Electroosmotic flow, ionic current rectification, and selectivity of a conical nanopore modified with a pH-regulated polyelectrolyte layer: Influence of functional groups profile

Nanopores functionalized with polyelectrolyte (PE) brushes on their surface are widely used to improve the performance of solid-state nanopore-based devices. Since the profile of the functional groups, and therefore, fixed charge density, inside PE layer can be influenced by surrounding conditions and the way a nanopore is modified, it is usually nonuniformly distributed. However, a theoretical model taking account of the presence of a PE layer on the nanopore surface assumes that its functional groups are uniformly distributed. In this study we consider a conical nanopore surface modified with a pH-regulated PE layer. Two types of PE layer are considered: the functional groups are clustered near the PE-liquid interface (type 1), and near the nanopore surface (type 2). We theoretically study the influence of pH, types of PE layer, and bulk salt concentration on the ion transport behavior of the nanopore, and the associated mechanisms are discussed. The dependence of the average electroosmotic flow on relevant driving force and resistive force, including shear stress and drag, is analyzed. The influence of the types of PE layer on the I - V, rectification, and selectivity behaviors of the nanopore depends highly on the levels of pH and applied potential. The larger diffusion flux of type2 polyelectrolyte layer is the most important factor affecting current performance. Knowing the rectification behavior of a nanopore is helpful for estimating the solution pH and understanding the profile of the functional groups in PE. A reversal in selectivity is observed when both pH and bulk salt concentration are low, which can be attributed to that the ionic flux is dominated by H+.