  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.
