Barrier properties of organic monolayers on glassy carbon electrodes

The barrier properties of phenyl layers covalently attached to glassy carbon electrodes by the aryldiazonium reduction method have been interpreted using a model of electron transfer at defect sites and closely spaced microscopic pinholes. The surface coverage of phenyl groups determines the effective average thickness of the modifying layer which is most likely less than that of a closely packed monolayer of phenyl groups. The voltammetric responses of eight redox probes in aqueous media at polished and modified glassy carbon electrodes were examined in order to evaluate the barrier properties. With the exception of the MV+/0 couple, the films are much more blocking toward electron-transfer reactions of solution species than is predicted on the basis of the average film thickness. Comparisons of pairs of redox couples show that the electron-transfer kinetics of hydrophobic probes are slowed less than those of hydrophilic probes at the modified electrodes. This finding supports the notion that hydrophobic/hydrophilic interactions between solution species and the monolayer restricts the approach of redox probes to the monolayer surface, forcing electron transfer at the modified electrodes to occur over a distance significantly greater than that defined by the monolayer. Under experimental conditions where adsorption of MV+ and MV0 is not important, electrode modification has no apparent effect on the kinetics of the MV+/0 couple, indicating that these redox species interact closely with the phenyl layer.