(232aa) Rectifying Ionic Current in Polyelectrolyte-Functionalized Bipolar Nanopores

Nanopore-based devices have emerged as promising tools for regulating the transport of nanoentities, sensing single molecules, and rectifying ion current. Inspired by biological ion channel, synthetic nanopores functionalized with biomimetic structures or polyelectrolyte (PE) brushes attracted lots of scientific interests in using them as novel platforms for both biosensors and nanofluidic systems. One of these phenomena is ionic current rectification (ICR), a diode-like current-voltage behavior, in a nanopore with bipolar charge. The specific distribution of the surface charge in such a nanopore brings about an asymmetric transport of ions through it, thereby exhibiting a strong ICR behavior. In this work, the ion transport in a nanopore functionalized with bipolar PE brushes is theoretically investigated by adopting a continuum-based model comprising a set of Poisson-Nernst-Planck, Stokes, and Brinkman equations. We show that the degree of ICR decreases with increasing bulk salt concentration, and increases with increasing repeat unit of PE chain. In addition, the higher the charge density the more significant the ion enrichment/depletion, resulting in a larger degree of ICR. However, if the positive and negative charges are asymmetrically distributed on either side of a nanopore, the degree of ICR has a local maximum as charge density varies. The results obtained suggest that the ICR behavior in a PE-functionalized nanopore with bipolar charges can be regulated by its surface modification.