On the Dynamic Stability of Gold Electrodes Exposed to Alternative Voltages in Microfluidic Systems

Abstract

While gold is a stable metal in water, it is not uncommon for microfluidic experimenters using biologically-relevant fluids such as phosphate-buffered saline (PBS) to witness their precious gold electrodes quickly vanish from the microchannel once the voltage exceeds a few volts. This stability issue concerns multiple fields where high voltage provides superior actuator or sensor performance, such as resistive pulse sensing (RPS), electroosmosis, electrowetting and so on. One solution to protect metallic electrodes is using alternative voltages (AV) as opposed to continuous voltages. After recalling that gold dissolution is enabled by the chloride ions present in most biologically-relevant solutions, we explore the stability conditions of the electrodes for voltages from 1 to 20 V pp (Peak to Peak voltage amplitude), actuation frequencies between 0 and 5 kHz, and for various pH and electrolytes (NaCl, Na2SO4, HCl). We find that the dissolution threshold voltage depends on the ratio of reaction to diffusion rate given by the Damkhöler number Da. In mass-transfer limited regime, the dissolution threshold is independent of the frequency, whereas the dissolution voltage is observed to grow as Da−1/2 in the reaction limited regime. These findings provide guidelines to design more reliable electrowetting, electroosmosis, dielectrophoresis and resistive pulse sensing devices.