Direct electron transfer, explanation

One difficulty in testing for direct electrochemical oxidation of the enzyme by a polymer is that small amounts of mediator species can be very efficiently cycled within the film, giving the appearance of a direct electron transfer mechanism. It is therefore necessary to be very careful to consider all other possibilities before assuming that such a mechanism is the only explanation available. 

In order to understand the observed shift in oxidation potentials and the stabilization mechanism two possible explanations were forwarded by Kotz and Stucki [83], Either a direct electronic interaction of the two oxide components via formation of a common 4-band, involving possible charge transfer, gives rise to an electrode with new homogeneous properties or an indirect interaction between Ru and Ir sites and the electrolyte phase via surface dipoles creates improved surface properties. These two models will certainly be difficult to distinguish. As is demonstrated in Fig. 25, XPS valence band spectroscopy could give some evidence for the formation of a common 4-band in the mixed oxides prepared by reactive sputtering [83],... 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

An alternative explanation for the positive shift in the current-voltage curve onset potential upon illumination is that an overpotential exists for electron transfer directly out of the conduction band, which would be in keeping with the observation that /3 has a value of 0.45. However, the origin of such an overpotential is not readily explained and requires further study. 

The reader may desire an explanation of the low values of y derived by Barton and coworkers [30, 131, 137-140] from fluorescence quenching data for systems in which the dynamics of electron transfer have not been directly measured. In most cases, the absolute efficiency (quantum yield) of the quenching processes studied in these systems is rather low, and thus they may represent long-range electron transfer by mechanisms other than superexchange, such as to those described by Felts et al. [125], Davis and colleagues [126], and Okada et al. [127]. However, the author considers that it is highly unlikely that such processes occur with rate constants > 10 s . In view of the complex nature of these systems, the author is loath to offer a detailed interpretation, and refers the reader to commentaries by others who have been directly involved in this research [13, 15]. 

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