Stabilization of Silver and Gold Nanoparticles: Preservation and Improvement of

Plasmonic Functionalities

Hyunho Kang, Joseph T. Buchman, Rebeca S. Rodriguez, Hattie L. Ring, Jiayi He, Kyle

C. Bantz and Christy L. Haynes *

Department of Chemistry, University of Minnesota, 207 Pleasant Street SE,

Minneapolis, MN 55455, United States

*Corresponding author, chaynes@umn.edu

Abstract

Noble metal nanoparticles have been extensively studied to understand and apply their plasmonic

responses, upon coupling with electromagnetic radiation, to research areas such as sensing, photocatalysis,

electronics, and biomedicine. The plasmonic properties of metal nanoparticles can change significantly with

changes in particle size, shape, composition, and arrangement. Thus, stabilization of the fabricated

nanoparticles is crucial for preservation of the desired plasmonic behavior. Because plasmonic

nanoparticles find application in diverse fields, a variety of different stabilization strategies have been

developed. Often, stabilizers also function to enhance or improve the plasmonic properties of the

nanoparticles. This review provides a representative overview of how gold and silver nanoparticles, the

most frequently used materials in current plasmonic applications, are stabilized in different application

platforms and how the stabilizing agents improve their plasmonic properties at the same time. Specifically,

this review focuses on the roles and effects of stabilizing agents such as surfactants, silica, biomolecules,

polymers, and metal shells in colloidal nanoparticle suspensions. Stability strategies for other types of

plasmonic nanomaterials, lithographic plasmonic nanoparticle arrays, are discussed as well.

Contents

1. Introduction

2. Synthesis of Ag and AuNPs and Stabilization with Adsorbed/Covalently-attached Ligands in Solution

Phase

2.1. Theoretical Background of Colloidal Stability of the Plasmonic Nanoparticles

2.2. Conventional Surfactants for In-Solution Synthesis and Stabilization

2.3. PEG Ligands-Stabilized Plasmonic Nanoparticles in Complex Matrix

2.4. Biomolecular Ligands-Stabilized Plasmonic Nanoparticles

2.5. DNA and Other Highly Functional Ligands: Stabilization and Assembly

3. Plasmonic Nanoparticles with Shell-Coating

3.1. Silica Shell-Stabilized Plasmonic Nanoparticles

3.2. Plasmonic Nanoparticles with Organic-Shell Coating

3.3. Plasmonic Nanoparticles Coated by Metal Shells

4. Two-dimensional Plasmonic Nanoparticle Arrays

4.1. Plasmonic Nanoparticles Arrays via Lithography Technique and Related Stabilization Strategies

4.2. Fixation of Plasmonic Nanoparticles on Solid Substrates via Non-Lithographic Technique

5. Conclusion and Prospective

Author Information

Corresponding Author

ORCID

Notes

Biographies

Acknowledgements

References

1. Introduction

The phenomenon of the surface plasmon resonance (SPR) was first reported by Wood in 1902.1 More

than a hundred years ago, he observed a form of abnormal incident, angle-dependent bands on a metal-

supported diffraction grating shed by polarized light. Since this introduction, the SPR phenomenon has

been explained more explicitly based on the work of many researchers including Kretschemann and Otto.2,3

Numerous scientific fields have taken advantage of the SPR either directly or indirectly, from simple optical

sensing techniques to solar energy conversion technology.4–7 Along with extensive and intensive work on

the development of nanotechnologies,8–11 a sub-area of SPR research has attracted a lot of attention: the

localized surface plasmon resonance (LSPR).

The SPR is a coherent oscillation of surface conduction electrons upon excitation by electromagnetic

radiation at the interfaces between, for example, metal and dielectric media. The LSPR occurs when this

surface plasmon is restricted to smaller volumes, that is, to nanoparticles, which are comparable in size to

the wavelength of incident light. The dimensions of the nanostructures allow the plasmon to oscillate locally,

within the near metal surface. The LSPR and plasmonic nanoparticles can provide a couple of advantages

over traditional SPR. First, the LSPR measurement platform needs no prism, and the angle of incident light

is not as important as in the SPR platform; this means that the design of a plasmonic device can be much

more affordable and flexible, and it is not susceptible to vibration or mechanical noise. The LSPR shows

relatively less sensitivity to bulk refractive index changes than SPR due to the short range of the enhanced

electromagnetic field, so more focused studies on reactions or sites of interest are available without much

interference from bulk solvent.

The LSPR is dependent on the size, shape, and composition of the nanoparticles as well as other

external factors; therefore, different types of plasmonic nanoparticles can be designed based on the needs

of specific studies or applications. Both theoretical and empirical research have demonstrated various kinds

of plasmonic nanoparticles over the last few decades.12,13 The most frequently and widely used metals are

silver and gold nanoparticles (AgNPs and AuNPs), though metal plasmonic nanoparticles can also be

fabricated from aluminum, copper, palladium, and platinum.14 Based on dielectric properties, copper should

also have good plasmonic performance, but its propensity to oxidize limits the use of plasmonic Cu NPs.

Typically, the surface plasmonic qualities of transition metals, such as titanium, cobalt, and nickel, are less

compelling than those of the coinage metals.15 There are multiple reasons for the dominance of AgNPs and

AuNPs in plasmonic nanoparticle research. AgNPs and AuNPs can be tuned to absorb and scatter light

throughout the visible and near-infrared regions (i.e. Ag LSPRs can range from 300 to 1200 nm). AuNPs

are chemically inert and oxidation-free and also show high biocompatibility, which is critical for biomedical

applications.16 Moreover, various synthetic strategies to produce different shapes and sizes of AuNPs and

AgNPs have been explored, resulting in the ability to tune their plasmonic functionalities,17 and their

application in the fields of energy, catalysis, sensing, and biotherapy.18–22

The life of nanoparticles, including plasmonic nanomaterials, can be divided into three stages:

preparation, storage, and application. Especially for plasmonic nanoparticles, whose size, morphology, and

chemical stability determine the overall level of plasmonic and application performance, conservation of

particles’ physical and chemical characteristics is critical and must be carefully controlled. In most cases,

how the nanoparticles are prepared is deeply associated with how the nanoparticles are stabilized. The

methods for generating Ag and AuNPs can be categorized into two major classes: wet-chemical synthesis

and lithographic fabrication. In the wet-chemical synthesis method, nanoparticle size and morphology can

be tuned by various reaction parameters, such as chemical precursor choice, temperature, pH, or reaction

times. Stabilizing agents must be present during and after nucleation and growth to imbue the nanoparticles

with colloidal stability. Without suitable stabilizers, neither Ag nor AuNPs can maintain their structures and

will aggregate or dissolve, resulting in loss of plasmonic functionality.23–27 These initial stabilizing agents

can be replaced by other more robust stabilizers specific to an applications’ needs. Choosing appropriate

protecting materials is especially important for in vivo applications, where the nanoparticles must maintain

their plasmonic properties until the nanoparticle arrives at a site of action and performs the desired function

within a complex biological matrix. In current research, many studies exploit more than one stabilizing

approach to set up a platform that can maximize the nanoparticles’ plasmonic abilities and perform other

critical functions (Figure 1). In lithographic fabrication, metal nanoparticles are usually deposited, using

various patterning methods, under vacu26um and immobilized on supporting substrates to form nanoparticle

arrays. Different from colloidally synthesized Ag or AuNPs, these plasmonic nanoparticle arrays don’t

always require stabilizing agents during the preparation step, but their stability during long-term storage

and use are still important.

The main focus of this review is how Ag and AuNPs, the most commonly used plasmonic nanoparticles,

are stably prepared and applied in specific plasmonic applications without significant damage to their

original chemical and physical properties. Three different types of nanoparticles are discussed based on

the types of protecting materials and the synthetic strategies, since these two factors are deeply related to

the origins of the particles’ stability, the media to which they are exposed, and the involved applications.

Where relevant, this review also discusses the role of stabilizers that enhance plasmonic nanoparticles to

achieve specific morphologies/arrangements or to incorporate other functional chemicals to achieve more

complex plasmonic designs.

Figure 1. A description of different stabilizing agents in colloidal plasmonic nanoparticle preparations and

related functions/characteristics. The sizes of the nanoparticles and ligands/shells are not drawn to scale.

2. Synthesis of Ag and AuNPs and Stabilization with Adsorbed/Covalently-attached Ligands in

Solution Phase

The importance of controlled synthesis and fabrication of plasmonic nanostructures has grown along

with development of increased applications for plasmonic materials. It is now well-appreciated that the

plasmonic properties and, thus, the performance in various applications are largely determined by size,

shape, and composition of nanoparticles.28 Solution phase synthesis of Ag and AuNPs is the most common

way to generate monodisperse particles with intentionally varied size and shape.14,29 In this variety of

solution phase synthesis methods, the stabilizer (also known as the capping agent) must be present to

control the size and morphology, prevent aggregation, and facilitate long-term storage. Both Ag and AuNPs

have shared some common and popular stabilizers for synthesis, and each particle’s synthetic history is

discussed herein. AuNPs were first introduced into the research field in 1857 when Michael Faraday

reported the preparation of colloidal AuNPs by the reduction of chloroauric acid by phosphorous.30 Since

that discovery, 20th century scientists have made large efforts to control nanoparticle size and shape with

tailored synthetic designs. Among various experimental explorations, the work done by Turkevitch and

Frens has offered one of the most important breakthroughs in AuNP synthesis, by pioneering and further

improving the citrate reduction of HAuCl4.31,32 This method is very often used for colloidal gold nanomaterial

synthesis, where citrate plays the role of both the reducing and stabilizing agents. In 1993, Mulvaney and

Giersing reported the stabilization of AuNPs with alkenethiols of various chain lengths.33 This two-phase,

thiolate-stabilized method was more clearly illustrated by the Schiffrin group in 1994,34 and it has enabled

researchers to synthesize AuNPs at lower temperatures, with relatively high stability and facile size control.

In the same year, Reetz et al. also reported an electrochemical synthetic strategy for metal nanoparticles.35

This electrochemical technique involves non-aqueous media where the dissolved metals from the anode

and the intermediate metal salts are reduced at the cathode. The stabilizer, usually a tetraalkylammonium

salt, is required to avoid indiscriminate aggregation in solution as well as to prevent all particles from plating

at the surface of the cathode.36 Due to the advantages of low cost, modest equipment, and ease of

controlling the yield and size of the nanoparticles by adjusting the current density,37,38 solution-phase

electrochemical synthesis of plasmonic metal nanoparticles is an important route to keep in mind.39–41

Anisotropic Au nanorods have been extensively synthesized and developed since the late 1990s due

to their distinct optical properties compared to common spherical nanoparticles, including multiple plasmon

modes in the visible region of the spectrum, LSPR tunability into the infrared, and concentration of excited

electromagnetic fields at the nanorod tips.42 Starting from electrochemical reduction preparation methods

in earlier years, now the most widely used method is a silver-assisted seed-mediated growth where

preformed small AuNPs act as a seed for further reduction of Au ions to generate anisotropic Au shapes in

the presence of silver nitrate and surfactants.43 In nearly all Au nanorod syntheses, cetyltrimethylammonium

bromide (CTAB) has been predominately used as a shape controller as well as stabilizer; this method was

first introduced in 2001 by the Murphy group, and has been widely used for the production of anisotropic

Au nanorods.44

In colloidal synthesis, also commonly called chemical synthesis, of AgNPs, the basic synthetic approach

is similar to that of AuNPs. The synthesis of AgNPs generally requires three chemical functional compounds:

a silver precursor, solvent, and a reducing/stabilizing agent. Like the synthesis of AuNPs, the reduction of

AgNO3 with citrate in water was first reported in 1982.45 However, relying on citrate-stabilized AgNP

synthesis usually produces nanoparticles with poor control of size and shape.14 Rather than this citrate

reduction method, the reduction of the silver precursor in multivalent alcohols – so-called polyols – is a

more popular chemical approach to synthesize various shapes of monodisperse silver nanoparticles.46 In

a typical synthetic process, ethylene glycol (EG), AgNO3, and poly(vinyl pyrrolidone) (PVP) serve as the

solvent/reducing agent, silver precursor, and stabilizing/capping agent, respectively. This polyol method

can achieve a high degree of control over the morphology of the final products by controlling the types and

amounts of capping agents and oxidative etchants, the availability of Ag+ ions, or reaction kinetics with

temperature.47 Other methods, such as seed-mediated growth or light-mediated growth have received the

great attention as well.48–51

The general synthetic strategies described above for both AuNPs and AgNPs mainly focus on solution-

based synthesis. These are “bottom-up” methods whereby particles are produced by chemical reductions.

Other fabrication methods for producing plasmonic gold and silver nanoparticles include mechanical

grinding of bulk metals, thermal decomposition, and evaporation; these methods will be discussed in later

sections. The solution-based chemical approaches are advantageous due to their low cost, high yield, and

ease of production. As described, under these chemical approaches, the metal precursors or seeds are

treated with surfactants or other molecules as stabilizing agents during growth, such as citrate, CTAB, and

PVP in previous examples. Those loosely bound molecules are somewhat limited in their ability to maintain

the colloidal stability of plasmonic nanoparticles, so in the final products, these stabilizing agents can still

present in their original role or they can be replaced with other functional groups via substitution. However,

the roles of aforementioned stabilizing surfactants should not be undervalued as they have critical impact

in determining the basic plasmonic properties of the final products by controlling the size and morphology

of the nanoparticles. In this section, the roles of those stabilizing surfactants and other replacement

stabilizing molecules and ligands will be discussed. In this review, to have a clear distinct definition between

ligands and shells (described in Section 2.2), the term ligand will be limited to general, small molecules, or

compounds which form a coordination complex, mainly via metal-sulfur bonds, with the metal core but do

not have strong intermolecular interactions between ligands.

