Quantification of Asymmetric Ion Transport in Glass Nanopipettes near Charged Substrates

Taking advantage of asymmetric ion transport in nanopipettes, the surface charges of various substrates have been successfully imaged by scanning ion conductance microscopy (SICM). The predominant electrostatic interaction between charged nanopipettes and substrates thus needs to be better elucidated and quantified, as traditional SICM theory is not adequate to describe the resulting ion current. Herein, we use both experimental and simulation methods to study the asymmetric ion-transport processes in nanopipettes near charged substrates, and correlate surface charge and geometry parameters to the resulting ionic current. It is found that for negatively charged substrates and nanopipettes, positive feedback responses are displayed at the high conductivity states, and they are sensitive to the applied voltages and surface charges. While at the low conductivity states, comparable negative feedback responses are largely independent of experimental conditions. The elucidated and quantified asymmetric ion transport features would allow more accurate measurements and analysis for approaching and surface charge mapping in SICM applications.