Single-Molucule Car Goes for Spin

The tiniest car in the world has gone for a drive. Made of a single molecule, the "vehicle" has four wheel-like paddles that rotate in the same direction when zapped with a beam of electrons.

"The molecule is autonomous," says Syuzanna Harutyunyan, a chemist at the University of Groningen in the Netherlands who worked on the mini motor vehicle. "You don't need to touch it. Just give it energy and it's capable of converting that energy into movement."

To create the nanocar, Harutyunyan and her team designed a molecule with a long central body and one pivoted paddle at each of four corners. The paddles are free to swing around in circles, not unlike wheels.

Ordinarily they arrange themselves so as to minimise crowding with the central body, as this costs the molecule the least amount of energy. But when the team applies a pulse of electrons to the "wheels", some gain energy and move a quarter turn.

In this new position, the wheels experience overcrowding against the body of the nanocar and will move to a more spacious position as soon as possible. They get this opportunity when the bonds holding the wheels to the body stretch, prompting the wheels to move another quarter turn in the same direction, to a more "comfortable" position.

Frigid temperatures of 7 kelvin (-266 °C) help this clunky forward motion by effectively freezing the wheels in place except when excited by the electron pulse and during their subsequent self-adjustment. This keeps them from rolling backwards.

A nanocar had been built before but its wheels only spun in place, equivalent to placing a car on blocks to test it. By contrast, the new vehicle moves in a straight line (Nature, DOI: 10.1038). It's a slow road trip: it takes 10 pulses of electric fuel to move the vehicle 6 nanometres. The head of a pin is about a million nanometres wide.

Nonetheless, nanocar team member Karl-Heinz Ernst at the University of Zurich, Switzerland, is anxious to put it to work. "We have a locomotive," he says, but it's time to put some cars at the back and pull them along." Paul Weiss at the University of California, Los Angeles, says the car can help us unravel why tiny motors in nature, such as the motor proteins that move material around in cells, are so highly efficient. "The reason we work at these small scales is so that we can really understand the motion and efficient energy conversion." He hopes this will lead to more efficient large-scale motors.

SOURCE : NEW SCIENTIST NOVEMBER 2011