A fast voltammetric pH microsensor based on electropolymerization of methylene blue

Microsensors for resolving local pH are versatile analytical tools for studying reactions at solid-liquid interfaces in applications such as electrocatalysis and corrosion. However, the achievable temporal resolution is limited, hindering the operando investigation of local pH at surfaces in electrochemical reactions. Here, we present a methodology to achieve a high time resolution by deploying AC voltammetry with a pH microsensor. The sensor is composed of a gold ultramicroelectrode functionalized with electropolymerized methylene blue at its tip, allowing for continuous monitoring of pH in aqueous solutions in the neutral and acidic regime at a subsecond time resolution. As a proof-of-principle, we show that our sensors can be used for detecting interfacial pH at an electrified platinum nanoparticle-modified carbon electrode during the hydrogen and oxygen evolution reactions. The fast pH sensing capability is expected to open an avenue for routine operando studies of local pH at electrified interfaces.