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Physics, Chemistry and Materials Science at the Nanoscale

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1. Introduction

Gold nanoparticles appeared first in recorded history as the “soluble” gold around the 5th century B.C. in Egypt and China [1], but scientific research on colloidal gold started in 1857 with Michael Faraday’s report on the formation of deep red solutions of gold in water by the reduction of an aqueous solution of chloroaurate (AuCl4-) using phosphorus in CS2 (a two-phase system) [2]. Since then, especially in the 20th century, many different methods for the preparation of gold nanoparticles have been reported, including top-down techniques [3–5] such as photolithography [6], electron beam lithography [7, 8] or laser-ablation techniques [3, 9], as well as bottom-up methods, mainly comprising various types of reduction in solution (chemical [10–12], electrochemical [13, 14], sonochemical [15–17], and photochemical [18–21] reduction) and few other approaches such as templating and seeded-growth methods [11, 22–25]. Among all these different approaches developed for the fabrication of gold nanoparticles, wet chemistry promises to become the preferred choice, because of its relative simplicity, possible scale-up and use of inexpensive resources.

The goal of all such syntheses comprises the preparation of nanoparticles with controlled parameters such as size, shape and uniformity, which are ultimately the prevailing factors behind their physical, chemical, optical, electronic and catalytic properties. However, the particles so prepared invariably carry a corona of organic or inorganic material that provides them with the colloidal stability required to maintain their individual identity. Additionally, this coating material plays an essential role to determine the nanoparticles physical, chemical, optical, electronic and catalytic properties, which in turn designate their final applications. It is in this context where “coating matters” and the choice of the right capping agent/coating material becomes fundamental. Since the applications of gold nanoparticles in various fields have substantially increased during the past few decades, it is of primary importance to understand the prevailing influence of the coating material during synthesis and on the final properties of interest.

Probably the most relevant properties of gold nanoparticles are based on the presence of intense absorption and scattering in the visible and NIR spectral regions, which are the origin of the observed brilliant colors of gold nanoparticles in solution. Such special optical response results from the collective oscillation of conduction electrons, in resonance with the frequency of the incident electromagnetic radiation, which is known as localized surface plasmon resonance (LSPR). The LSPR frequency, and thus the color of the gold nanoparticles mainly depend on particle size and shape, the dielectric nature of the surrounding medium and inter-particle distance. Whereas gold nanoparticles with core diameters below 2 nm do not display LSPRs, the LSPR shifts toward longer wavelengths as particle size is increased. For example, gold nanoparticles in water with mean diameters of 9, 15, 22, 48, and 99 nm, display LSPR maxima that are centered at 517, 520, 521, 533, and 575 nm, respectively [1].

The influence of shape on the optical properties of gold nanoparticles is even greater than that of size [26, 27]. The anisotropy of non-spherical particles leads to multiple possible LSPR modes. For example, small gold nanorods can accommodate two well differentiated resonance modes due to electron oscillation across and along the long axis of the nanorod and are commonly labeled as the transverse and the longitudinal modes, respectively, the latter being extremely sensitive to the aspect ratio of the rod [28]. Interestingly, three main resonance modes have been identified for triangular gold nanoplates [29], which have been recently defined as “stationary short range surface plasmons” [30]. In these and other non-spherical nanostructures (also including nanoshells), a common feature is that the main plasmon resonance is red-shifted with respect to that of spherical nanoparticles, at 520 nm [31]. Hence, by tuning the morphology of gold nanoparticles, one can readily set the LSPR frequency and mode multiplicity of the nanoparticles in a wide spectral range, as schematically shown in Figure 1.

Another important parameter toward the determination of the optical response is the inter-particle distance. When two (or more) nanoparticles are in close proximity to each other, usually within 2.5 times the particle diameter [32], their localized surface plasmon resonances can couple, leading to broadening and red-shift of the single particle LSPR band [27]. Importantly, such coupling also leads to the formation of so-called “hot-spots”, which are small gaps between adjacent particles exhibiting large electromagnetic field enhancements. This coupling has been theoretically modeled [33], but also practically exploited in a number of applications, including surface-enhanced Raman scattering (SERS) [34–36], optoelectronics [37] and bio-/chemosensors [38–40]. In the latter case, aggregation of nanoparticles induced by interactions between the analyte and the particle coating have been specifically employed for detection. Therefore, it is clear that the particle coating does play a crucial role in detection methods based on (either reversible or irreversible) nanoparticle aggregation, as we discuss in detail in this review article.

Figure 1

Indicative ranges of the LSPR of gold nanoparticles, as a function of shape and morphology. Scheme adapted with permission from ref. [31].

Finally, the fourth main parameter affecting the optical properties of gold nanoparticles is the dielectric nature of the immediate surroundings, including the dispersion medium (solvent) and the coating material on the nanoparticle [41]. Early work by Papavassiliou et al. analyzed the optical properties of nanoparticle metal films immersed in solvents of various refractive index, finding a strong dependence of the position, intensity, and shape of absorption peaks on the dielectric constant of the medium surrounding the metal deposits [42]. For instance, gold films appeared red in air but red-blue in CS2 and linear relationships were found between the square of the maximum wavelength and the dielectric constant of the medium. While refractive index changes in the dispersion medium usually have moderate effects on the properties of gold nanoparticles, the refractive index of the coating materials may have a larger influence. Depending on the nature of the coating material, including its physical properties, chemical composition and architecture on the particle surface, different effects or combination of effects can be differentiated, which will be discussed below with respect to the different types of coating materials.

Apart from its influence on the physical properties of gold nanoparticles, the coating material is ultimately the limiting factor toward applications in various fields, in particular due to colloidal stability and/or bio-compatibility issues. The large extinction coefficients, as well as the ability of tuning the optical properties with particle size and shape render these particles very interesting as compared to traditional organic dye molecules. However, it is again the coating material or the stabilizing agent during the synthesis that actually plays the fundamental role in tuning the size, shape and morphology, and therefore also the LSPR wavelength range and mode multiplicity, as well as the colloidal stability and functional versatility of the nanoparticles, determining their final properties. Especially in the case of non-spherical nanoparticles, the resonance frequency can be tuned over a wide spectral range from the blue to the near-infrared (NIR). Interestingly, the resonance of anisotropic nanoparticles can be driven into the so-called “water transparency window” in the NIR (800–1300 nm), where absorption by biomatter is low, thus showing potential in a wide variety of biological applications (e.g., bio-labeling or hyperthermia). Although gold nanoparticles can indeed be synthesized in various shapes including cubes [43], rods [28], plates [44, 45], polyhedrons [43] and wires [46], their application in bio-medicine is restricted by the frequent use of toxic or non-biocompatible coating materials, which are necessary to achieve such morphological shapes and structures.

Multiple reviews [1, 25, 28] and books [47, 48] have been published regarding the synthesis of gold nanoparticles, where their different chemical and physical properties are discussed, but often in relation to size and shape related effects. Therefore, the scope of this review has been set at the consequences and effects of various types of coating materials on the optical properties of gold nanoparticles, depending on their own chemical and physical properties. In this context, the main criterion on which this review is structured is the chemical nature of the coating material. Hence, in the first part, different types of coatings comprising organic materials are reviewed, while the second part deals with different types of inorganic coating materials. Whereas the organic coating materials span a wide range of organic molecules, which can be classified as monomeric (small molecules, MW<1000 g/mol) and polymeric (macromolecules MW>1000 g/mol), the inorganic coating materials can be divided as dielectrics (insulators, semi-conductors and metal oxides and salts) and conductors i.e., elemental metals.

