A versatile and robust surface-poison-resisting Scanning Amperometric Proton Microscopy

A sensitive and accurate measurement of localized pH with improving spatial resolution is highly desirable to the scientific understanding of many processes ranging from clean energy production to biological cellular mechanisms. In this study, we report the observation that the diffusion-limited current (DLC) of hydrogen evolution reaction (HER) on Pt surface is largely surface poison-resisting to carbon monoxide and thiol, two well-known Pt surface poisons. Building upon this observation, we proposed and tested a technique for amperometrically measuring localized proton concentration with a sub-mu m spatial resolution through the utilization of a linear pH dependence of HER DLC on a Pt ultramicroelectrode, hereby referred to as Scanning Amperometric Proton Microscopy (SAPM). By bringing the Pt ultramicroelectrode close to a proton generating/consuming substrate through resonance feedback on an atomic force microscopy (AFM) tuning fork, SAPM can be used to measure and image localized proton concentrations and changes in pH caused by an active catalytic surface during reactions, largely free of concerns about the presence of surface-poisoning species. This was demonstrated through three proof-of-concept cases: methanol oxidation reaction (MOR) on a Pt thin film substrate, active proton-coupled electron transfer by a membrane-tethered protein (cytochrome c oxidase) in biological compatible electrolyte, and SAPM imaging of PtRu islands electrodeposited onto a Pt film substrate during MOR. The development of this surface-poison-resisting scanning probe technique through this study shows the potential of a broadly applicable way to study proton-generating or -consuming reactions for catalytically-active substrates with a sub-mu m spatial resolution, including biological processes involving localized pH changes in a variety of salts, buffers or broth-type reaction media at near-neutral pH values.