  With the ability to selectively control ionic flux, biological protein ion
channels perform a fundamental role in many physiological processes. For
practical applications that require the functionality of a biological ion
channel, graphene provides a promising solid-state alternative, due to its
atomic thinness and mechanical strength. Here, we demonstrate that nanopores
introduced into graphene membranes, as large as 50 nm in diameter, exhibit
inter-cation selectivity with a ~20x preference for K+ over divalent cations
and can be modulated by an applied gate voltage. Liquid atomic force microscopy
of the graphene devices reveals surface nanobubbles near the pore to be
responsible for the observed selective behavior. Molecular dynamics simulations
indicate that translocation of ions across the pore likely occurs via a thin
water layer at the edge of the pore and the nanobubble. Our results demonstrate
a significant improvement in the inter-cation selectivity displayed by a
solid-state nanopore device and by utilizing the pores in a de-wetted state,
offers an approach to fabricating selective graphene membranes that does not
rely on the fabrication of sub-nm pores.
