Metals exhibiting SERS

In order to obtain high-quality SERS on silver (as well as on other metals exhibiting SERS) the electrodes have to be electro-chemically pretreated. As Schultz et and others have shown, this is not an absolute requirement, but it certainly helps in producing extremely large signals. 

A very large number of molecules adsorbed on or near the surface of metals exhibit SERS, but their SEE values could be very different. For example, the polarizabilities of CO and N2 are... 

SERS is used to study monolayers of materials adsorbed on metal substrates. Although the substrates are often used as electrodes, a wide variety of substrate formats have been found to exhibit SERS electrochemically modified electrodes [64], colloids, island films, particles grafted on silanized glasses [65-67], and more... 

Since most biomolecules normally exhibit medium or low Raman cross sections, an enhancement of the signal intensity for the ability to characterize even low concentrations would be preferable. Besides the application of resonance Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS) is a promising alternative. In doing so the vicinity of molecules to rough noble metal surfaces leads to Raman enhancement factors of 106-108 and even up to 1014 leading to a single molecule detection limit [9]. 

An example of the results for such an approach is shown in Fig. 2.5 (from Ref [1]), which we now discuss. Methylene Blue (MB) is used here as a probe molecule and is transferred onto gold NP arrays by dipping them into a 10 // M MB solution for 5 minutes. The arrays are plasma-cleaned before any dipping, to ensure that no contaminants are previously adsorbed on the NPs. The MB molecules are therefore adsorbed directly onto the gold surfaces, with possibly some molecules further away from the surface if several monolayers are present. Three NP arrays are used with distinct LSP resonances at 612 nm (array Al), 667 nm (A2) and 713 nm (A3), as evidenced in the extinction spectra of Fig. 2.5(b). The resonance of A2 is close to the peak absorption and fluorescence of MB and would be considered as the standard situation for most MEF experiments (except for the direct adsorption onto the metal). Al and A3 have resonances much further away on either side of the MB fluorescence spectrum. The MEF spectra (corrected for the ITO background), shown in Fig. 2.5(c), exhibit a broad spectrum underneath the SERS (Raman) peaks. The SERS peaks clearly confirm the presence of MB on the NPs surface. The accompanying broad signal is attributed to the modified MB fluorescence (MEF), initially for two reasons ... 

Thiophenol, also known as benzene thiol, is a small molecule that does not exhibit the pre-resonance issues of dye molecules. Thiophenol has been shown to produce relatively stable monolayers on silver and gold surfaces which make it a convenient reference compound for SERS substrates [28, 29, 38, 39]. When the SERS substrate is immersed in an ethanolic solution of thiophenol, the sulfur groups form covalent bonds with the metal surface, forming self-assembled mono-layers over time, as illustrated in Fig. 4.1. 

SIERA Surface-enhanced infrared absorption As in the case of surface-enhanced Raman scattering (SERS), molecules adsorbed on metal island films or particles exhibit intense infrared absorption several folds higher than what one would expect from conventional measurements without the metal. This effect is referred to as surface-enhanced infrared absorption (SEIRA). 

In spite of the sometimes conflicting experimental results, the four questions which were posed at the beginning of Section II.5 have been answered in full. It was unanimously found that the excitation spectra in SERS are significantly different than those of the molecules in solution. Even for the case of dyes it was found that the surface affects the profile. There is evidence that in many systems the SERS excitation profile does not follow the optical behavior of the system (as observed in absorption or reflection), let alone that of the bare metal. On the other hand, there are systems where such a correlation is apparent. At times the various modes of the same adsorbed molecule exhibit the same dependence on the exciting frequency, while in many other cases, different modes behave differently. The modes may be distinguished on the basis of the vibrational frequency (large, small) or their symmetry. 

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