Modified electrodes Raman spectroscopy

Jiang, X., Wang, Y.L, Qu, X.H., and Dong, S.L (2006) Surface-enhanced resonance Raman spectroscopy and spectroscopy study of redox-induced conformational equilibrium of cytochrome c adsorbed on DNA-modified metal electrode. Biosensors Bioelectronics, 22,49-55. 

While electrochemical methods provide powerful and sensitive ways of studying modified electrodes to provide information about electron-transfer kinetics and film porosity, they cannot provide information about structure or elemental composition. Thus complete characterization requires application of many of the nonelectrochemical methods described in Chapter 17. These encompass microscopy, high vacuum surface analysis, Raman and IR spectroscopy, and methods based on scanning probes, the quartz crystal microbalance, and measurements of contact angles. 

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. 

It is clear from this concluding discussion that the theory of simple electrode reactions must be modified considerably to deal with the majority of electrode processes. From the experimental point of view, it is important to measure the anodic and cathodic Tafel slopes as well as the reaction order with respect to all possible reactants (including the supporting electrolyte and the solvent). At the same time, non-electrochemical methods such as infrared and Raman spectroscopy [31,32] can provide direct molecular information which is essential for the proper understanding of electrode processes. 

Kania S, Holze R (1998) Surface enhanced Raman spectroscopy of anions adsorbed on foreign metal modified gold electrodes. Surf Sci 408 252... 

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