2.1. Theoretical Background of Colloidal Stability of the Plasmonic Nanoparticles

Before exploration of various stabilizing agents in colloidal states, it is valuable to discuss the theoretical

background which has supported the colloidal nanoparticles’ behaviors. Derjaquin-Landau-Verway-

Overbeek (DLVO) theory has been widely used to study the behavior of colloidal particles, and is therefore

very applicable to this review. According to DLVO theory, the interparticle behavior in colloidal science is

dominated by the interplay of attractive van der Waals forces and repulsive Coulombic forces.52 The

classical DLVO theory calculates the total interaction potentials between two particles as the sum of the

van der Waals attraction and the electrostatic repulsion. The size and the electrical double layer of the two

particles are used to express the electrostatic repulsion potential along with other parameters. For the

attraction force, the Hamaker constant plays a crucial role in the description of attraction energy between

the particles. The Hamaker constant, which can be calculated based on the Lifshitz theory, refers to the

relative strength of the attractive forces between the two surfaces.53 For the colloidal particles, the Hamaker

constant can be used to estimate the forces between two particles of the same material separated by a

continuous medium.54 More details and the full set of related equations can be found in previous studies.53

DLVO theory has often been used to interpret experimental results. In 2005, Kim et al. manipulated the

interparticle interaction of the AuNPs to control the size of AuNP aggregates.55 The citrate-capped AuNPs

underwent ligand exchange with the addition of benzylmercaptan ions, resulting in increased particle size

due to the aggregation. With the experimental results and the calculated interaction potentials based on

DLVO theory, the authors suggested that the addition of benzylmercaptan ions lowered the energy barrier

and reduced the colloidal stability of AuNPs via decreased surface potential and increased ionic strength,

resulting in destabilization of the nanoparticles and aggregation. In another study, aggregation kinetics of

the citrated-capped AgNPs were investigated in aqueous sodium chloride solutions.56 As the concentration

of the sodium chloride increased, the attachment efficiency of aggregating particles also increased. The

attachment efficiency, which can describe the aggregation kinetics of AgNPs, were both experimentally

determined and theoretically calculated by using a Hamaker constant for citrate-coated aqueous AgNPs in

varied ionic strength. The results from experimentally obtained attachment efficiencies showed remarkable

agreement with the values from DLVO predictions. Furthermore, this study compared the aggregation

kinetics of PVP-coated AgNPs with citrated-coated AgNPs, and a significantly higher stability was found for

PVP-coated AgNPs, probably due to steric repulsion imparted by the adsorbed PVP molecules. Recently,

Anand et al. examined the solvation force between two adjacent CTAB-coated AuNPs by using in situ

transmission electron microscopy and pairwise interaction forces derived from a fit function of repulsive

forces, hydration forces, and van der Waals forces.57 The combined analysis of measured distance between

particles from TEM and calculated pairwise interaction forces suggested that the hydration forces become

effective only when the nanoparticles are separated by a few water molecules, and in other situations

electrostatic repulsions and van der Waals attractions dominate the pairwise interaction. This metastable

transient nanoparticle pairs occur when the distance between the particles is around 0.5 nm, and the

particles proximity induces vanishing hydration forces and resulting attachment.

2.2. Conventional Surfactants for In-Solution Synthesis and Stabilization

In the citrate reduction method, citrate anions reduce metal ions to atoms and stabilize clustered atoms,

resulting in colloidal nanoparticles. Citrate-stabilized metal nanoparticles have played a crucial role as a

fundamental material in a number of gold nanoparticle-based plasmonic applications. These days, as the

applications of synthesized nanoparticles require more robust and versatile platforms, citrate-stabilized

AuNPs are usually an intermediate product before further treatments. However, the citrate reduction

method remains the most popular strategy to produce noble metal colloids with easily exchanged surface

species.

Until recently, many studies have relied on this facile synthesis method despite the fact that the exact

structure/orientation of citrate anions on gold and other metal surfaces were not well known. Recent studies

have more closely analyzed the interaction between metal nanoparticles and citrate ions.58–60 AuNPs are

positively charged during the gold ion reduction reaction. Charges on gold nanoparticle surface are then

neutralized, and the AuNPs are negatively charged due to the adsorbed citrate ions.61 The adsorbed citrate

layers stabilize the AuNPs via electrostatic repulsions. There are a small number of published manuscripts

that focus deeply on the citrate-metal interaction. In one example, Park et al. investigated the structure of

citrate layers on gold nanoparticles via attenuated total reflectance infrared spectroscopy and X-ray

photoelectron spectroscopy and concluded that, on a Au surface, η2-COO− coordination of the central

carboxylate group of the dihydrogen citrate anions is dominant.62 The adsorbed citrate anions interact with

adjacent citrate molecules through hydrogen bonds and van der Waals interactions, thus forming self-

assembled layer of 8−10 A in thickness; steric repulsions between citrate anions provide dispersion stability

of the particles in solution. To use the citrate during the nanoparticle synthesis more efficiently, many

studies have introduced a secondary reducing agent along with the citrate and have widened the range of

nanoparticles from the citrate-reduction solution synthesis. For example, in 2014, Bastus et al. synthesized

highly monodisperse sodium citrate-coated AgNPs with varying diameters by kinetically controlling the

seed-growth method with sodium citrate and tannic acid as reducing agents.63 The researchers suggest

that the functions of citrate as both a stabilizing agent and weak reducing agent disturb the efficient and

fast nucleation and growth of AgNPs, wherein a monomer of silver ions (Ag42+) and oxidized citrates led to

rather slow and heterogeneous nucleation and finally to a polydisperse AgNP product. Tannic acid was

added to enhance the reduction reaction performance and achieve improved size control. The amount of

tannic acid was carefully controlled to induce fast reduction and to avoid the formation of intermediate

complexes so that homogeneous growth was possible. The synthesized particles showed improvement in

size control (with a range from 16- to 118-nm-diameter) and narrow size distributions. By changing the ratio

of the two reducing agents and using the seed-mediated growth method, these nanoparticles also showed

long-term colloidal stability, similar to that achieved for AgNPs stabilized using PEG, PVP, or bovine serum

albumin (BSA). Interestingly, despite their similar colloidal stabilities, citrate/tannic acid-coated AgNPs

exhibited improved ability as a catalyst in the electron transfer reaction between Rhodamine B and

borohydride ions, compared to PVP-coated AgNPs, likely due to the less dense AgNP surface coating with

layers of citrate/tannic acid. The same synthetic strategy has been applied to Au ion reductions to obtain

AuNPs smaller than 10 nm.26 The enhancement in monodisperse AgNPs synthesis by focusing citrate as

a stabilizing agent can be found in other studies such as the work of Haber et al., where the production of

Ag nanoprisms with high stability and reproducibility was achieved with sodium borohydride and L-ascorbic

acid as reducing agents and trisodium citrate as a capping agent.64

As mentioned above, the role of CTAB cannot be neglected when reviewing synthesis methods for

AuNPs. The Murphy group showed that anisotropic AuNPs could be obtained when the surfactant CTAB is

coordinated with another mild reducing agent .65 The exact role of CTAB in the synthesis is still under

debate,65,66 but it is obvious that in the final product, CTAB plays a role as a stabilizer in colloidal dispersion

by protecting the gold from aggregation or dissolution. It is generally accepted that CTAB is present as a

bilayer on the gold surface via electrostatic interactions, where the ammonium headgroups in each layer

are facing the Au surface and bulk solvent media respectively, and long hydrocarbon chains in both layers

are located between the two sets of the headgroups.42 One impactful aspect of this CTAB adsorption is that

the packing density of bilayers on the side and the tips of the Au nanorods are different due to the curvature

at the tips, allowing site selective further shape modification or surface functionalization.67,68 For instance,

to take advantage of how nanoparticle morphology impacts the LSPR, bipyramidal AuNPs were

synthesized with CTAB surfactant as a stabilizer and shape-guiding agent.69 Furthermore, with variation of

the ratio of binary surfactants including two of the following: CTAB, cetyltrimethylammonium chloride, and

benzyldimethyl-hexadecylammonium chloride, the colloidal stability of bipyramidal AuNPs was finely

controlled, resulting in AuNPs with multiple novel morphologies via site-selective regrowth and etching

(Figure 2a). When only one surfactant was used in the growth step, only size augmentation of the

bipyramidal AuNP was observed. However, with the combination of two different surfactants, tip regions of

the AuNPs overgrew, likely because of the exposure of less protected crystalline features based on the

different binding affinity of the surfactants. This study reinforced the vital role of CTAB in stabilizing colloidal

AuNPs and the application of surfactants to induce desirable AuNP morphology, which is critical for

potential applications in optics or surface-enhanced Raman spectroscopy (SERS). In another study, citrate-

capped AuNPs were investigated for adjacent particle interactions. Yang et al. measured emission

polarization from close AuNP dimers with varied internanoparticle gap widths.70 The samples were prepared

by dropping AuNP dispersions onto a sample grid; since AuNPs were citrate-stabilized, varying gap

distances were obtained randomly during solvent evaporation (Figure 2b). The authors argue that gap

sizes less than a nanometer were feasible because particles were citrate-coated, where van der Waals

squeezing and capillary forces reduced the inter-particle distances, thus, gap distances this small would be

hard to achieve with more robust stabilizers, such as oxide shells. These varying gap distances between

AuNPs dimers enabled the investigation of polarization states of scattering, and even the quantum effects

from dimers forming quantum range gap distances.

Figure 2. (a) Transmission electron microscopic (TEM) images of regrown AuNPs from bipyramidal seeds

in different conditions. (1-5: singular surfactant, 2-10: binary surfactants. Scale bars: 200 nm for low

magnification, 50 nm for high magnification) Different colors of arrow indicate the detailed condition for the

regrowth. Reproduced with permission from ref 69. Reproduced with permission under Creative Commons

Attribution 4.0 International License http://creativecommons.org/licenses/by/4.0/. (b) TEM images of citrate-

stabilized gold dimers. The particle diameter is 80 ± 2 nm. The distances in the figures indicate the gap

between the particles. Scale bars are 100 nm. Reprinted with permission from ref 70. Copyright 2015

American Chemical Society.

PVP is a non-ionic polymer widely used in nanoparticle synthesis, especially for AgNPs. Possessing

both a highly polar amide group in the pyrrolidone ring and a non-polar alkyl backbone makes PVP highly

soluble both in water and in non-aqueous solvents.71 Generally, PVP can act as a stabilizing agent in

colloidal metal nanoparticle dispersion via the repulsive forces from its hydrophobic carbon chains and

benefits from inert physicochemical properties over a broad pH range.72 Even though a nanoparticle-

adsorbed PVP layer is often categorized as a shell, it is included in this section due to its common use as

a stabilizer during the synthesis step and relatively weak adsorption on the metal surface, which is similar

to other popular molecular stabilizers, such as citrate and CTAB. The polyol synthesis, which is the most

popular method to produce AgNPs in solution, was introduced by Xia et.al. in 2002 as a system for the

preparation of Ag polyhedra, where a diol solvent reduces the Ag salt at high temperature with PVP as a

capping agent.73 Significant studies over the last decade have studied the interaction between PVP and Ag

nanocrystals during and after synthesis, and it is clear that PVP plays a critical role in the Ag polyol method

via stabilization of lowest-energy crystal {100} facets.71 This surface-selective adsorption of PVP on AgNPs

has been examined in many ways, such as Raman, IR, and X-ray photoelectron spectroscopy.39,74,75

Among the various approaches, Saidi et al investigated the PVP-AgNP interaction through density

functional theory.76 In the study, the authors found that the interaction between PVP and {100} and {111}

Ag crystal facets occurs via direct binding and van der Waals force. The study clearly demonstrated that

the PVP molecules bind to Ag in a flat conformation, and the binding energy of oxygen atoms in the carbonyl

group is stronger with Ag(100) than Ag(111). The surface-selective stabilization of PVP on AgNPs can be

used to alter the morphology of the nanoparticles during synthesis. Xia and co-workers showed that with

different concentration and molecular weight PVP led to different AgNP shapes, including cubes, truncated

cubes, and octahedra.77 The authors suggest that the concentration of PVP changes the surface free

energy of Ag facets and that the molecular weight can affect the effective coverage of PVP on the Ag

surface, both resulting in altered final morphologies of the particles. Other morphologies, such as

pentagonal wires, bipyramids, and decahedral AgNPs, have been demonstrated with the polyol

synthesis.14,78,79 The PVP-stabilized Ag nanocube synthesis method has been investigated and further

refined in many ways, such as in seed-mediated growth.46 More recently, PVP has also been applied to

AuNP synthesis, where PVP adsorption enables the formation of triangular plates, octahedra, and other

morphologies of AuNPs.80 Interestingly, for AuNPs, PVP binds primarily to {111} facets. A recent

computational study by Liu et al. reasoned that in the presence of PVP, {111}-faceted Au nanostructures

are thermodynamically more favorable.81 In addition, researchers suggest that, different from Ag(100) which

doesn’t reconstruct, Au(100) shows surface reconstruction, resulting in PVP binding preference to Au {111}

facets.

As shown, due to its excellent ability in shape control and colloidal stabilization, PVP-coated Ag and

AuNPs with novel morphologies are commonly used. For example, in 2016, Zhang et al. synthesized Au-

coated Ag concave cuboctahedra as a SERS monitoring platform.82 In this study, Au was first deposited on

the colloidal PVP-stabilized Ag cuboctahedra; during the initial deposition, Au atoms covered the entire

surface of the Ag nanoparticles. In the continuing and subsequent Au deposition steps, Au was preferably

deposited onto the {100} Au facets due to the selective passivation of the {111} Au facets by PVP, leading

to concave cuboctahedral structure. This alloyed nanoparticle proved to be a more efficient SERS probe

than the original Ag cuboctahedral nanoparticles, with a 70-fold higher SERS intensity for the same analyte

(Figure 3a). The particle also exhibited fine performance for in situ SERS monitoring of 4-nitrothiophenol

reduction, with colloidal stability maintained throughout the reaction. The particles showed high stability

against hydrogen peroxide etching as Ag was fully coated by Au. In another example, Zhai et al. observed

the effect of PVP in a plasmon-driven synthesis of gold nanoprisms.83 The authors revealed a unique

function of PVP, that of preferentially adsorbing on the perimeter of Au nanoprisms, inducing the anisotropic

growth of Au nanoprisms (Figure 3b). Under photochemical irradiation, it was observed that the PVP

adsorbed onto the AuNPs prolonged the hot-electron lifetime to expedite the reduction of Au ions—not a

usual capability of PVP in normal conditions. The authors suggested that adlayered PVP molecules were

capable of stabilizing electrons generated through plasmon excitation. As described here, PVP can act as

not only as a stabilizing agent but also as a morphology-inducing agent like other weakly binding molecules

such as CTAB or citrate.

Figure 3. (a) (i) Schematic illustration of the process of fabricating Ag@Au cuboctahedra and Ag@Au

concave cuboctahedra from Ag cuboctahedron template. (ii) SERS spectra of 1,4-benzenedithiol adsorbed

on each three different nanoparticles at the excitation of 785 nm: (blue: Ag@Au concave cuboctahedra,

red: Ag@Au cuboctahedra, black: Ag cuboctahedra). The intensity from Ag cuboctahedra substrate was

20 times multiplied. (bottom). Reprinted with permission from ref 82. Copyright 2016 American Chemical

Society. (b) Scanning electron microscopic (SEM) images of Au hexagonal nanoprisms (left) and Au

triangular nanoprisms fabricated by PVP-induced photochemical irradiation-reduction method. The insets

in each image show high-magnification SEM images (left insets from each) and the elemental distributions

of nanoscale secondary-ion mass spectrometric images showing the 12C14N− signals from PVP (green)

and 127I− signals (blue), respectively (right insets from each). The iodide ions were included to facilitate

the production of sharp triangular shape. Scale bars in all insets are 200 nm. Reprinted by permission from

Springer Customer Service Centre GmbH: Springer Nature, ref 83. Copyright 2016.

