Rectification of Ion Current in Nanopores Depends on the Type of Monovalent Cations: Experiments and Modeling

Abstract

Rectifying nanopores feature ion currents that are higher for voltages of one polarity compared to the currents recorded for corresponding voltages of the opposite polarity. Rectification of nanopores has been found to depend on the pore opening diameter and distribution of surface charges on the pore walls as well as pore geometry. Very little is known, however, on the dependence of ionic rectification on the type of transported ions of the same charge. We performed experiments with single conically shaped nanopores in a polymer film and recorded current–voltage curves in three electrolytes: LiCl, NaCl, and KCl. Rectification degrees of the pores, quantified as the ratio of currents recorded for voltages of opposite polarities, were the highest for KCl and the lowest for LiCl. The experimental observations could not be explained by a continuum modeling based on the Poisson–Nernst–Planck equations. All-atom molecular dynamics simulations revealed differential binding between Li+, Na+, and K+ ions and carboxyl groups on the pore walls, resulting in changes to both the effective surface charge of the nanopore and cation mobility within the pore.

Multiscale modeling of a rectifying bipolar nanopore: explicit-water versus implicit-water simulations

“…Although bipolar nanopores have been studied with the PNP theory extensively [54,36,23,55,56,2,57,58,59,60,61,62,63,64], we are not aware of any paper where this system is examined by molecular simulations apart from our previous study for a rectifying bipolar ion channel [65] and our parallel study where we compare to PNP [66]. We are aware only of a few MD studies [67,68,69,70,71,72] for other types of nanopores.…”

Ion current rectification behavior of a nanochannel having nonuniform cross‐section

“…Through these methods, various types of nanochannels have been fabricated such as cylindrical [27], conical [22], hourglass‐shaped [26], cigar‐shaped [20], bullet‐shaped [25], dumbbell‐shaped [21], and funnel‐shaped nanochannels [23]. The ICR behavior of an asymmetric nanochannel can be influenced by factors including ionic species [40], ion concentration [35, 41], applied electric potential [40], pH [42, 43], temperature [44], and nanochannel length and its opening radii [24, 45, 46]. The cone angle [24] and the ratio of cylindrical segment/conical segment [23] are important to conical and funnel‐shaped nanochannels, respectively.…”

Conductivity-based detection techniques in nanofluidic devices

“…The degree of rectification also depends on the identity of the cations in solution and is shown experimentally to increase with increasing cation diameter. 118 ICR is highly dependent on local surface charge, 119 which can be controlled by pH and surface functionalization to build nanofluidic sensors with a high degree of sensitivity and selectivity. Furthermore, diode-like behaviour permits use of nanofluidics in simple logic circuits for enhanced device functionality.…”