Barrier properties of organic monolayers on glassy carbon electrodes

The barrier properties of phenyl layers covalently attached to glassy carbo n electrodes by the aryldiazonium reduction method have been interpreted us ing a model of electron transfer at defect sites and closely spaced microsc opic pinholes. The surface coverage of phenyl groups determines the effecti ve average thickness of the modifying layer which is most likely less than that of a closely packed monolayer of phenyl groups. The voltammetric respo nses of eight redox probes in aqueous media at polished and modified glassy carbon electrodes were examined in order to evaluate the barrier propertie s. With the exception of the MV+/0 couple, the films are much more blocking toward electron-transfer reactions of solution species than is predicted o n the basis of the average film thickness. Comparisons of pairs of redox co uples show that the electron-transfer kinetics of hydrophobic probes are sl owed less than those of hydrophilic probes at the modified electrodes. This finding supports the notion that hydrophobic/hydrophilic interactions betw een solution species and the monolayer restricts the approach of redox prob es to the monolayer surface, forcing electron transfer at the modified elec trodes to occur over a distance significantly greater than that defined by the monolayer. Under experimental conditions where adsorption of MV+ and MV 0 is not important, electrode modification has no apparent effect on the ki netics of the MV+/0 couple, indicating that these redox species interact cl osely with the phenyl layer.