Rapid fluorescent mapping of electrochemically induced local pH changes

We present a fluorescent microscopic method using an ultra-pH-sensitive polymeric probe to rapidly map within subsecond the pH distribution resulting from oxygen reduction reaction electrocatalysed by an array of platinum nanoparticles. Upon voltammetry of the surface-supported Pt catalysts, fluorescent quenching waves are observed to depend on the electrode potential. The spatiotemporal fluorescent evolution is then confirmed under a constant potential control to be due to the local pH change as a function of diffusing time by an estimation of the proton diffusion coefficient $\left(L\alpha t^{1/2}\right)$. On these bases, the fluorescent measurements at short reaction times can provide quantitative information regarding the one and two dimensional pH distributions, which are shown to exhibit the expected shape of a typical diffusion-driven concentration gradient. Such imaging of proton/pH profiles may find important applications such as efficient screening of different micro/nanoscale electrocatalysts.