Understanding charge transfer on the clinically used conical Utah electrode array: charge storage capacity, electrochemical impedance spectroscopy and effective electrode area

Abstract

Objective. The Utah electrode is used for pre/clinical studies on neural recording and stimulation. Anecdotal and empirical reports on their performance have been made, resulting in variable testing methods. An in depth investigation was performed to understand the electrochemical behaviour and charge transfer mechanisms occurring on these clinically important electrodes. The impact of electrode geometry and material on performance was determined. Approach. Platinum and iridium electrodes were assessed by cyclic voltammetry and electrochemical impedance spectroscopy. The effective electrode area was measured by reduction of Ru(NH3)6 3+. Main results. Pristine Utah electrodes have little to no oxide present and the surface roughness is less than the diffusion length of Ru(NH3)6 3+ during voltammetry, which was ∼30 µm. Pristine iridium electrodes pass charge through capacitance and oxide formation. Hydride and anion adsorption occurs on the platinum electrode. Anodic current oxidises both metal surfaces, altering the charge transfer mechanisms at the electrode-solution interface. Charge storage capacity depends on measurement technique and electrode structure, this simplified number ignores more detailed information on charge transfer mechanisms that can be obtained from cyclic voltammetry. Electrode oxidation increases pseudocapacitance, reducing impedance. Charge transfer was non-homogeneous, most likely due to the electrode geometry enhancing charge density at the electrode tip and base. Oxidation of the electrode surface enhanced charge transfer inhomogeneity. The effective electrode area could be measured by reduction of Ru(NH3)6 3+ and calculated with a finite cone geometry. Significance. Increasing electrode pseudocapacitance, demonstrated by metal oxidation, reduces impedance. Increasing electrode capacitance offers a potential route to reducing thermal noise and increasing signal-to-noise ratio of neural recording. The effective electrode area of conical electrodes can be measured. The charge density of the conical electrode was greater than expected compared to a planar disc electrode, indicating modification of electrode geometry can increase an electrodes safe charge injection capacity. in vivo electrochemical measurements often do not include sufficient details to understand the electrode behaviour. Electrode oxidation most likely accounts for a significant amount of variation in previously published Utah electrode impedance data.