Ion current rectification in asymmetric nanochannels: effects of nanochannel shape and surface charge

Ion current rectification (ICR) in nanochannels has attracted increasing attention for its great potential in the applications of ionic circuits and biological sensors. Herein, the influence of nanochannel shape and surface charge on the ion transport and ICR performance of asymmetric nanochannels was numerically investigated. Firstly, three asymmetric nanochannels with different shapes (bullet, conical, and trumpet) were constructed to investigate the spatial size effect on the ICR behavior. And then selected the best-performing one to further analyze the effect of the surface charge on the ICR. In the investigation of spatial size effect, it is found that the increases and the decreases of spatial size will not be beneficial for improving the ICR performance and the conical nanochannel exhibits the best performance. In the investigation of surface charge effect, it is found that specified asymmetric surface charge on the outer wall surface enhances the corresponding ICR performance, revealing the cooperative role with inner wall surface charge. Besides, it is found that the outer wall surface charge is dominant when the nanochannel length is small and the inner wall surface charge is dominant when the nanochannel length is large. Moreover, the finite length enhancement effect for ICR performance is illustrated by tuning the range of outer wall surface charged zone, indicating the charged zone near the entrance and exit plays the dominant role on ion transport. The obtained results provide essential insights on the ion transport and useful guidelines for the design and performance optimization of biological sensors and nanofluidic devices.