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Characterization of the electrochemically oxidized glassy carbon electrodes (GCEs-ox) by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and electrochemical methods suggested that a negatively charged and densely compact oxide film was formed on GCEs, which could block the electrochemical reaction of negatively charged Fe(CN)63-/4- and ascorbic acid (AA) and enhance the electrochemical response of dopamine (DA) due to electrostatic interaction. Electrochemical reduction of the GCEs-ox made the oxide film porous but still negatively charged. The porous oxide film on the reduced GCEs (GCEs-re) allowed negatively charged Fe(CN)63-/4- to reach the electrode surface for reaction because the pores with an average diameter of no less than 200 nm were big enough for Fe(CN)63-/4- to free from the electrostatic repulsion. It could further improve the sensitivity of positively charged DA because of their larger effective surface area than densely compact oxide film, but decrease the reproducibility of the GCEs-re owing to DA adsorption on the electrode substrate surface to poison the electrodes. DA as low as 8 × 10−9 M could be detected at a GCE-ox in the presence of 2 × 10−5 M AA.
Zhao et al. (Mon,) studied this question.