Tapping Tiny Pores: Nanovalves control chemical releases

Cells readily manufacture the nanoscale valves, pumps, and other gadgets that make life work. For human researchers, fabricating devices in the nanometer range is anything but easy.

That didn’t stop chemist Thoi D. Nguyen and his colleagues at the University of California, Los Angeles (UCLA) from building arrays of nanovalves, each made from a single molecule. If used to control minute flows of liquids, such nanovalves might lead to technologies capable of delivering drugs to individual cells.

The UCLA team, led by J. Fraser Stoddart and Jeffrey I. Zink, describes the new valves and their operation in the July 19 Proceedings of the National Academy of Sciences.

Each of the valves is an organic molecule known as a rotaxane (SN: 2/7/04, p. 87: Virtual Nanotech). The molecule consists of a 4-nm-long rod poking through a 1-nm-wide ring. By controlling the composition of the solution containing the rotaxanes, researchers can induce each molecule’s ring to move along the rod between two fixed positions.

In previous attempts to make tiny valvelike mechanisms, other groups have fabricated sealed nanopores that can subsequently be unsealed by, for instance, corroding away overlying metal films. With such systems, Zink says, “you can remove the cork and open the bottle,” but reinstalling the cork requires rebuilding the original structure.

In contrast, the rotaxane valves operate as spigots on wine barrels do. This development “demonstrates that reversible opening and closing … of nanopores in a synthetic system can be achieved,” comments Ben L. Feringa of the University of Groningen in the Netherlands.

To build the spigots, Nguyen and his colleagues first used established methods to create pore-riddled glass balls, each about the size of a large virus. Then, they coated each ball with a film of thousands of the rotaxane molecules, whose rodlike components stuck out like tiny bristles all over the balls’ surfaces. “It’s just a skin of organic machinery,” Stoddart says.

The team next immersed the balls in an organic solvent containing luminescent chemicals. When the valves started in their open positions—with the ring component away from the glass balls—the luminescent chemicals infiltrated the pores. Adding an iron salt to the mixture caused the rings to slide down onto the balls’ surfaces, sealing in the light-emitting cargo.

Finally, when the scientists moved the balls into a fresh solvent and added ascorbic acid, or vitamin C, the valves reopened, permitting the luminescent molecules in the pores to escape.

Nguyen told Science News that the group has also used light, instead of a changed chemical environment, to control the valves.

“The real dream,” adds Zink, is to manipulate just a handful of valves, or even a single valve, rather than thousands together. Such precision could open new ways to provide minute amounts of chemicals for sensitive reactions.

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