Coulometric Detection of Irreversible Electrochemical Reactions Occurring at Pt Microelectrodes Used for Neural Stimulation

The electrochemistry of 50 μm diameter Pt electrodes used for neural stimulation was studied in vitro by reciprocal derivative chronopotentiometry. This differential method provides well-defined electrochemical signatures of the various polarization phenomena that occur at Pt microelectrodes and are generally obscured in voltage transients. In combination with a novel in situ coulometric approach, irreversible H2 and O2 evolution, Pt dissolution and reduction of dissolved O2 were detected. Measurements were performed with biphasic, charge-balanced, cathodic-first and anodic-first current pulses at charge densities ranging from 0.07 to 1.41 mC/cm2 (real surface area) in phosphate buffered saline (PBS) with and without bovine serum albumin (BSA). The extent to which O2 reduction occurs under the different stimulation conditions was compared in O2-saturated and deoxygenated PBS. Adsorption of BSA inhibited Pt dissolution as well as Pt oxidation and oxide reduction by blocking reactive sites on the electrode surface. This inhibitory effect promoted the onset of irreversible H2 and O2 evolution, which occurred at lower charge densities than those in PBS. Reduction of dissolved O2 on Pt electrodes accounted for 19–34% of the total injected charge in O2-saturated PBS, while a contribution of 0.4–12% was estimated for in vivo stimulation. These result may prove important for the interpretation of histological damage induced by neural stimulation and therefore help define safer operational limits.