When a fluid flows through a pipe, the interactions between the fluid and the walls of the pipe become increasingly important as the diameter of the pipe decreases. And in the narrow channels found in biological nanopores, the chemistry of the walls may block the fluid flow completely. However, these effects can also be engineered to control the flow of fluid through nanopores, as researchers from New Mexico State University and Arizona State University in the US have demonstrated.

Sergei Smirnov and co-workers coated porous alumina membranes with hydrophobic molecules that respond to light. Being hydrophobic, the molecules keep the walls of the nanopore dry, and stop the flow completely for diameters below 70 nm. However, when the same molecules are exposed to ultraviolet radiation, they become more polar and less hydrophobic, which allows the fluid to flow. Exposing the molecules to visible light drives the molecular transformation backwards. The only way to block the flow completely is to dry the nanopores, but Smirnov and colleagues suggest that the optical approach might work on its own for pore radii below 3 nm.

On–off valves in cellular membranes have a crucial role in controlling ion transport and other essential functions in most living organisms. Light-activated valves could prove equally useful in the laboratory.