Surface-enhanced Raman spectroscopy (SERS) inherits the rich chemical fingerprint information on Raman spectroscopy and gains sensitivity by plasmon-enhanced excitation and scattering. In particular, most Raman peaks have a narrow width suitable for multiplex analysis, and the measurements can be conveniently made under ambient and aqueous conditions. These merits make SERS a very promising technique for studying complex biological systems, and SERS has attracted increasing interest in biorelated analysis. However, there are still great challenges that need to be addressed until it can be widely accepted by the biorelated communities, answer interesting biological questions, and solve fatal clinical problems. SERS applications in bioanalysis involve the complex interactions of plasmonic nanomaterials with biological systems and their environments. The reliability becomes the key issue of bioanalytical SERS in order to extract meaningful information from SERS data. This review provides a comprehensive over...

Surface-Enhanced Raman Spectroscopy for Bioanalysis: Reliability and Challenges

Metallic nanostructures attract much interest as an efficient media for surface-enhanced Raman scattering (SERS). Significant progress has been made on the synthesis of metal nanoparticles with various shapes, composition, and controlled plasmonic properties, all critical for an efficient SERS response. For practical applications, efficient strategies of assembling metal nanoparticles into organized nanostructures are paramount for the fabrication of reproducible, stable, and highly active SERS substrates. Recent progress in the synthesis of novel plasmonic nanoparticles, fabrication of highly ordered one-, two-, and three-dimensional SERS substrates, and some applications of corresponding SERS effects are discussed.

Nanostructured Surfaces and Assemblies as SERS Media

After over 30 years of development, surface-enhanced Raman spectroscopy (SERS) is now facing a very important stage in its history. The explosive development of nanoscience and nanotechnology has assisted the rapid development of SERS, especially during the last 5 years. Further development of surface-enhanced Raman spectroscopy is mainly limited by the reproducible preparation of clean and highly surface enhanced Raman scattering (SERS) active substrates. This review deals with some substrate-related issues. Various methods will be introduced for preparing SERS substrates of Ag and Au for analytical purposes, from SERS substrates prepared by electrochemical or vacuum methods, to well-dispersed Au or Ag nanoparticle sols, to nanoparticle thin film substrates, and finally to ordered nanostructured substrates. Emphasis is placed on the analysis of the advantages and weaknesses of different methods in preparing SERS substrates. Closely related to the application of SERS in the analysis of trace sample and unknown systems, the existing cleaning methods for SERS substrates are analyzed and a combined chemical adsorption and electrochemical oxidation method is proposed to eliminate the interference of contaminants. A defocusing method is proposed to deal with the laser-induced sample decomposition problem frequently met in SERS measurement to obtain strong signals. The existing methods to estimate the surface enhancement factor, a criterion to characterize the SERS activity of a substrate, are analyzed and some guidelines are proposed to obtain the correct enhancement factor.

Surface-enhanced Raman spectroscopy: substrate-related issues

A Review on Surface-Enhanced Raman Scattering.

Direct alcohol fuel cells (DAFCs) based on liquid fuels have attracted enormous attention as power sources for portable electronic devices and fuel-cell vehicles owing to the much higher energy density of liquid fuels than gaseous fuels such as hydrogen (e.g., the energy densities of ethanol and methanol are 6.34 kWh L and 4.82 kWh L, respectively, as compared to 0.53 kWh L for gaseous hydrogen at 20 MPa. Among various liquid fuels, ethanol is particularly attractive because it is less toxic than methanol and can be produced in large quantities from agricultural products. Ethanol is also the major renewable biofuel from the fermentation of biomass. Pt and Pt-based catalysts such as PtRu/C have been extensively investigated as electrocatalysts for the electrooxidation of liquid fuels such as methanol and ethanol. However, the high cost and limited supply of Pt constitute a major barrier to the development of DAFCs. We studied recently Ptfree electrocatalysts for the electrooxidation reactions of ethanol and methanol, and the results revealed that Pd is a good electrocatalyst for ethanol oxidation in alkaline media. Metal nanowire arrays (NWAs) have attracted much attention and interest owing to their excellent physical and chemical properties and have been extensively investigated for applications such as high-density magnetic recording devices and sensors. Ordered nanowire or nanotube arrays have also been applied for methanol oxidation and H2O2 electrocatalytic reduction because of their high active surface area. Among various techniques to synthesize NWAs, the anodized aluminum oxide (AAO) template method is probably the simplest and most versatile approach to creating highly ordered metal NWAs with uniform and tunable porous structure and good mechanical and thermal stability. Electrodeposition has been shown to be an efficient method for the growth of uniform and continuous metallic nanowire arrays. Despite the exceptional physical, chemical, and electrical properties of metal NWAs, there is little information on the electrocatalytic properties of Pd NWAs. Here, we report the fabrication of highly ordered Pd NWA electrodes by the AAO templateelectrodeposition method, and the results show that Pd NWAs are highly active for ethanol oxidation in alkaline media, demonstrating the potential of applying Pd NWAs as effective electrocatalysts for DAFCs. Figure 1 shows typical scanning electron microscopy (SEM) images of Pd NWAs after the AAO template has been fully dissolved. The Pd nanowires (NWs) are highly ordered with uniform diameter and length. The average length and diameter of the Pd NWs are ca. 800 and ca. 80 nm, respectively. The NWs are uniform, well isolated, parallel to one another, and standing vertically to the electrode substrate surface. The hexagonal shape of the Pd NWs is due to the AAO porous structure during anodization. The X-ray diffraction (XRD) pattern (inset in Fig. 1a) indicates that the Pd NWAs exhibit a typical face-centered cubic (fcc) lattice structure. The strong diffraction peaks at 40.10°, 46.49°, and 68.08° correspond to the (111), (200), and (220) facets of Pd. This indicates that Pd NWAs have been successfully fabricated. Figure 2a shows cyclic voltammograms (CVs) of ethanol oxidation in a 1.0 M KOH + 1.0 M C2H5OH solution on a Pd film electrode (curve a, Pd loading: 1.10 mg cm), a Pd NWA electrode (curve c, Pd loading: 0.24 mg cm), and an E-TEK PtRu/C electrode (curve b, Pt loading: 0.24 mg cm). The cyclic voltammograms of the Pd film, E-TEK PtRu/C, and Pd NWA electrodes in 1.0 M KOH solution without ethanol are shown in Figure 2b. In the CVs obtained in 1.0 M KOH electrolyte solution, the anodic peaks appearing between –0.73 and –0.53 V versus Hg/HgO on Pd and –0.8 and –0.5 V versus Hg/HgO on Pt originate from the desorption of atomic hydrogen on the electrocatalysts (Fig. 2b). Thus the area of H desorption after the deduction of the double layer region on the CV curves represents the charge passed for the H desorption, QH, and is proportional to the electrochemically active area (EAA) of the electrocatalysts. The value QH = 10.6 mC cm –2 for the Pd NWA electrode is much higher than 3.4 mC cm for the Pd film electrode and 4.6 mC cm for the E-TEK PtRu/C electrode. This shows that the Pd NWA electrode has high EAA, most likely due to the well-defined and uniform porous structure of the nanowires in the arrays (Fig. 1). Such well-defined nanowire structure enhances the active sites for the electrooxidation reaction of ethanol. The high electrocatalytic activity of the Pd NWA electrode is also indicated by its superior performance for the electrooxC O M M U N IC A TI O N

