Surface-enhanced Raman spectroscopy pyridine

Intensity enhancement takes place on rough silver surfaces. Under such conditions, Raman scattering can be measured from monolayers of molecular substances adsorbed on the silver (pyridine was the original test case), a technique known as surface-enhanced Raman spectroscopy. More recendy it has been found that sur-fiice enhancement also occurs when a thin layer of silver is sputtered onto a solid sample and the Raman scattering is observed through the silver. 

Ever since Albrecht and Creighton [85], and Jenmarie and van Duyne [86], observed that the Raman cross-section for pyridine absorbed on a roughened silver electrode was larger than that in solution by six orders of magnitude, surface enhanced Raman spectroscopy (SERS) has steadily gained ground in analytical instrumentation. The sensitivity of this technique... 

An investigation of the adsorption of pyrazine and pyridine on nickel electrodes by in situ surface-enhanced Raman spectroscopy was reported in [44]. The result suggests that both pyrazine and pyridine were strongly adsorbed onto the substrates. It also implies that pyridine was adsorbed perpendicularly onto the substrate, while pyrazine adsorbed onto the substrate in a slightly tilted vertical configuration. 

A related technique is based on the fact that signals from adsorbed species are much larger than from the same species in solution (surface enhanced Raman spectroscopy, SERS)23,24. The phenomenon was first noted in a study of the adsorption of pyridine on silver electrodes25, and has been extended to the investigation of the adsorption of many species such as, for example, porphyrins. 

Wu, D.Y., Ren, B., Xu, X. et al. (2003) Periodic trends in the bonding and vibrational couphng pyridine interacting with transition metals and noble metals studied by surface-enhanced Raman spectroscopy and density-functional theory. The Journal of Physical Chemistry, 119, 1701-1709. 

Mayer P, Holze R (2003) Pyridine as a probe molecule for surface enhanced Raman spectroscopy of the silver-modified glassy carbon/solution interface. Surf Sci 522 55... 

Besides infrared reflection spectroscopy, Raman spectroscopy, particularly surface-enhanced Raman spectroscopy (SERS), is able to provide information about the structure of the electrode/electrolyte interface through the identification of adsorbed species from their vibrational spectra/ However, Raman spectroscopy is not of as general use as infrared spectroscopy, and the basis of the enhancement mechanism, which is observed only for a limited number of systems (e.g., adsorption of pyridine on a silver electrode), is not yet very clear/ Surface-enhanced Raman scattering is discussed by Birke and Lombardi in Chapter 6 of this book. 

SERS. A phenomenon that certainly involves the adsorbent-adsorbate interaction is that of surface-enhanced resonance Raman spectroscopy, or SERS. The basic observation is that for pyridine adsorbed on surface-roughened silver, there is an amazing enhancement of the resonance Raman intensity (see Refs. 124—128). More recent work has involved other adsorbates and colloidal... 

Zeolites. The weak Raman signals arising from the aluminosilicate zeolite framework allow for the detection of vibrational bands of adsorbates, especially below 1200 cm which are not readily accessible to infrared absorption techniques. Raman spectroscopy is an extremely effective characterization method when two or more colored species coexist on the surface, since the spectrum of one of the species may be enhanced selectively by a careful choice of the exciting line. A wide range of adsorbate/zeolite systems have been examined by Raman spectroscopy and include SO2, NO2, acety-lene/polyacetylene, dimethylacetylene, benzene, pyridine, pyrazine, cyclopropane, and halogens. Extensive discussions of these absorbate/zeolite studies are found in a review article by Bartlett and Cooney. ... 

Using standard Raman spectroscopy for surface studies leads to rather tedious experiments because of its generally very low sensitivity. In contrast, SERS often provides high sensitivity due to a giant enhancement. The first paper on SERS pubhshed by the Van Duyne group reported a million-fold enhanced Raman intensity for pyridine molecules adsorbed on a silver electrode compared with the Raman intensity for unbound pyridine molecules [21]. Hence, SERS makes it possible to detect submonolayer quantities of adsorbates. 

Pyridine and pyrazine have received considerable attention in Raman spectroscopy because of their tremendously enhanced Raman scattering cross section when adsorbed on activated Ag surfaces. The activation consisted of an oxidation-reduction cycle in the presence of the Raman-active... 

As discussed before in the case of nucleic acids the authors have also considered the incidence of the interfacial conformation of the hemoproteins on the appearance of the SERRS signals from the chromophores. Although under their Raman conditions no protein vibration can be observed, the possibility of heme loss or protein denatura-tion are envisaged to explain a direct interaction of the heme chromophores with the electrode surface in the case of the adsorl Mb. extensive denaturation of Cytc at the electrode appears unlikely to the authors on the basis of the close correspondence of the surface and solution spectra. Furthermore, the sluggish electron transfer kinetics measured by cyclic voltammetry in the case of Cytc is also an argument in favour of some structural hindrance for the accessibility to the heme chromophore in the adsorbed state of Cytc. This electrochemical aspect of the behaviour of Cytc has very recently incited Cotton et al. and Tanigushi et al. to modify the silver and gold electrode surface in order to accelerate the electron transfer. The authors show that in the presence of 4,4-bipyridine bis (4-pyridyl)disulfide and purine an enhancement of the quasi-reversible redox process is possible. The SERRS spectroscopy has also permitted the characterization of the surface of the modified silver electrode. It has teen thus shown, that in presence of both pyridine derivates the direct adsorption of the heme chromophore is not detected while in presence of purine a coadsorption of Cytc and purine occurs In the case of the Ag-bipyridyl modified electrode the cyclicvoltammetric and SERRS data indicate that the bipyridyl forms an Ag(I) complex on Ag electrodes with the appropriate redox potential to mediate electron transfer between the electrode and cytochrome c. 

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