| Abstract
| - The electrochemical oxidation of dopamine, 4-methylcatechol, dihydroxyphenylacetic acid, dihydroxyphenyl ethylene glycol, and hydroquinone was examined on severalnative and modified glassy carbon (GC) surfaces. Treatment of polished GC with pyridine yielded small ΔEpvalues for cyclic voltammetry of all systems studied,implying fast electron-transfer kinetics. Changes in surfaceoxide coverage had little effect on kinetics, nor did thecharge of the catechol species or the solution pH. SmallΔEp values correlated with catechol adsorption, andsurface pretreatments that decreased adsorption alsoincreased ΔEp. Electron transfer from catechols wasprofoundly inhibited by a monolayer of nitrophenyl or(trifluoromethyl)phenyl (TFMP) groups on the GC surface,so that voltammetric waves were not observed. The ΔEpincreased monotonically with surface coverage of TFMPgroups. The results indicate that catechol adsorption toGC is required for fast electron transfer for the redoxsystems studied. Unlike Ru(NH3)63+/2+, chlorpromazine,methyl viologen, and several others, electron tunnelingthrough 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 siteson the GC surface. The vital role of adsorption in theelectron-transfer process is currently under study butappears to involve changes in the inner-sphere reorganization energy.
|
