1.D.69 The Conical Nanopore (ConNP) Family
Ion current rectification (ICR) refers to the asymmetric potential-dependent rate of the passage of solution ions through a nanopore, giving rise to electrical current-voltage characteristics that mimic those of a solid-state electrical diode. Synthetic nanopores of various geometries, fabricated in membranes, have been investigated to understand fundamental aspects of ion transport. ICR requires an asymmetric electrical double layer within the nanopore, producing an accumulation or depletion of charge-carrying ions at opposite voltage polarities. Lan et al. 2016 reviewed investigations based on experiments using conical nanopores (10-300 nm tip opening). Measurable fluid flow in nanopores can be induced either using external pressure forces, electrically via electroosmotic forces, or by a combination of these two forces. Pressure-driven flow can alter the electrical properties of nanopores and vice versa. Nonlinear electrical properties of conical nanopores can impart novel and useful flow phenomena. Electroosmotic flow, which depends on the magnitude of the ion fluxes within the double layer of the nanopore, is coupled to the accumulation/depletion of ions. Rectification can be used to electrically pump fluids with precise control across membranes containing conical pores via the application of a symmetric sinusoidal voltage. Nanopore-based negative differential resistance is sensitive to the surface charge near the nanopore opening, suggesting applications in chemical sensing (Lan et al. 2016). Scalable nanocone membranes enable ultra-fast cation permeation (Na+ = 8.4x vs. Mg2+ = 1.4x) and high ion charge selectivity (Na+ /Mg2+ =6x) compared to the commercial state-of-the-art permselective membrane owing to negligible surface resistance and positively charged conical pore walls (Shehzad et al. 2019).