Stability of Substituted Phenyl Groups Electrochemically Grafted at Carbon Electrode Surface

The electrochemical reduction of an aryl diazonium tetrafluoroborate salt, dissolved in acetonitrile, at a carbon electrode surface allowed the grafting of aryl groups with the formation of a carbon-carbon bond. Groups such as 4-carboxyphenyl, 4-nitrophenyl, 4-diethylaniline (DEA), and 4-bromophenyl were grafted at a glassy carbon electrode surface. The stability of these grafted groups, present at the glassy carbon electrode surface, was studied at various electrode potentials in aqueous media. In appropriate experimental conditions, the as-grafted groups severely inhibit the cyclic voltammetry response of selected redox probes. Thus, the reappearance and/or increase of an electrochemical response, after polarization, was taken as an indication that a modification of the grafted layer occurred. Our results demonstrated that polarization at very positive (ca. 1.8 V) and negative (ca. -2 V) potentials is needed to observe an electrochemical response. Electrochemical impedance and X-ray photoelectron spectroscopies were also used to investigate the stability of the grafted layers. The impedance data usually tracks fairly well the cyclic voltammetry results, although the former appears to be more sensitive to changes that are occurring upon polarization of the modified electrode. Interestingly, the XPS data indicate clearly that the grafted layer is not always completely removed at the extreme positive and negative potentials investigated. A mechanism was proposed to explain the transformation occurring during polarization of the modified electrode and involves desorption of the substituted aryl groups during the concomitant hydrogen, oxygen, or chlorine evolution and finally leaving close to a covalently bonded monolayer of the grafted species at the electrode surface.