Modeling of a pH–tunable dual–response nanopore sensor

Abstract

We consider a nanopore sensor model proposed previously (Mádai et al. J. Mol. Liq. 283 (2019) 391–398) by extending our study to the effect of pH that is an experimentally controllable parameter. With pH, we can tune the protonation and deprotonation states of the functional groups on the nanopore's wall, and, thus, the charge pattern that strongly influences the ionic current. Sensing is based on the selective binding of analyte ions in the right-hand-side binding region of the pore. The left-hand-side region is a buffer region that determines the main charge carrier. We show that we can optimize the sensitivity of the sensor with pH by creating a bipolar pore, where the buffer region is positive and the binding region is negative. Anions that carry the current are attracted into the pore by the positive analyte ions. The device is dual response, because the nanopore is rectifying: we can use the relative current (in relation to the current in the absence of analyte) and rectification as device functions. We model the electrolyte with charge hard spheres in implicit water, and the binding potential with the square-well potential. The Nernst-Planck equation coupled to Local Equilibrium Monte Carlo simulations is used to compute the ionic flux.