THE smallest motor ever built has been chugging away merrily in a German lab. Its mechanism consists of a single molecule, and it鈥檚 powered by nothing more than a beam of light. The team who built it think that such photon-powered motors could be the next little thing in nanotechnology.
Hermann Gaub, a biophysicist at the University of Munich, and his colleagues built their nanomotor from a synthetic azobenzene polymer. This molecule contains pairs of nitrogen atoms with a benzene ring attached on either side. The nitrogen 鈥渂ridge鈥� between the rings is kinked, but when exposed to a particular wavelength of light, it straightens out and makes the molecule longer. A different wavelength returns it to the kinked form again. The researchers wondered if they could harness this motion to do useful work.
First they chemically tethered molecules of the azobenzene polymer to a coating on the surface of a glass slide. They also coated a tiny lever with chemicals that bind to the other end of the polymer molecule and attached it to an atomic force microscope (AFM). Then they lowered the end of the lever toward the slide until it bound to just one molecule. They checked that they鈥檇 hooked just one by using the AFM to measure the force needed to stretch it.
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The team shone UV light with a wavelength of 420 nanometres on the apparatus to switch the azobenzene molecule to its longer form. Then, when they bathed it in UV with a wavelength of 350 nanometres, the molecule shortened by about 5 per cent.
The researchers found that they could switch repeatedly between the short and long forms in this way. And by adding a load to the molecule as it shortened and removing the load as it lengthened, the researchers were able to make the motor do work on each stroke.
鈥淚t鈥檚 pretty much the same way your car engine runs,鈥� says Gaub. It鈥檚 a lot less powerful, though: each stroke did only 4.5 脳 10-20 joules of work. That鈥檚 about 5 millionths of a billionth of a billionth of the output of one cycle of a Ferrari engine. Yet it could still be enough to be useful at the nanoscale, perhaps to power microscopic pumps in future body implants. Gaub says light is probably the best power source for such a motor, because it鈥檚 easy to control and doesn鈥檛 require any fiddly wiring.
So far, the nanomotor has only idled-the researchers haven鈥檛 used its output to run any kind of nanomachine. But it proves that molecular motors can work. 鈥淚t鈥檚 the first demonstration of such controlled mechanical action in a single-molecule system,鈥� says Jim Gimzewski, who specialises in nanotechnology at the University of California at Los Angeles. 鈥淚t shows what you can do in nanoscale science at the ultimate limits of fabrication and measurement.鈥�
But the motor is a long way from being practical. It needs to be made more durable, and probably more efficient as well. 鈥淵ou鈥檇 need deep pockets and time to get money out of this,鈥� says Gaub. 鈥淏ut then it鈥檚 probably big money.鈥�
