Electron transfer kinetics of catechols on modified glassy carbon electrodes

Catechols have been studied extensively for their biological significance and for fundamental electrochemical issues, and undergo a two electron, two proton oxidation on glassy carbon electrodes in the pH range 1 to 8. The oxidation rate of catechols on solid electrodes is dependent on surface condition. This work represents and effort to characterize catechol electron transfer on glassy carbon electrodes. Catechol oxidation kinetics on glassy carbon electrodes was studied by targeted surface modifications in order to gain insight into what factors control catechol oxidation. Catechol electron transfer kinetics were found to be enhanced on electrodes that had been cleaned with certain solvents and surfaces that had low oxide content from vacuum heat treatment. Modification of the glassy carbon electrode with an adsorbed monolayer of methylene blue, nitrophenyl, and trifluoromethylphenyl groups severely inhibited catechol electron transfer rates. Nitrophenyl and trifluoromethylphenyl were chemisorbed to the electrode surface via electrochemical reduction of the corresponding diazonium salt, producing a compact monolayer. Adsorption of the catechol to the electrode surface is necessary for fast catechol electron transfer.; The relationship between catechol adsorption and fast catechol electron transfer kinetics was examined. Two general mechanisms were considered: a stepwise adsorption, electron transfer, desorption mechanism and a mechanism whereby catalysis of catechol electron transfer occurs through an adsorbed layer of catechol. A stepwise mechanism was found not to be consistent with desorption studies. Different quinones were adsorbed to the surface to investigate the effect of changing the identity of the adsorbed quinone species on catechol electron transfer. While non-quinone monolayers blocked catechol adsorption and catechol electron transfer, an adsorbed layer of quinone can catalyze solution catechol electron transfer even though catechol adsorption is suppressed. This observation is discussed in light of several possible mechanisms for catechol electron transfer.; Diazonium modified surfaces were used on redox systems that represent commonly encountered problems in electroanalytical applications. The stability of diazonium modified surfaces with air exposure and upon potential cycling and potentiostatting was examined with cyclic voltammetry and x-ray photoelectron spectroscopy. Diazonium modified surfaces were found to be significantly more stable than bare glassy carbon surfaces.