Heterogeneous sub-continuum ionic transport in statistically isolated graphene nanopores

Abstract

Graphene and other two-dimensional materials offer a new class of ultrathin membranes that can have atomically defined nanopores with diameters approaching those of hydrated ions. These nanopores have the smallest possible pore volumes of any ion channel, which, due to ionic dehydration and electrokinetic effects, places them in a novel transport regime and allows membranes to be created that combine selective ionic transport with ultimate permeance and could lead to separations and sensing applications. However, experimental characterization and understanding of sub-continuum ionic transport in nanopores below 2 nm is limited. Here we show that isolated sub-2 nm pores in graphene exhibit, in contrast to larger pores, diverse transport behaviours consistent with ion transport over a free-energy barrier arising from ion dehydration and electrostatic interactions. Current-voltage measurements reveal that the conductance of graphene nanopores spans three orders of magnitude and that they display distinct linear, voltage-activated or rectified current-voltage characteristics and different cation-selectivity profiles. In rare cases, rapid, voltage-dependent stochastic switching is observed, consistent with the presence of a dissociable group in the pore vicinity. A modified Nernst-Planck model incorporating ion hydration and electrostatic effects quantitatively matches the observed behaviours.

Observation of ionic Coulomb blockade in nanopores

“…Instead, ionic transport through nanochannels/nanopores typically exhibits ohmic behaviour. Voltage-activated nonlinear ion transport was also observed in various systems, such as hydrophobic wetting and ion dehydration 10,11 . Here, we report for the first time the experimental observation of single-ion transport through the ionic junction of a sub-nm MoS 2 pore.…”

Relation between selectivity and conductivity in narrow ion channels

“…Finally, we comment that the results obtained are applicable far beyond the KcsA channel, and may be expected to describe conduction in a wide class of narrow, ion-selective, biological and artificial nano-filters [34], [46].…”

Maxwell-Hall access resistance in graphene nanopores

“…In particular, from DNA sequencing [19][20][21][22] to filtration [23][24][25][26][27], graphene nanopores and porous membranes are one of the most promising materials for applications. Novel fabrication strategies and designs are under development to create large-scale, controllable porous membranes [25,26,28] and graphene laminate devices [23,24].…”

“…Novel fabrication strategies and designs are under development to create large-scale, controllable porous membranes [25,26,28] and graphene laminate devices [23,24]. Moreover, their single atom thickness makes these systems ideal for interrogating ion dehydration [29,30], which both sheds light on recent experiments on ion selectivity in porous graphene [25,26,28] and will help analyze the behavior of biological pores [29,30].…”

“…Novel fabrication strategies and designs are under development to create large-scale, controllable porous membranes [25,26,28] and graphene laminate devices [23,24]. Moreover, their single atom thickness makes these systems ideal for interrogating ion dehydration [29,30], which both sheds light on recent experiments on ion selectivity in porous graphene [25,26,28] and will help analyze the behavior of biological pores [29,30]. Dehydration has been predicted to give rise to ion selectivity and quantized conductance in long, narrow pores [31][32][33][34][35] but the energy barriers are typically so large that the currents are minuscule, which is rectified by the use of membranes with single-atom thickness [29,30].…”