Effect of the surface charge on ion transport through nanoslits

A description of ion transport through geometrically defined nanoslits is presented. It is characterized by the effective surface charge density and was obtained by impedance spectroscopy measurements of electrolytes with different physico-chemical properties. The fluid channels were fabricated in a Pyrex-Pyrex field assisted bonding process with an intermediate layer of amorphous silicon. The height of the nanoslits was defined by the 50 nm thickness of the amorphous silicon layer. Two microfluidic channels, containing electrodes for the characterization of the nanoslits, maintained fresh liquid on both sides of the nanoapertures. By changing the KCl concentration of the electrolyte, a conductance plateau (in log-log scale) was observed due to the dominance of the effective surface charge density, resulting in an excess of mobile counterions in the nanoslits at low salt concentrations. The effective surface charge density of the Pyrex nanoslits could be modified by changing the pH of the solution. It was verified that at higher pH values the nanoslit conductance increased. Field-effect experiments allowed changing the effective surface charge density as well. The polarity of the external voltage could be chosen such that the effective surface charge density was increased or decreased, resulting in a higher or lower nanoslit conductance. This regulation of ionic flow can be exploited for the fabrication of nanofluidic devices