Highly Ordered Pd Nanowire Arrays as Effective Electrocatalysts for Ethanol Oxidation in Direct Alcohol Fuel Cells

Raman spectroscopy, which is based on the inelastic scattering of photons by chemical entities, has been successfully utilized for the investigation of adsorbed molecules on surfaces, although the low cross section limits its applications. Surface-enhanced Raman scattering (SERS) has drawn a lot of attention since its discovery in 1974, primarily because it can greatly enhance the normally weak Raman signal and thereby facilitate the convenient identification of the vibrational signatures of molecules in chemical and biological systems. Recently, the observation of single-molecule Raman scattering has further enhanced the Raman detection sensitivity limit and widened the scope of SERS for sensor applications. Although SERS effects can be achieved simply by exploiting the electromagnetic resonance properties of roughened surfaces or nanoparticles of Au or Ag, the fabrication of reliable SERS substrates with uniformly high enhancement factors remains the focus of much research. Spraying Au or Ag colloids on a substrate leads to an extremely high SERS signal at some local ‘hot-junctions’; however, it is not easy to achieve a reliable, stable, and uniform SERS signal spanning a wide dynamical range using this method. Van Duyne and coworkers have used nanosphere lithography, while Liu and Lee exploited soft lithography, in order to fabricate Ag nanoparticle arrays with high SERS activity and improved uniformity. Kall and co-workers have shown theoretically that the effective Raman cross section of a molecule placed between two metal nanoparticles can be enhanced by more than 12 orders of magnitude. Such enhancement is likely to be related to the ‘hot-junctions’ observed in some SERS experiments. Several theoretical groups have also investigated field enhancement for SERS from metal nanoparticle arrays. Specifically, Garcia–Vidal and Pendry proposed that very localized plasmon modes, created by strong electromagnetic coupling between two adjacent metallic objects, dominate the SERS response in an array of nanostructures. The interparticle-coupling-induced enhancement was attributed to the broadening of the plasmon resonance peak because the probability of the resonance covering both the excitation wavelength and the Raman peak increases with its width. They calculated the average enhancement factor over the surfaces of an array of infinitely long Ag nanorods with semicircular cross sections, and showed that significant near-field interaction occurs between adjacent nanorods when the gap between the nanorods reaches half the value of their diameter. Other groups have studied the dependence of the enhancement factor on the gap between adjacent nanoparticles on a SERS active substrate. For example, Gunnarsson et al. investigated SERS on ordered Ag nanoparticle arrays with an interparticle gap above 75 nm. Lee and co-workers were able to achieve the temperature-controlled variation of interparticle gaps between Ag nanoparticles embedded in a polymer membrane. Wei et al. performed SERS on self-organized Au nanoparticle arrays with narrow interparticle gaps, although they have not carried out a detailed investigation of the dependence of the SERS signal on the interparticle gap. Sauer et al. investigated SERS from nanowire arrays embedded in an alumina matrix with interparticle gaps of ∼ 110 nm, but no gap-related enhancement was observed in their experiment. These theoretical and experimental studies indicate that the precise control of gaps between nanostructures on a SERS-active substrate in the sub-10 nm regime, which is extremely difficult to obtain by existing nanofabrication methods, is likely to be critical for the fabrication of substrates with uniformly high enhancement factors, and for understanding collective surface plasmons existing inside the gaps. C O M M U N IC A IO N S

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Tsu-Hsin Chan

Papers

Highly Raman‐Enhancing Substrates Based on Silver Nanoparticle Arrays with Tunable Sub‐10 nm Gaps