Electron Transfer at Self-Assembled Monolayers Measured by Scanning Electrochemical Microscopy

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Abstract
New approaches have been developed for measuring the rates of electron transfer (ET) across self-assembled molecular monolayers by scanning electrochemical microscopy (SECM). The developed models can be used to independently measure the rates of ET mediated by monolayer-attached redox moieties and direct ET through the film as well as the rate of a bimolecular ET reaction between the attached and dissolved redox species. By using a high concentration of redox mediator in solution, very fast heterogeneous (108 s-1) and bimolecular (1011 mol-1 cm3 s-1) ET rate constants can be measured. The ET rate constants measured for ferrocene/alkanethiol on gold were in agreement with previously published data. The rates of bimolecular heterogeneous electron transfer between the monolayer-bound ferrocene and water-soluble redox species were measured. SECM was also used to measure the rate of ET through nonelectroactive alkanethiol molecules between substrate gold electrodes and a redox probe (Ru(NH3)63+) freely diffusing in the solution, yielding a tunneling decay constant, β, of 1.0 per methylene group.

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