Scanning gel electrochemical microscopy: Combination with quartz crystal microbalance for studying the electrolyte residue

Abstract

The residue of electrolyte is a main challenge for local electrochemical measurements based on spatially localized contact between the electrolyte and the sample surface, such as scanning droplet cell or scanning electrochemical cell microscopy. By immobilizing electrolyte in the form of gel, scanning gel electrochemical microscopy (SGECM) is expected to overcome this challenge. This work proposes an analytical method for studying the issue of electrolyte residue from gel probes in SGECM. It is based on measuring approach-retract curves of gel probes on a quartz crystal microbalance (QCM), where the resonance frequency is recorded and synchronized with the current response. Verified by SEM observation, the frequency shift after one approach-retract cycle is taken as a measure for systematically compare the extent of electrolyte residue on the Au-coated quartz crystal surface. The results show that the electrolyte residue from freshly prepared gel probes, especially after soaking redox probes, could be reduced to negligible after a few approach-retract cycles. It can also be inhibited by co-electrodeposition of chitosan with tetraethoxysilane for preparing composite gel probe. Electrolyte residue could increase when over-pressing the gel against the sample. The residue of two types of gel probes and microcapillaries is also compared. The combination of QCM with SGECM offers a quality control method for gel probes before quantitative local electrochemical measurements.