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The oxygen reduction reaction (ORR) in neural stimulation electrochemically generates reactive oxygen species, which if diffused away into the implanted tissue, might cause tissue damage. Therefore the ORR should be as minimized in neural stimulation (JT.Morton, 1994; S.F Morgan, 2010; EM Hudak, 2017). Boron-doped diamond electrodes have attracted considerable interest as a bio-interfacing electrode material, because of their biocompatibility, wide water window, and electrochemical stability; they also have been reported to have slow kinetics for the ORR. However, few investigations have focused on how its sp² carbon content, or non-diamond content, can contribute to oxygen reduction. Moreover, ultranano-crystaline diamond (sp² abundant) was recently reported in neural interface applications, justifying the need for further understanding the effect of sp² carbon.

In this work, we investigate how sp² coverage affects the ORR on polycrystalline diamond film electrodes, and also how other factors such as anodic oxidation increase ORR activity. Electrochemical redox activity of Fe^2+/3+ and Raman spectroscopy were used to characterize sp² content, and X-ray Photo electron spectroscopy to probe surface oxygen functional groups, before and after anodic oxidation. The electrode’s cathodic charge storage capacity (cCSC), as a measure of the relative contribution of oxygen electrochemistry in charge injection, was compared with cyclic voltammetry (200mV/s scan rate) in N2 and O2 saturated phosphate buffered saline (PBS). Preliminary results showed that diamond electrodes containing surface sp² carbon showed more activity towards oxygen reduction than those containing less sp² carbon. In addition, we found that if sp² containing electrode was anodically oxidized by potential scanning in dilute H₂SO₄, the cathodic current (presumably oxygen reduction current) increased. The increase in ORR activity after anodic oxidation could be explained by surface chemistry change or increased surface roughness (thus effective surface area). Since anodic polarization can oxidize an as-grown, primarily hydrogen-terminated diamond surface, and reaching anodic potentials during neural stimulation is inevitable, the effect of anodic oxidation should also be considered when evaluating the effect of sp² carbon. In the future, possible products of ORR will be investigated, and the ORR process investigated using faster scan rates to match the time scale of neural stimulation methods.