Minimotor: Single molecule does some work

Scientists have coerced a single molecule to act as a tiny, light-powered motor. The molecule, which stretches and contracts when exposed to light, performed mechanical work that can be harnessed, researchers report.

The achievement is another step toward the construction of nanoscale machines, says Hermann E. Gaub of the Center for Nanoscience at Ludwig-Maximilians University in Munich. Nanoscale motors, as well as levers, pumps, valves, and other minuscule parts, might be used someday in tiny laboratories-on-a-chip or other devices, says Gaub, who with his German colleagues reports the work in the May 10 Science.

“This report is an important milestone along the path to making nanomachines that we can control externally,” comments Paul Hansma of the University of California, Santa Barbara.

Although scientists had created many macroscopic materials that change shape in response to light, making a single molecule do work remained an important goal, says Gaub. “Single-molecule devices are the ultimate limit of miniaturization,” he says. “A frontier was reached in our experiments.”

Gaub and his colleagues used a polymer molecule, 50 to 100 nanometers long, made of light-sensitive units called azobenzenes. The angles between the chemical bonds in each unit shift when it’s exposed to certain wavelengths of light. A wavelength of 420 nm causes a unit to take on an extended shape, while light with a wavelength of 365 nm leads to a tighter conformation.

The researchers chemically bonded one end of the molecule to a bendable cantilever similar to the tip of an atomic-force microscope. They bonded the other end to a glass plate, akin to a microscope slide. Flashes of 365-nm light directed through the glass made the molecule contract, pulling down the cantilever and storing energy in it. Pulses of 420-nm light then extended the molecule, causing the cantilever to rise and release the stored energy.

If machine parts were attached to the cantilever, the system might bridge the gap between the nanoscale world and the visible one, says Gaub. Or the molecule might be directly attached to other nanoscale components.

Gaub cautions, “It will require several years of research to develop the necessary understanding of the basics and many more years to build up the technology in order to implement these effects in applications.”

“I think it’s the first kind of clear-cut example of a machine action . . . generated in a controlled way using a single-molecule system,” comments James Gimzewski of the University of California, Los Angeles.

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