SERS effect

Plenary 13. Andreas Otto, e-mail address otto rz.imi-duesseldorf.de (SERS). A survey of problems and models that underlie the SERS effect, now two decades old. Understanding the role of surface rouglmess in the enliancement. 

Zhang, L., Fang, Y. and Zhang, P. (2008) Experimental and DFT theorehcal studies of SERS effect on gold nanowires array. Chemical Physics Letters, 451, 102-105. 

Surface-enhanced Raman scattering (SERS) is a candidates for resolving this issue. Since the SERS effect is observed only at metal surfaces with nanosized curvature, this technique can also be used to investigate nanoscale morphological structures of metal surfaces. It is thus worth investigating SERS under oscillatory electrodeposition conditions. The author of this chapter and coworkers recently reported that... 

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... 

Figure 2.53 Schematic representation of the SERS effect. For details see text.

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. 

For trace analysis in fluids, some Raman sensors (try to) make use of the SERS effect to increase their sensitivity. While the basic sensor layout for SERS sensors is similar to non-enhanced Raman sensors, somehow the metal particles have to be added. Other than in the laboratory, where the necessary metal particles can be added as colloidal solution to the sample, for sensor applications the particles must be suitably immobilised. In most cases, this is achieved by depositing the metal particles onto the surfaces of the excitation waveguide or the interface window and covering them with a suitable protection layer. The additional layer is required as otherwise washout effects or chemical reactions between e.g. sulphur-compounds and the particles reduce the enhancement effect. Alternatively, it is also possible to disperse the metal particles in the layer material before coating and apply them in one step with the coating. Suitable protection or matrix materials for SERS substrates could be e.g. sol-gel layers or polymer coatings. In either... 

Nevertheless, there has been a renewed interest in Raman techniques in the past two decades due to the discovery of the surface-enhanced Raman scattering (SERS) effect, which results from the adsorption of molecules on specially textured metallic surfaces. This large enhancement was first... 

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. 

Recently, the SERS effect has been used increasingly for highly sensitive sensor devices for biological and medical applications as well as in practical analytical chemistry (Alivisatos, 2004 Emory and Nie, 1998 Faulds et al., 2004 Ishikawa et al., 2002 Kneipp et al., 1995 Wang et al., 2003), and the field of catalysis certainly will see a comparable increase in SERS investigations of active metal catalysts (vide infra). 

The metallic nanocrystals are remarkable due to their localized surface plasmon resonance (SPR) phenomenon, that is, the excitation of surface plasma by light. It ensures these nanocrystals to be color based sensors (Homola et al., 1999 Kelly et al., 2003). The metallic nanocrystals could also sensitize the Raman signals from their adsorbed organic molecules. This surface enhanced Raman scattering (SERS) effect potentially raises the detection sensitivity to single molecule level (Kneipp et al., 1997 Nie and Emery, 1997). 

Tang, X. L., Jiang, P., Ge, G. L., Tsuji, M., Xie, S. S. and Guo, Y. J. (2008). Poly(N-vinyl-2-pyrrolidone) (PVP)-capped dendritic gold nanoparticles by a one-step hydrothermal route and their high SERS effect. Langmuir 24 1763-1768. 

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... 

For the past two decades, SERS has been widely studied due to its high sensitivity and specificity [1-5]. It is a powerful tool for determining vibrational spectra of probe molecules on roughened metal substrates [6, 7]. In general, there are two simultaneously operative mechanisms to describe the predominantly SERS effect ... 

The SERS method is therefore often used for a variety of applications in medicine, biophysics, and microelectronics. As the SERS effect seems to be a local phenomenon, SERS as a method delivering chemical or molecular information should be combined with spatial microscopy (Fig. 8.1). 

Although electromagnetic enhancement can explain important features of the SERS effect, it does not take into account in any way the chemical structure of the analyte species. Therefore, chemical enhancement models take into account the structural properties of the analyte and it is thought to operate independently from electromagnetic enhancement. Chemical enhancement can result from charge-transfer or bond formation between the metal and the analyte molecule which can result in an increase in the polarizability, ot, of the molecule. Therefore, a surface complex between the analyte and the metal must form before chemical enhancement can occur. 

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