Non-Equilibrium Ionic Charging and Discharging During Molecular Translocation Through Nanopores: A Non-Equilibrium Capacitive Spectral Assay for Single Molecules

We show that induced ionic dipoles appear at the ends of nanopores due to corner field leakage (Zhang et Al., J Phys Chem C, 2011; Thamida and Chang, Phys of Fluids, 2002). The opposite space charge of these dipoles across the end corners can charge and discharge molecules translocating through the nanopore. With an asymmetric KCl electrolyte, whose cation and anion have disparate diffusivities due to hydration cage effects, the dynamics of the least mobile ions as well as the length and charge density of the translocating molecules lead to unique non-equilibrium capacitive signals, which can augment the classical DC resistive signal to identify the translocating molecules---particularly those with specific Hoffmeister counter-ions. Under proper conditions, these charging and discharging events are distinguishable during translocation, leading to unusual molecule-dependent capacitive displacement current signals, such as the biphasic wave shapes observed experimentally (Wang et Al., Anal Chem, 2015).