2. Organic coatings

The preparative reports dealing with gold nanoparticles capped with organic materials can be divided into two main catagories: monomeric coatings and polymeric coatings, which in both cases can be linked to the gold cores either via covalent binding or by physical adsorption. Monomeric coatings can be classified into small charged molecules (prime example: citric acid) [10, 49] and amphiphilic molecules, comprising a hydrophilic head group and a hydrophobic tail [prominent example: cetyltrimethylammonium bromide (CTAB) [50]]. In the case of polymeric coatings, many classifications are possible, due to the large variety of molecular designs existing for polymers. We decided to categorize the polymer coatings according to the binding architecture of the polymers onto the gold surface, namely: 1) physical adsorption of polymers (polyelectrolytes) in a patchy structure or in a layer-by-layer (LbL) organization; 2) one-end covalent binding in polymer brush structure [51]; and 3) crosslinked polymer microgels [52, 53]. In the following sub-sections, the main chemical and physical differences between these molecules will be highlighted, as well as their effect on the optical properties of the nanoparticles, based on their influence on particle size and shape, aggregation- and assembly-states (ordered or disordered aggregation), during and after the particle synthesis.

2.1. Monomeric coatings

2.1.1. Small charged molecules

The classical example for small capping molecules is citrate, which was first used by Turkevich for the reduction and simultaneous stabilization of spherical gold nanoparticles in size ranges of 10–20 nm [10]. With its three carboxylic groups, it adsorbs onto the gold surface, at least with one carboxylic group facing the solvent, therefore providing sufficient electrostatic stabilization to the particles in water. Due to the rather loose binding properties, citrate-coated gold nanoparticles have often been used as precursors for the synthesis of gold nanoparticles with different morphologies, but also for many other gold nanoparticle based materials.

Other small charged molecules that have been largely used to stabilize gold nanoparticles in water are organic thiols containing one or multiple charged groups, such as mercaptopropionic acid [54], thioglycolic acid [55], mercaptosuccinic acid [56], cysteine [57, 58], penicillamine [59], N-acetyl-DL-penicillamine [59], etc. These molecules bind covalently with the gold surface, forming a strong gold-sulfur bond while the charged groups lead to formation of an electric double layer that provides colloidal stability. These gold nanoparticles can be sensitive to the pH and ionic strength of the solvent, exhibiting pH-dependent optical properties, through reversible aggregation. Upon aggregation, the inter-particle distance between the nanoparticles drastically decreases, and this leads to LSPR coupling as described above, resulting in a color change of the dispersion, i.e., LSPR redshift and formation of electromagnetic hot-spots. A recent example is the

Figure 2

(A) Representative TEM images of gold nanoparticles and photographs of their corresponding dispersions, at different pH as indicated in the labels. (B) Variation of UV-Vis spectra of Au@penicillamine upon successive decrease and increase of pH, as indicated in the labels. (C) Variation of the absorbance at 650 nm when the pH was cycled between 7 and 3. Reprinted with permission from ref. [59].

demonstration of the reversible aggregation of penicillamine and N-acetyl-DL-penicillamine coated gold nanoparticles at acidic pH, as shown in Figure 2. The protonation of the carboxylic groups at low pH, causes a decrease in the electrostatic stability and the advent of hydrogen bonding interactions, promoting interparticle linkage, resulting in pH-dependent optical/electronic properties and formation of SERS hot spots [59].

2.1.2. Small amphiphilic molecules (monomeric surfactants)

Small amphiphilic molecules, consisting of a polar and/or aurophilic head group (e.g., thiolates, dithiolates, amines, ammoniums, carboxylates, cyanides, isothiocyanates, phosphines, etc.) and a hydrocarbon chain can self-assemble into monolayers (SAMs) or bilayers to allow the stable dispersion of particles in both aqueous and organic media. The complexation of such moieties with gold ions usually occurs spontaneously over the millisecond to minute time scale, at room temperature, for commonly used surfactants such as CTAB or alkanethiols. Direct evidence on the existence of a bilayer on the surface of Au nanorods has been recently provided not only for the particular case of CTAB, but also for gemini surfactants [60]. Packing and reordering mechanisms in combination with kinetics and/or oxidative etching power [61] of different surfactant molecules allow the design of synthesis strategies for gold nanoparticles of different shapes such as nanorods (CTAB) [28], nanowires (oleylamine [46], CTAB [62]), nanoplates (CTAC) [63], nanostars (CTAB [64–66], SDS [67]), concave polyhedra (CTAC) [68–70] and some other unusual shapes (CTAC) [71]. However, the chemical nature of the surfactant molecules plays a crucial role in determining the final particle shape, and thus on the corresponding optical properties. For example, the aspect ratio of gold nanorods can be controlled by varying the hydrophobic chain length (i.e., alkyltrimethylammonium bromides (CnTAB) with n=10–16) [72] or head groups (i.e., cetyltrialkylammonium bromide (CTCmAB) with m=1–4) [73, 74] of cationic surfactants used in seeded-growth synthesis (Figure 3 and Table 1), resulting in fine tuning of the longitudinal LSPR of these particles [72–74]. Jian et al. reported the formation of concave cubes using CTAC but convex cubes (tetrahexahedrons) using CTAB, where just the counter-anion is different (CTAC, Cl- and CTAB, Br-) [68]. Gonzalez et al. reported on the synthesis of gold-silver nanoparticles with two different architectures, namely alloys or Aucore-Agshell nanoparticles, simply by choosing SDS or CTAC, respectively, as stabilizing agent [75].

Moreover, such surfactants can be employed to fabricate 2- and 3-dimensionally ordered arrays of nanoparticles, known as nanocrystal superlattices (NCSLs), which display novel chemical and physical properties arising from the collective behavior of the constituting units. Such nanocrystal superlattices, in particular those comprising plasmonic nanoparticles may have interesting applications in various fields like sensors, optical waveguides, photo-electronic devices and metamaterials. For example, alkanethiol-stabilized gold nanoparticles have been shown to self-assemble into fcc or hcp nanocrystal superlattices depending upon the method

Figure 3

TEM micrographs of gold nanoparticles prepared in the presence of (A) C10TAB, (B) C12TAB, (C) C14TAB, and (D) C16TAB. Scale bars are 500 nm (B and D) and 100 nm (A and C). Reprinted with permission from ref. [72].

Table 1

Micellar properties of CTAB (CTC1AB), CTEAB (CTC2AB), CTPAB (CTC3AB), and CTBAB (CTC4AB) and their nanorod-growth behaviors. Reprinted with permission from ref. [74].

of their preparation [76] or the hydrocarbon tail length [77]. Also gemini surfactants, made of two hydrophobic tails and two hydrophilic headgroups linked by a spacer chain, have been employed not only to influence the growth [78, 79] of gold nanorods, but also to facilitate self-assembly into highly ordered, robust 2D and 3D gold NR superlattices with directional optical properties [80]. Therefore, the nature of the capping surfactant molecule substantially influences the shape or the surface architecture and therefore also the final optical properties of the gold nanoparticles. For interested readers, the recently published feature article by Xiao et al. on surfactant-assisted synthesis of gold nanoparticles of different shapes provides a comprehensive state of the art [81].

