Fluorescent pH-Nanosensors for the in-Operando Visualization of Fast Dynamics, Interfacial pH in Electrochemical Reactions

Abstract

The in-operando visualization of interfacial pH distribution, at electrode-electrolyte interface, provides crucial insights into the mechanism and performance of many electrochemical processes. However, due to the fast dynamics and wide pH range, the spatiotemporal visualization of such interfacial pH changes requires the development of appropriate probes capable of capturing evolution of pH changes in the harsh chemical environments that normally occur in electrochemical processes. In this study, ratiometric, fluorescent, pH-sensitive nanosensors were fabricated with a wide pH measurement range (pH 3.5 to 11), for the in-operando visualisation of fast-dynamic, interfacial pH changes during an electrocoagulation process. The developed nanosensors were synthesised from the crosslinked polyacrylamide matrix, covalently linked to three pH-sensitive dyes and a reference fluorophore, to provide ratiometric nanosensors with protective but proton permeable matrix and comparable sensitivity and responsiveness to the free dyes. Using the developed nanosensors and an electrochemically coupled laser confocal fluorescence microscope, the in-operando visualization of the dynamic interfacial pH distribution during electrocoagulation of synthetic and real oil-sands produced water was achieved for the first time. The results (Figure 1) revealed the differences between the real and synthetic produced water and reflected the crucial roles of factors such as the rate of water electrolysis and chemical composition of the system on the evolution of the pH boundary layer and the interfacial pH. Overall, this study provides powerful probes (nanosensors) that can be utilized to study interfacial pH changes in a wide range of electrochemical reactions such as batteries and electrolyzers. Figure 1: Selected time series, processed images shows the differences in the interfacial pH boundary layers evolved during the electrocoagulation treatment of synthetic and real produced water. In-operando imaging was conducted using the developed nanosensors and an electrochemically coupled laser scanning confocal microscopy. Figure 1