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 ∆Ep 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 ∆Ep values correlated with catechol adsorption, and surface pretreatments that decreased adsorption also increased ∆Ep. 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 ∆Ep 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, chlorpromazine, methyl viologen, and several others, electron tunneling through monolayer films was not observed for the catechols. The results are not consistent with an electrontransfer 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.