2.2. Polymeric coatings

Polymers represent the most diverse and widely employed coating materials for particulate systems, and therefore they are also indispensable in the fabrication of advanced nanoscale materials based on gold nanoparticles [82]. Polymer chains/layers (grafted/coated) on the surfaces of gold nanoparticles not only enhance their stability, but also confer further functionalities and properties to the nanoparticles, which are characteristic of the nature of the polymer, i.e., of its chemical and physical properties. In many cases, the morphology and physical properties of the core particle can be tuned and controlled by adding specific functional polymers, the so-called shape regulating polymers, during or after synthesis. Given the wide variety of polymer coatings, we necessarily restrict ourselves in this section to discuss those that can affect the optical properties of gold nanoparticles by either regulating their size and morphology or inducing plasmon coupling through reversible changes in the colloidal stability or wettability, which influence inter-particle distance due to the reversible aggregation or packing density. Besides the chemical nature, also the architecture of the polymer layer around the particles plays an important role toward the final properties and applications of the nanoparticles. The main architectural designs can be divided into the following categories, 1) polymer layer(s) physically adsorbed on particle cores 2) polymer brushes, i.e., polymer chains tethered with one end to the particles surface, and 3) polymer gels, i.e., polymer network grown around the particle.

2.2.1. Physically adsorbed polymers

Many types of natural and synthetic polymers can physically adsorb or covalently bind to gold nanoparticles, which is determined by their molecular structure and functional groups. Physical adsorption (physisorption) of polymers on gold nanoparticles is based on relatively weak interactions between the polymer and the gold surface rather than on covalent links, thus allowing for straightforward post-synthetic surface modifications. The polymer can either wrap around the particle or adsorb in a patchy manner [83, 84] on the particle surface, depending on the size of the particles, the chain length (and structure) of the polymer and the solvent. When the polymers are charged (polyelectrolytes), those with a charge opposite to that on the particle surface will adsorb onto it in a relatively flat orientation and may neutralize or overcompensate the surface charge of the particle. Subsequent adsorption of oppositely charged polyelectrolytes on nanoparticles, leads to stable core-shell systems [85, 86]. This method is known as the LbL (Layerby-Layer) technique [87], and is widely applied not only to stabilize gold nanoparticle dispersions, but also to confer them with a desired surface charge, which is determined by the charge of the outmost polyelectrolyte layer [85]. Besides synthetic charged polymers, also bio-polymers [88] such as DNA [89–93], dextran [94], cellulose [95, 96], chitosan [97, 98] and various peptides and proteins can adsorb onto the nanoparticle surface, and have been widely used to make gold nanoparticles suitable for bio-applications. Moreover, polyelectrolyte multilayers can also serve as primers to make gold nanoparticles suitable for further surface modifications or coatings, such as silica [99] or pNIPAM microgels [100]. The latter coating materials in particular can have an important influence on the optical properties of the gold nanoparticles, as discussed below in detail.

In some cases, partial coating rather than complete coverage can be achieved, especially for anisotropic nanoparticles such as Au NRs, due to the large variations in surface curvature and ligand density. Such partial coating has been shown to favor assembly into side-by-side or end-to-end geometries, leading to superstructures with different degree of organization, complexity of collective behavior and unique optical properties. By taking advantage of interactions between small organic molecules [101], polymers [102], or biomolecules such as DNA [103], oligonucleotides [104] or proteins (antibody–antigen [105] or biotin–strepavidin interactions [106]), end-to-end and side-to-side assemblies of gold NRs have been successfully used for sensing applications [104, 107].

However, some amphiphilic polymers such as poly(vinylpyrrolidone) [PVP, not to be confused with poly(vinylpyridine)] have been often claimed to exhibit shape directing/regulating properties [25, 108, 109]. The application of such adsorbing polymers as stabilizers or additives during the synthesis actively influences the optical properties of the resulting gold nanoparticles, through morphology control, as shown in Figure 4. It is believed that the selective adsorption of such polymers (or surfactants, ions and other ligands) on certain crystalline facets can inhibit the growth along specific directions, leading to anisotropic growth [110]. Therefore, by selecting a stabilizing agent with a certain selective adsorption or introducing an additive together with the stabilizing agent in the reaction that displays surface-regulating properties, anisotropic nanocrystals can be synthesized in a solution phase. Gold nanoparticles of different shapes have been synthesized in non-aqueous media, mainly based on the socalled “polyol process” [111]. This process involves the thermal reduction of a metal salt in liquid polyols or diols that act as both solvent for the metal precursor and mild reducing reagent. Introduction of shape-regulating polymers, foreign ions and seeds, combined with careful regulation of reaction temperature during the synthesis provides metal nanocrystals with well-defined geometrical shapes. One-pot synthesis procedures for gold nanoparticles with platonic shapes synthesized via a modified polyol process in the presence of the shape-regulating polymer PVP in ethylene glycol were proposed by Yang et al. [112]. The ratio of gold salt to PVP is crucial toward controlling the shape of polyhedral particles. While tetrahedral particles were formed at higher gold/PVP ratios, icosahedra resulted from lower ratios. Seo et al. reported a similar approach where polyhedral shapes of gold nanocrystals could be tuned by altering PVP concentrations in a simple one-pot polyol process containing multi-twinned decahedral or single crystalline seeds [113]. At high PVP concentration, decahedral seeds were sufficiently stabilized to maintain their structure during particle growth. Lower PVP concentration yielded icosahedra in order to reduce the total surface energy of the crystals. Single-crystalline truncated tetrahedra and octahedra were obtained at low PVP concentration, mainly due to decreased blocking of the particle surface by PVP from oxidative environments with Cl-/O2 in the reaction mixture [113].

Although the above-mentioned polyol approach provides a versatile method for synthesizing polyhedral gold particles, it has a drawback that requires a complex and time-consuming workup procedure. Therefore, Cho et al. introduced a straightforward polyol process using the polyelectrolyte PDDA as stabilizer rather than the traditional PVP [110]. High-yield (>95%, without purification), monodisperse Au octahedra were obtained in a one-pot reaction with a short reaction time. PDDA is a cationic polyelectrolyte, which is believed to form stable ion pairs, due to the initial strong electrostatic interaction between PDDA and AuCl4 ions. Consequently, the reduction rate of AuCl4 ions in the polyol synthesis of Au nanocrystals is decreased in the presence of PDDA, which favors the formation of high-quality anisotropic metal nanocrystals. Also, the presence of silver ions together with such shape regulating polymers induces the formation of uniform anisotropic gold nanocrystals. For example, octahedral, truncated octahedral, cubo-octahedral and cubic particles (hexahedrons) were synthesized by Seo et al. by conversion of cubo-octahedral particles into particles with different shapes using silver ions, which seems to be related to selective blocking of the growth along certain crystallographic facets [114]. Also, gold nanorods, synthesized in aqueous media, can be completely reshaped

Figure 4

Electron micrographs of Au nanoparticles with various shapes, synthesized through DMF/polyol reduction in the presence of the shape-regulating polymer PVP. In the upper panel, particles synthesized in a seeded growth process are shown, while the particles in the lower panel were fabricated in the absence of seed particles. Reprinted with permission from ref. [25].

into perfect, single-crystalline octahedrons in a DMF–PVP solution as reported by Liz-Marzán and coworkers [115].

