Imaging electrochemically regulated water–air nanointerfaces with single-molecule fluorescence†

Water–air nanointerfaces are essential components of multiphase electrochemical processes in various energy-related applications, including water electrolysis, hydrogen fuel cells, and CO₂ electrochemical reduction. Deep insights into the critical properties of the interfaces are much sought after but very challenging to obtain due to their highly dynamic, transparent, and nanoscopic nature. A new approach has been proposed for constructing stable water–air nanointerfaces using FIB-fabricated Janus nanopore electrodes. The curvature of the nanointerfaces can be controlled electrochemically, ranging from positive (nanodroplets) to negative (nanoconcaves/nanobubbles) ones. The morphologies of different nanointerfaces were fully characterized with AFM. Single-molecule collision events of charged dye molecules, recorded with fluorescence imaging, were used to probe the intrinsic properties of the nanointerfaces. A unique phenomenon of charged dye rejection was discovered for isoelectric nanointerfaces. The role of surface curvature in the collision frequency was also elucidated. We believe that using this platform could be highly beneficial for deepening our understanding of the interfaces, thus guiding the rational design of various energy-related systems.