions are reviewed in detail in ref 172. It is important to identify the changes imposed by the biological systems on the NPs and distinguish between the NP’s initial synthetic identity and the biological identity of the particles inside the organism; see Figure 28. The changes in the biological identity, with respect to the synthetic identity, can include both altered surface chemistry and particle agglomeration. It is

which was introduced in 2014.161 Moreover, SThM is invasive, thus limiting its applications.

Table 1. Overview of Nanothermometric Techniques, the Probes Employed, Setup Requirements, the Accuracy of the Methods, and the Maximum Temperature Reached method fluorescence probes

environment

Raman spect. thermal spect.

measure

probe

setup requirements

polarization intensity luminescence spectrum luminescence spectrum spin resonance spectroscopy wavefront distortion particle positions fluorescence intensity luminescence spectrum radiation spectrum

fluorophore fluorophore quantum dot UCNP nanodiamond none none lipid bilayer AuNP none

2 lasers + polarization cube 2 lasers 2 lasers + spectrometer 2 lasers + spectrometer 2 lasers + 2D phase grading 2 lasers + QPD 1 laser laser + spectrometer

X

accuracy (C) Tmax (C)

Plasmonic Heating of Nanostructures | Chemical Reviews

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