Surface-enhanced Raman spectroscopy pyridine adsorption

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. 

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. 

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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