Ionic current rectification behaviors in quartz nanopipettes with ionic liquids/aqueous solution systems

Abstract

Understanding ion transport behaviors in confined ionic liquid (IL)/aqueous solution systems is crucial for advancing nanofluidic device applications. In this study, we investigate the ionic current behaviors of such systems using quartz nanopipettes, revealing significant ionic current rectification (ICR) phenomena at the nanoscale, which are absent at the microscale. Experimental results show that in the absence of an external bias voltage, diffusion currents in nanopipettes are negative due to the liquid-junction potential at the IL/aqueous interface. The rectification ratio (R) increases from 1.47 to 11.64 as the KCl electrolyte concentration increases from 0.01 M to 1 M, indicating a unique behavior distinct from conventional aqueous systems. Additionally, different ILs exhibit varying rectification strengths, following the sequence: [Bmim][BF₄] > [Bmim][NTf₂] > [Bmim][PF₆] > [Bmim][N(CN)₂]. To further elucidate the underlying mechanisms, we conducted finite element simulation using a Poisson-Nernst-Planck model. The simulation results further demonstrate that IL diffusion, electrostatic interactions, and electric double layer effects collectively influence the observed ICR behavior. These findings provide new insights into ion transport in IL/aqueous systems and offer valuable guidelines for designing nanopipette-based ion sensors and nanofluidic devices.