2.3. PEG Ligands-Stabilized Plasmonic Nanoparticles in Complex Matrix Another commonly used molecular functionalization of plasmonic nanoparticles is the use of

polyethylene glycol (PEG)-stabilized nanoparticles for a variety of applications.84–86 PEG-based stabilization

offers two major advantages, especially in vivo where steric repulsions inhibit colloidal aggregation and

imbue resistance to protein adsorption and uptake by the mononuclear phagocytic system.87 In many

biomedical or therapeutic applications, where nanoparticles need to be dispersed in highly complex media,

Ag and AuNPs are surface-functionalized with thiolated-PEG ligands through strong metal-S bonds to

stabilize and exploit the plasmonic properties of Ag and AuNPs more effectively.88 In 2013, Kang et al. used

PEG stabilization when performing plasmon-tunable Raman/fluorescence imaging spectroscopy with

anticancer drug-loaded AuNPs.89 When doxorubicin was attached to peptide-functionalized PEGylated

AuNPs via peptide-drug conjugation, the SERS spectrum of the doxorubicin could be detected while its

fluorescence was quenched, indicating the short distance between the drug and AuNPs. But upon the

release, a reduction in Raman enhancement was observed, and the fluorescence signal became apparent

(Figure 4a). This selective “on”/“off” behavior took place inside the lysosomes of a malignant epithelial cell,

where high colloidal stability is required. In another example, Cheng et al. developed a SERS-based

immunoassay for prostate cancer;90 two types of prostate specific antigens (PSAs) were simultaneously

detected, since the ratio of the two antigens is crucial for accurate analysis and diagnosis. Two different

SERS nano-tag molecules were adsorbed onto a single AuNP, which were further functionalized with

thiolate-PEG-COOH ligands. The carboxyl groups of these ligands were conjugated with antibodies for the

two antigens (Figure 4b). By measuring the SERS spectra of the two SERS tags, the quantification of each

antigen was achieved in the SERS-based assay, opening a strong potential for more accurate diagnosis of

prostate cancer. AuNPs played a crucial role in this SERS-based assay, and PEG on the surface enabled

the detection of proteins in clinical samples by maintaining their plasmonic properties and supporting

antibody conjugation.

Figure 4. (a) (i) Raman and SERS spectra of DOX molecules in four different conditions. Normal Raman

spectrum of DOX molecules (1), SERS spectrum of DOX when bound to AuNPs at pH 7.4 (2)/pH 5.4 (3).

SERS spectra of AuNPs without close DOX (4). (ii) Schematic diagram of DOX chemical structure (1) and

DOX conjugated, PEG-functionalized AuNPs at pH 7.4 (2) pH 5.0 (3). (iii) Fluorescence spectra of Free

DOX without AuNPs (1), bound to AuNPs (2), when DOX molecules are released from AuNPs (3), and

AuNPs without DOX (4). Reprinted with permission from ref 89. Copyright 2013 American Chemical Society.

(b) Schematic diagram describing the process of fabricating PEG-functionalized AuNPs. One type of the

particles is conjugated with malachite green isothiocyanate (MGITC) and f-PSA antibody, and the other

type conjugated with X-rhodamine-5-(and-6)-isothiocyanate (XRITC) and c-PSA antibody. Reprinted with

permission from ref 90. Copyright 2017 American Chemical Society.

2.4. Biomolecular Ligands-Stabilized Plasmonic Nanoparticles

The physiological fate of plasmonic nanoparticles is of great interest to many scientists since the

behavior of nanoparticles are related not only to their functionalities, but also to their toxicity and induction

of inflammation.91–93 Because the usual access for many intended biomedical applications, such as disease

diagnosis and drug delivery are intravenous, the interaction and the stabilization of the nanoparticle with

blood plasma is very important. Achieving nanoparticles that will be colloidally stable, biocompatible, and

functional in blood is difficult when using stabilizing agents such as citrate, CTAB, or PVP, which have

insufficient stability in high ionic strength and can be cytotoxic.94,95 Thus, proteins hold great promise as

coating agents for biomedical applications.96 Their high molecular weights, charges, complex but well-

defined structures, retained multifunctional-chemical groups, and high affinity for metal surfaces have

attracted many researchers who seek stable metal nanoparticle coating materials for use in biological

matrices. The most widely used protein in this context is bovine serum albumin (BSA). However, due to the

structural complexities and difficulties in controlling BSA during functionalization, this protein has been used

mainly as a secondary stabilizing agent to enhance the functionalities of nanoparticles, and a number of

researchers debate the stabilizing quality of albumin on colloidal AuNPs.97 However, many reports have

shown use of BSA as a coating material for plasmonic nanoparticles and the enhanced stability of BSA-

coated AuNPs. One such study in 2012 by Khullar et al., demonstrated synthesis of BSA-conjugated AuNPs

for biomedical applications.98 The BSA was unfolded for efficient coating of AuNPs via interactions with

anionic, cationic and zwitterionic surfactants and temperature control during synthesis. The unfolded state

enabled the reduction of gold ions due to the presence of several reducing amino acids such as cysteine.

Additionally, BSA can act as an excellent capping/stabilization agent for growing Au nucleation centers,

which is rather hard to achieve with folded BSA structures. The BSA-coated AuNPs showed colloidal

stability in various pHs and low cytotoxicity with no hemolytic response, a stark contrast to traditional

surfactant-capped AuNPs. The decreased hemolytic response indicates the nearly complete passivation of

crystal planes of AuNPs, which is also hard to achieve with a common surfactant coating.

As mentioned above, anisotropic AuNPs and their resultant plasmonic properties are of great interest

but the cationic surfactant CTAB usually plays a crucial role in synthesis and stabilization. However, CTAB

displays significant cytotoxicity, and must be removed before use in biomedical applications. In one

example, Tebbe, et al. nearly completely removed CTAB from Au nanorods and replaced it with BSA

(Figure 5a).99 Fast and efficient ligand exchange for BSA showed no LSPR shift, and the protein-coated

Au nanorods showed higher colloidal stability in high ionic strength conditions such as phosphate-buffered

saline or cell culture medium. The particles were also able to be lyophilized to powder and re-dispersed in

media with the same optical and colloidal properties as before lyophilization, due to the robust protein

coating. In another study, BSA-coated small AuNPs/AgNPs formed clusters on larger AuNPs, inducing

colloidal stability as well as satellite-to-satellite and core-to-satellite plasmon coupling.100 This was

accomplished when Holler et al. synthesized citrate- or tannic acid-stabilized AuNPs/AgNPs (5-21 nm) and

then coated those particles with BSA via ligand-exchange. The BSA-coated particles were adsorbed on the

surface of larger, citrate-coated AuNPs (84 nm) randomly, leading to a disordered distribution of small

particles on the larger cores (Figure 5b). The authors reasoned that the high colloidal stability of the protein-

coated satellite nanoparticles enabled the highly concentrated particle suspensions, which is essential in

core-satellite cluster formation. The weakly bound initial citrate ligands provided sufficient colloidal stability,

and they could be easily removed during the adsorption of BSA-coated satellite nanoparticles. Thus, BSA

played an important role in small/large particle stabilization in colloidal states and as a soft spacer to achieve

plasmon coupling between small/large plasmonic particles. Of course, BSA is not the only protein that acts

as a ligand for plasmonic nanoparticle surface stabilization. In 2015, Chapman et al. designed a lateral flow

bioassay for phospholipase A2, a potential biomarker for diagnosing diseases such as pancreatitis and

prostate cancer.101 For the assay, polystreptavidin-coated AuNPs were synthesized, forming an inter-

particle aggregation via PEG-biotin linkers, which are released upon the enzymatic activity of phospholipase

A2 to liposome. These polystreptavidin-coated AuNPs, aggregated by streptavidin-biotin affinity, can induce

interparticle surface plasmon coupling, which can be read with the naked eyes when loaded on a

nitrocellulose membrane lateral flow strip (Figure 5c).

Figure 5. Protein-stabilized plasmonic nanoparticles. (a) Surfactant-free, protein-coated colloidally stable

Au nanorods. BSA-coated AuNPs with three different aspect ratios dispersed in DMEM + 10% newborn

bovine calf serum were able to be dispersed in the same media after freeze-dried and were stable in high

concentration of Au nanorods of 20 mg/mL. Reprinted with permission from ref 99. Copyright 2015

American Chemical Society. (b) Citrate-stabilized small AuNPs are first coated with BSA, and then

adsorbed onto large citrate-stabilized AuNPs. Reprinted with permission from ref 100. Copyright 2016

American Chemical Society. (c) Schematic illustration of the later flow assay for phospholipase A2 (PLA2).

Liposomes containing biotyinlated PEG linkers were incubated with phospholipase A2, cleaved, and the

PEG linkers were released. The polystreptavidin-coated AuNPs were added to the mixture, and the solution

was transferred to a lateral flow strip. The green line preprinted with streptavidin on the strip turned to red

due to the biotin-streptavidin affinity and forming multivalent AuNPs networks. Reprinted with permission

from ref 101. Copyright 2015 American Chemical Society.

2.5. DNA and Other Highly Functional Ligands: Stabilization and Assembly

As shown in the previous section, biopolymers possess high potential as stabilizing agents for colloidal

plasmonic nanoparticles based on their biocompatibility that will allow various applications in biological

diagnostics or sensing. DNA is another biomolecule which has been widely used as a ligand to stabilize

nanoparticles in different colloidal conditions. The strong negative charge on the phosphate backbone in

DNA contributes high electrostatic repulsion forces to the nanoparticles, and the DNA layer also provides

steric stabilization. In most cases, DNA or oligonucleotides are functionalized with alkylthiols at the ends to

covalently bind to the metal surfaces.102 In previous research, DNA-stabilized AuNPs showed noticeable

colloidal stability in high ionic strength103 or in complex media such as sea water.104 Beyond thiol-driven

modification, adsorption-driven stabilization has also been reported using DNA, especially for the synthesis

and stabilization of fluorescent DNA-Ag nanoclusters.105–107 Thus, DNA surface-modified nanoparticles

have attracted many researchers’ attention due to their great potential in analytical, materials, and medical

applications.108 While DNA itself has great stabilizing potential when covalently attached to plasmonic

nanoparticles, the utility of nanoparticle-bound DNA hybridization/assembly presents an exciting range of

potential applications for these functionalized nanoparticles, taking advantage of programmable DNA base

pairing interactions.108,109 In 1996, Mirkin et al. first demonstrated the use of DNA-functionalized AuNPs for

the detection of target DNAs via programmable assembly of two complementary DNA strands.110 Since this

report, DNA-nanoparticle studies in various fields have resulted in a number of interesting applications in

SERS, optics, energy transfer, etc.108 Before discussing how DNA-coated AuNPs assemble, the basic

synthetic scheme and related properties will be reviewed here. For DNA conjugation on the nanoparticle

surface, single-stranded DNA oligonucleotides are most frequently used. Since pre-synthesized AuNPs

and DNA strands are both negatively charged, the synthetic solution requires a certain ionic strength to

overcome the electrostatic repulsion forces between the DNA and AuNP surfaces. At this step, a colloidal

stability problem arises due to the aggregation of AuNPs because of charge screening high ionic strength

media. Thus, fine control of salt concentration is necessary for successful DNA coating of AuNPs without

aggregation. To bypass this synthetic difficulty, other methods have been proposed,111,112 but this salt-

mediated DNA functionalization method is still the most widely used.113 Typically, once 5’-thiolated DNA

strands have been layered on the AuNPs, particles can be made colloidally stable via steric stabilization

and electrostatic repulsions of the DNA. Due to these dual stabilization effects, DNA-coated AuNPs showed

enhanced colloidal stability in high ionic strength solutions compared to citrate-coated AuNPs.103

The single-stranded DNA functionalized plasmonic nanoparticles enable various application

opportunities to utilize the plasmonic properties of the core metal with DNA-driven assembly. In the last

decade, plasmonic has been adopting classical antennae concepts from the field of optics. Within optics, it

is known that carefully fabricated metal nanoparticles with sophisticated assembly of two or more

nanoparticles have great potential to decrease luminescence life times and maintain a high emission

quantum yield.114 In 2011, Busson et al. demonstrated AuNP dimers linked by a single DNA double strand,

producing a substantial scattering cross-section and plasmon coupling.115 This research revealed the

important relationship between colloidal stability during the DNA conjugation on the AuNPs and the resulting

plasmon coupling effect from particular geometric conformations. Electrophoresis has been proven to be

an efficient ways to purify AuNPs conjugated with a known number of DNA single strands,116 but in typical

electrophoretic purifications, the diameter of the AuNP generally has to be small enough (less than 20 nm)

and the grafted DNA strands need to be between 90 and 100 bases long.117,118 To satisfy the need in the

application of AuNP groupings where larger diameter AuNPs and smaller interparticle gaps are

required,119,120 researchers have studied efficient purification and synthesis methods to generate AuNPs

as large as 36 nm in diameter with grafted DNA strands as short as 19 nm. To maintain colloidal stability

of these AuNPs, they were first coated with bis(p-sulfonatophenyl)phenylphosphine. This labile stabilizing

agents shows weaker affinity to the gold surface than DNA but stronger binding affinity than the citrate

surfactant, so it can not only achieve colloidal stability, but also be displaced by DNA strands in the presence

of an ionic strength sufficient for charge screening (which would be too harsh an environment for citrate-

coated AuNPs). The particles were further stabilized by passivation via thiolated-PEG oligomers before

electrophoretic purification, which relies on surface charges and a different size of DNA-AuNP conjugates

to induce separated bands. During the synthesis, thiolated-DNA strands on the AuNPs were lengthened by

hybridization to overcome mid-purification aggregation and to achieve a good separation of the products.

After removal of the lengthening DNA strands and further purification, 36-nm–diameter AuNPs conjugated

to a single DNA strand of 10 nm in length were obtained. Upon assembly, the inter-particle gaps measured

were in good agreement with the lengths of the linker DNAs between the particles. A few years later, Bidault

et al. fabricated DNA-templated colloidal gold dimer nanostructures that behave as single-photon emitters

with short lifetimes and maintained quantum yield.121 The same synthetic scheme with slight modification

was applied as in the work from Busson et al., and the final products were 80-nm-diameter AuNP-based

dimers linked by a single DNA double strand and one dye molecule in the center (Figure 6a). The

interparticle gap was found to be 14 nm, in excellent agreement with the length of the DNA molecule. This

dimer system achieved up to an estimated 70% quantum yield and average luminescence lifetimes on the

order of 50 ps, proving the benefits of the programmed DNA-assembly designed from stabilized

nanoparticles. In another study, CdSe/ZnS quantum dots were assembled around the AuNPs via the DNA

assembly method.122 The quantum dots and AuNPs were functionalized with aminated and thiolated single-

stranded DNA, respectively. Hetero-nanoclusters of AuNPs-DNA-quantum dots with satellite-like structures

were fabricated (Figure 6b). All spectroscopic experiments were performed in high ionic strength buffer,

and no aggregation or loss of plasmonic properties of AuNPs were reported, indicating the high colloidal

stability of AuNPs/quantum dot nanoclusters. In this study, the length of DNA ligands played an important

role in controlling and tuning the plasmon-exciton interaction and the optical behavior, including

photoluminescence quenching and enhancement. These colloidal nanoclusters represent plasmon-

assisted light harvesting systems to transfer light collected by quantum dots to the plasmonic reaction

center of AuNP cores. DNA-driven dimer AuNP formation can be found in other types of applications as

well. For example, in phototherapy, the engineering of multifunctional nanoparticle platforms for

simultaneous imaging and therapeutic treatment holds great promising tools for enhancing current imaging

techniques such as MRI, X-ray, or CT.123,124 In 2016, Sun et al. synthesized nanorod dimers further

functionalized by the chlorin e6-attached upconversion nanoparticles, NaGdF4.125 In this research, a

nanorod dimer functioned as a photothermal therapeutic agent, and NaGdF4 acted as a photodynamic

therapeutic agent. DNA played a crucial role in forming the gold nanorod dimer. With careful conjugation

design, two complementary oligonucleotides were selectively bound to sides of two nanorods respectively,

and the nanorod dimers formed via DNA hybridization (Figure 6c). Even though the entire cluster platform

was stabilized by a layer of dense polymer, the DNA-driven dimer was able to maintain its geometric

conformation throughout the experiment due to the precise gap formed by DNA hybridization. The Au

nanorod dimer nanoclusters showed higher photothermal conversion efficiency and photostability than the

Au nanorod monomer clusters due to plasmonic coupling and formation of an electromagnetic “hot spot”,

and they also showed fine performance for in vivo tumor therapy and as imaging agents.

