Controlling Droplet Cell Environment in Scanning Electrochemical Cell Microscopy (SECCM) via Migration and Electroosmotic Flow

Abstract

Scanning electrochemical cell microscopy (SECCM) is a powerful nanoscale electrochemical technique that advances our understanding of heterogeneity at the electrode-electrolyte interface. Dual-channel nanopipettes can often serve as the probe, and a voltage bias between the channels can control the local electrolyte environment via migration and electroosmotic flow (EOF). The ability to elucidate and predict the contribution of each transport is desirable. In this work, we measured the limiting current of different redox molecules to experimentally elucidate the contribution of migration and EOF at the droplet-substrate interface in SECCM. The results were further supported by fluorescence imaging and finite element modeling. We showed that redox mediators with high charge, such as Ru(NH3)63+, migration contributes 5× as much mass transport limiting current compared to EOF when a bias voltage is applied. The exact contribution of each mode at a given potential bias depends on the electrical double layer structure, which can be tuned by the surface charge and solution composition. The contribution can be quantitatively predicted in the finite element model. Our findings will enable the precise control of mass transport in dual-channel SECCM and potentially open new scanning modes in SECCM via precise control of reaction flux.