Measurements of the capacitance and the response time of solid-state potentiometric sensors by an electrochemical time-of-flight method

Abstract

A micro-electrochemical time-of-flight technique relying on galvanostatic ion generation and potentiometric sensing (P-ETOF) has been developed in order to characterize dynamic properties of solid-state potentiometric sensors and to measure their capacitance. Lithographically-fabricated generator-sensor devices featuring 10 μm-wide micro-electrodes spaced by 20–50 μm are used in either an open face mode, where hemi-cylindrical diffusion governs mass transport of generated ions, or in a narrow channel mode, where linear diffusion describes transport of ions between the generator and sensor micro-electrodes in a thin (~3 μm) layer of electrolyte. P-ETOF-open face mode is capable of generating silver ions or protons at a rate equivalent to a current density of 10 A/cm2. This induces a maximum, mass transport controlled rate of a sensor potential rise of ~80 V/s. P-ETOF was used to characterize anodically electrodeposited iridium oxide film pH micro-sensors. Their maximum rate of potential change was determined to depend inversely on the oxide film’s thickness and varied from 34 V/s for 40 nm-thick films to 9 V/s for the 650 nm-thick iridium oxide films. The capacitances of these pH micro-sensors were determined to depend linearly on their thicknesses, with a slope of 3 μF cm−2 nm−1 and a specific capacitance of 30 F/cm3. These results point to slow proton transport in the bulk of the oxide film as a mechanistic element of their pH response.