Nanoparticle assembly using DNA has expanded to three-dimensional arrangements, known as

crystallization of nanoparticles, whereby a single nanoparticle itself can be viewed as analogous to an

atom.126,127 DNA-mediated assembly can be an ideal tool for the systematic crystallization of nanoparticles,

since different DNA designs and interactions among DNA-functionalized nanoparticles can be tuned to

produce a number of unique crystalline structures.128,129 The challenge in this work is to control and fabricate

a structure with a desired crystal symmetry and lattice spacing similar to that of atomic crystallizations.

Among the different types of nanoparticles, plasmonic nanoparticles are surely promising candidates since

the optical properties of the plasmonic nanoparticle can evolve via plasmonic coupling within periodic layers

of nanoparticle structures. For three-dimensional DNA assembly, energy-related stability is key in

synthetically programmable colloidal crystallization. After appropriate single-stranded DNA ligands have

been functionalized on AuNPs, sequence-programmed linker oligonucleotides are introduced into the

colloidal system. From the energy minimization standpoint, each AuNP adopts the conformation that will

maximize the number of neighboring particles via DNA ligand hybridization. A well-defined and close-

packed crystal can be formed by slow-cooling methods whereby the crystal formation is driven by

thermodynamic forces, not kinetic energy. The balance between entropic and enthalpic energy must be

properly managed by temperature, size of particles, and length and binding strength of DNA.130 Recently,

Auyeung et al. explored the packing behaviors of DNA-AuNP superlattices and discovered that, in the

described system, the rhombic dodecahedron structure is the most thermodynamically favorable form for a

range of different particle sizes (Figure 6d).131 The conclusion was that, similar to the formation of atomic

crystallization, DNA assembly-driven AuNP crystallizations also have the structure of Wulff polyhedra in

which the crystal form of minimizing surface energy can be favorably shaped. This work is an interesting

example of how DNA-functionalization of plasmonic nanoparticles drives assembly and gap control due to

the strong steric hindrance and electrostatic repulsion from DNA ligands, using control of DNA length to

determine the physical gaps between adjacent nanoparticles.

Figure 6. DNA-functionalized AuNPs. (a) Schematic representation of a AuNPs dimer linked by long double

stranded DNA featuring a single ATTO647N fluorescent molecule (left). Single-emitter lifetime spectra of

the 80 nm AuNP dimer featuring the single dye. Experimental data (red) and estimated value from the

instrument response function (IRF, black dots). The life time was estimated below 10 ps (center). Rotational

averaged fluorescence enhancement factors for three different dimer antennas in solution, extracted from

the measurement and Mie theory calculations. (right). Reprinted with permission from ref 121. Copyright

2016 American Chemical Society. (b) Schematic illustration of nanoclusters of AuNPs linked with CdSe/ZnS

quantum dots via DNA strands and the spectra of photoluminescence when nanoclusters were excited by

530 nm laser (circle dots) and 440 nm laser (square dots). Reprinted with permission from ref 122. Copyright

2015 American Chemical Society. (c) Schematic illustration of DNA-based Au nanorod dimers and chlorin

e6-attached upconversion NP assemblies for multifunctional biotherapy applications and TEM images of

the nanostructures. Scale bars represent 50 nm. Reprinted with permission from ref 125. Copyright 2015

John Wiley and Sons. (d) Illustration of the molecular dynamics simulation of a colloid model predicted a

rhombic dodecahedron equilibrium crystal structure for the DNA–gold nanoparticle system (left).

Experimental result of SEM image of microcrystal with well-defined facets. Small AuNPs are 20 nm (right).

Scale bar is 1 m. Reprinted by permission from Springer Customer Service Centre GmbH: Springer Nature,

ref 131. Copyright 2014.

Along with DNA, there are other ligands that are able to induce the assembly of plasmonic nanoparticles

for a desirable research platform with appropriate particle stabilization. Cheng et al. focused on the weak

points of ligand-dependent assembly in previous research where controllable aggregates of AuNPs in vivo

are not very viable.132 In such environments, light-triggered assembly has great potential since light

application can be localized, and assembly of nanoparticles at a specific site is feasible. In one example,

AuNPs were synthesized and functionalized with a PEG ligand having diazirine terminal groups. The

diazirine groups were triggered by 405 nm laser irradiation and transformed to carbenes, which eventually

formed covalent bonds with ligands of nanoparticles nearby (Figure 7a). The covalently crosslinked AuNPs

showed the surface plasmon resonance peak at longer wavelength due to their strong coupling behavior.

The controlled redshift in the LSPR was achieved because the colloidal stability of individual AuNPs was

maintained upon aggregation due to the existing PEG ligands on the gold surfaces. This photo-initiated

assembly was successful in in vivo experimentation, proving the potential for an effective photothermal

treatment platform for malignant tumors. Amphiphilic ligands can also be utilized to induce assembled

plasmonic nanostructures. In 2015, Song et al. fabricated biocompatible and degradable plasmonic vesicles

of assembled Au nanorods coated with hydrophilic PEG ligands and hydrophobic poly(lactic-co-glycolic

acid) (PLGA) ligands (Figure 7b).133 This work demonstrated that each Au nanorod can be stabilized by a

PEG coating, and the entire assembly can also be stabilized by extended PEG on the outside and inside

of the vesicles. The formation of the vesicles was derived from an oil-in-water emulsion. The evaporations

of oil and the resulting assembly showed significant red-shifts in the LSPR (from around 790 nm to longer

than 1000 nm) due to strong plasmonic coupling. The photothermal conversion efficiency of the assembled

vesicles reached 51%, which is two-fold higher than the 23% of uncoupled Au nanorods.

Not ligand-driven, but a solvent-evaporation-driven distinct orientational packing assembly was also

reported in 2016.134 Bipyramidal AuNPs were synthesized and functionalized with hydrophobic, thiolated

polystyrene via a ligand exchange method. The polystyrene-coated bipyramidal AuNPs were dispersed in

chloroform and drop-cast onto the surface of a convex-shaped water droplet on a silicon wafer. The

evaporation of chloroform and water droplets led the formation of two-dimensional nanoparticle liquid

crystalline superstructures. The fabricated superstructures showed collective plasmonic coupling and

different Raman enhancement factors depending on the orientational packing orders (Figure 7c).

Interestingly, the length of the polystyrene ligands proved crucial in determining the nanoparticle packing

behavior. The van der Waals forces among AuNPs and the steric hindrance force of the ligands needed to

be balanced correctly to achieve the desired assembly. For example, if the length of the ligand was too

long, the dominant force becomes steric hindrance and ordered lattices fail.

As described above, many research efforts have worked to exploit the plasmonic properties of Au/Ag

nanoparticles via systematic assembly, but sometimes assembly-driven functionalization of the

nanoparticles can be burdensome and induce adverse effects on the performance.135,136 A new

methodology was envisioned by Kundu in 2015 to overcome these limitations, whereby a medium

consisting of light-switchable molecules drives colloidal stability in AuNPs functionalized with a pH-sensitive

ligand, resulting in reversible aggregation and dispersion.137 In this system, AuNPs were functionalized with

11-mercaptoundecanoic acid, suspended in methanolic solution, and stabilized with small amount of HCl.

A photo-switchable molecule was synthesized and dispersed in the same medium and competed for

protons with the -COOH group of 11-mercaptoundecanoic acid (Figure 7d). The suspension medium was

comprised of this photoresponsive molecule and was capable of releasing or capturing protons. Due to the

pH-sensitive nature of the ligand on the AuNPs, their colloidal stability was directly affected by the behavior

of the photo-responsive molecules in the medium. The AuNPs were stable only under continuous light

exposure, and without the light they aggregated and precipitated. However, the existing ligands on the

aggregated AuNPs could be disassembled again, and the colloidal stability was restored. Depending on

the state of the AuNPs, different LSPRs were achieved. This reversible assembly-disassembly system was

conducted over 100 cycles with no appreciable change found from the particles, proving the solid stability

and functionality of this novel approach to controlled nanoparticle assembly.

Figure 7. Ligand-assisted assembly of various plasmonic NPs. (a) Schematic illustration of light-induced

AuNPs assembly functionalized with PEG-diazirine ligands. Reprinted with permission from ref 132.

Copyright 2017 John Wiley and Sons. (b) The illustrations of AuNRs functionalized with PEG and PLGA

ligands and assembled small vesicle. Reprinted with permission from ref 133. Copyright 2015 John Wiley

and Sons. (c) SEM images and 3D illustrations of bypiramidal AuNPs formed on Si waters with different

orientational packing orders. The scale bars indicate 100 nm. Reprinted with permission from ref 134.

Copyright 2016 American Chemical Society. (d) Schematic representation of light-controlled reversible

assembly of AuNPs. The solution contains light responsive molecule, protonated merocyanine. The

particles were functionalized with ligands terminated with carboxyl groups. Blue light irradiation increases

the acidity of the solution, triggering the disassembly of the AuNPs assembled by hydrogen bond. The

solution in ambient conditions or the dark the acidity drops, and particles reassembles. Reprinted by

permission from Springer Customer Service Centre GmbH: Springer Nature, ref 137. Copyright 2015.

3. Plasmonic Nanoparticles with Shell-Coating

Ligand- or adsorbate-stabilized nanoparticles can be formed when ligand molecules are chemically or

physically bound to the surfaces of the nanoparticle core. Often, the nomenclature of ligand or shell is used

without distinction to describe the coating materials around the nanoparticle core. In this review,

nanoparticles shells are defined as chemically and physically robust structures homogeneously covering

the nanoparticle core. Shells are not easily removed or exchangeable as ligands are, even though no

covalent links, such as Au-S bonds, may exist between cores and shells. Furthermore, shells provide

another chemical environment not only to stabilize the cores but also to add desired functionality on the

designed nanoparticles. In most cases, the shell is formed by strong chemical interactions or bonds among

the shell chemical compositional units. In many cases, ligand-stabilized nanoparticles are synthesized first

and then shell structures are formed via ligand exchange method. The thickness of the shell sometimes

can determine the distance between and plasmonic cores and the outer environments, and this distance

gap can be used to tune the interactions between plasmonic cores and other molecules, such as fluorescent

dyes. This section will discuss recent trends and applications in plasmonic nanoparticles, focusing on three

major types of shells: silica, organic polymer, and metal.

3.1. Silica Shell-Stabilized Plasmonic Nanoparticles

Silica nanoparticles themselves have high colloidal stability and dispersibility, and these properties are

conserved when silica is used for coating materials.138,139 Silica is optically transparent in the visible region

of the spectrum, and due to its chemical inertness, it is able to coat the core particle’s surface without

sacrificing the ability to perform reduction-oxidation reactions at the surface of the plasmonic cores.140 Silica

is known to be biocompatible; therefore, the cytotoxicity of the core nanoparticle is often reduced with the

addition of a silica shell. Silica shell surface properties can easily be modified due to reactive silanol groups

on the surface of the silica. These silanol groups also contribute to negative zeta potentials in aqueous

solutions and thus, good colloidal stability due to electrostatic repulsion. Silica-shelled nanoparticle stability

can be achieved through a relatively large pH range via surface modifications with species such as PEG-

silane to add steric hindrance.138 Coating plasmonic nanoparticles, such as AuNPs, with a nonporous silica

coating improves their use in various applications such as for photothermal treatment,141 photoacoustic

imaging142–144 and for DNA quantification.145

Besides imbuing colloidal stability with a silica shell, if interaction of solution-phase analytes with the

plasmonic nanoparticle surface is desired, silica shells can be synthesized to have sufficient porosity to

allow analyte transfer to the plasmonic core. A porous silica coating still provides protective benefits and

also allows access of the external media to the core structure. Since the early 1990s when

micro/mesoporous structures of silicate (MCM-41) were observed with an electron microscope,146

mesoporous silica nanoparticles have been synthesized in various formats and applied in many different

fields.147 Pore structures can play an important role in plasmonic catalysis or sensing, where the reactants

or analytes must be placed within several nanometers of the plasmonic core.148,149 Effective roles for

mesoporous silica have been frequently reported, especially for colloidal AuNPs. Zhang et al. studied the

effects of porosity on the catalytic activity of silica-coated AuNPs.150 The permeation levels of the reactant,

4-nitrophenol, were tuned by the degrees of porosity within silica shells via etching time control. The

fabricated silica-coated AuNPs showed more improved activity as catalysts for the reduction of 4-

nitrophenol to 4-aminophenol as the porosity increased. The unetched silica shells were impenetrable for

the reactant molecules to the Au cores, resulting in no catalytic conversion. Even though the changed shell

morphologies increased the exposure level of the AuNPs to the outside media, the silica shell still

maintained the stability of the AuNPs during the catalytic activity. Furthermore, the stability was proven to

provide consistent catalytic behavior after 12 continuous cycles, with a nearly 100% conversion rate, while

bare AuNPs underwent coalescence and aggregation after the 1st catalytic cycle. This porosity-dependent

plasmonic performance of silica shell-stabilized AuNPs is also relevant for solution-phase SERS sensing.

For example, Gao et al. investigated how the size of the pores and the size of various analyte molecules

impacted SERS detection of those analytes.148 In 2015, Sanz-Oritz et al. synthesized novel mesoporous

silica-coated branched AuNPs for colloidal SERS sensing.151 The authors optimized a synthesis method to

obtain pore structures in the silica shells that are oriented radially from the core AuNPs to the outer bulk in

such a way that analytes entering the pores will be adsorbed or placed very near to the surface of the

AuNPs. The authors also improved the colloidal stability of the nanoparticles by heat treating dried

nanoparticles; a lower temperature than is conventionally used was employed in this study so as to cause

minimal deformation of the shape of the AuNP core. The silica shell in this study had three major roles:

shells for AuNP cores, passageway for the analytes, and as structural templates for branched gold growth.

To improve SERS performance, the Au nanorods, preformed within radial silica shells, were overgrown and

transformed to branched structures, during the Au reduction catalyzed by Au nanorod cores through the

silica shell pores (Figure 8a). The further reduction of the Au overgrowth and the formation of final

structures occurred in the silica shell, thus the aggregation was prevented, and controlled shape change

was achieved. In application, the limit of detection for crystal violet in ethanol was lowered by four orders

of magnitude when branched AuNPs were formed within the silica shells, compared to the silica-coated

spherical AuNPs. Due to the structural changes, where the sharp Au tip features render high electric field

enhancement, mesoporous silica-coated branched AuNPs showed enhanced Raman signal intensity

suitable for colloidal SERS measurements.

