Tiny rotary motors made of spinning molecules hold the promise of driving microscopic devices of the future. But so far, scientists have had a difficult time controlling which direction tiny artificial cogs spin. New interlocking rings designed by a team of researchers may solve that problem, bringing the vision of clockwork machinery on a molecular scale one step closer to reality.
One leading set of microrotor designs is made with catenanes, which are organic molecular rings. Zaps of ultraviolet (UV) light or electric fields could speed up the motors, but their rings were free to randomly whirl either clockwise or counterclockwise. To gain some directional control, organic chemists Jenny Wong and David Leigh of the University of Edinburgh, U.K., and their colleagues at the University of Bologna, Italy, reasoned that if a ring was blocked from spinning in one direction, it would have no choice but to go the other direction.
Their solution was to form a catenane made of a large ring with two smaller rings and four kinds of hydrogen-bonding sites. Illuminating the rings with long-wavelength UV light causes the bonding site to let go of the first small ring. Meanwhile, the other small ring stays in place and blocks the first from rolling in one direction. So instead, the detached ring slides all the way around in the opposite direction to get hydrogen bonded to the site on the other side of the small, stationary ring. Short-wavelength UV light then frees the second small ring, and it does the same thing as the first, sliding around to the next available bonding site. White light and a bromine solution reset the motor, returning the rings to their original positions. The team's findings appear in the 10 July issue of Nature.
In experiments, this prototype spun fairly slowly, taking 70 minutes for both small rings to complete a circuit along the large ring. Still, Leigh said that UV laser bursts could easily make the rings spin at millions or billions of revolutions per minute if needed. Chemical tinkering with the rings could also get them spun by visible light, he added.
Molecular biologist Richard Pomerantz of the State University of New York Downstate Medical Center in Brooklyn suggests that such artificial machinery could prove more robust than enzymatic systems motors. "Do I wish I had thought of this and pulled it off? Yes!" says organic chemist Harry Gibson of Virginia Polytechnic Institute and State University in Blacksburg.