Another interesting and important group of gold particles are the so-called “branched” nanoparticles or nanostars [116]. The fabrication of gold nanostars has generated interest because of their sharp tips and edges, where light can be highly concentrated, corresponding with high localization of surface plasmon modes. This makes them potential candidates for a number of applications, including SERS-based detection and analysis. A wide variety of wet chemistry-based synthetic methods is currently available for their preparation with different size and shape distributions and varying degree of branching. Either surfactants (CTAB, CTAC, SDS, oleylamine) [46, 62–70] or polymers such as PVP [108, 109] have been used as shape directing agents. Although PVP has been typically employed in prior works for the production of particles with smooth surfaces and well-defined geometries (spheres, wires, decahedrons, and octahedrons), it has been reported that highly branched nanostructures can be obtained upon addition of HAuCl4 in the presence of 15 nm PVP-coated Au seeds, with a significantly higher concentration of PVP in solution, where PVP acts as a stabilizer and reducing agent [22, 117]. This process is highly efficient and it can be carried out either in a seed-mediated or in a seedless growth process at room temperature, in a very short time, resulting in 100% production of nanoparticles with multiple sharp spikes. The fascinating feature of these particles is that each tip is a single crystal, so the challenge remains to understand the underlying growth mechanism of the selective tip growth on the seed particle’s surface, in particular in connection with the previous claims regarding the influence of PVP on the selective growth of certain crystal facets. However, due to the biological incompatibility of PVP, gelatin has been proposed as an alternative capping ligand to obtain branched gold nanoparticles that exhibit tunable SPR bands and excellent surface-enhanced Raman scattering (SERS) activity, thus making them suitable for further use in biological systems [118]. Recently, Tan et al. have reviewed all kind of possible shapes for plasmonic nanoparticles, synthesized via wet chemistry and have composed a “periodic table” of metallic nanoparticles that highlights the substantial progress in their synthesis, also reflecting the diversity of “plasmonic elements” in terms of shape and dimensionality [119].

Although the mechanisms underlying these syntheses are obviously very complex and not yet well understood, the main element responsible for the final morphology of such nanoparticles seems to be the capping agent (PVP in most of cases), which not only influences the intrinsic reactivity of the metal precursor and the reduction kinetics, but also may preferentially adsorb on certain crystallographic faces, thus modulating their relative growth rates. Hence, it has to be stressed, that the critical prerequisite for the production of high-quality metallic (gold) nanoparticles with tunable shapes and distinct plasmonic signatures is the organic (polymers, surfactants) component in the synthesis procedure.

2.2.2. Polymer brushes

Various methods and techniques are available for grafting polymer brushes on colloidal particles [120]. They can be fabricated either by physisorption of block copolymers [121] or by covalent attachment of polymer chains. For physisorption, block copolymers adsorb onto the particle with one block interacting strongly with the surface whilst the other block, weakly interacting, dangles freely in solution. However, the most promising method is the covalent attachment of polymers to the gold surface, commonly using end-thiolated polymers, due to the strong bonding between gold and sulfur atoms. Such covalently bonded polymer brushes are less vulnerable to desorption, partial damage or environmental impacts and can be classified according to their synthetic strategy, i.e.: 1) “grafting-to” method [122]; 2) in-situ “grafting to” method [12]; and 3) “grafting-from” strategy [123, 124], as illustrated in Figure 5. In the “graftingto” method, preformed end-functionalized polymers are grafted onto the surface of pre-synthesized gold nanoparticles, usually in a ligand exchange process where the polymer chains replace (partially or completely) the initial stabilizing agent such as citrate. In fact, the end-groups of the polymer should have a higher binding affinity to the particle surface than the stabilizing ligand, to facilitate the ligand exchange process, which is the case for the end-thiolated polymers [126, 127]. In the “in-situ grafting to” method, the gold nanoparticles are synthesized in the presence of the pre-synthesized polymer. Both, the “grafting to” and the “in-situ grafting to” techniques are very flexible strategies, because the polymer chains can be precisely tailored and their properties can be tuned prior to the coating process. In the “grafting from” approach, the polymer chains are grown from the particle surface by using surface-initiated polymerization techniques. Therefore, gold particles are first coated with an initiator molecule to generate suitable polymerization moieties for the subsequent synthesis of the polymer brush from the surface. Surface-initiated living/controlled polymerizations, especially atom transfer radical polymerization (ATRP) [124], reversible addition fragmentation chain transfer polymerization (RAFT) [128], and ring-opening metathesis polymerization [129], have enabled production of polymer brushes with predetermined molecular weights, narrow size distributions and controlled architectures [130, 131]. In contrast to the former two grafting techniques, the “grafting from” approach leads to polymer brushes with high grafting densities, which influence the chemical and physical properties of the nanoparticles, especially the wettability properties. Several reports were published where various kinds of polymers such as poly(ethylene glycol) [132, 133], poly(styrene) [134, 135], poly(methacrylamides) [136, 137], poly(methacrylates), [123] poly(vinylpyridines) [129] are tethered by one end to gold nanoparticles by either “grafting to” [122, 138], the in-situ “grafting to” [12] or the “grafting from” [128] technique [82].

Perhaps the most popular polymer for coating gold nanoparticles is the FDA approved polyethylene glycol (PEG), because of the high colloidal stability, biocompatibility and stealth behavior of the resulting nanoparticles. End-thiolated PEG is usually employed so that the polymer is attached to the gold surfaces through the sulfur-gold bond [133]. Nevertheless, PEG is chemically and physically considered a very inert molecule and therefore, it does not impart any other kind of remarkable chemical or physical properties

Figure 5

Schematic illustration of various polymer grafting techniques on nanoparticles. In the case of gold nanoparticles the common anchoring group is a thiol or disulfide. Modified scheme reprinted with permission from ref. [125].

to the nanoparticles. However, PEG has been successfully employed as an intermediate coating for gold nanoparticles stemming from different synthetic methods, allowing for their transfer into Stöber reaction mixtures [139], and facilitating their subsequent coating with SiO2, as described below. However, among all the polymers that can be grafted on gold nanoparticles, the most interesting polymer systems in regard of optical properties of gold nanoparticles are the stimuli-responsive polymers. Stimuli-responsive polymers undergo reversible but significant changes in their properties, in response to environmental variations such as temperature [53, 140–143], pH [143–145] or light [146, 147], and thus, also influence (among others) the optical properties of gold nanoparticles, through changes in either local refractive index or inter-particle distance. The responsive behavior of such systems strongly depends on the chemical constitution of the polymeric coating. For example, molecules/polymers bearing weak acidic or basic functionalities, such as carboxylic or amino groups, impart pH-sensitivity to the particles, due to changes in ionization degree (protonation/deprotonation), induced by pH changes. On the other hand, thermo-responsive molecules, such as non-ionic hydrophilic polymers, confer temperature-sensitivity to the particles, due to the hydrophilic/hydrophobic transition, induced by temperature changes.

