In addition, a surface bound quasi-reversible process appeared with a CV peak separation potential of 150 mV (Fig. 3b). The existence of such a process has been observed on glassy carbon electrodes and carbon fibre microelectrodes following electrochemical pretreatment [11,14]. It has usually been attributed to the presence of a surface quinone-hydroquinone system. In this study, the process did not appear until the upper potential reached ca. 1.6 V. The amplitude of the anodic and cathodic peaks also increased as the applied potential range was extended anoditally. The potential required to generate the surface group corresponds to the potential required to activate the microelectrode towards folic acid. The coincidence of these results suggest that these two processes are related. However, while the
appearance of surface groups is indicative of activation, contrary reports on their significance appear in the literature. Edmonds and Guoliang [23] support the view that surface quinoidal functionalities generated by electrochemical pretreatment were responsible for the increased sensitivity observed for carbon fibre microelectrodes. Other workers [24] have reported that quinone groups introduced onto the surface of glassy carbon electrodes do not serve as electron mediators for oxygen reduction.