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Enhanced water permeability and tunable ion selectivity in subnanometer carbon nanotube porins

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Science  25 Aug 2017:
Vol. 357, Issue 6353, pp. 792-796
DOI: 10.1126/science.aan2438

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Go with the flow

Enhanced water transport occurs in a number of narrow pore channels, such as biological aquaporins. Tunuguntla et al. thoroughly characterized molecular transport through narrow carbon nanotubes (CNTs) (see the Perspective by Siwy and Fornasiero). In contrast to previous studies, the authors focused on water and ion transport through relatively short (∼10-nm) fragments of CNTs embedded in lipid bilayer membranes. Strong confinement generated highly accelerated water flow compared with that observed in biological water transporters. A key factor in the transport rate was the tunable rearrangement of intermolecular hydrogen bonding. Furthermore, by changing the charges at the mouth of the nanotube, the authors were able to alter the ion selectivity.

Science, this issue p. 792; see also p. 753

Abstract

Fast water transport through carbon nanotube pores has raised the possibility to use them in the next generation of water treatment technologies. We report that water permeability in 0.8-nanometer-diameter carbon nanotube porins (CNTPs), which confine water down to a single-file chain, exceeds that of biological water transporters and of wider CNT pores by an order of magnitude. Intermolecular hydrogen-bond rearrangement, required for entry into the nanotube, dominates the energy barrier and can be manipulated to enhance water transport rates. CNTPs block anion transport, even at salinities that exceed seawater levels, and their ion selectivity can be tuned to configure them into switchable ionic diodes. These properties make CNTPs a promising material for developing membrane separation technologies.

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