Monolayer-gated Ion Transport in Artificial Ion Channels Based on A Nanoporous Gold Membrane

Biological ion channels that intelligently control the transport of ions or molecules through the cell membrane in response to external stimuli can maintain the balance between the extracellular and intracellular substances, which ensures the normal life activities of the organism. The development of artificial ion channels with analogous function to the biological counterparts is of great significance because of their possible applications as ion switches and sensors. In this paper, we describe a new type of artificial ion channels based on de-alloyed nanoporous gold membrane with three-dimensional nanochannels. The nanochannels were built by electrochemical etching of gold-silver alloy in concentrated nitric acid. The surface morphology and component of nanoporous gold membrane were characterized by scanning electron microscope (SEM) and energy dispersive X-ray spectrum (EDX). The ion transport properties of artificial ion channels were characterized with current-voltage curves which was measured by a picoammeter. Our results indicate that this nanoporous gold membrane demonstrates an ion rectification phenomenon because of the electrochemical polarization of gold under an electric field. Subsequently, a layer of hydrophobic molecules was assembled on the surface of nanoporous gold membrane by strong Au-thiol bonds after immersed in the solution of 1-dodecanethiol. The measurements of water contact angles (CAs) indicated that the modification of 1-dodecanethiol molecules converted the surface water CA of nanoporous gold membrane from 36.5° to 120.6°. This hydrophobic monolayer prevents the transport of water-soluble ions, which makes the channels exist in an "off" state. The stimulus of surfactant in the electrolyte is favorable for the wetting of channel surface by aqueous electrolyte, which makes the channels exist in an "on" state for water-soluble ions. Therefore, the monolayer-modified nanoporous gold membrane can serve as a surfactant-gated ion switch. Our work provides a new idea for the preparation of artificial ion channels, which can be applied for intelligently responsive artificial system.