A thin-film electrode exhibiting Nernstian pH response via a proton-coupled electron-transfer redox couple.

Accurate pH monitoring in flexible thin films is essential for biochemical sensing applications. However, in conventional soft-material-based film-type pH sensors, the apparent pH response is often influenced by variations in thin-film morphology, such as swelling, microstructural disorder, and structural inhomogeneity. Here, we adopt a different strategy by confining a small-molecule proton-coupled electron-transfer (PCET) redox couple, traditionally used in homogeneous aqueous solutions, within an ion gel. A benzoquinone/hydroquinone (BQ/HQ) redox couple is incorporated into an ion-gel matrix deposited on a gold electrode and covered with a Nafion overlayer. The open-circuit potential of the Au/ion-gel interface, measured against an Ag/AgCl reference electrode, exhibits a Nernstian dependence on pH, indicating that the proton-coupled BQ/HQ redox equilibrium is preserved in the thin-film device. These results demonstrate that PCET redox couples homogeneously dissolved in ion gels provide a viable route toward compact and reliable pH-sensing films.