Control of catechol and hydroquinone electron-transfer kinetics on native and modified glassy carbon electrodes

The electrochemical oxidation of dopamine, 4-methylcatechol, dihydroxyphenylacetic acid, dihydroxyphenyl ethylene glycol, and hydroquinone was examined on several native and modified glassy carbon (GC) surfaces. Treatment of polished GC with pyridine yielded small Delta E-p values for cyclic voltammetry of all systems studied, implying fast electron-transfer kinetics. Changes in surface oxide coverage had little effect on kinetics, nor did the charge of the catechol species or the solution pH. Small Delta E-p values correlated with catechol adsorption, and surface pretreatments that decreased adsorption also increased Delta E-p. Electron transfer from catechols was profoundly inhibited by a monolayer of nitrophenyl or (trifluoromethyl)phenyl (TFMP) groups on the GC surface, so that voltammetric waves were not observed. The Delta E-p increased monotonically with surface coverage of TFMP groups. The results indicate that catechol adsorption to GC is required for fast electron transfer for the redox systems studied. Unlike Ru(NH3)(6)(3+/2+), chlorpromazine, methyl viologen, and several others, electron tunneling through monolayer films was not observed for the catechols. The results are not consistent with an electron-transfer mechanism involving proton transfer or electrostatic interactions between the catechols and surface sites on the GC surface. The vital role of adsorption in the electron-transfer process is currently under study but appears to involve changes in the inner-sphere reorganization energy.