Ions and Water Dancing through Atom-Scale Holes: A Perspective toward “Size Zero”

Abstract

We provide an overview of atom-scale apertures in solid-state membranes, from “pores” and “tubes” to “channels”, with characteristic sizes comparable to the sizes of ions and water molecules. In this regime of ∼1 nm diameter pores, water molecules and ions are strongly geometrically confined: the size of water molecules (∼0.3 nm) and the size of “hydrated” ions in water (∼0.7–1 nm) are similar to the pore diameters, physically limiting the ion flow through the hole. The pore sizes are comparable to the classical Debye screening length governing the spatial range of electrostatic interaction, ∼0.3 to 1 nm for 1 to 0.1 M KCl. In such small structures, charges can be unscreened, leading to new effects. We discuss experiments on ∼1 nm diameter nanopores, with a focus on carbon nanotube pores and ion transport studies. Finally, we present an outlook for artificial “size zero” pores in the regime of small diameters and small thicknesses. Beyond mimicking protein channels in nature, solid-state pores may offer additional possibilities where sensing and control are performed at the pore, such as in electrically and optically addressable solid-state materials.

From Water Solutions to Ionic Liquids with Solid State Nanopores as a Perspective to Study Transport and Translocation Phenomena

“…This is because the water molecules were able to penetrate the interfacial layer to form many-body interactions (Keesom) with both anions of [EMim][BF 4 ] and the interface itself. [147,154,155] If, however, ions condensate on the surface of the solid due to attractive van der Waals interactions, as in the case of hydrophobic carbon electrodes, graphene supercapacitors, or carbon nanotubes, [156] water will be displaced toward the bulk. [147,157] Stabilization of water in wider nanopores, especially in hydrophobic ILs is then achieved by its aggregation into condense nanodomains or networks of hydrogen bonds.…”

Ultrafast rectifying counter-directional transport of proton and metal ions in metal-organic framework–based nanochannels

“…Inspired by these functions, artificial macroscopic nanochannel (NC) devices constructed from porous materials have been widely studied for the experimental investigation of nanofluidic ion (i.e., nanoionic) transport at the sub-1-nm scale dimension to achieve the ion-specific transport properties observed in biological ion channels (7)(8)(9). For instance, carbon nanotubes (CNTs), graphene, polymers, and metal-organic frameworks (MOFs) have been used to construct nanometer-sized pores to mimic atomic-scale ionic and molecular transport of biological ion channels (10)(11)(12)(13)(14)(15)(16)(17)(18). The promising ion transport properties analogous to biological ion channels such as ultrafast ion transport, high ion-ion selectivity, and quasi-unipolar ion transport have been demonstrated by tuning the effective channel size down to below 1 nm of two-dimensional (2D) layered materials-based (graphene, graphene oxide, MoS 2 , h-BN, etc.)…”

Single‐Molecule Frequency Fingerprint for Ion Interaction Networks in a Confined Nanopore

“…In nature,t he transport of molecules and ions through biological nanopores is af undamental process.I nt he confined regime,t he transport single molecules and ions are only afew times smaller or even comparable to the constrict size of the channel. [1][2][3] As aresult, their transportations were controlled by the characteristic interaction networks among the restricted ions,transported molecules and residue moiety at pore walls.T he microscopic view of this interaction network is important to understand the fundamental principles of ion and molecule transport through ananopore down to atom-scale. [4] Especially,t he enhanced sensitivity in the nanopore-based single-molecule analysis is often achieved by strengthening the interaction between individual molecule and nanopore interface.…”

“…In nature, the transport of molecules and ions through biological nanopores is a fundamental process. In the confined regime, the transport single molecules and ions are only a few times smaller or even comparable to the constrict size of the channel [1–3] . As a result, their transportations were controlled by the characteristic interaction networks among the restricted ions, transported molecules and residue moiety at pore walls.…”