Interfacial Charge Regulation by Phosphate Adsorption on Indium Tin Oxide for Charge-Selective Electrochemical Sensing

Controlling interfacial charge without compromising mass transport remains a challenge in charge-selective electrochemical sensing. Here, we use phosphate adsorption on indium tin oxide (ITO) as a model system to investigate how specifically adsorbed anions regulate the apparent electron-transfer kinetics and enable charge-selective electrochemical sensing. Using three ferrocene derivatives with distinct charge states, we show that phosphate adsorption enhances the voltammetric response of cationic species while suppressing that of anionic species, in a manner strongly dependent on pH and phosphate speciation. To elucidate the underlying mechanism, we directly quantify shifts in the potential of zero charge using scanning electrochemical cell microscopy (SECCM) and correlate these measurements with the observed kinetic trends. Comparison of orthophosphate, pyrophosphate, and tripolyphosphate demonstrates that multidentate adsorption and higher anion charge produce progressively stronger electrostatic modulation. Leveraging this effect, we achieve charge-selective electro-oxidation of dopamine in the presence of ascorbic acid and uric acid, with oligophosphate adsorption further enhancing the discrimination of dopamine and yielding detection limits of 0.15 and 0.11 μM for pyrophosphate and tripolyphosphate, respectively. The reversibility of phosphate adsorption under mild alkaline treatment further establishes this approach as a simple and regenerable alternative to conventional film-based interface engineering.