chemists have created what they say is the first nano valve that can be opened and closed at will to trap and release molecules. The discovery, federally funded by the US National Science Foundation, is published in the July 19 issue of the Proceedings of the National Academy of Sciences.
"This paper demonstrates unequivocally that the machine works," said Jeffrey I. Zink, a UCLA professor of chemistry and biochemistry, a member of the California NanoSystems Institute at UCLA, and a member of the research team. "With the nano valve, we can trap and release molecules on demand. A nano valve potentially could be used as a drug delivery system."
"The valve is like a mechanical system that we can control like a water faucet," said UCLA graduate student Thoi Nguyen, lead author on the paper. "Trapping the molecule inside and shutting the valve tightly was a challenge. The first valves we produced leaked slightly."
The nano valve consists of moving parts - switchable rotaxane molecules that resemble linear motors designed by California NanoSystems Institute director Fraser Stoddart's team - attached to a tiny piece of glass (porous silica), which measures about 500 nanometres, and which Nguyen is currently reducing in size. Tiny pores in the glass are only a few nanometres in size.
"It's big enough to let molecules in and out, but small enough so that the switchable rotaxane molecules can block the hole," Zink said.
The valve is designed so one end attaches to the opening of the hole that will be blocked and unblocked, and the other end has the switchable rotaxanes whose movable component blocks the hole in the down position and leaves it open in the up position.
The researchers used chemical energy involving a single electron as the power supply to open and shut the valve, and a luminescent molecule that allows them to tell from emitted light whether a molecule is trapped or has been released.
Switchable rotaxanes are molecules composed of a dumbbell component with two stations between which a ring component can be made to move back and forth in a linear fashion. Stoddart, who holds UCLA's Fred Kavli Chair in nanosystems sciences, has already shown how these switchable rotaxanes can be used in molecular electronics. Stoddart's team is now adapting them for use in the construction of artificial molecular machinery.
"The fact that we can take a bistable molecule that behaves as a switch in a silicon-based electronic device at the nanoscale level and fabricate it differently to work as part of a nano valve on porous silica is something I find really satisfying about this piece of research," Stoddart, said. "It shows that these little pieces of molecular machinery are highly adaptable and resourceful, and means that we can move around in the nanoworld with the same molecular tool kit and adapt it to different needs on demand."
In future research, they will test how large a hole they can block, to see whether they can get larger molecules, like enzymes, inside the container.
“a” (top left) shows the structural formula of the rotaxane molecule and the procedure for tethering it to the surface of a tiny piece of glass; “b” shows how the nano valve opens and closes. Credit: J. Zink, T. Nguyen, F. Stoddart/UCLA Chemistry
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