Charge Induced Rectification in Single Nanopores

Abstract

Ion current rectification is one of the most exciting transport properties observed with both conically shaped nanopores as well as pores containing surface charge patterns. Ion current rectification can also be achieved by voltage-controlled electric potential of the pore walls, but these systems require building a multi-terminal set-up. The device reported here presents a simple two-electrode system whose surface charge properties are controlled by the transmembrane potential via polarization of gold. As reported before, when the applied electric field had a component in the direction parallel to the gold surface, a bipolar charge pattern was created on the gold so that one part contained excess electrons, and the other part excess holes. Thus, when placed in an electrolyte solution, the polarized surface can modify local ionic concentrations, forming regions with enhanced cation and anion concentrations. In a nanopore system with an integrated gold layer, polarization of gold is expected to create a diode surface charge pattern and ion current rectification. Experiments were performed with single silicon nitride nanopores drilled in a 50 nm thick films by e-beam in the transmission electron microscope. Evaporation of a gold film on one membrane surface did induce ion current rectification, suggesting the formation of a voltage-induced bipolar surface charge pattern. Direction of the rectification is determined by the structure of the gold in the pore, which determines the local electric field and ionic distributions. Passivation of the metal layer via chemisorption of thiols eliminated the rectification further, supporting the leading role of gold polarization in our system.