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)62+/3+, Fe(CN6)3-/4-, ascorbic acid, and Feaq3+/2+.
The nine systems were categorized according to their kinetic sensitivity
to surface modification. Several, including Ru(NH3)62+/3+, are insensitive
to surface modifications and are considered outer sphere. Feaq3+/2+,
Vaq2+/3+, and Euaq2+/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)63-/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.
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