Graphene oxide membranes allow only very small hydrated molecules and ions to pass with an accelerated transport rate. [Also see Perspective by Mi ]

Squeezing through a hole: Transport of an ion is usually directly related to its hydrated radius and assumed to be nonflexible. Either a hydrated ion fits through an aperture or it does not, and shape should play a dominant role rather than charge. Esfandiar et al. created nanofluidic devices by stacking structured bulk materials, including graphite, boron nitride, and molybdenum disulfide. They investigated the transport of ions in aqueous solutions through the nanochannels in the devices. Unexpectedly, they observed different behavior for ions of similar hydrated size but opposite charge. Science , this issue p. 511