Salt Dependence of Ion Transport and DNA Translocation Through Solid-state Nanopores

We report experimental measurements of the salt dependence of ion transport and DNA translocation through solid-state nanopores. The ionic conductance shows a three-order-of-magnitude decrease with decreasing salt concentrations from 1 M to 1 muM, strongly deviating from bulk linear behavior. The data are described by a model that accounts for a salt-dependent surface charge of the pore. Subsequently, we measure translocation of 16.5-mum-long dsDNA for 50 mM to 1 M salt concentrations. DNA translocation is shown to result in either a decrease ([KCl] > 0.4 M) or increase of the ionic current ([KCl] < 0.4 M). The data are described by a model where current decreases result from the partial blocking of the pore and current increases are attributed to motion of the counterions that screen the charge of the DNA backbone. We demonstrate that the two competing effects cancel at a KCl concentration of 370 +/- 40 mM.