Control of nanopore wetting by a photochromic spiropyran: a light-controlled valve and electrical switch.

By modifying the surface of nanoporous alumina membranes using mixtures of a photochromic spiropyran and hydrophobic molecules, it is possible to control the admission of water into the membrane using light. When the spiropyran is in the thermally stable, relatively hydrophobic closed form, the membrane is not wet by an aqueous solution. Upon exposure to UV light, the spiropyran photoisomerizes to the more polar merocyanine form, allowing water to enter the pores and cross the membrane. Thus, the photosensitive membrane acts as a burst valve, allowing the transport of water and ions across the membrane. If the aqueous solution contains ions, then the membrane acts as an electrical switch; photoisomerization leads to a two-order-of-magnitude increase in ionic conductance, allowing a current to flow across the membrane. Exposure to visible light causes photoisomerization of the merocyanine back to the closed, spiro form, but dewetting of the membrane does not occur spontaneously, due to a high activation barrier.