Opening and closing an atom-sized valve may be as simple as turning on a light. Using molecular switches that expand and contract in response to different wavelengths of light, two teams of scientists have constructed nanoscale gateways that could someday be used for everything from precisely-controlled drug delivery to brewing super-cooled nanofluids.
The valves are located within ion channels in cell membranes. Cells shuttle all sorts of materials through these channels, which consist of proteins packed around a tiny opening that is often less than a few atoms across. By installing chemical nanoswitches within these openings, scientists can control the flow of material into the cell. But finding an easy way to open and close these switches has been a challenge.
Two groups of researchers have taken an illuminating approach to the problem, using light-activated molecules as sentries at the gate. Scientists at the University of California at Berkeley designed a system based on the structure of ion channels in neurons, which allow a single ion to pass at a time. Led by chemist Dirk Trauner, the group tethered a photosensitive molecule at one end of the channel so that it blocked the opening to the pore. When irradiated with ultraviolet light, the molecule retracted, allowing flow into the cell, the researchers reported 1 December 2004 in Nature Neuroscience. Light at visible wavelengths caused the molecule to expand back into the opening and block the channel.
This week, another team of scientists led by Ben Feringa of the University of Groningen in the Netherlands presents a light-activated nanovalve design based on a larger, 3-nanometer-wide Escherichia coli protein channel. The channel is a safety valve for the bacteria, designed to protect the cell from bursting due to external changes in pressure. Though it stays tightly closed under most conditions, it opens with sufficient pressure. The scientists installed photosensitive molecules around the entrance of the channel to harness energy from ultraviolet light. Under irradiation, charge builds up around the channel, and the pore springs open. Light at visible wavelengths closes the valve, the team reports today in Science. The bacterially inspired nanovalve, large enough to admit small proteins, could become an important tool for future nanotechnology, the authors say.
Ching Tung, a molecular chemist at Massachusetts General Hospital in Boston who works with optical imaging for disease detection, says that while the light-activated approach is interesting, it's a long way from being commercially viable. "At these wavelengths, light can only penetrate the surface area of the skin," Tung says. Therefore, the challenge will be how to activate release of the drug in other parts of the body, such as internal organs, he adds.