RELATIONSHIP BETWEEN ELECTRONIC TUNNELING COEFFICIENT AND ELECTRODE POTENTIAL INVESTIGATED USING SELF-ASSEMBLED ALKANETHIOL MONOLAYERS ON GOLD ELECTRODES

We investigated the relationship between the electronic tunneling coefficient and the electrode potential. Monolayers of saturated long-chain alkanethiols, SH(CH2)NCH3 (n = 11, 13,15,17), were self-assembled from acetonitrile solutions onto gold electrodes. The closely packed and ordered monolayers were insulating and diminished the rate of electron transfer at the modified gold surface. The rate of electron transfer was the rate-limiting step in the overall electrode reaction pathway. The electronic tunneling coefficient was calculated by varing the thickness of the insulating layer (changing the number of carbon atoms in long-chain thiol) and measuring the change in the heterogeneous electron-transfer rate of the selected redox couple at a given potential. To get a more precise measure of the tunneling coefficient, the observed current was corrected using convolution techniques for diffusional depletion of the surface concentrations of both the oxidized and reduced forms of the redox probe. The electronic tunneling coefficient was determined to be 1.02 +/- 0.20 per methylene unit in the long-chain alkanethiol with the Fe(CN)63-/Fe(CN)64- redox couple. This same value was obtained when the Fe3+/Fe2+ redox couple was used. Least-squares analysis of the data showed that the electronic tunneling coefficient was nearly independent of the electrode potential but may be dependent upon the functional group at the long-chain alkanethiol terminus.