In addition, the thickness of the silica shell can be well controlled and applied in the study of plasmonic

nanoparticle properties. Among the many different morphologies of AuNPs, anisotropic AuNPs, especially

Au nanorods, have been heavily researched in recent years due to their distinct optical properties based

on longitudinal plasmon resonances.42 Abadeer et al. used mesoporous silica-coated Au nanorods to

elucidate the relationship between gold-fluorophore distance and plasmonic-fluorescence enhancement

and quenching.152 Silica shells with a range of thicknesses from 11-26 nm were coated onto a series of Au

nanorods that had a plasmonic extinction maximum ranging from 530-850 nm. An IR-dye was covalently

attached onto the silica shells, with the distances between the dyes and Au nanorod cores tuned by the

range of shell thicknesses used. Steady-state fluorescence measurements were carried out with Au

nanorods coated with dye-conjugated silica shells suspended in methanol. During the measurements, the

plasmonic properties of Au nanorods and the distance between the dyes and the cores were maintained

by the stable silica shell, to reveal the fluorescence enhancement/quenching behaviors that resulted from

the plasmon-generate electromagnetic fields. As was shown in this study, the thickness of inorganic shells

can be a crucial factor for the application of plasmonic nanoparticles, and silica shell thickness can be tuned

without detracting from the stability of the Au cores. Recently the thickness of the silica shell was tuned

down to around 2 nm with a large-scale synthesis, producing about 190 mg of the nanoparticles.153

The enhanced thermal stability of shell-coated AuNPs is critical in application spaces such as

hyperthermic cancer treatment therapy, photoacoustic molecular imaging, and image-guided drug

delivery.154–156 With appropriate excitation, plasmonic nanoparticles can produce heat following targeted

delivery to particular in vivo destinations. Thus, the thermal stability of the plasmonic nanoparticles in a

complex medium is critical. Several researchers have addressed this issue and shown that AuNPs have

much lower melting temperatures than their bulk counterpart, depending on NP size and structure.157,158

Silica coatings have been shown to increase the thermal stability of AuNP cores. Previous studies

demonstrated that silica-coated AuNPs possess higher photothermal stability than CTAB or PEG-coated

AuNPs, under the influence of nanosecond higher-energy laser pulses.159,160 In some studies, this silica

shell-induced enhanced photothermal stability has been used to tune the LSPR and morphology of the

AuNPs while they reside within the silica shell via oxidative etching or femtosecond laser pulse excitation;

this method allows modification of aspect ratio of AuNPs without irreversible dissolution or aggregation. For

another in vivo application, Liu et al. designed and fabricated silica-coated AuNPs conjugated to an antibody

for the chemokine receptor known as CXCR4.163 The prepared nanoparticles were loaded into human-

induced pluripotent stem cells, which serve as the delivery agent, since they are capable of targeting tumor

cells (Figure 8b). The performance of this platform was evaluated via photoacoustic tomography, two-

photon luminescence for tracking the nanoparticle localization in the tumor tissue, and by in vivo

photothermal therapy toward gastric cancer cells in mice. Plasmon-related evaluations showed a prolonged

retention time, good biodistribution characteristics, and therapeutic efficacy against the growth of tumors in

mice, proving that the colloidal stability of the injected particles was maintained during the journey. In

another recent biomedical application of silica-coated plasmonic nanomaterials, Wang et al. synthesized a

single nanocomposite capable of photothermal heating, in vivo tracing, and tumor-targeting drug delivery.164

In this nanocomposite, Au nanorods were first coated by mesoporous silica, and gadolinium was then

loaded in and onto the pores/surfaces of the silica shells for high sensitivity bio-imaging by enabling the

simultaneous use of CT and MRI (Figure 8c). Further, the anti-cancer drug, doxorubicin, was loaded on

the outside of the silica shell via electrostatic interactions, and finally, the composite was layered with

hyaluronic acid for effective tumor targeting. The gadolinium doping onto the silica helped to avoid direct

modification on Au cores, enhance the gadolinium loading capacity, and prevent undesirable toxicity from

gadolinium ion leakage.165–167 Laser irradiation of the injected nanocomposite in vivo increased the

penetrability of the nanocomposites into the inner regions of the tumors and induced disruption of the

electrostatic interaction between the drug and the silica shell via the plasmonic-heating effect from the Au

nanorod cores. Overall, the silica shells in this nanocomposite fulfilled multiple roles while protecting the

colloidal stability of the inner Au cores.

Figure 8. (a) Visualized 3D reconstructed TEM images of silica-coated Au nanorod (left) and Au branched

(right). The silica shells form the mesoporous radial channels. Reprinted with permission from ref 151.

Copyright 2015 American Chemical Society. (b) Schematic illustration of formation of silica-coated, CXCR4-

functionalized Au nanorods. The particles are loaded in human-induced pluripotent stem cells and injected

in vivo for photothermal tumor target therapy. Reprinted with permission from ref 163. Copyright 2016

American Chemical Society. (c) TEM image of silica -coated, gadolinium-hybridized Au nanorods (left).

High-angle annular dark-field scanning TEM elemental mapping (HAADF-STEM) images of silica, oxygen,

gold, and gadolinium localization in the silica-coated, gadolinium-hybridized Au nanorods (right). Reprinted

with permission from ref 164. Copyright 2016 John Wiley and Sons.

3.2. Plasmonic Nanoparticles with Organic-Shell Coating

Due to the variety of chemical and physical characteristics of the organic polymers, they are applied to

fabricate advanced and stable plasmonic nanocomposites for a variety of application areas.140 Grafting the

metal nanoparticle surfaces with polymers can be categorized broadly into three groups, differentiated by

how the polymers graft onto the metal surfaces during preparation: 1) in situ grafting methods, 2) “grafting-

to” methods, and 3) “grafting-from” methods.168 For the in situ grafting method, either already-prepared

polymers or monomers are introduced to the metal salt reduction solutions and serve as a shape-guiding

and stabilizing agent. PVP, which has been discussed in previous sections is one of the most widely used

polymers for Ag and AuNP modification. Other polymers such as cationic polyelectrolytes can be introduced

during synthesis to stabilize the final products via in situ grafting. Poly(2-(methacryloyloxy) ethyl

phosphorylcholine), poly(diallyl dimethylammonium) chloride, and poly(2-hydroxy-3-methacryloxy-

propyltrimethyl ammonium chloride) have all been commonly used in situ to produce Au or AgNPs coated

by these charged polymers.169,170 Sulfur-terminated polymers can also be applied during the metal salt

reduction steps. McMormick and coworkers in 2002 mixed Au salts with various polymers bearing

dithioester end groups and reduced the Au salts to AuNPs.171 The reduction of both the metal salt to metal

nanoparticles and the dithioester groups of the polymers to thiol groups occurred simultaneously, leading

to polymer-stabilized AuNPs. The high affinity of the thiol for Au surfaces promoted facile surface

functionalization in situ. Thiol-functionalized polymers such as poly(N-isopropylacrylamide) (pNIPAM) and

polystyrene (PS) can be prepared first and then mixed with Au precursors in a solution.172,173

The “grafting-to” method is sometimes used interchangeably with the in situ grafting synthesis method,

but in this review the meaning is limited to methods where AuNPs or AgNPs are first prepared with loosely

bound capping agents such as CTAB or citrate, and then those surfactants are replaced with polymers via

ligand exchange. During the exchange, the polymers can bind to the metal surface covalently or via

chemi/physisorption, depending on the properties of the polymers. In contrast, the “grafting-from” method

first synthesizes metal nanoparticles functionalized with monomers, and polymerization reactions are done

on the nanoparticle surface. Fabrication schemes can vary depending on the application intended for the

nanoparticles. Many of the previously-mentioned polymers, such as PS or pNIPAM, can coat the metal

cores by either the grafting-to or grating-from method.174,175 Particular polymers are chosen as the shell

materials based on the chemical or physical characteristics that they will impart onto the plasmonic core.

For example, creating a nanoparticle shell from conducting polymers such as polyaniline (PANI),

polypyrrole (PPy), or polythiophene (PTh), has great potential for electrical or electrochemical applications

based on their conductivity as well high stability.176 In 2016, Lu et al. synthesized PANI-coated colloidal Au

nanocrystals by seed-mediated growth wherein aniline monomers underwent oxidative polymerization to

form a polymer shell around the Au cores with the help of the surfactant, SDS.177 It was expected that PANI

would induce effective plasmonic switching with encapsulated Au cores and function to control the spaces

between Au nanocrystals to avoid aggregation. The synthesized particles were placed in an electrochemical

set up, and applied potential could drive PANI in the nanoparticle shell from half-oxidized to fully reduced

states, resulting in a change in the refractive index and significant shifts in the longitudinal and transverse

LSPR of the Au nanoparticle cores (Figure 9a). The plasmon shifts of the Au cores could be reversibly

controlled via the PANI shells, and the particles showed remarkable stability over 200 cycles of reversible

plasmonic shifts.

Thermal-responsive behavior is another characteristic of some polymers that can be applied in

plasmon-related applications. pNIPAM has a property of temperature-responsive coil-to-globule transition

in which the polymer above the lower critical solution temperature (LCST) undergoes dehydration and

conformational change.178 In 2016, Ding et al. investigated the combined activity of the temperature-

responsive pNIPAM and the plasmonic heating ability of AuNPs to design light-induced actuating

nanotransducers.179 The citrate-stabilized AuNPs were functionalized with amine-terminated pNIPAM.

Upon the irradiation with a resonant laser at 532 nm, the extinction peaks of colloidal AuNPs shifted from

535 nm to 645 nm, indicating that the conformational changes of the polymer shells induced particle

aggregation (Figure 9b). The optical heating caused the AuNPs to aggregate within a microsecond.

Interestingly, when the irradiation ceased, and the temperature dropped below the LCST, the clusters

disassembled within less than a second due to strong elastic forces from the hydrating and swelling

polymers. These temperature-dependent LSPR changes did not occur when AuNPs were suspended with

-COOH terminated pNIPAM which don’t attach as strongly to the Au surface as amine-terminated pNIPAM,

showing the importance of proper binding of the polymer shells on the AuNP surfaces. The laser-induced

aggregation and isolation behaviors of the pNIPAM-coated AuNPs could be performed over many

continuous cycles. This reversible aggregation-isolation behavior was achieved by balancing the forces

between van der Waals attractions and strong elastic repulsions by controlling the thickness of the polymer

shells which also colloidally stabilized AuNPs cores.

Polymer shells can also be used to drive plasmonic nanoparticle assembly. For example, amphiphilic

polymers can drive nanoparticle assembly based on the segregation of hydrophobic and hydrophilic

polymer components in polar or non-polar solvents. The interactions between amphiphilic polymer shells

and solvents can be programmed to design various assembled plasmonic nanoparticle platforms.168 One

recent focus area in self-assembly is specifically anisotropic nanoparticle assembly, where the tailored

arrangements of non-spherical plasmonic nanoparticles generate complex plasmonic phenomena such as

chiral nanoparticle assemblies and plasmonic circular dichroism.180 The assembly of anisotropic

nanoparticles requires a delicate approach since both the distances between and orientation of the

nanoparticles are important, and different parts of the nanoparticle may be subjected to different physical

or chemical forces. The poly(acrylic acid)-block-polystyrene (PAA-b-PS) diblock copolymer is popular

polymer that has been widely used for plasmonic nanoparticle assembly.181–183 In 2012, Grzelczak et al.

manipulated the combination of attraction forces and steric hindrance to assemble Au “nano dumbbells” by

using PAA-b-PS as a stabilizing and orientation-guiding agent.184 CTAB-coated Au nanorods were first

synthesized, and then the tips were selectively overgrown to produce the dumbbell shape and enhance

anisotropy. Then, the tips were further functionalized by thiol-terminated polystyrene (PS). At this point, the

amount of the polymer was optimized to cover the tip area only and to maintain colloidal stability in the

solvent mixture of tetrahydrofuran/dimethylformamide. As the polarity of the solvent increased upon the

addition of water, the solubility of both CTAB at the sides of the particles and PS at the ends became poor,

forming a side-to-side dimer that eventually aggregated to larger clusters. To impede further clustering after

the dimer formation and to orient the dimers into a cross-like formation, PAA-b-PS polymers were added to

encapsulate the dimers by forming polymer shells with the PAA block at the outer parts. Furthermore, to

overcome the steric hindrance between the Au nano dumbbells, temperature and water amount were

increased to force organic solvents away from the hydrophobic core. Then, the mobilities of PS shell and

CTAB surfactants lowered, putting more mechanical stress on the dimers; this resulted in the transformation

from side-to-side dimers into cross-like structures (Figure 9c). The final products were stable in water due

to the electrostatic repulsion forces of the hydrophilic PAA corona. In each step, PS and PAA-b-PS

copolymers prevented the aggregation of the Au nano dumbbells and guided the dimers into specific

formations while protecting the particle and interacting with solvent mixtures. In subsequent research, Smith

et al. used similar shell formation methodology to explore the effects of different dimer geometries on

scattering properties by fabricating achiral and chiral Au nanodumbell dimers.185

For biomedical fields, not only is colloidal stability important, but the biocompatibility of the nanoparticles

must also be considered for in vivo applications. Polydopamine (PDA) is an analogue to the pigment,

eumelanin, a type of melanin. PDA is known to be biocompatible, exhibiting low cytotoxicity, and is therefore,

id capable of attenuating the adverse biological effects of materials when used as a coating.186 More

importantly, PDA is best known for its ability to act as an adhesive with virtually any solid surface, so it has

been widely used to coat a number of materials through covalent and non-covalent interactions.187–189 In

2015, the Duan group suspended AuNPs in a dopamine solution to deposit PDA onto the Au surfaces.190

The self-polymerized PDA on the Au surface gave the nanoparticles excellent colloidal stability and drove

further nucleation and growth of heterogeneous metal-organic frameworks on the PDA surfaces. Another

polymer, poly(sodium 4-styrenesulfonate) (PSS), also contributed to enhanced colloidal stability and low

cytotoxicity as a coating for Au cores in work by Ye et al. The authors synthesized Au nanotubes capable

of absorbing near-infrared (NIR) that therefore have potential for various photothermal therapy and

photoacoustic imaging applications.191 Contrary to CTAB-coated Au nanotubes, PSS-coated Au nanotubes

showed negative zeta potentials and colloidal stability in a serum-containing medium over seven days

without significant loss in NIR absorbance. In contrast, CTAB-coated Au nanotubes without the PSS

aggregated within 30 minutes in the same medium.

