(5dd) "Gated" Ion Transport through Dense Carbon Nnaotube Membrane

?Gated? ion transport through hydrophobic protein domains in various cellular membranes was found widely, while these hydrophobic protein channels regulate ions transport through these membranes with both high selectivity and high flux. Hydrophobic and narrow carbon nanotubes (CNT) may serve as a simplified model to mimic these biological pores and thus reveal main important properties of these hydrophobic nanochannels. In our previous study, structurally well defined dense CNT membranes with both CNT pores and space between CNTs available for transport were prepared by a simple capillary-driven shrinking process. The average pore size of these two types of pores in dense CNT membranes was 3 nm, as indicated by nano-gold filtration and N2 porosimetry. In this study, we show that ion diffusion through dense CNT membranes with 3 nm hydrophobic pores has a ?gated? behavior, possibly due to the poor water wettability on CNTs, as suggested by water vapor adsorption isotherms at room temperature and zero-current potential measurements. At 298 K, the ion diffusion channel was gradually closed gradually due to the discontinuity of the water phase in dense CNT membranes and thus ions cannot continuously diffuse through dense CNT membranes. Small perturbations, such as ultrasonication or small temperature change (< 20 K), open the gate for ions to diffuse. These small perturbations may change the water filling state in and around CNTs and create a continuous or temporarily continuous ion diffusion channel. When the ion diffusion channel was open, ions diffused through dense CNT membranes at a rate similar to their diffusion rate in bulk water. This water wettablity induced ?gated? ion diffusion behavior in dense CNT membranes may have significant implications on ?gated? ion diffusion in hydrophobic biological pores.