Anomalous low impedance for small platinum microelectrodes

Electrical measurement of the activity of individual neurons is a primary goal for many invasive neural electrodes. Making these “single unit” measurements requires that we fabricate electrodes small enough so that only a few neurons contribute to the signal, but not so small that the impedance of the electrode creates overwhelming noise or signal attenuation. Thus, neural electrode design often must strike a balance between electrode size and electrode impedance, where the impedance is often assumed to scale linearly with electrode area. Here we test this assumption by measuring the impedance at 1 kHz for differently sized electrodes. Surprisingly, we find that for Pt electrodes (but not Au electrodes) this assumption breaks down for electrodes with diameters of less than 10 microns. For these small sizes, Pt electrodes have impedance values that are up to 3-fold lower than expected. By investigating the impedance spectrum of Pt and Au electrodes we find a transition between planar and spherical diffusion for small electrodes combined with the pseudo-capacitance of proton adsorption at the Pt surface can explain this anomalous low impedance. These results provide important intuition for designing small, single unit recording electrodes. Specifically, for materials that have a pseudo-capacitance or when diffusional capacitance dominates the total impedance, we should expect small electrodes will have lower-than-expected impedance values allowing us to scale these devices down further than previously thought before thermal noise or voltage division limits the ability to acquire high-quality single-unit recordings.