Probably the most prominent polymers in this case are the thermo-responsive polymers based on poly(NIPAM) [53, 140–143] or poly(ethylene glycol)methacrylates [148, 149]. These polymers exhibit a sharp water soluble-insoluble (hydrophilic/hydrophobic) phase transition at a critical temperature known as the lowest critical solution temperature (LCST). Therefore, gold nanoparticles coated with such thermoresponsive polymers are usually well dispersed at temperatures below the LCST of the polymer and aggregate at temperatures above the LCST, which leads to a drastic color change, due to LSPR coupling of the particle cores. In the case of gold nanoparticles coated with pH-responsive polymers, the particles aggregate and disaggregate reversibly by changes in the solution pH. For polymer brushes with carboxylic acid groups, for example polyacrylic acid brushes, the particles aggregate by acidification of the solution, i.e., at low pH values [120]. On the other hand, polymer brushes with amine or imine groups, for example poly(2-(dimethylaminoethyl) methacrylate) PDMA [150] or poly(vinyl pyridine) [144, 151] respectively, aggregate at high pH values. Interestingly, proteins are another interesting class of pH responsive coatings, due to their pH-dependent U-shape solubility behavior with a minimum at the isoelectric point (pI) [152]. Proteins are natural co-polymers made of polar, nonpolar, and ionic monomers (in total 21 aminoacids, including selenocysteine), and depending on the primary structure of the proteins, they can exhibit dissimilar sensitivity toward pH and different pI. Chanana et al. have reported that gold nanoparticles coated with insulin [153] and BSA [143] exhibit not only reversible pH responsive behavior, but also high colloidal stability due to covalent bonding of proteins to gold nanoparticles through the thiol/disulfide groups from the cystein residues (Figure 6). Especially the BSA coated gold nanoparticles were shown to display bidirectional pH-sensitivity, where the particles aggregated reversibly at pH=pIBSA (pIBSA∼4.6–4.9) [143], but upon increasing or decreasing the pH away from the pI, the particles disaggregated again completely as indicated by the

Figure 6

pH-responsive optical properties of protein coated gold nanoparticles. (A) Photographs at different pH and (B) pH-responsive zetapotential and LSPR variation of Au@insulin nanoparticles [153]. (C) Photographs at different pH and (D) pH-responsive zeta-potential and LSPR variation of Au@BSA nanoparticles [143]. Reprinted with permission from refs. [143, 153].

LSPR shift back to its original position. Such stimuli-responsive changes are usually reversible, thus providing access to switchable optical properties.

2.2.3. Microgels/nanogels

Networks composed of crosslinked polymers have rised great interest [52, 53], which has increased by the development of polymer networks in the colloidal size range (10–1000 nm). These are known as microor nanogels and have become very interesting due to their promising applications as micro/nano reactors in catalysis, sensing, or in chemical and biological separation [52, 53, 154–156]. Similar to polymer brushes, stimuli responsive microgels that change with temperature or pH variations are of great interest, since they open new perspectives toward the fabrication of various materials with advanced properties. For example, microgels consisting of cross-linked poly(N-isopropylacrylamide) (pNIPAM) are thermo-responsive materials that swell at low temperatures by taking up water and shrink at elevated temperatures (∼32°C) by expelling the water. Similarly, pH-responsive microgels consisting of poly(methacrylic acid) networks e.g., swell at high pH and shrink at low pH values [52, 53]. Hence, precise inclusion of gold nanoparticles in such systems [157, 158], can lead to novel plasmonic (photonic) materials with interesting optical properties and novel effects.

Two main synthetic routes have been reported for the fabrication of gold nanoparticles coated with a microgel, namely by generating gold nanoparticles in-situ inside the pre-synthesized microgel particles or growing microgel around pre-synthesized particles. In the former case, gold metal ions are first immobilized in the microgel network and subsequently reduced using a chemical agent such as NaBH4, which leads to the formation of gold nanoparticles only within the network pores, due to the localization of the AuCl4 in relatively higher concentrations within the network by complexation of the gold ions by amines [159] or thiols [160]. In the case of charged microgels, the electrostatic interactions between metal nanoparticles and the charged groups also facilitate the immobilization of metal nanoparticles inside the microgel particles [159]. Antonietti et al. were the first to employ charged microgels consisting of crosslinked polystyrenesulfonate as “exotemplates” for the preparation of gold nanoparticles [161]. Variation of cross-linking density, reducing agent, and reducing conditions resulted in a variety of stable colloidal solutions with very different colors and shades were obtained, such as the classical Faraday colors, Barolo red and purple, as well as bengal rose, apricot and black shades [161].

For the second method, pre-synthesized gold nanoparticles are functionalized with a polymerization initiating or propagating moiety, which is subsequently used to grow a polymer network around the particle. The main concerns in this method are usually the colloidal stability of the pre-synthesized nanoparticles in the reaction mixture, which have been solved in several different ways. For example, Contreras-Cáraces et al. applied a two-step synthetic approach by growing first a thin layer of cross-linked poly(styrene) around the CTAB-stabilized gold nanoparticles to ensure colloidal stability of the nanoparticles to further grow pNIPAM microgel on the nanoparticles [158]. Through careful control of the experimental conditions, the authors were not only able to vary the pNIPAM shell cross-linking density, but also to tune its porosity and stiffness, as well as the shell thickness from few to a few hundred nanometers. Later, the same group presented a one-step procedure where butenoic acid was used not only as reducing agent but also as a polymerization propagating agent (monomer), because it readily provides the particles with a vinyl functionality, which was applied for the direct polymerization of pNIPAM around the particles, while avoiding complicated surface functionalization steps. The authors could demonstrate that this procedure was versatile for coating gold nanoparticles of different shapes, including Au spheres [162] and nanorods [155]. Similar approach was also employed by Karg et al. to grow an pNIPAM microgel on smaller gold nanoparticles (10–15 nm, citrate synthesis) by functionalizing them with butenylamine prior to polymerization [163]. In a very recent approach, Liz-Marzán et al. presented a general method to encapsulate gold nanoparticles within pNIPAM microgels is, using first the LbL strategy to provide the gold nanoparticles surface with vinyl functional groups, from which pNIPAM could be readily grown. They demonstrated that the method could be applied for gold nanoparticles of a wide range of sizes, shapes and capping agents, such as CTAB stabilized Au spheres and PVP stabilized Au decahedra and nanostars, exemplifying the fabrication of nanocomposites with tailored optical properties (Figure 7). Additionally, the total diameter of the Au@pNIPAM microgels, as well as the number of encapsulated metal cores, was varied through controlled addition of sodium dodecyl sulfate (SDS). Hence, one of the main advantages of this technique is that nanoparticles of designed size and shape can be employed, defining the optical properties from the very beginning. Furthermore, the gold cores encapsulated with the porous microgel shells can also be further grown in situ into different shapes [162], but also with different metals, such as silver [155], platinum [164] or nickel [164], due to the favored catalytic reduction of the metal salt complexes selectively at the gold particle surface (the cores).

Figure 7

TEM images of Au@pNIPAM nanocomposite particles based on Au spheres (A), decahedra (B) and nanostars (C). The scale bar in the insets is 200 nm. (D) Temperature dependence of the hydrodynamic diameter (measured with PCS) for the different samples: (•) spheres, (○) decahedra and (■) nanostars. (E) Vis-NIR spectra of the metal@pNIPAM aqueous colloids, below (22°C, solid lines) and above (44°C, dotted lines) the LCST, for spheres (black), decahedra (blue) and nanostars (red). (F) Surface plasmon band shift as a function of sample and temperature (same symbols as in D). Reproduced with permission from ref. [100].

