New Unidirectional Molecular Rotor May Lead To Tiny Sensors, Pumps, Switches

The synthetic molecule features a chemical axle with two attached"paddles" carrying opposite electrical charges, which is mountedparallel to a gold substrate surface, said Professor Josef Michl ofCU-Boulder's chemistry and biochemistry department. The researchersfound that the microscopic rotor -- constructed with a few hundredatoms -- will turn in a desired direction at a selected frequency usingan oscillating electrical field concentrated in a tiny area above themolecule.

Such molecular rotors may someday function as nanotechnology machinesand be used as chemical sensors, cell-phone switches, miniature pumpsor even laser-blocking goggles, he said. A paper by Michl and formerCU-Boulder postdoctoral student Dominik Horinek, the Feodr Lynen Fellowof the German Humboldt Foundation, appeared in the Oct. 4 issue of theProceedings of the National Academy of Sciences.

In March 2004, the CU-Boulder research group led by Michl reported thesynthesis of these molecules and their mounting on a gold surface --the world's first surface-mounted artificial molecular rotor, whichturned spontaneously in random directions at room temperatures. Whilethe team was able to make the rotor "flip" using electricity, the newcomputer model indicates such rotors can be harnessed to turn in one,desired direction at varying, prescribed speeds, he said.

"We are very pleased," said Michl. "The computer model tells us we willbe able to manipulate the frequency of rotor revolutions by changingthe strength of the outside electrical field."

The researchers were able to make the new molecular rotor model turn atthree different speeds by adjusting the electrical field strength at agiven oscillation frequency, he said. The behavior of the rotorresponds both to the imposed electrical field and frictional dragwithin the gold substrate on which the device is anchored, as well asthe natural thermal movements of molecules, known as Brownian motion.

The molecular rotors designed and constructed by Michl and hiscolleagues are an outgrowth of a "Molecular Tinkertoy Kit" the groupdeveloped in the 1990s. Made up of chemical rods and connectors tens ofthousands of times smaller than the width of a human hair, the parts --which are made primarily of carbon atoms-- have been used to assemble avariety of simple nanostructures over the past decade.

Complex molecular motors, including the protein, ATPase -- which fuelsmost cellular processes in living things -- are found throughout thenatural world, Michl said. "Ours is much more primitive and one hundredtimes smaller, and is but a first step."

Michl's group hopes to design a rotor with larger "paddles"and to power it with either a liquid or gas fluid rather thanelectricity. "Ultimately, we would like to use light pulses to drivethe rotor and make it pump fluid. At that point we would have a motor,which is something that actually does useful work, rather than a rotor,which merely idles."

Michl said modeling the behavior of molecular rotors with powerfulcomputers saves a significant amount of time and money in the researchprocess. "Modeling allows us to discard designs that are not fruitful,"he said. "We can save a lot of labor and cost by modeling them in thecomputer first, and only then synthesizing them in the laboratory."

Michl is collaborating with several others in CU-Boulder's chemistryand biochemistry department, including research associates ThomasMagnera and Jaroslav Vacek and graduate students Debra Casher and MaryMulcahy. He also works closely with Professors Charles Rogers and JohnPrice of the CU-Boulder physics department, as well as faculty membersat Northwestern University.

Funded primarily by the U.S. Army Research Office and theNational Science Foundation, the research could lead to new technologyto produce goggle coatings that would shield human eyes from blindinglasers, said Michl. Arrays of rotors laid down in a protective coatingwould rest perpendicular to the goggle surface and allow light through.But when a laser pulse arrived at the goggles, the rotors would pushthe paddles into a parallel position to block incoming light.

Michl is one of 19 CU-Boulder faculty members who have been elected tothe National Academy of Sciences, which publishes the Proceedings ofthe National Academy of Sciences.

A movie animation/ still image, courtesy of PNAS/CU-Boulder, is available at: http://www.pnas.org/content/vol0/issue2005/images/data/0506183102/DC1/06183Movie1.mpg