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, [email protected]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
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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,
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.
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.