Such hybrid microgels with embedded plasmonic cores exhibit reversibly tunable optical properties triggered by the thermosensitive swelling/deswelling property of the pNIPAM microgel shells. The LSPR band of the gold nanoparticles inside the microgels shifts to higher wavelengths at higher temperature, caused by the shrinkage of the shell, which increases the local refractive index around the gold cores [155, 158, 162–165]. Hence, the thickness of the microgels and thus the distance of closest approach between neighboring nanocrystals can be tuned through temperature [140]. Also, different cross-linker densities in the microgel can be used to tune the responsive character and the local refractive index environment of the cores, and thus the optical properties of such systems [163].

Liz Marzán et al. reported that such optically active Au nanoparticles immobilized in responsive microgels, can be applied as molecular traps for surface-enhanced Raman scattering (SERS) [154, 156]. The SERS performance of the gold particles can be significantly improved by either increasing their size or by growing silver shells on Au cores as demonstrated in the case of Au@Ag@pNIPAM colloids with much higher SERS intensities than their Au@pNIPAM counterparts. The shape of the gold particle cores also has a strong effect on the SERS intensity. This is particularly important for Au@Ag@pNIPAM nanorods, where silver not only increases the optical efficiency but blue shifts the longitudinal LSPR, which leads to strong electric field at the ends of the rods [155]. Apart from the SERS enhancement, the fluorescence intensity of adsorbed chromophores can be modulated as a function of temperature as well [154].

Optical properties of gold nanoparticle/microgel composites can also be easily controlled and manipulated by applying a different structural geometry, as reported by Karg et al. [166], where gold nanorods were assembled on the periphery of a dual-responsive poly-(NIPAM-co-allylacetic acid) microgel. These multiresponsive hybrid colloids combined successfully the interesting optical properties of gold nanorods (with two well-differentiated plasmon modes) with the sensitivity of the copolymer microgel toward external stimuli. It was shown that the collapse of the microgel core, induced by changes in either temperature or pH, enhances the LSPR coupling between the gold nanorods on the gel surface, as a result of the subsequent increase in packing density arising from the volume decrease of the collapsed microgel. At high nanorod density, such interactions lead to significant redshifts of the longitudinal plasmon resonance of the order of 55 nm from the swollen state, at 15°C, to the collapsed state, at 60°C [166].

Gold nanoparticles coated with well-defined microgels can also be assembled into 2D or 3D macroscopic supercrystals (NCSL), enabling the coupling of the optical response of the nanoparticles to the optical modes of the superlattice, and thus allowing for exploiting and understanding the unique optical and electronic properties of nanocrystals and their assemblies. Karg et al. demonstrated the preparation of NCSLs with spacings of 50–500 nm assembled from Au@pNIPAM core-shell particles with fascinating diffraction behavior [140]. These NCSLs exhibit pronounced diffraction in the visible (440–560 nm) with peak half-widths of the order of 10 nm. The position of the Bragg peak was simply tuned by adjusting the particle volume fraction. Due to the thermoresponsive nature of the polymer shell, temperature was used to initiate reversible crystallization or melting of the superlattice.

3. Inorganic coatings

3.1. Dielectric coatings

Among all the dielectric coating materials, silica is the most prominent and preferred coating material for nearly all kinds of colloidal particle systems, such as metal-, metal oxide-, semiconductor-, or ceramic nanoparticles, mainly because of its well-known high stability, especially in aqueous media. However, other reasons including easy regulation of the coating process, chemical inertness, controlled porosity, processability, and optical transparency also play a big role in terms of its wide application range [167]. Silica shells can confer both steric and electrostatic protection to almost any kind of particulate system, and act as dispersing agent of many electrostatic colloids. Moreover, silica is a low-cost material endowing the particle cores with several further beneficial properties, such as the possibility of subsequent functionalization and biocompatibility, which allow the use of these nanomaterials, especially in biomedical fields. Guerrero-Martinez et al. recently published a comprehensive review on the recent advances in the synthesis of silica-coated nanomaterials and their impact in different areas such as spectroscopy, magnetism, catalysis, and biology [167].

A number of reports have been devoted to silica coating of gold nanoparticles by various methods such as the Stöber method [168], the sodium silicate water-glass methodology [169, 170], two-step precipitation method [171] and water-in-oil (W/O) microemulsions [172, 173]. In the simplest and more extensive applied methodology, previously synthesized gold cores stabilized by surfactants or polymers are coated with silica using one of abovementioned procedures. However, one-step approaches based on (W/O) microemulsions, where both the synthesis of the cores and the silanization process take place in-situ have been successfully employed using a wide variety of surfactants, such as Igepal CO-520 [poly(5)oxyethylene-4-nonylphenyl-ether] [173] or CTAB [174]. It is worth noting that the choice of surfactant is a highly relevant factor in the design of a microemulsion, because it determines the size of the emulsion droplets, which in turn has an influence on the homogeneity and the thickness of the coating. The selection of a suitable silica coating method is mainly determined by the chemical affinity of the surface material for silica as well as by the stability of the particles in the coating reaction medium. W/O microemulsion methods are mostly employed for the silica coating of nanoparticles that are unstable in the classical Stöber reaction medium (tetraethyl orthosilicate (TEOS) hydrolysis and condensation in a water–ammonia–ethanol mixture). Han et al. [173] coated gold nanoparticles synthesized in cyclohexane with silica, by transferring them into reverse Igepal CO-520 microemulsions and then using TEOS under basic conditions. However, post-synthesis surface modification of gold nanoparticles with amphiphilic stabilizers (polymers) greatly assists the transfer of nanoparticles from water into various solvents (such as ethanol), where silica coating can be achieved through controlled hydrolysis and condensation of TEOS [167]. Various approaches were developed for the silica coating of both spherical and anisotropic gold nanoparticles, including CTAB capped gold nanoparticles. Because CTAB coated nanoparticles are not stable in Stöber reaction mixtures and CTAB cannot be easily removed from the particle surface, the authors wrapped them in the LbL fashion with negative (polystyrene sulfonate, PSS) and positive (PAH) polyelectrolytes, followed by the amphiphilic polymer PVP. Subsequent hydrolysis and condensation of TEOS in a 2-propanol/water mixture leads to formation of silica shells with well-controlled thickness [99, 175]. More recently, a much simpler and less time-consuming method has been reported for the coating of citrate and CTAB stabilized gold nanoparticles with silica, by transferring them into the Stöber reaction mixture by means of thiolated PEG [176].

Besides its most basic advantages such as the colloidal, chemical and biological stability, particularly in the case of plasmonic particles silica coatings bring the advantage that they can also be used to modulate the position and intensity of the LSPR band of the nanoparticles because silica is an optically transparent dielectric and can cause significant local refractive index changes. By controlling the experimental parameters such as coating time, concentration of reactants, catalyst, and particle concentration, the shell thickness can be tuned from 20 to 100 nm, leading to increase and redshift of the LSPR band with increasing shell thickness [99, 167, 175, 176], as shown in Figure 8. Gold nanoparticles coated with well-defined and tunable silica shells can also be assembled into 2D or 3D NCSLs, again enabling the coupling of the optical response of the nanoparticles to the optical modes of the superlattice, and thus tuning the distance between the plasmonic units and fabricating novel optical and electronic materials [177].

