Ionic Current Rectification: A Result of the Series Connection of Nanochannels with Different Dukhin Numbers

Originating from the ionic concentration polarization, ionic current rectification (ICR) is closely related to ion selectivity. Since it is the region with the greatest selectivity, the tip of the conical nanopore became the focus of research. However, even if the characteristic of the tip is fixed, the changes of pore length and cone angle still affect the magnitude of ICR─rectification factor (RF). This shows that only focusing on the selectivity of the tip is not comprehensive. Through the simulations based on the one-dimensional Poisson-Nernst-Planck model, it is found that the pore length and cone angle can influence the RF by changing the Dukhin number of pore base (Du_base). Here, Du is a parameter describing the ratio of excess ion concentration and bulk ion concentration. In addition, it is proved that the RF is determined by Du_tip (Du of the pore tip) and Du_base together. On the basis of the results, we suggest that a uniformly charged conical nanopore can be equivalent to the series connection of many ultrashort nanochannels with different Du. The differences in Du between adjacent channels lead to unbalanced ion transport, ultimately leading to enrichment or depletion of ion concentration under different polarities. Besides, ICR in bipolar diodes also exhibits Du dependence. We anticipate that this work will provide help to understand the mechanism behind ICR.