Hydrophobic Gating in Single Synthetic Nanopores

Abstract

In nature, nanopores play a critical role in a number of vital biological functions. These pores can be ion selective based on their size and/or surface charge, but further functionality is achieved by modulating, or gating, their conductance state. The conductivity of a particular nanochannel can be controlled in a number of ways, including mechanically, chemically and electrically. By studying these phenomena in model systems, we may be able to create abiotic analogues of these biostructures with similar transport properties. Here, we present what we believe is first study to show the spontaneous opening and closing of a synthetic hydrophobic nanopore with no leakage current in the closed state. Hydrophobic gating has been observed before in biological channels.Single conically shaped nanopores in polyethylene terephthalate prepared by the track-etching technique were functionalized with hydrophobic groups either throughout the pore or on the outside of the tip region. These hydrophobic pores would be closed at low voltages below ∼1V but would open up for ionic transport when a transmembrane potential was increased above a threshold value. At the threshold voltages the pores would fluctuate between conducting and non-conducting states, which we attributed to reversible wetting and dewetting of the pores. Prior to the hydrophobic modification, aqueous electrolyte solutions were able to conduct readily through the structures for all voltages. Another feature of nanopores with a local hydrophobic layer is a strong dependence of their gating behavior on pH. At pH 8 the pore would typically conduct the current, but at pH 3 the pore was either closed or rectifying in the opposite direction. Transport properties of hydrophobic nanopores also provide information on hydrophobic interactions at the nanoscale.