Revisiting ion transport through micropores: significant and non-negligible surface transport

From a theoretical perspective, ion transport through micrometer or nanometer-sized pores under a cross-pore electric field can be described well by the Hall equation, involving only the bulk conductivity, if the solution is not too dilute. For dilute solutions, it is predicted that the surface conduction will become important, especially in nanopores. Nonetheless, this remains unsupported by experiments, especially for micropores, where the experimentally observed ion conductance is intuitively thought to be dominated by bulk conduction. Herein, our electrical measurements of ion transport through silicon nitride pores having diameters ranging from sub-µm up to a few µm show that the surface conduction can be significant and non-negligible in such large pore systems, especially at solution concentrations lower than 1 mM. In the latter case, the observed surface conductivity of the order of 1 nS can dominate over the bulk contribution, yielding a Dukhin length comparable to or even larger than the pore size and a Dukhin number up to 10. The surface conduction can be further enhanced by covering the silicon nitride surface with two-dimensional (2D) crystals such as graphene, graphene oxide, or monolayer titania sheets. The resulting surface conductivity is seen to increase upon increasing the solution concentration and can be increased by up to one or two orders of magnitude. Our observations provide insights into ion transport in micropore systems and suggest the possibility of exploiting surface conduction in such large pores for new technologies that were previously believed to apply only to nanopores.