Changing a surface from sticky to slippery could now be as easy as flipping a molecular light switch. Researchers have created an "optically switchable" material that alters its surface characteristics when exposed to ultraviolet (UV) light. The new material could have a variety of applications, from a protein filter for biological mixtures to a tiny valve on a "lab-on-a-chip." Synthetic polymer membranes are used in a variety of applications based on the science of "bioseparation" -- filtering specific proteins from complex liquid mixtures of biological molecules. But proteins often stick to these membranes, clogging their pores and severely limiting their performance, said Georges Belfort, a professor at Rensselaer Polytechnic Institute. "We asked ourselves, can one use light to help the proteins hop on and hop off? We have shown that when one changes light, the proteins don’t stick as well," Belfort said. Operators need an inexpensive way to clean these membranes while they are still in place, rather than periodically removing them from the application environment, Belfort said. But the only available cleaning options involve expensive chemicals or labor-intensive procedures that result in significant process downtime. To make the new materials, Belfort and his coworkers attached spiropyran molecules to a widely used industrial polymer, poly(ether sulfone). Spiropyrans are a group of light-switchable organic molecules that exist in a colorless, "closed" form under visible light but switch to a reddish-purple, "open" form when exposed to UV light. This change leads to an alteration of the new material’s polarity, or the chemical structure of its atoms. In switching from nonpolar to polar, the material becomes less attractive to proteins that might stick to its surface, Belfort said. Exposing the material to UV light is like flipping a molecular switch, causing sticky proteins to detach from the surface and wash away in the liquid. Not only is the switching mechanism uncomplicated, so is the patented procedure required to graft spiropyran molecules to poly(ether sulfone). "We used a relatively simple two-step process that could be easily incorporated into a commercial manufacturing process," Belfort said. "The relative ease of this grafting and switching process suggests many industrial opportunities." In addition to bioseparations, Belfort said he envisions a number of potential applications for the materials, from new membranes for treating polluted water to the targeted release of drugs in the body. For example, in recent years researchers have developed "lab-on-a-chip" technology for automating laboratory processes on extremely small scales. Belfort said the new material could be employed as a surface valve that can be opened and closed by applying light, offering the ability to control liquid flow in a chip’s ultratiny channels. And in sensors designed to detect biological agents, the ability to control the polarity of the membrane could help reduce the attachment of unwanted proteins, providing more accurate readings, Belfort said. Two other Rensselaer researchers contributed to the project: Arpan Nayak, a graduate student in chemical and biological engineering; and Hongwei Liu, a postdoctoral research associate in chemical and biological engineering. The research was funded by the US Department of Energy and the National Science Foundation. The paper, "An Optically Reversible Switching Membrane Surface," was published in the June 19 issue of Angewandte Chemie International Edition. For more information, visit: www.rpi.edu