Figure 9. (a) (i) Schematic illustration of PANI coating on Au nanorods via surfactant-assisted, oxidative

polymerization. (ii) Oxidation-reduction reaction for PANI. (iii) Plasmon peak wavelengths during 200

switching cycles of PANI-coated Au nanorods at the oxidized and reduced states (bottom right). Reprinted

with permission from ref 177. Copyright 2016 John Wiley and Sons. (b) Schematic illustration of aggregation

and disassembly behavior pNIPAM-coated AuNPs at the critical temperature, Tc induced by collapsing and

swelling pNIPAM. Adapted figure and figure caption reproduced with permission from ref 179. (c)

Description of inducing crosslike arrangement of Au nanodumbbell dimer via controlling steric hindrance

and encapsulation of the dimer in PS403-b-PAA62 polymeric micelles. Reprinted with permission from ref

184. Copyright 2015 American Chemical Society.

3.3. Plasmonic Nanoparticles Coated by Metal Shells

While a large majority of plasmonics research has focused on nanostructures made of silver, gold and

copper,15 the skew towards these three materials is perhaps most pronounced when it comes to using

plasmonic materials for SERS. The enhancement factors that can be achieved in the visible part of the

spectrum where there are good detectors with other metals such as Pt, Pd, and Co are far lower than those

that can be achieved with Au or Ag.15,192 However, these same low-enhancing metals are very important in

application areas such as catalysis.193,194 From the various efforts to improve the plasmonic properties of

transition metals, the “borrowing SERS” strategy was born.195–197 In this method, the Au or Ag is coated by

transition metals, and the Raman signals of the target molecules or analytes adsorbed on the transition

metal surfaces can still be enhanced without direct contact since the Ag or Au can produce enhancement

in the electromagnetic fields a few nanometers away from the surfaces.198 To make this approach work,

the catalytic metal shells must be ultrathin so that the electromagnetic fields extend through those layers.

Additionally, the thin shell must be pinhole-free to avoid direct interactions of the target molecules with the

plasmonic core surfaces. Thus, in the “borrowing SERS” strategy, unlike other stabilizing agents that coat

the Au or Ag cores to protect the core, the main purpose of coating plasmonic nanoparticles with transition

metals has been to take advantage of the outstanding plasmonic properties of core metals and to fuse the

distinctive capabilities of transition metal shells into the overall plasmonic nanoplatforms.

Generally, the stability of transition metal-coated plasmonic nanoparticles is lower than that of particles

coated with other shells such as silica or polymers, and the synthesis requires more complex

instrumentation to perform atomic layer depositions or galvanic replacement.199 Thus, much of the research

into metal-coated plasmonic nanoparticles has employed the help of other stabilizing agents such as CTAB,

citrate, or PVP during experimental preparation.184,200,201 Even though the aggregation behavior of the

AuNPs were found to be somewhat attenuated when transition metal-coated particles were suspended in

aqueous solutions without stabilizers, the metal-coated particles are often applied in applications which

require the use of a substrate to immobilize the particles to avoid colloidal instability. Interestingly, transition

metal-coated plasmonic nanoparticles demonstrate overall enhanced durability and stability during

repeated use,202,203 however, the improved performance is not attributed to the shells but from structural

interactions between the plasmonic cores and metal shells. For example, Chen et al. demonstrated stability

and enhanced catalytic activity of Pd-coated AuNPs for oxygen reduction and attributed this stable

performance to the lattice tensile effect in the Pd shell induced by the Au core.203

A similar synergy of behaviors of bimetals can be found in the work of Wang and co-workers in 2015.204

The authors focused on improving the refractive index sensitivity and plasmon-enabled field enhancement

of AgNPs by controlling nanoparticle size/morphology and tuning the bimetallic character of the

nanostructures.205,206 Generally, it is more difficult to control of size and shape of AgNPs, a critical parameter

for achieving strong and narrow plasmon bands, compared to AuNPs. Furthermore, bare AgNPs are

thermodynamically unstable, and the morphology of AgNPs changes easily when stored in aqueous

solutions, even with PVP.207 To overcome these drawbacks, Wang and his co-workers overgrew Ag on

bipyramidal AuNPs, creating Ag-coated AuNPs.204 The bipyramidal AuNPs were synthesized first, then

AgNO3 was reduced on the Au surface in the presence of cetyltrimethylammonium chloride. The study with

the final product showed that Ag-coated AuNPs possessed narrow longitudinal plasmon peak linewidths in

ensemble and single particle measurements due to high monodispersity. Moreover, the structural and

optical colloidal stability of the particles were investigated by checking and comparing the extinction spectra

of the Ag-coated AuNPs and that of triangular Ag nanoplates as both particles aged in aqueous solutions

(Figure 10). Even though Ag nanoplates have been known for narrower plasmon line widths than other Ag

nanomaterials159, the Ag-coated AuNPs demonstrated narrower extinction peaks and improvement in the

maintenance of the structures and the LSPR peak wavelength during storage. The authors posited two

reasons for this higher colloidal stability: (1) the reduction in the electron density of Ag due to the large

electronegativity of Au cores or (2) charge redistribution in the Ag atom orbitals during synthesis, resulting

in increased resistance to oxidation. This research is a fine example of the enhancement of plasmonic

ability and chemical stability of outer shells that result from the synergy of combining different plasmonic

inner cores.

Improved chemical stability due to bimetallic formation can also be found in the work of Huang et al.208

The authors synthesized Au nanorods coated with a AuAg alloy and tested the chemical stabilities of the

nanoparticles suspended in a strong oxidizing environment. For comparison, four different particles (Au

nanorods coated with AuAg alloy, Au, Ag, and Ag/Au (not alloy)) were incubated in an aqueous solution

containing H2O2. The particles coated with Ag and Ag/Au and incubated in 0.5 M H2O2 showed

disappearance of the LSPR after 1 hour, but the particles coated with Au only and AuAg alloy maintained

LSPR peak wavelength positions and intensities (Figure 11) The Ag in AuAg alloy showed higher

resistance to oxidative etching due to electron redistribution upon alloy formation.

Figure 10. (a) Schematic illustration of Ag growing over bipyramidal AuNPs. (b) Extinction peak wavelength

changes of triangular Ag nanoplates (green) and Ag overgrown on bipyramidal AuNPs (pink) incubated in

aqueous solutions. (c) Extinction peak wavelength intensity changes of triangular Ag nanoplates (green)

and Ag overgrown on bipyramidal AuNPs (pink) incubated in aqueous solutions. Reprinted with permission

from ref 204. Copyright 2015 John Wiley and Sons.

Figure 11. (a) HAADF-STEM image of AuAg alloy-coated Au nanorods. (b) The extinction wavelength

spectra of AuAg alloy-coated Au nanorods in varying concentrations of aqueous solutions of H2O2.

Reprinted with permission from ref 208. Copyright 2015 John Wiley and Sons.

4. Two-dimensional Plasmonic Nanoparticle Arrays

In the chemical synthesis for colloidal plasmonic nanoparticles discussed in previous sections, Au or Ag

precursors are presented and reduced in solution with capping agents that chemically and/or physically

block possible sources of destabilization such as contaminants, oxidants, and aggregation due to other

nearby plasmonic nanoparticles. Thus, each isolated particle is formed in solution and coated by ligands or

more rigid shells which protect the inner plasmonic cores. However, particles produced from chemical wet

synthesis may have limitations. First, due to the thickness of coating materials, the plasmonic performance

can be impacted as direct contact with the plasmonic metal nanostructure is limited. Second, since particles

are in suspension and continually move in solution, precise control of plasmonic properties is difficult to

achieve. In some application areas, such as solar energy conversion or electrochemical catalysis, it would

be beneficial if plasmonic nanoparticles could be fixed in a small area with a particular formation. Based on

this need, there has been significant focus on fabrication of plasmonic nanoparticle arrays using a variety

of methods.209

4.1. Plasmonic Nanoparticles Arrays via Lithography Technique and Related Stabilization

Strategies

Lithographic techniques have been utilized to make plasmonic nanoparticle arrays for over 30 years,

including use of electron beam lithography (EBL),210 focused ion beam lithography (FIB),211 and nanosphere

lithography (NSL).212 The basic schematic description of each technique is described in Figure 12. In EBL,

an electron-sensitive resist is deposited on a sample via spin coating, and an electron beam induces a

specific pattern on the resist. After exposure to the beam, the solubility of the exposed resist changes to

selectively dissolve in a suitable solvent. A layer of plasmonic metal is deposited on the resist via vapor

deposition, and the resist and the metal deposited on it are lifted off to reveal the metal nanoparticles arrays

as defined during patterning (Figure 13). FIB is similar, but it is capable of both locally depositing213 or

milling away material.214 Both methods offer high fidelity fabricated nanostructures but require expensive

and time-consuming sample preparation. Nanosphere lithography (NSL) is a simpler, cheaper patterning

method where nanospheres are first deposited in a single layer on a supporting substrate to act as a

deposition mask. When plasmonic metals are deposited onto the nanosphere mask, a portion of the metal

deposits through the voids in the assembled pattern. The nanospheres are then removed to leave a

nanopatterned array. While NSL is an effective way to create a nanoparticle array, researchers are limited

by the void pattern created as nanosphere assembly; arbitrary nanoparticle patterns can only be achieved

by using EBL or FIB.

Figure 12. A schematic description of the preparation steps for plasmonic nanoparticle arrays in three

different lithographic techniques.

Regardless of which lithographic method is used, the lithographically defined nanoparticles are

different from nanoparticles in suspension because nanoparticles on a solid substrate are not likely to

aggregate. However, unlike colloidal suspensions of nanoparticles where the liquid media helps dissipate

thermal energy, 2D nanoparticle arrays on substrates can be used in air or aqueous media. Arrays used

with air as the surrounding media can suffer changes in their plasmons due to thermal degradation and

adsorption of contaminants from the air.215,216 Also, since lithographically defined nanoparticles without a

protective shell may be directly exposed to the media and laser irradiation, the plasmonic nanostructures

and properties may degrade with use.

Figure 13. SEM images of nanoarrays fabricated by lithographic techniques. (a) The array of the Au in the

third-order Cayley trees structures fabricated by EBL. The scale bar for the array is 2 m, and 300 nm for

the inset. Reprinted with permission from ref 217. Copyright 2015 American Chemical Society. (b) The array

of the Au in “L”-shaped chiral structures fabricated by FIB. The scale bar represents 1 m. Reprinted with

permission from ref 218. Copyright 2016 American Chemical Society. (c) The array of the Au in nanotriangle

structures fabricated by NSL. The inset shows the monolayer of polystyrene nanospheres. Reprinted with

permission from ref 219. Copyright 2012 American Chemical Society.

In the fabrication and application of plasmonic nanoparticle arrays, where direct exposures of the arrays

to air is common, the stability of the nanoscale metals in air must be considered. Among the various

plasmonic metals, Ag has many advantages even over Au. Ag has the highest thermal and electrical

conductivity over all metals. It is known to support a LSPR in the widest range of visible to near infrared

regions, from 300 to 1200 nm.47 Simulated calculations28 and experimental results220 show that AgNPs can

possess sharper and more intense LSPR and stronger plasmon field intensity than AuNPs, and thus are

more suitable for plasmon sensing. However, Ag has not been studied in plasmonic applications as much

as Au, mainly due to its chemical instability especially in applications involving air exposure. Ag is likely to

be oxidized upon exposure to the atmosphere,221 resulting in an attenuation of plasmonic properties.222

Moreover, sulfidation of the bare Ag surface occurs frequently at room temperature in air; this can occur in

a single day and significantly influences conductivity and the extinction wavelength.223

Au is chemically much more stable at room temperature in air and doesn’t suffer from oxidation.224

However, photothermal stability is another issue in two dimensional plasmonic nanoparticle arrays, and Au

is not free from this issue. Generally, structural changes of AuNPs are found to occur at lower temperatures

than the melting temperature of the bulk.225,226 Exposure to laser irradiation can also induce shape

deformation. Petrova et. al. exposed Au nanorods to pulsed 400 nm laser with energies between 0.1 to 20

J/pulse, and Au nanorods maintained their structures up to around 700 oC, the heat induced by the laser.157

It was found that this temperature was much higher than 250 oC, the temperature at which thermally heated

Au nanorods showed rapid transformation. The authors reasoned that the thermal diffusion between pulses

induced higher deformation temperatures for Au nanorods exposed to laser. Similar work was done by El-

Sayed and co-workers, where a femtosecond pulsed laser showed much more efficient photothermal

reshaping of Au nanorods than nanosecond pulsed laser irradiation.227 In more recent research, Hentschel

and coworkers examined the thermal stability of Au nanostructures deposited on a substrate through EBL

in ambient atmosphere by imposing high temperatures and intense laser pulses.228 60-nm-wide Au

nanorods of various aspect ratios showed significant shape deformation and LSPR shifts when the

temperature exceeded 600 oC. Albrecht also measured the third harmonic generation (THG, i.e. third-order

hyper-Rayleigh scattering) intensity, emitted light from nanostructures where the optical frequency is three

times that of the irradiating laser beam; THG is an emerging technique in nonlinear optical imaging.229,230

The investigation of THG was chosen specifically as THG signal is known to be crucially dependent on the

exact size and shape of the nanoparticles.229 During the stability investigation of Au nanorods exposed to

10 GW/cm2 of laser radiation (a pulse duration of 16fs, 44 MHz repetition rate, and 180 mW average power),

the nanorods showed a continuous decrease in THG intensity due to cumulative damage at the Au surface.

The authors reasoned that the change of the linear spectra overlapped with the laser spectrum reduced the

nonlinear signal resulting in THG intensity decrease. A similar thermal study on Ag nanorods was performed

as well in 2018 by Albrecht et al.231 The 60-nm-wide Ag nanorods showed significant loss of plasmonic

response at 500 oC.

The substrates on which plasmonic arrays are deposited also impact the overall optical properties, in

part because the anisotropic environment can induce LSPR peak splitting, LSPR peak shifting, and other

unusual effects.232–234 Among the many possible substrates available, transparent conducting oxides such

as indium tin oxide (ITO) are commonly used layer due to its electrical conductivity, optical transparency,

and ease of thin film formation.235 For this reason, the effects of ITO substrates on plasmonic nanoparticles

have received much attention, and a number of studies of Au or Ag-ITO hybrid plasmonic platforms have

been reported.19,236–238 Glass substrates such as fused silica or borosilicate are also popular, and plasmonic

nanoparticles deposited on different types of glass showed different LSPR peak wavelength positions and

varying sensitivities to bulk environments.239,240 Halas and co-workers studied the influence of substrates

with varied dielectric properties, including glass, sapphire, and ZnSe, on deposited plasmonic AuNPs.241

The substrates beneath the nanoparticles induced the splitting of plasmonic dipolar peaks, which are

dependent on the dipoles parallel or perpendicular to the surfaces. This splitting was proven to be

influenced by increasing substrate permittivity. Other studies have emphasized the importance of adhesion

layers on deposited metal nanoparticles because these layers can impact the stability and plasmonic

response of the nanomaterials on a substrate.242,243 Carson and co-workers showed that the adhesion layer

composition and thickness between Au and a Pyrex substrate had effects on the optical resonance

properties of Au film.242 The Au films on chromium or titanium adhesion layers showed larger optical

resonance bandwidths than the films on etched adhesion layers, and the increased thickness of the

adhesion layers induced a reduction in the magnitude of the optical resonance peaks.

