Silver SERS effect

The sensitivity limitations of TLC-FT-Raman spectroscopy may be overcome by applying the SERS effect [782]. Unlike infrared, a major gain in Raman signal can be achieved by utilising surface activation and/or resonance effects. Surface-enhanced Raman (SER) spectra can be observed for compounds adsorbed on (rough) metahic surfaces, usually silver or gold colloids [783,784], while resonance Raman (RR) spectra... 

The fact that the CT model is not sufficient to account for all SERS effects is most clearly illustrated by the work of Murray and Allara (1982), who used well-defined polymer films as separators between suitable test molecules and a roughened silver surface. 

Normal Raman laser excitation in the visible and NIR region (52) can be used to obtain the SERS effect. The substrate surface is extremely important in providing the necessary enhancement to make the technique as valuable as it has become. A number of substrates have been used (53). These include evaporated silver films deposited on a cold surface at elevated temperature ( 390 K) on a glass substrate, photochemically roughened surfaces (e.g., silver single crystals subjected to iodine vapor, which roughens the surface), grating surfaces, and mechanically abraded and ion-bombarded silver surfaces. 

Rycenga M, Camargo PHC, Li W, Moran CH, Xia Y (2010) Understanding the SERS effects of single silver nanoparticles and their dimers, one at a time. J Phys ChemLett 1 696... 

Stable silver sols were prepared by reduction of AgNOs (Aldrich, purity 99.998%) with excess NaBH4 (Aldrich, purity 99%), aged a week to prevent the formation of reduction products. The usual pH value of the aqueous suspension was about nine. NaCl (Aldrich purity 99.999%) was added in a small amount (10 M) to the Ag colloids to improve the SERS effect, without altering the sol stability. 

Recently, novel nanomaterials have become a new frontier for SERS experiments, where different metals are collected together to form, for example, bimetallic particles. Thus, the same nanoparticle could be responsible for both SERS effect and catalytic activity. This is the case of the Ag/Pd colloids synthesized by chemical reduction with sodium borohydride (NaBH4) of silver nitrate (AgNOs) and palladium nitrate (Pd(N03)2), with a 96 4 Ag/Pd molar ratio [11]. The silver nanoparticles provide the SERS enhancement for the ligand molecules, while palladium may induce catalytic reactions. Also, in this case, TEM microscopy provides an important help to characterize these composite materials. In Fig. 20.6 TEM images at different magnifications are reported for bimetallic Ag/Pd particles, in comparison with those constituted by pure silver. While these latter present spheroidal shapes, bimetallic particles show more irregularities, due to palladium clusters in contact with the silver core surface. 

The use of solid silver nanoparticles indicates the 10-fold enhancement of Raman signals due to presence of art pigments. The enhancement of the order of magnitude is not SERS effect. However it is high enough to identify very small quantities of such specific samples as art pigment from canvas paintings. The proposed technique does not require any complex procedures for sample preparation. 

Campbell and Creighton discussed the excitation profile for several dyes adsorbed on a silver electrode. They analyze the data in terms of a product of two Lorentzians one for the resonance enhancement (RR) and the other for the SERS. The assumption is that one can have both RR and SERS effects simultaneously. Their conclusion is that the excitation band of the dye on the surface is shifted compared to solution. The effect of the surface is indeed clearly seen. However, the same experimental spectra can be analyzed equally well also in terms of a sum of two Lorentzians, meaning that one has either RR or SERS effects. Only a careful estimate can distinguish between these two interpretations of the experimental results. 

McMahon showed that M,2-bis-(4-pyridyl)ethylene, (t-BPE), adsorbed on a silver electrode exhibits an excitation profile with a definite maximum at about 600 nm, and then increases again toward the red. The intensities were normalized to perchlorate ion scattering. These maxima have no counterpart in the solution absorption spectrum. This is a case of an off-resonance SERS effect. 

For a limited number of metal surfaces, adsorption of a molecular species in a thin (monomolecular layer) film results in a huge increase in the effective vibrational Raman scattering cross-section (again, as with RR scattering, up to ca. 106 times) of the adsorbate species. The SERS effect was discovered more than ten years ago for pyridine adsorbed at a silver electrode surface in contact with an aqueous electrolyte [1, 2]. In the intervening period, many hundreds of papers devoted to SERS phenomena have been published, extending the studies to other metals than silver, to non-aqueous as well as aqueous electrolytes, to colloidal dispersions of metals as well as metal electrodes, and even to vacuum-deposited thin film systems under UHV conditions. This review will concentrate on studies of metal-electrolyte interfaces. 

Since colloidal particles of silver and gold also give large SERS effects, it is useful to consider these as disconnected microelectrodes. Controlled variations in the potential at the interface between silver sol particles and electrolyte solution have been achieved by adding the Eu3+/Eu2+ redox system and varying the concentration ratio [22]. From measurements of SER spectra of pyridine adsorbed on the silver sol, the potential-dependent effects have been found to be identical with those given by bulk silver electrodes. These experiments also demonstrated that the intensity ratio of the pyridine Raman bands at ca. 1010 and 1040 cm-1 may be used as a simple but effective measure of sin-face potential for colloidal metals. 

Raman spectroscopy is not particularly surface sensitive but a surface enhanced Raman scattering (SERS) effect is observed on some metals (copper, gold, silver, nickel etc). On an appropriately prepared (roughened) surface or on metal colloids the surface coverage of molecules can be measured by Raman spectroscopy with high sensitivity. 

Since the discovery of the SERS effect in 1974, the number of publication on this topic increased exponentially. Of major interest is the development of suitable SERS-active substrates. On the one hand, research focuses on the development of reproducible SERS substrates. On the other hand, SERS substrates are specifically designed for a particular application. Figure 4 presents different kinds of SERS-active substrates. Widely used SERS substrates are colloid solutions as shown in Fig. 4a, b. Figure 4a presents aggregated silver colloid prepared according to Lee and Meisel. In cluster arranged gold nanoparticles are shown in Fig. 4b. Colloids are easy to prepare and easy to... 

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