Nonlinear dependence of the ion current rectification factor on bias voltage in conical nanopipettes

Ion current rectification (ICR), which is the departure of the experimentally measured current-voltage curves from ohmic behavior, has recently drawn intensive interests from both a fundamental point of view and microfluidic-based applications. In this case, the rectification factor (RF), which is defined as the absolute value of the quotient between the currents recorded for one voltage polarity and the currents recorded for the same absolute value of voltage at the opposite polarity, is one of the most important parameters to describe the rectification behavior. Herein, we interestingly observed that RF is strongly dependent on the bias voltage from both the experimental and theoretically simulated results, and this dependence exhibits nonlinear deviation with increasing the rectification degree. At the same surface charge density, the linear deviation increases with decreasing the electrolyte concentration. When the electrolyte concentration remains the same, the linear deviation increases with increasing the surface charge density. The simulated results demonstrate that the electro-osmotic flow is not the dominating factor to this nonlinear phenomenon. This nonlinear deviation is considered to possibly originate from the inhomogeneous conductivity and the nonlinear change of conductance with the bias potential. This study is not only useful to understanding the ICR behavior at a conical glass nanopipette, but also potentially offers a new parameter to describe the rectification degree with this nonlinear relationship.