Many researchers have fabricated different metal nanoparticle arrays and studied their corresponding

plasmonic capabilities/related stabilities. The critical role of the substrate was thoroughly investigated in

2013 when Sivis et al. measured nanostructure-enhanced high-harmonic generation in plasmonic bow-tie

antennas.244 To study the specifics of extreme-ultraviolet generated by high-harmonic conversions from

plasmonic nanostructures irradiated by high-energy pulses, the authors fabricated arrays of Au bow-tie

antennas via FIB lithography. Each triangle had lengths of 200 to 240 nm, and the gap distance between

two triangles was 20 nm. An 8 fs light pulse with 800 nm center wavelength illuminated two-dimensional

plasmonic devices placed in a vacuum chamber, and extreme-ultraviolet fluorescence and third and fifth-

harmonic generations were observed. Studies of the durability of the devices under high-energy irradiation

and achievable maximum local intensities followed. The same Au bow-tie antennas were fabricated on both

mica and sapphire substrates. The long-term extreme-ultraviolet yield was measured from both devices,

and the results showed that the Au nanostructures on mica gradually lost ultraviolet generation capacity,

but the structures on sapphire were maintained during the exposure (Figure 14a). It was more obvious

from SEM images that, unlike the Au on mica which clearly showed the effect of cumulative damage, the

Au structures on sapphire remained intact. Figure 12a shows SEM images that depict rapid and significant

degradation of Au structures due to high photon energy on the substrates, which could result in a loss of

plasmonic enhancement and extreme-ultraviolet generation. This study shows the importance and effect of

the substrate on two-dimensional plasmonic devices fabricated by lithography techniques.

Figure 14. (a) (i) Long term extreme-ultraviolet intensity measurements of bow-tie Au nanoantennas

fabricated on sapphire (red) and on mica (black). Nanostructures on sapphire substrates show much more

stable and consistent yields and structures than nominally identical nanostructures on mica substrates

(insets). (ii) SEM images of the structures right after the sample preparation (left), after several hours of

exposure to incident less than 0.15 TW cm-2 intensities (middle), after a few minutes of exposure to incident

higher than 0.15 TW cm-2, upto 0.3 TW cm-2 (right). The scale bars represent 200 nm. Reprinted by

permission from Springer Customer Service Centre GmbH: Springer Nature, ref 244. Copyright 2013. (b)

(i) Cross-sectional SEM images of PZT-sandwiched Au nanostructures on TIO substrates. (ii) Time-

dependent short circuit photocurrent for sample PZT-sandwiched Au nanostructures array in three different

states: as-deposited, after +10 V and –10 V poling. The structures show high stability and reproducibility

on photoelectrochemical performance. (c) Schematic designs of the electronic band structures when the

injected hot-electron transfer from PZT films to the electrolyte for the two poling configurations, (i) +10 V

and (ii) –10 V. Reprinted with permission from ref 245. Reproduced with permission under Creative

Commons Attribution 4.0 International License http://creativecommons.org/licenses/by/4.0/.

To enhance the thermal and chemical stability of two-dimensional plasmonic nanoarray platforms,

deposition of thin protecting layers of dielectric oxide layers such as titania or alumina is the most popular

strategy. The enhanced stability of AgNPs fabricated by NSL was observed when the Al2O3-coated

nanoparticles maintained their LSPR wavelength despite exposure to femtosecond laser pulses.246 Albrecht

et al. studied the enhanced thermal stabilities of various Al2O3-coated plasmonic nanoparticles, including

Ag and Au fabricated by electron lithography on glass substrates.231 All metals except copper and

magnesium showed increased thermal stability with 4-nm-thick layers of Al2O3. Photostability was also

investigated by looking at third-harmonic generation. Bare Au and Ag nanostructures clearly showed a

steady decrease in third harmonic generation intensity, indicating the weakening of plasmonic responses

from local heating or structural deformation. The nanostructures with layers of Al2O3 generated steady third-

harmonic generation signals upon exposure to incident lasers. This improved stability facilitated by Al2O3

can be explained by the suppressed deformation of the plasmonic nanostructures. The surface melting

temperature is increased by decreasing the mean square displacement of Ag atoms on the surface upon

the oxide layer coating.247 Adibi et al. fabricated a platform for in operando plasmonic nanospectroscopy,

where Au nanodisks coated with mesoporous Al2O3 layers impregnated with Pt nanoparticles could sense

sintering kinetics of the Pt nanoparticles.248 Al2O3 layers protected Au from possible thermal deformation

and prevented direct contact between Au and catalyst materials. TiO2 layers have also been known to

enhance the thermal and chemical stabilities in harsh oxidative conditions and to improve the catalytic

activities of two-dimensional plasmonic catalytic platforms.249,250 A rather unique oxide layer was used by

Wang et al. in 2015 for a plasmonic photoelectrochemical device. Instead of using a conventional

photoelectrochemical semiconductor, such as TiO2, they used ferroelectric Pb(Zr,Ti)O3 (PZT) to make

tuning the band bending at the ferroelectric and electrolyte interface possible. An array of square Au dots

(270 X 270 nm2) were deposited between PZT layers on ITO substrates (Figure 14b).245 The steady-state

external quantum efficiency spectra showed hot-electron injection from excited Au dots to PZT layers via

higher and more distinctive external quantum efficiency spectra than that of PZT without Au dots. This result

qualitatively matches with the Au dot-PZT layer absorbance spectrum. The ferroelectric domains in PZT

films are known to be poled, resulting in the capability to switch the direction of depolarization of electric

fields.251 This characteristic enables the manipulation of hot-electron injection and transfer (Figure 14c).

However, then the Au dots were not sandwiched between two PZT layers, and a degradation of

performance was observed when the electrodes were poled with different potentials in a propylene

carbonate solution. The performance could be maintained only when the structure had double layers of

PZT film, indicating the positive effects of coating layers on stability and reproducibility (Figure 14b).

There are a few other methods also known to improve nanoparticle array stability. For example, Bosman

et al. focused on the damping effects of grain boundaries or surface roughness underneath lithographically-

fabricated plasmonic nanomaterials, reducing plasmonic response.252 The researchers worked to reduce

grain boundaries by applying a thermally stable, but removable, 30-nm-thick layer of hydrogen

silsesquioxane during the annealing on the EBL-fabricated Au nanoparticle array. Au nanostructures with

this encapsulated annealing experienced fewer grain boundaries while preserving the designed shapes.

Scuderi et al. used a hexanethiol monolayer for the passivation of EBL-fabricated Ag nanodisk surfaces.253

This monolayer overcomes drawbacks of metal oxide coatings, which can alter the achievable plasmonic

shifts due to the larger refractive index and blocking of plasmonic interactions on the targets due to the thick

oxide layers. To achieve a similar result, Losurdo et al. imposed a low temperature hydrogen processing

on the EBL-fabricated Ag surface, rather than coating them, to inhibit oxidization and maintain the optical

properties under ambient conditions.254 The authors argue that treating Ag nanostructures with low

temperature hydrogen processing not only can make the surface chemically passive, but can also avoid

the additional heating process that would change the morphology or grain sizes of the original Ag

nanostructures.

Thick layers of oxide or other coating layers can provide plasmonic nanoparticles with protection from

shape deformation or chemical contamination, but this protection can also reduce or hinder the functionality

of the plasmonic nanomaterials since many plasmonic applications require interactions within a few

nanometers of the plasmonic surface.255 Graphene is an alternative protective layer that displays many

advantages over other types of protective layers. A layer of graphene has an average thickness of 0.35 nm,

which is much thinner than most applied oxide layers.256 Even though it is quite thin, graphene monolayers

have a great degree of impermeability, making it possible to block gas molecules as small as helium.257,258

Thus, passivating the surface of a metal such as Ag with graphene can conserve the plasmonic ability of

the metal and effectively protect it from external contaminant sources such as hydrogen sulfide or carbonyl

sulfide in the air. Graphene -protected AgNPs were fabricated in 2012 by Reed, et al. On glass substrates,

EBL-defined Ag nanoantenna arrays were deposited followed by coating with chemical vapor deposition-

grown graphene.259 Two nanoparticle array substrates, one with graphene and one without, were stored in

an ambient laboratory environment for a month. As shown in Figure 15a, it was obvious that significant

morphological deformation had occurred on the sample without the graphene layer, but the sample covered

by graphene maintained its morphology, indicating no sign of sulfidation. This observation was supported

with data from X-ray spectroscopy and optical measurements (Figure 15b). When refractive index

sensitivity of the two nanoparticle arrays was evaluated, the graphene-protected Ag nanoparticle arrays

showed a higher sensitivity to refractive index changes. Based on this work, it is clear that there will be

many applications where a graphene protective layer is superior to traditional oxide layers.

Figure 15. (a) SEM images of (i) graphene-coated Ag nanostructures and (ii) bare Ag nanostructures after

30 days; scale bars represent 200 nm. (b) Normalized LSPR peak extinction spectra of (i) graphene-coated

Ag nanostructures and (ii) bare Ag nanostructures over 30 days. Reprinted with permission from ref 259.

Copyright 2012 American Chemical Society. (c) XPS spectra of the Ag3d photoelectron (i) for Ag deposited

on glass and (ii) for the graphene-coated Ag on glass after 1 month of air exposure. Reprinted with

permission from ref 260. Copyright 2013 John Wiley and Sons. (d) (i) Schematic illustration of AgCo NPs.

Ag oxidation can be suppressed by hydroxide formation on the Co surface via electron injection from Co

into Ag. (ii) The plasmonic degradation graphs measured by the normalized inverse full-width-at-half-

maximum of the LSPR peak of NP arrays, AgNPs and AgCO NPs. Reprinted with permission from ref 261.

Copyright 2013 John Wiley and Sons.

In 2015, Losurdo evaluated another benefit of the graphene layer on plasmonic nanomaterials. The

authors argue that a graphene layer on AgNPs can act as an electron shuttle and deoxidize the Ag surface,

making the plasmonic platform more chemically stable.260 Generally, when a graphene layer is deposited

or transferred to another material, the occurrence of a defect is inevitable. The defect sites of graphene

layers chemisorb more oxygen, become more chemically unstable, and increase the carbonaceous

material reactivity.262 Since the work function of Ag is known to be slightly different from that of the graphene

monolayer,263 electron transfer from graphene to Ag happens during the graphene layer deposition. Finally,

the reduction potential difference between Ag oxide is known to be higher than that of graphene264, allowing

the reduction of Ag oxide to Ag initiated by injected electrons from graphene. To prove this deoxidization

process on the Ag surface, the authors fabricated Ag nanostructures via EBL, and graphene layers were

formed on Ag surfaces. X-ray photoelectron spectroscopy results were consistent with the authors’

arguments: an increase in Ag metal, a reduction in Ag oxide, and a decrease in oxygen amount on the

surface during exposure in air, indicating that the Ag deoxidization occurs after graphene layers are

deposited onto the Ag nanostructures (Figure 15c).260 This study proves the stability induced by graphene

layers on plasmonic Ag nanostructures, not only through passivation but also through induction of Ag

deoxidization via electron transfer. The similar enhanced stability behavior can be found when Ag is in

contact with cobalt, resulting in oxidation-resistant Ag nanostructures (Figure 15d).261

4.2. Fixation of Plasmonic Nanoparticles on Solid Substrates via Non-Lithographic Technique

As shown, two-dimensional lithographic fabrication of AgNPs or AuNPs enables various plasmonic

applications. Fixing plasmonic nanoparticles on solid substrates can also be accomplished by first

chemically synthesizing the nanoparticles, followed by adsorption onto a substrates of interest.265,266,267 For

example, Wang et al. designed a plasmonic assembled AuNP thin film for efficient solar-enabled

evaporation. The idea was similar to biological evaporation in plant leaves: plasmonic nanoparticles can

absorb sunlight and induce heating, acting as a light-to-heat converter. With the help of plasmonic AuNPs,

vapor bubbles do not lose heat to bulk water during travel to the air-water interface because there is more

intense formation of bubbles near the interface upon plasmonic heating.268 While this system demonstrated

efficient evaporation, the system was not reusable and there was thermal diffusion to the non-evaporative

portion of the liquid. To address these limitations, the authors employed an air-laid porous paper as a

substrate to provide mechanical stability and low thermal conductivity.269 AuNPs were synthesized via

citrate reduction and allowed to self-assemble onto the paper by incubating particles in water and under

formic acid vapor. The acid vapor diffusion to water neutralized, destabilized, and trapped the AuNPs at the

water-air interface, resulting in a thin film. This film was transferred to the airlaid paper and dried. This

relatively simple AuNP thin film preparation generated plasmonic paper in which each AuNP was separately

sitting on fibers, resulting in non-aggregation and a robust plasmonic platform for an efficient, sunlight-

enabled evaporation system (Figure 16a).269 The surface temperature of the paper floating on liquid under

sunlight illumination rose to 80 oC, and the hot zone showed power density of 4.5kWm-2 after 15 minutes

of illumination. Furthermore, due to the robustness of the fibers, the evaporation experiment showed

consistent rates for 30 continuous cycles with the same paper, proving reusability and sustained plasmonic

function (Figure 16b).

In 2015, Wu et al. took a similar nanoparticle-on-fiber approach with AgNPs, taking advantage of the

antibacterial properties of AgNPs as they release of Ag ions.270,271 To create antibacterial fabrics, AgNPs

were used both as the coloring and the antibacterial agents. To solve the chronic problem of poor color

fastness of this natural AgNP dyes, the authors dipped cotton fabric into a poly(ethylenimine) solution to

create a layer of positive charge on the fabric, which would interact with carboxylate groups of citrate-

stabilized AgNPs via hydrogen bonding and electrostatic attractions, leading to successful deposition of

AgNPs on the fabric. Next, the fabric was transferred to a fluorinated-decyl polyhedral oligomeric

silsesquioxane (F-POSS) solution to give it superhydrophobic characteristics. Each product showed

distinctive colors with superhydrophobicity, and the deposition of F-POSS enhanced the AgNP adherence

to the fabric despite continuous washing and rubbing (Figure 17a).272

As is clear from the examples given here, nanoparticle fabrication via lithography or deposition enables

precise control of nanoparticle size, morphology, and gap distances within arrays; however, their structural

qualities can suffer due to grain boundaries and defects during preparation. In the case of wet-chemical

synthesis, particles can display excellent plasmonic properties, but sophisticated arrangements and

architectures over a large area or volume are limited. To harness the advantages of each method effectively,

Flauraud et al. proposed topographical control of plasmonic nanoparticles via capillary force (Figure

17b).273 By dividing the dynamics of the capillary assembly of nanoparticles into three stages, insertion,

resistance and drying, a colloidal suspension of Au nanorods could be placed onto solid substrates with

specific topographic patterns with 1 nm resolution. This study is meaningful in that scalable and

lithographically accurate control of colloidal nanoparticles was achieved that are independent of the surface

patterns and types of nanoparticles.

Figure 16. (a) Schematic illustration of the structure of AuNPs films on paper. (b) (i) The maximum,

minimum, and average weight change of water over 800 seconds by using AuNPs films on paper. (ii) Total

weight changes after 15 min illumination in each cycle; the average weight change is 1.25 g. Reprinted with

permission from ref 269. Copyright 2015 John Wiley and Sons.

Figure 17. (a) Cotton fabrics dyed with different morphologies of AgNPs, resulting in different colors. The

inset graph shows water contact-angle and sliding angle changes of the fabricated AgNPs cotton fabrics

during 80 cycles of dry cleaning. Reprinted with permission from ref 272. Copyright 2015 John Wiley and

Sons. (b) (i) Schematic illustration of of the capillary force-driven assembly of AuNPs onto topographical

traps. Dimension not to scale. (ii) the SEM image of the accumulated AuNPs and AuNPs trapped in the

fabricated holes. Reprinted by permission from Springer Customer Service Centre GmbH: Springer Nature,

ref 273. Copyright 2017.

