Tunable Ion Selectivity in Sub-Nanometer Diameter Carbon Nanotube Porins

Abstract

Carbon nanotube porins (CNTPs), ultra-short carbon nanotubes (5-20nm) that can self-insert into lipid bilayers, allow single-channel planar lipid bilayer measurements of ionic transport through carbon nanotube (CNT) pores. CNTs with sub-nanometer diameters have been predicted to display ultra-fast water transport and high levels of ionic exclusion resulting from one-dimensional water wire transport through their hydrophobic cores. Here, we show that ultra-short 0.8-nm-diameter CNTPs, can spontaneously insert into lipid bilayers to form highly cation selective channels that exhibit low ionic conductance. Using reversal potential in various asymmetric salt conditions, we observe that these CNTPs exhibit large permselectivity values, corresponding to a high selectivity for both potassium and sodium over chloride. We also show evidence that neutralization of charges on the CNTs enhances ion selectivity. We also show that by using a solid-state nanopore to act as a support and form a completely solvent-free bilayer system, we can observe extremely stable ionic currents through these nanotubes, in contrast to previous reports with solvent containing bilayers. These results establish CNTPs as a promising biomimetic platform for developing cell interfaces, creating stochastic sensors, and water desalination.