To further characterize the anodic process, pH studies of
insulin electrooxidation at CNT-based electrodes were performed.
Figure 4 shows that increasing the pH of the solution facilitated
the electrooxidation of insulin. This was consistent with the
facilitation of the proton-release electrode process at higher pH.
The plot of the onset potential for the oxidation of insulin versus
solution pH had a slope of -61 mV ¢pH-1 (R2 ) 0.987, pH 2.5-
12.0), which implied the electrode process with an H+/e- ratio
equal to 1. Such behavior can be accounted for by assuming that
the oxidation process involves redox-active tyrosine residues in
the insulin molecule. The oxidation of the amino acid tyrosine
involves its hydroxyl group and results in a release of a proton;
i.e., C-OH f CdO + H+ + e-.