Figure 8

(A–D) TEM images of Au spherical particles coated with a homogeneous silica shell, with increasing core size from a to d [176]. The average diameters of the Au cores are in (A) 15.5 nm, in (B) 60.4 nm, in (C) 98.5 nm and in (D) 142.8 nm. The average silica shell thicknesses are 6.1, 21.0, 20.9, and 24.2 nm, respectively. (E–F) TEM images of silica-coated gold nanorods, with increasing silica shell thickness from E to F [99]. The scale is 100 nm for images E-F. (I) Vis-NIR spectra (dashed) of PEG-capped Au spheres (in ethanol) of various diameters: 15.5 (black), 60.4 (red), 98.5 (blue), and 142.8 (green) nm. Vis-NIR spectra of the same particles upon silica coating (solid) [176]. All of the spectra were normalized to 1 at 400 nm to facilitate comparison. (Inset) Plot of the dipolar λmax of PEG-coated and silica-coated Au nanoparticles vs. core diameter. (J–K) Normalized experimental (J) and calculated (K) UV-visible-NIR spectra of Au@SiO2 gold nanorods in 2-propanol, with increasing silica shell thickness. The insets summarize the shift of the longitudinal surface plasmon with increasing shell thickness. Modified and reprinted with permission from refs. [99, 176].

3.2. Metallic coatings

Perhaps the most interesting coating materials regarding the active enhancement or modulation of the plasmon resonance of gold nanoparticles are metallic coatings, due to the strong interaction of the conduction band electrons of the coating metal with those of gold. Due to the differences in the electronic structure and properties of the different possible metal coatings for gold nanoparticles, their diversity has a significant and different effect on optical properties of the nanoparticles [178, 179]. Depending on parameters such as surface properties, crystal lattices, redox potentials, crystal morphologies, surface charge and nature of the interface between the core and the coating material, the successful overgrowth of various metallic materials in the form of a discrete metal sheath for the gold nanoparticles could present some synthetic challenges. A number of comprehensive reviews are available, dealing with the nucleation and growth processes for the fabrication of bimetallic and other hybrid core-shell nanoparticles, which the reader is referred to [178, 179]. Many synthesis methods were reported for the fabrication of bimetallic structures, including galvanic replacement [180], laser ablation [6], electrochemistry [181], sonochemical [182–184], or radiolytic [185] reduction methods [178]. However, to produce true bimetallic core-shell nanoparticles with gold cores, the wet chemistry route applying deposition-precipitation strategy has been the most relevant one. In this strategy, the desired core gold nanoparticles are first produced separately according to existing synthetic procedures. Subsequently, the reaction conditions are changed so that the metallic coating material can nucleate and grow (homogenously or heterogeneously) onto the gold surface producing core-shell nanoparticles [178].

Although silver is the most frequently used metal in the context of the metallic coatings, a number of other metals have been successfully coated on gold particles. Besides the group 11 metals (coinage metals) such as copper and silver, the group 10 elements such as nickel, palladium and platinum have also been successfully employed (Figure 9). Other unusual metal coatings comprising single metal coatings such as cobalt Au/Co [187] and tin (Au@Sn) [188] or multishell coatings such as Ag/Hg [189] or Pb/Cd [190] coatings have been reported too. Apart from the elemental metal coatings, also metal oxides of titanium (Au@TiO2) [191], iron (Au@Fe2O3) [192] and nickel (NiO) [164], metal-semiconductors consisting of metal halogenides (Au@CuI) [193], metal sulfides (Au@CdS) [193] and metal selenides (Au@CdSe) [194] have been successfully coated on gold nanoparticles in coreshell structure.

As a result of the interband transitions, the coinage metals absorb light in the higher energy region of the visible spectrum through to the long-wave UV, for example with plasmon resonance peaks at about 550–600, 360–400, and 520 nm corresponding to Cu, Ag, and Au nanospheres, respectively [178]. Although the optical properties of copper and silver are both well suited for sustaining the plasmon resonance of gold nanoparticles in the visible region of the spectrum, silver has become a more popular metal coating for gold nanoparticles

Figure 9

Schematic illustration of coating gold nanooctahedrons with Pd, Ag and Pt with their respective TEM images and UV-Vis-NIR spectra. Modified and reprinted with permission from ref. [186].

than copper, first because of the relatively straightforward chemistry of silver required for the preparation of Au@Ag nanoparticles (aqueous chemistry and ambient conditions), which is quite difficult and complex for copper, due to its relatively high chemical reactivity and low stability. Second, silver is more stable against oxidation than copper. The third reason for its popularity is the increased activity of silver in SERS enhancement, which makes silver highly interesting for many sensing and detection applications [178].

Since the pioneer work of Morriss and Collins [195], who prepared first colloidal Au@Ag particles in 1964, Au/Ag hybrid nanoparticles, i.e., single shell Au@Ag and Ag@Au nanoparticles, but also multi-shell Au/Ag nanoparticles, for example with Au@Ag@Au structure have been the subject of intense research, because of the interesting optical properties as a result of the strong plasmon resonances of the two constituent elements. Whereas in the case of Au/Ag alloy particles the plasmon resonance peak can be smoothly interpolated between that of Ag (∼380 nm) and that of Au (∼520 nm) by changing the stoichiometry [178], the optical properties of Au/Ag core-shell nanoparticles are more complicated. The theoretical background for optical properties of such system was given by Aden and Kerker in 1951 [196], describing how a concentric core-shell particle might interact with incident light when the core and the shell are assumed to have a different complex propagation constant, complex dielectric factor, complex characteristic velocity, and permeability [196]. In general, the plasmon resonance of the core particle is rapidly attenuated by that of the growing shell, and after passing through an intermediate regime with two plasmon resonances, the shell resonance dominates. However, since the plasmon resonance of silver is superior to that of gold [178], the resonance of gold core is rapidly attenuated and blue-shifted until the two resonances merge into a single peak that lies rather closer to the resonance wavelength of pure silver particles. In some cases, however, it has been observed that the Ag shell initially alloys into an existing Au nanoparticle, leading to minimal blue-shift in the plasmon resonance wavelength of the hybrid with no separate resonant peak of Ag, but more importantly significantly enhancing the intensity of the primary Au resonance. Although this phenomenon has been successfully reproduced, it remains unclear. Apart from homogeneous coating, anisotropic coating of Ag was observed, when single crystalline Au NRs were employed as seeds, resulting in an orange slice-like shape for the Au@Ag nanocrystals as shown in Figure 10[197]. This unusual growth behavior is again ascribed to the unique properties of the stabilizing surfactant, CTAB, namely as a consequence of competition between its strong stabilization of certain lateral facets of Ag and the minimization of the overall surface energy. Also here, although the reason for the anisotropic coating still remains quite unclear, the results strongly suggest that interaction of CTAB and metal can be utilized to tune the shapes of bimetallic structures. It should also be noted that the actual crystalline structure and in particular the nature of the lateral facets of Au NRs is still a matter of debate [198].

