Diffusion limitations and translocation barriers in atomically thin biomimetic pores
Subin Sahu, Michael P. Zwolak
Ionic transport in nano- to subnano-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, ion dynamics both inside and outside the geometric volume of the pore can play a major role 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, there are regimes where transport is highly sensitive to the pore size due to the interplay of dehydration and interaction with pore charge. Outside of these regimes, other factors come into play. The small pore size itself gives a large resistance even when electrostatic factors and dehydration compensate each other to give a relatively flat fee 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 entrance effects dominating the conductance. The current thus plateaus and becomes effectively independent of pore characteristics.