Among engineers who make nanoscale membranes, the holy grail is air separation; its main ingredients, oxygen and nitrogen, are only 0.02 nanometer different in size. "People would love a filter that could separate oxygen from nitrogen," says Jeff Brinker, a senior scientist at Sandia National Laboratories and professor at the University of New Mexico. "Industrialists have invested hundreds of millions of dollars to do that." Lucky for Brinker, it looks like he's come up with the membrane that will let them do that. His latest resultsa follow-up on four earlier papers published by Brinker's group in Natureappear in the October 6th issue of Science.

Brinker and his colleagues have created what is, in effect, a tunable filter: a self-assembled thin-film silica with pores, arranged in a honeycomb-like structure, that change size when exposed to ultraviolet light. The material's seeming magic comes from photoacid molecules, which ordinarily self-assemble and snap into a uniform, nanoscale structure. When exposed to light, though, they break apart and form an acid that solidifies the silica nearby. More light means more acid, more solidification and smaller pore sizes. "Using light to change pore size is a kind of nanostructural engineering," Brinker says. "In addition to creating an overall pattern as achieved by conventional lithography, this kind of lithography also can help us define the internal structure of the films on the nanoscale. Even though the overal shapes we create are at the high end of the nanostructure regime, the light influences pore size and connectivity in the heart of nanoland."