Control of Electron Transfer Kinetics at Glassy Carbon Electrodes by Specific Surface Modification

Various well-established and novel surface modification procedures were used on glassy carbon (GC) electrodes to yield surfaces with low oxide content or which lack specific oxide functional groups. In addition, monolayers of several different adsorbates were formed on GC surfaces before electrochemical evaluation. Both the nonspecific monolayer adsorbates and reagents which chemisorb to specific functional groups were observed on the surface with Raman and photoelectron spectroscopy. The various GC surfaces were then evaluated for their electron transfer reactivity with nine redox systems in aqueous electrolyte, including Ru(NH₃)₆^2+/3+, Fe(CN₆)^3-/4-, ascorbic acid, and Fe_aq^3+/2+. The nine systems were categorized according to their kinetic sensitivity to surface modification. Several, including Ru(NH₃)₆^2+/3+, are insensitive to surface modifications and are considered outer sphere. Fe_aq^3+/2+, V_aq^2+/3+, and Eu_aq^2+/3+ are catalyzed by surface carbonyl groups and are very sensitive to the removal of surface oxides or derivatization of C═O groups. Ascorbic acid and Fe(CN)₆^3-/4- constitute a third group which are not catalyzed by oxides but which do require a specific surface interaction. A procedure for classifying redox systems by their kinetics on modified carbon surfaces is proposed.