Redox couples, surface-enhanced Raman spectroscopy

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

The active site on the surface of selective propylene anmioxidation catalyst contains three critical functionalities associated with the specific metal components of the catalyst (37—39) an CC-H abstraction component such as Bi3+, Sb3+, or Te4+ an olefin chemisorption and oxygen or nitrogen insertion component such as Mo6+ or Sb5+ and a redox couple such as Fe2+/Fe3+ or Ce3+/ Ce4+ to enhance transfer of lattice oxygen between the bulk and surface of the catalyst. The surface and solid-state mechanisms of propylene ammoxidation catalysis have been determined using Raman spectroscopy (40,41), neutron diffraction (42—44), x-ray absorption spectroscopy (45,46), x-ray diffraction (47—49), pulse kinetic studies (36), and probe molecule investigations (50). 

While the redox chemistry of metal- and flavin-based cofactors may be readily detected by voltammetry, that associated with thiol-disulfides and amino acids is not. It is also important to be aware that voltammetry by itself provides no direct insight into the chemical identity of the redox couple. This can be overcome by using electrodes that allow for simultaneous spectroscopic and voltammetric analysis of adsorbed proteins. Examples include Ag electrodes that allow for surface-enhanced resonance Raman spectroscopy (SERRS) and mesoporous nanocrystalline Sn02 electrodes that allow for electronic absorption or magnetic circular dichro-ism spectroscopies [3]. Another consideration is the need for a redox center to be positioned within ca. 14 A of the electrode, and so the surface of the protein, for facile interfacial electron exchange. As a consequence, proteins with only buried redox centers are not routinely addressed by direct electrochemical methods. 

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