Anomalous ion transport in 2-nm hydrophilic nanochannels

Transmembrane proteins often contain nanoscale channels through which ions and molecules can pass either passively (by diffusion) or actively (by means of forced transport). These proteins play important roles in selective mass transport and electrical signalling in many biological processes. Fluidic nanochannels that are 1-2 nm in diameter act as functional mimics of protein channels, and have been used to explore the transport of ions and molecules in confined liquids. Here we report ion transport in 2-nm-deep nanochannels fabricated by standard semiconductor manufacturing processes. Ion transport in these nanochannels is dominated by surface charge until the ion concentration exceeds 100 mM. At low concentrations, proton mobility increases by a factor of four over the bulk value, possibly due to overlapping of the hydrogen-bonding network of the two hydration layers adjacent to the hydrophilic surfaces. The mobility of K+/Na+ ions also increases as the bulk concentration decreases, although the reasons for this are not completely understood.