Complete steric exclusion of ions and proton transport through confined monolayer water

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Science  11 Jan 2019:
Vol. 363, Issue 6423, pp. 145-148
DOI: 10.1126/science.aau6771

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Only the water may pass

Removing objects by size to keep only the smallest ones is simple in theory, but it requires a sieve, membrane, or filter with holes small enough to allow only the smallest objects to pass. Gopinadhan et al. engineered two-dimensional water channels by removing an atomic plane of atoms from a sandwich made from bulk crystal with a graphene spacer. Water flowed through the channels without resistance, but the channels excluded all ions except for protons because the ion hydration shells could not squeeze through the channels.

Science, this issue p. 145


It has long been an aspirational goal to create artificial structures that allow fast permeation of water but reject even the smallest hydrated ions, replicating the feat achieved by nature in protein channels (e.g., aquaporins). Despite recent progress in creating nanoscale pores and capillaries, these structures still remain distinctly larger than protein channels. We report capillaries made by effectively extracting one atomic plane from bulk crystals, which leaves a two-dimensional slit of a few angstroms in height. Water moves through these capillaries with little resistance, whereas no permeation could be detected even for such small ions as Na+ and Cl. Only protons (H+) can diffuse through monolayer water inside the capillaries. These observations improve our understanding of molecular transport at the atomic scale.

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