The ability to control the size, shape, and material of a surface has reinvigorated the field of surface-enhanced Raman spectroscopy (SERS). Because excitation of the localized surface plasmon resonance of a nanostructured surface or nanoparticle lies at the heart of SERS, the ability to reliably control the surface characteristics has taken SERS from an interesting surface phenomenon to a rapidly developing analytical tool. This article first explains many fundamental features of SERS and then describes the use of nanosphere lithography for the fabrication of highly reproducible and robust SERS substrates. In particular, we review metal film over nanosphere surfaces as excellent candidates for several experiments that were once impossible with more primitive SERS substrates (e.g., metal island films). The article also describes progress in applying SERS to the detection of chemical warfare agents and several biological molecules.

Surface-Enhanced Raman Spectroscopy

Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

Surface-enhanced Raman spectroscopy.

Coinage metals, such as Au, Ag, and Cu, have been important materials throughout history.1 While in ancient cultures they were admired primarily for their ability to reflect light, their applications have become far more sophisticated with our increased understanding and control of the atomic world. Today, these metals are widely used in electronics, catalysis, and as structural materials, but when they are fashioned into structures with nanometer-sized dimensions, they also become enablers for a completely different set of applications that involve light. These new applications go far beyond merely reflecting light, and have renewed our interest in maneuvering the interactions between metals and light in a field known as plasmonics.2–6

In plasmonics, the metal nanostructures can serve as antennas to convert light into localized electric fields (E-fields) or as waveguides to route light to desired locations with nanometer precision. These applications are made possible through a strong interaction between incident light and free electrons in the nanostructures. With a tight control over the nanostructures in terms of size and shape, light can be effectively manipulated and controlled with unprecedented accuracy.3,7 While many new technologies stand to be realized from plasmonics, with notable examples including superlenses,8 invisible cloaks,9 and quantum computing,10,11 conventional technologies like microprocessors and photovoltaic devices could also be made significantly faster and more efficient with the integration of plasmonic nanostructures.12–15 Of the metals, Ag has probably played the most important role in the development of plasmonics, and its unique properties make it well-suited for most of the next-generation plasmonic technologies.16–18

1.1. What is Plasmonics?
Plasmonics is related to the localization, guiding, and manipulation of electromagnetic waves beyond the diffraction limit and down to the nanometer length scale.4,6 The key component of plasmonics is a metal, because it supports surface plasmon polariton modes (indicated as surface plasmons or SPs throughout this review), which are electromagnetic waves coupled to the collective oscillations of free electrons in the metal.

While there are a rich variety of plasmonic metal nanostructures, they can be differentiated based on the plasmonic modes they support: localized surface plasmons (LSPs) or propagating surface plasmons (PSPs).5,19 In LSPs, the time-varying electric field associated with the light (Eo) exerts a force on the gas of negatively charged electrons in the conduction band of the metal and drives them to oscillate collectively. At a certain excitation frequency (w), this oscillation will be in resonance with the incident light, resulting in a strong oscillation of the surface electrons, commonly known as a localized surface plasmon resonance (LSPR) mode.20 This phenomenon is illustrated in Figure 1A. Structures that support LSPRs experience a uniform Eo when excited by light as their dimensions are much smaller than the wavelength of the light.



Figure 1

Schematic illustration of the two types of plasmonic nanostructures discussed in this article as excited by the electric field (Eo) of incident light with wavevector (k). In (A) the nanostructure is smaller than the wavelength of light and the free electrons ...



In contrast, PSPs are supported by structures that have at least one dimension that approaches the excitation wavelength, as shown in Figure 1B.4 In this case, the Eo is not uniform across the structure and other effects must be considered. In such a structure, like a nanowire for example, SPs propagate back and forth between the ends of the structure. This can be described as a Fabry-Perot resonator with resonance condition l=nλsp, where l is the length of the nanowire, n is an integer, and λsp is the wavelength of the PSP mode.21,22 Reflection from the ends of the structure must also be considered, which can change the phase and resonant length. Propagation lengths can be in the tens of micrometers (for nanowires) and the PSP waves can be manipulated by controlling the geometrical parameters of the structure.23

/pdf/controlling-the-synthesis-and-assembly-of-silver-1ktlgomyt6.pdf

Controlling the synthesis and assembly of silver nanostructures for plasmonic applications

