<i>Operando</i> Electrochemical and Optical Characterization of the Meniscus of Scanning Electrochemical Cell Microscopy (SECCM) Probes

We present a thorough description of the scanning electrochemical cell microscopy (SECCM) meniscus probe, in operation, by combining dual-channel SECCM measurements with <i>in situ</i> interference reflection microscopy (IRM). SECCM is a pipette-based nanoscale characterization tool with an unparalleled capacity for mapping the electrochemical activity of material surfaces, with high precision and at high throughput. In hopping mode, it operates by bringing the electrolyte meniscus, at the scanned pipette tip, in contact with the sample, restricting the probed area each time to a separate, newly wetted site, and forming a small-scale reactor. Each contact area can normally be imaged post-experiment, to inform on the wetted area stability and enable quantitative data interpretation (e.g., to calculate current density). However, the description of meniscus behavior during measurements would be beneficial. Herein, we utilize semi-transparent electrode substrates, to enable the direct optical observation, by IRM, of the meniscus status, with high spatial and temporal resolution, and synchronously to SECCM operation. The surface-sensitive optical method allows us to accurately capture the nature of the miniature electrochemical cell during all phases of the experiment-during approach, meniscus contact, wetting, and pipette withdrawal-and to follow subtle changes while in contact with the electrode substrate. Through the use of a dual-channel probe, we are able to monitor both the ionic current across the meniscus, between quasi-reference counter electrodes (QRCEs) under bias, and between the working electrode surface and the QRCEs. Correlating these electrochemical data and <i>operando</i> optical information via the hybrid SECCM-IRM approach aids the design of experimental protocols, streamlines the interpretation of results, and paints a comprehensive picture of meniscus wetting behavior.