Gating of Hydrophobic Nanopores with Large Anions

Abstract

Understanding ion transport in nanoporous materials is critical to a wide variety of energy and environmental technologies, ranging from ion-selective membranes, drug delivery, and biosensing, to ion batteries and supercapacitors. While nanoscale transport is often described by continuum models that rely on a point charge description for ions and a homogeneous dielectric medium for the solvent, here, we show that transport of aqueous solutions at a hydrophobic interface can be highly dependent on the size and hydration strength of the solvated ions. Specifically, measurements of ion current through single silicon nitride nanopores that contain a hydrophobic–hydrophilic junction show that transport properties are dependent not only on applied voltage but also on the type of anion. We find that in Cl–-containing solutions the nanopores only conducted ionic current above a negative voltage threshold. On the other hand, introduction of large polarizable anions, such as Br– and I–, facilitated the pore wetting, making the pore conductive at all examined voltages. Molecular dynamics simulations revealed that the large anions, Br– and I–, have a weaker solvation shell compared to that of Cl– and consequently were prone to migrate from the aqueous solution to the hydrophobic surface, leading to the anion accumulation responsible for pore wetting. The results are essential for designing nanoporous systems that are selective to ions of the same charge, for realization of ion-induced wetting in hydrophobic pores, as well as for a fundamental understanding on the role of ion hydration shell on the properties of solid/liquid interfaces.

Water Nanoconfined in a Hydrophobic Pore: Molecular Dynamics Simulations of Transmembrane Protein 175 and the Influence of Water Models

“…g . 61 for a recent example) when designing hydrophobic gates or comparable structures into nanopores. 14 , 62 Relatively small changes in detailed nanoscale structure (and hence in the local dielectric/polarizability properties) could be used to ‘fine tune’ a hydrophobic gate to e .…”

“…However, quantitative details alter when electronic polarizability is included in the simulations. This in turn suggests the need to include consideration of polarizability (see, e.g ., ref for a recent example) when designing hydrophobic gates or comparable structures into nanopores. , Relatively small changes in detailed nanoscale structure (and hence in the local dielectric/polarizability properties) could be used to “fine-tune” a hydrophobic gate to, for example, electrowetting.…”

Electric Field Induced Wetting of a Hydrophobic Gate in a Model Nanopore Based on the 5-HT3Receptor Channel

“…Our results suggest that an excess of only five elementary charges (from amino acid side chains) near one opening of the pore is sufficient to create a substantial intrinsic electric field acting in the transmembrane direction. Therefore, by functionalizing the openings of a hydrophobic pore with static charges, it should be possible to design a nanopore that permits ion transport in only one direction, thus rectifying ionic currents. Importantly, this rectifying property would apply not only to ions but also to water molecules.…”

Molecular Simulations of Hydrophobic Gating of Pentameric Ligand Gated Ion Channels: Insights into Water and Ions

“…As can be seen from Figure D, this model fits simulation data for the effect of the applied electric field on hydration probability for both closed (PDB id 4PIR) and open (PDB id 6DG8) states of the 5-HT 3 R M2 5 model pore. Although, given the high fields involved, such effects are unlikely to result in a switch between dry/wet and closed/open states of biological ion channels under physiological conditions, they may provide the basis of electric field switchable biomimetic pores, as has been demonstrated experimentally. , …”