Table 1. The summary chart describing the types of the stabilizing agents in this review and related stability

methods, features, and related research fields.

1 Stabilizing agents Specific examples Stability methods Features Main application area

colloidal nanoparticle

ligand

conventional surfactant or ligand

citrate,63,70

PVP,82,83

CTAB69

mainly via electrostatic repulsion

and/or steric hindrance

can be replaced with other stabilizing agents,

controls nanoparticle morphologies in colloidal synthesis (spheres, prism, bipyramidal, etc)

nanoparticle preparation steps in colloidal

synthesis

polyethylene glycol (PEG) thiol-PEG,

89

thiol-PEG-COOH90

resistant to protein adsorption

biomedical sensing, imaging, or therapeutic

applications protein

bovine serum albumin

(BSA),98–100

streptavidin101

biocompatible, stable at various pH levels

assembly-inducing ligands DNA,

121,122,125,131

diazirine132

programmable assembly via DNA base pairing interactions

(dimer, hetero-nanocluster, nanoparticle crystal),

photo-responsive assembly

quantum-level or single molecular interaction

study, photonics, sensing

shell

silica mesoporous or nonporous

shell148,151,152,163,164

tunable porosity, facile surface modification

biomedical field, sensing, catalysis

organic shell

poly(N-

isopropylacrylamide),179

polyaniline,177

polydopamine,190

PAA-b-PS184

temperature-responsive conformational change,

high conductivity, adhesive properties, amphiphilic

electrochemical applications,

sensing

metal Pd,203

Ag,204

AgAu alloy208

core-shell chemical

interaction increased resistance to oxidation,

enhanced catalytic activity sensing, catalysis

two dimensional nanoparticle

array

conventional lithographic technique

substrate or adhesion layer sapphire,

244 titanium, and

chromium242

enhanced durability of the particles under high

photon energy

affecting optical resonance property

photovoltaics, photoelectrochemistry,

sensing

coating

metal oxide

(Al2O3,231,248

TiO2,249,250

PZT245

),

graphene,259,260

hydrogen silsesquioxane252

re-distribution of the metal atoms

at the surface layers, more facile electron

transfer

tuning the band bending at the electrolyte interface,

increased external quantum efficiency,

thin but great degree of impermeability

non-conventional lithographical

technique

none or additional coating on array

air-laid paper,269

fabric,272

specific topographic

patterned substrate273

physical separation of nanoparticles

on specific substrates

pre-prepared nanoparticles are transferred to

specific substrates

photocatalysis, sensing, toxicity related research

2

5. Conclusion and Prospective

This review aims to survey the stabilization strategies for plasmonic Au and Ag nanoparticles utilized in

various fields. As shown, the type of stabilizing approach applied to preserve plasmonic properties depends

on the nanoparticle’s bulk environment, how nanoparticles are fabricated, and the types of plasmonic

applications. The most robust and thorough stabilization strategies are not always the most advantageous

routes to take, due to their impacts on the refractive index and distance between the analyte and core,

which are vital for catalysis and SERS detection. Thus, researchers are always working to obtain a balance

between achieving efficacious plasmonic properties and maintaining nanoparticle stability. In-solution

nanoparticle preparations are often used because the nanoparticle crystallinity, and thus plasmonic

behavior, is usually superior to that achieved using lithographic techniques. In colloidal synthesis and

applications, initially AgNPs and AuNPs are mostly surrounded by stabilizing surfactants or ligands which

also play roles in nanoparticle morphology. Even though their roles in determining morphologies are crucial,

loosely bound stabilizers are often not sufficient to protect the nanoparticles in complex media or in vivo

applications. Further functionalization of the colloidal nanoparticles with more strongly-bound ligands, such

as thiolated-PEG, can enhance colloidal stability, resulting in improved plasmonic performance.

Analysis of current studies reveals that near future directions for plasmonic AgNPs/AuNPs research will

be investigation of controlled nanoparticle self-assembly. During nanoparticle preparation, it is desirable to

separate each nanoparticle to avoid aggregation and reach the targeted morphologies. However, when

nanoparticles are placed in close proximity to each other without aggregation, the gaps between the

nanoparticles can be filled with coupled plasmonic electric fields. These short gaps enable unique

plasmonic-related applications in a variety of fields such as biosensing, photovoltaics, photocatalysis, and

photothermal therapeutics.274 This synergistic plasmonic amplification can be maximized when

nanoparticles are very closely packed and arranged, thus recent work has focused on decreasing the gap

distance between the particles and designing unique plasmonic materials that are assembled using the

nanoparticles as nano-building blocks. Among different assembly techniques, DNA-based nanoparticle

assembly is particularly exciting as DNA enables very precise control over the distance between

nanoparticles owing to Watson-Crick base pairing.108 As a ligand, DNA has excellent biocompatibility and

feasible functionalization via nucleic acids, which make DNA a promising linker for plasmonic nanoparticle

self-assembly. From a stability point of view, due to the predictable lengths of DNA strands and strong thiol

bonds between modified DNA and the nanoparticle surface, aggregation can be prevented while the gap

distances can be controlled to the nanometer level. DNA origami, a nanoscale folding of DNA to create a

customized two or three-dimensional structure, is of particular interest these days, as various designed

templates enable a variety of plasmonic nanoparticle assemblies in controlled manners.275 From a basic

structure as AuNP dimers,276 AuNP helices277 or toroidal AuNP superstructures278 have been fabricated

based on different DNA origami structures. Considering the various morphologies of the AgNPs/AuNPs and

available designs of the DNA origami structures, more diverse plasmonic platforms and related synergistic

plasmonic properties will likely be studied. Another active and promising field enabled by DNA ligands is

the fabrication of chiral plasmonic nanostructures. Chirality is a geometric feature where a structure cannot

be superimposed with its mirror image.279 Nanomaterials with chirality have the capacity to rotate the

polarization of light and interact differently with left circularly polarized light and right circularly polarized

light.280 This phenomenon is recognized as circular dichroism (CD).281 Circular dichroism has a high

potential to be used in many applications such as the detection of subtle conformational changes of

biomolecules or proteins,282,283 measurement of circularly polarized light,284 and stimulate asymmetric

catalysis.285 Plasmonic nanoparticles assembled with a chiral geometry can exhibit strong optical activity

as well as enhanced chiroptical activity. DNA can be used to stably assemble achiral AuNPs/AgNPs into

overall chiral plasmonic nanostructures of helices,277 spirals,286 rod dimers,287 pyramids,288 etc. Peptides

and proteins are also promising stabilizing agents that can produce plasmonic chiral structures. The various

ranges of functional groups on the peptides and proteins can be used for the controlled nucleation and

stabilization of metal NPs during the association of the growing particles with their surfaces.279 Very recently,

Lee et al. fabricated amino acid and peptide-directed three dimensional chiral nanoparticles in an aqueous-

based synthesis.289 The pre-synthesized Au seed particles were mixed with chiral cysteine or cysteine-

based peptides in Au growth solution; since chiral cysteine was used, the Au helicoid nanoparticles that are

synthesized exhibit chirality. As introduced, DNA and biomolecules are expected to be employed actively

in the near future as stabilizing and structure-designing agents to achieve a clear goal: the programmatic

construction of highly effective plasmonic nanoplatforms. With DNA and biomolecule-assisted

assembly/synthesis, AgNPs and AuNPs can go beyond general plasmonic performance to allow

exploration of quantum-level phenomena and single-molecular or structural interactions.290,291

Inorganic, organic, and metal shells can provide AgNPs/AuNPs with stability against aggregation and

dissolution in complex media. Further, shell components can act as a completely different intermediate

environment from bulk media or the plasmonic cores, where further functionalization or pH/temperature-

responsive behavior can be achieved for specific applications such as imaging or cancer therapy.

Conventional silica shells are still being actively and widely utilized. However, in current studies, their use

is pushing toward fabricating hetero-complex structures for improved plasmonic performance. For example,

Wang and co-workers achieved selective deposition of Pd on Au nanobipyramids via pre-deposition of

silica.292 Before the deposition of Pd, the surfaces of Au were site-selectively coated with silica, then the

remaining exposed parts of Au surface, either tips or sides, were covered with Pd. Silica assisted the site-

selective depositions of Pd on two different positions of the Au nanobipyramids and also protected coated

regions during colloidal catalytic Suzuki coupling reactions under laser irradiation to examine the correlation

between the plasmonic photocatalytic activity and the positions of the deposited Pd. Currently, there is a

lot of research being performed on hybrid or hetero-nanostructures in the field of plasmonic photocatalysis

to achieve both stability and high performance.293 With metal shells, the properties of a transition metal shell

are affected by the inner plasmonic core. However, the fabrication of ultrathin (less than 1 nm thick) and

pinhole-free shells that allow use of the plasmonic properties of the inner cores is not easy to achieve, and

further functionalization is not straight-forward, limiting more efficient plasmonic applications. To overcome

these issues, non-traditional shells such as MnO2 have been applied to produce more stable and tunable

plasmonic nanoparticles.294 Au-Ag bimetallic alloy plasmonic nanoparticles have been an exciting platform

to maintain the excellent plasmonic properties of Ag while making use of the chemical inertness of Au.295,296

In lithographic plasmonic nanoparticle array fabrication, there are no issues with ligands or colloidal

stability, but the nanoparticles are much more likely to be exposed to air or thermally harsh conditions.

Silver is very susceptible to oxidation and sulfidation; therefore, coating it with oxide layers or graphene is

necessary for protection and deoxidation via electron transfer, respectively. AuNPs don’t suffer from

oxidation, but exposure to high photothermal energy can reduce their stability. Plasmon damping is a major

cause of plasmonic nanoparticles losing their optical properties; this loss can be attributed to the presence

of grain boundaries and surface roughness on the substrate or adhesion layers.252,297 For these reasons,

alternative plasmonic materials such as aluminum or hybrid plasmonic nanoparticles have recently received

significant attention.12,298,299 Whether future applications use new materials such as aluminum or traditional

plasmonic materials such as silver and gold, researchers will have to continue to consider appropriate

stabilization tactics to achieve performance stability without hindering the exciting and useful plasmonic

properties.

The overall rapid development of the plasmonic Ag/Au nanoplatforms has overcome many obstacles

and is pushing the boundaries toward more sophisticated and enhanced plasmonic systems, but there is

still need for further improvements. From the stability perspective, most research has been performed in

simplified or benign conditions which are far from a realistic environment. The stability, as well as the

preserved plasmonic properties, must be tested in complex systems such as cell matrices. DNA and

biomolecules show remarkable potential for enabling nanoparticle assemblies and chiral nanoparticles, but

at elevated temperatures or in complex media, DNA can be denatured and biomolecules can be unfolded,

resulting in loss of the plasmonic properties of the stabilized nanoparticles. For further practical applications,

more proper tests of stability, reversibility, and reproducibility of various plasmonic nanoplatforms specific

to each purpose must be performed and satisfied. Moreover, current plasmonic nanoplatforms mainly

reside within the proof-of-concept stage, considering the high cost of complex templates such as DNA

origami and relatively low yield of lithographically defined plasmonic noble metal nanoparticles.300 Thus,

future research will likely encompass the improvement and enhancement of the stabilities and viabilities of

the developed AgNPs/AuNPs systems and design of more simplified and market-friendly fabrication

methods for the production of practical plasmonic metal nanoparticle platforms.

Acknowledgements

This work was supported by the National Science Foundation through the Centers for Chemical Innovation

Program Award CHE-1503408 for the Center for Sustainable Nanotechnology. J.T.B. acknowledges

support by a National Science Foundation Graduate Research Fellowship (Grant number 00039202).

Portions of this work were conducted in the Minnesota Nano Center, which is supported by the National

Science Foundation through the National Nano Coordinated Infrastructure Network (NNCI) under Award

Number ECCS-1542202.

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ORCIDs of The Authors:

Hyunho Kang: 0000-0001-9258-7168

Joseph T. Buchman: 0000-0001-5827-8513

Rebeca S. Rodriguez: 0000-0002-8994-554X

Hattie L. Ring: 0000-0001-5779-2561

Jiayi He: 0000-0003-4361-3379

Kyle C. Bantz: 0000-0002-1732-2183

Christy L. Haynes: 0000-0002-5420-5867

Table of Contents:

Authors’ Biographies. Hyunho Kang received his B.S degree in chemistry from the University of Illinois at Urbana-Champaign in

2014. He is currently a Ph.D. candidate in chemistry at the University of Minnesota under the supervision

of Dr. Christy L. Haynes. His current research is on the design and investigation of colloidal SERS

substrates using silica-coated gold nanoparticles. He also does research as part of the Center for

Sustainable Nanotechnology, where his research focus is on the synthesis of silica nanoparticles for the

investigation of environmental impacts.

Joseph T. Buchman received his B.S. degrees in chemistry and biology from Augsburg University in 2013.

He is currently a Ph.D. candidate in the Department of Chemistry at the University of Minnesota, working

under the supervision of Dr. Christy L. Haynes. He currently does research as part of the Center for

Sustainable Nanotechnology, where he focuses on understanding the mechanisms of nanoparticle toxicity

to environmentally-relevant bacteria.

Rebeca S. Rodriguez received her B.S. in chemistry from American University in 2016. She is currently

pursuing her Ph.D. in chemistry at the University of Minnesota. Her research focuses on the design and

fabrication of polymer affinity agents to detect small molecule toxins found in food. Using surface-enhanced

Raman spectroscopy, this platform allows for molecular fingerprint identification as well as the possibility of

multiplex detection. Her current work has focused on mycotoxin detection and will move to other classes of

small molecules for food safety.

Hattie L. Ring received her B.S. degrees in physics and chemistry at Iowa State University. She then earned

her Ph.D. (2012) in physical chemistry at the University of California, Berkeley. Her postdoctoral training

was at the University of Minnesota in the Department of Chemistry and the Center for Magnetic Resonance

Research. Her research interests include biologically compatible nanoparticle coatings, iron-oxide

nanoparticles, magnetic resonance imaging contrast agents, and magnetic fluid hyperthermia. She is

currently a research associate at the University of Minnesota.

Jiayi He received her B.S. (2016) in Chemistry with honor from Wuhan University. Currently, she is a Ph.D.

candidate under the supervision of Prof. Haynes in the Department of Chemistry at the University of

Minnesota. Her research interest focused on single cell electrochemistry measurements and polymer

modified electrolyte-gated transistors for food safety applications.

Kyle C. Bantz received her B.A. in Chemistry in 2006 from Cornell College and her Ph.D. in Chemistry in

2011 from the University of Minnesota under the supervision of Prof. Christy Haynes on the development

of SERS sensors for detection in complex mixtures. She received postdoctoral training in SAMDI analysis

of phosphatase enzymes at Northwestern University with Prof. Milan Mrksich. She is currently a term-

assistant professor at the University of Minnesota.

Christy L. Haynes received her B.A. in Chemistry in 1998 from Macalester College and her Ph.D. in

Chemistry in 2003 from Northwestern University. As the Elmore H. Northey Professor of Chemistry, she

leads the Haynes research group at the University of Minnesota. Her group focuses on exciting research

questions at the intersection of analytical, biological, and materials chemistry. Prof. Haynes is also the

Associate Director of the NSF-funded Center for Sustainable Nanotechnology.