Diffusion Limitations and Translocation Barriers in Atomically Thin Biomimetic Pores

TL;DR

This paper investigates how atomically thin biomimetic pores, like graphene crown ethers, exhibit unique ionic transport behaviors influenced by pore size, dehydration, and electrostatic interactions, revealing regimes where conductance is highly sensitive or plateaued.

Contribution

It provides a detailed analysis of translocation barriers and diffusion effects in atomically thin biomimetic pores, highlighting the sensitivity of conductance to small structural changes.

Findings

Transport is highly sensitive to pore size due to dehydration and electrostatic interactions.

Small pore sizes can cause large resistance despite flat free energy landscapes.

Diffusion and drift dominate conductance when permeability is high, leading to conductance plateaus.

Abstract

Ionic transport in nano- to sub-nano-scale pores is highly dependent on translocation barriers and potential wells. These features in the free-energy landscape are primarily the result of ion dehydration and electrostatic interactions. For pores in atomically thin membranes, such as graphene, other factors come into play. Ion dynamics both inside and outside the geometric volume of the pore can be critical in determining the transport properties of the channel due to several commensurate length scales, such as the effective membrane thickness, radii of the first and the second hydration layers, pore radius, and Debye length. In particular, for biomimetic pores, such as the graphene crown ether we examine here, there are regimes where transport is highly sensitive to the pore size due to the interplay of dehydration and interaction with pore charge. Picometer changes in the size, e.g., due to a minute strain, can lead to a large change in conductance. Outside of these regimes, the small pore size itself gives a large resistance, even when electrostatic factors and dehydration compensate each other to give a relatively flat -- e.g., near barrierless -- free energy landscape. The permeability, though, can still be large and ions will translocate rapidly after they arrive within the capture radius of the pore. This, in turn, leads to diffusion and drift effects dominating the conductance. The current thus plateaus and becomes effectively independent of pore-free energy characteristics. Measurement of this effect will give an estimate of the magnitude of kinetically limiting features, and experimentally constrain the local electromechanical conditions.