Noble metal nanoparticles are capable of confining resonant photons in such a manner as to induce coherent surface plasmon oscillation of their conduction band electrons, a phenomenon leading to two important properties. Firstly, the confinement of the photon to the nanoparticle's dimensions leads to a large increase in its electromagnetic field and consequently great enhancement of all the nanoparticle's radiative properties, such as absorption and scattering. Moreover, by confining the photon's wavelength to the nanoparticle's small dimensions, there exists enhanced imaging resolving powers, which extend well below the diffraction limit, a property of considerable importance in potential device applications. Secondly, the strongly absorbed light by the nanoparticles is followed by a rapid dephasing of the coherent electron motion in tandem with an equally rapid energy transfer to the lattice, a process integral to the technologically relevant photothermal properties of plasmonic nanoparticles. Of all the possible nanoparticle shapes, gold nanorods are especially intriguing as they offer strong plasmonic fields while exhibiting excellent tunability and biocompatibility. We begin this review of gold nanorods by summarizing their radiative and nonradiative properties. Their various synthetic methods are then outlined with an emphasis on the seed-mediated chemical growth. In particular, we describe nanorod spontaneous self-assembly, chemically driven assembly, and polymer-based alignment. The final section details current studies aimed at applications in the biological and biomedical fields.

Gold Nanorods: From Synthesis and Properties to Biological and Biomedical Applications

Seed-mediated growth of large, monodisperse core–shell gold–silver nanoparticles with Ag-like optical properties

We report the synthesis and characterization of gold core palladium shell (Au@Pd) nanoparticles with thickness-controlled shell as an improved transition-metal substrate for surface-enhanced Raman scattering (SERS). By changing the molar ratio of H2PdCl4 to Au, the Pd shell thickness can be precisely controlled from a few nanometers down to ca. one monolayer. A series of characterizations were performed using transmission electron microscopy (TEM), UV−vis, SERS, and electrochemical techniques. The results confirmed the core−shell structure and the uniform and pinhole-free nature of the Pd shell, ensuring the properties of Pd without possible interference from Au. Consistent with theoretical prediction, the core−shell setting borrows high SERS activity from the Au core through the long-range electromagnetic enhancement in addition to the enhancement from the Pd shell itself. Moreover, their SERS activity can be optimized by the tunable shell thickness and core size. The nm-Au@Pd/Pd electrodes allow us to o...

Palladium-Coated Gold Nanoparticles with a Controlled Shell Thickness Used as Surface-Enhanced Raman Scattering Substrate

Surface-enhanced Raman spectroscopy study on the structure changes of 4-mercaptopyridine adsorbed on silver substrates and silver colloids

Silver nanowire (Ag-NW) thin films have emerged as a promising next-generation transparent electrode. However, the current Ag-NW thin films are often plagued by high NW–NW contact resistance and poor long-term stability, which can be largely attributed to the ill-defined polyvinylpyrrolidone (PVP) surface ligands and nonideal Ag–PVP–Ag contact at NW–NW junctions. Herein, we report a room temperature direct welding and chemical protection strategy to greatly improve the conductivity and stability of the Ag-NW thin films. Specifically, we use a sodium borohydride (NaBH4) treatment process to thoroughly remove the PVP ligands and produce a clean Ag–Ag interface that allows direct welding of NW–NW junctions at room temperature, thus greatly improving the conductivity of the Ag-NW films, outperforming those obtained by thermal or plasmonic thermal treatment. We further show that, by decorating the as-formed Ag-NW thin film with a dense, hydrophobic dodecanethiol layer, the stability of the Ag-NW film can be gr...

Direct Room Temperature Welding and Chemical Protection of Silver Nanowire Thin Films for High Performance Transparent Conductors

This discussion focuses on our recent approaches at aiming to optimize surface-enhanced Raman scattering (SERS) activity for transition metals (group VIII B elements), by intentionally fabricating desired surface nanostructures or synthesizing nanoparticles. The SERS activity of transition metals critically depends on the surface morphology of electrodes and on size, shape and aggregation form of nanoparticles. A correct surface roughening procedure for transition-metal electrodes is indispensable for fabricating cauliflower-like nanostructures that show a higher SERS activity. Two more methods have been explored to synthesize nanoparticles, i.e., cubic nanoparticles and gold-core palladium-shell nanostructures, respectively. Their SERS activities are considerably higher than those of normal spherical mono-metallic nanoparticles. To explain these observations, a preliminary theoretical calculation, using the three-dimensional finite difference time domain (3D-FDTD) method, was performed to evaluate the local electromagnetic field on transition metal surfaces. The result is in good agreement with the experimental data.

Jiawen Hu

Papers

Seed-mediated growth of large, monodisperse core–shell gold–silver nanoparticles with Ag-like optical properties

Palladium-Coated Gold Nanoparticles with a Controlled Shell Thickness Used as Surface-Enhanced Raman Scattering Substrate

Surface-enhanced Raman spectroscopy study on the structure changes of 4-mercaptopyridine adsorbed on silver substrates and silver colloids

Direct Room Temperature Welding and Chemical Protection of Silver Nanowire Thin Films for High Performance Transparent Conductors

Surface-enhanced Raman scattering from transition metals with special surface morphology and nanoparticle shape.