Besides the classical Au@Ag core-shell particles, also Au/Ag multishell structures, such as Au@Ag@Au or Au@Ag@Au@Ag nanoparticles with very interesting optical properties can be obtained [199]. Whereas Au@Ag nanoparticles are deep yellow in color, an additional Au shell (Au@Ag@Au) results in blue colloids, due to the development of a strong plasmon resonance at ∼620 nm, while a further layer of Ag, to make Au@Ag@Au@Ag, blue-shifts the resonance again, leading to dispersions with an orange color (Figure 11). However, the detailed characterization of this system indicates that silver oxidation occurs upon addition of AuCl4, thereby altering the multishell configuration. Similar to the spherical gold nanoparticles, the two-step synthesis technique can also be applied to overcoat gold nanorods with silver, first reported by Jang et al. in 2001 [200] and since then studied intensively, due to its unique optical properties [178, 201, 202]. The optical properties of the resulting AuNR@Ag particles depend strongly on the aspect ratio, shape, and distribution of silver and can be tuned by changing one of these parameters. Although the longitudinal LSPR peak of the bare gold NRs extend into the NIR region by increasing their aspect ratio [28], it can be blue-shifted back when coated with silver, due to the combined effects of decreasing the aspect ratio due to preferential deposition of Ag on the NR walls and the dielectric properties of silver. Moreover, a new peak at smaller wavelengths close to the LSPR position of a pure silver nanoparticle is observed by increasing Ag shell thickness. However, a thin shell of silver leads to a strong reduction of the ensemble linewidth, caused by a change in slope of the plasmon-shape relation,

Figure 10

Photographs, SEM images and UV-Vis-NIR spectra of Au NRs (A) and Au NRs coated with Ag (B–F) increasing Ag/Au molar ratios from B to F. The inset in the lower left panel shows the relationship between the longitudinal SPR position and the Ag/Au molar ratio. The scale bar for the SEM images is 60 nm. Insets are corresponding STEM images with the same scale bar. Modified and reprinted with permission from ref. [197].

Figure 11

(A) Photographs and TEM an (B) UV-visible spectra of the colloidal dispersions of Au-Ag Multishell nanoparticles with Au, Au@Ag, Au@Ag@Au, and Au@Ag@Au@Ag nanoparticles. Modified and reprinted with permission from ref. [199].

an effect observed by Sönnichsen et al. and termed as “plasmonic focusing” [203]. Furthermore, the blue-shifting effect of the silver shell with a certain thickness on the plasmon resonance of gold NRs is larger for longer nanorods than for shorter ones. If the Ag shell is not cylindrical and is instead a parallelepiped, more than one new peak from the Ag shell is observed due to loss of rotational symmetry [197, 204].

Besides spherical and elongated gold nanoparticles, silver also has been overgrown onto core shapes such as nanotriangles, nanopods, or octohedra [205, 206] in a similar fashion. Tsuji et al. [207] recently reported silver coated gold icosahedra, with absorption peaks corresponding to both Ag and Au, but much broader than the peaks of pure nanospheres of the two elements. In contrast, narrower LSPR peaks were obtained for silver coated Au nanotriangles, tunable in the range of 800–1300 nm by controlling the Ag content of the solution [206]. Again, more silver produced a greater blue shift, which was ascribed to the thickening of the nanotriangles with increased Ag, and thus to a geometric effect rather than to the different dielectric properties of the Ag shell. Moreover, such silver coated Au nanotriangles show stronger surface plasmon resonances than pure Au nanoprisms, making the particles useful for SERS [206]. The shape of the overgrown Ag nanocrystal can be controlled again by the use of shape-regulating polymers/surfactants or by controlling the reaction conditions. Thus, Au nanotriangle cores have been buried in silver cubes, triangular prisms, rods, wires and other shapes [205].

Compared to the relatively facile coating of silver on gold nanoparticles, coating copper on gold in a core-shell structure has been a greater challenge and is therefore a much less explored field, in particular, due to the chemical instability of copper. Merely one report exists in the literature, in which the existence of Au@Cu core-shell structures along with Cu@Au and Au-Cu alloy structures have been demonstrated [208]. However, such particles were not stable and transformed back to one of the other structures at a certain temperature. Therefore, no information can be found on the effect of copper coatings on the optical properties of gold nanoparticles.

Interestingly, the plasmon resonance of a gold metal core is rapidly damped by any platinum group metal shell, which is usually the reason for a lesser use of these structures in optical applications. Both Au@Pd and Au@Pt particles have been studied, primarily as catalysts, with their optical properties only utilized as tools for characterization. Li et al. [209] produced Au@Pd nanoparticles by thermal decomposition of PdCl2 on existing Au nanoparticle cores, whilst Nutt et al. [210] deposited Pd onto a colloid of Au nanoparticles. The precipitation of Pd onto Au nanorod cores has also been demonstrated, and as expected, the longitudinal and transverse resonances of the nanorod cores were attenuated by increasing Pd shell thickness until they completely disappeared. Similar to Au@Pd, the resonance in Au@Pt is attenuated with a reasonably thin shell of Pt [178, 211–213]. Damping was also observed when Pt was deposited onto Au nanorods. Besides, Pd and Pt, Ni can be also be deposited onto Au nanorods, which confers them with magnetic properties, and for example they can be aligned in an external magnetic field [214]. Controlled alignment of Au nanorods [80, 215, 216], in particular with external magnetic fields could provide switchable optical and electronic systems (e.g., switchable colors or SERS hotspots) [217], but coating the rods with Ni significantly attenuates the longitudinal plasmon of Au nanorods. However, retention of the resonance of the Au cores is in principle possible by controlling the thickness of the Ni shells. For example, Sanchez-Iglesias et al. have shown that the plasmon resonance of an Au nanospheres was still evident after coating with up to 8 nm of Ni and NiO [164]. Other metals, different to coinage or platinum group metals, such as lead [190] or cobalt [187, 218] have also been deposited on gold nanoparticles. Lead deposited on gold nanoparticles also caused blue-shift of the Au plasmon band and three monolayers of Pb were sufficient to produce the plasmon band of pure lead [190]. On the other hand, little is known about the effect of cobalt on the optical properties of gold, due to the dispersion instability as a result of magnetic properties of cobalt [187]. Multishell coatings of gold nanoparticles with more than one type of metal have also been reported, and the outcome strongly depends on the nature of employed metals, and the type of structure they build on the gold cores. For example gold NRs coated with shells of Ag and Hg have been prepared [189], where Hg coating caused a strong blue-shift of the longitudinal plasmon resonance of the rod. Au@Ag@Pd [219], Au@Pb@Cd [190] have also been reported, but their optical properties were unremarkable or not well investigated.

Table 2

An overview of different classes of coating materials with some examples and their effect on the optical properties of gold nanoparticles by modulation of parameters such as size, shape, refractive index, interparticle distance and interparticle orientation.

4. Conclusions

In conclusion, a wide variety of coating materials have been reported for stabilizing and modulating the properties of gold nanoparticles. However, since all these coatings have a prevailing influence on the physical, chemical, optical, electronic and catalytic properties of the nanoparticles, they need to be taken into account at the time of designing any products for final applications. The coating can influence these properties in a direct manner, such as shape-directing surfactants during the synthesis or the dielectric and metal coatings, actively affecting the LSPR of the gold nanoparticles, or indirectly, by providing additional functionalities, such as the stimuli-responsive colloidal stability, which in turn can influence the optical properties of gold nanoparticles under certain environmental conditions. Some of these coating materials and their influence on the optical properties of gold nanoparticles have been summarized in Table 2.

Although particle shape and size are the most prominent among all the parameters governing the optical properties of gold nanoparticles, the coating material is ultimately the limiting factor for their applications in various fields, especially due to its influence on colloidal stability, catalytic and/or bio-compatibility issues. Hence, it is the choice of the coating material which becomes fundamental regarding the fate of gold nanoparticles.

Acknowledgments

The authors acknowledge the EU (METACHEM, grant number CP-FP 228762-2; ERC Advanced Grant PLASMAQUO, 267867) for financial support and Dr. Jorge Pérez-Juste for fruitful discussions.

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