Solar-Powered Nanomotors

Inspired by natural molecular machines, such as those that assemble proteins or move materials inside cells, scientists have long worked to construct tiny machines of their own to further miniaturize, for example, computers and medical devices. Already, they have built basic parts for such machines, such as devices that walk along a strand of DNA powered by “batteries” that store chemical energy.

Eventually, however, these devices will run out of power and stop working. Now a team from UCLA and the University of Bologna in Italy have made a molecular device powered by an external, practically unlimited power source: sunlight. Further development of the device, which is described online this week in the Proceedings of the National Academy of Sciences, could lead to applications in computing and cancer treatment.

The new machine “is a step forward from the past, where we had to add components that would be used and produce waste products,” says J. Fraser Stoddart, professor of chemistry at UCLA and one of the lead researchers on the project. “That’s not happening in this case. We simply can switch on the light, get the movement, and when the light is off, the movement comes back in the other direction. What thrills us most is that it is autonomous and it doesn’t involve producing waste.”

[Click here to view an image of the machine.]  

The device consists of a ring that shuttles back and forth between two stations on a barbell-shaped molecule, making the complete trip as many as a thousand times in a second. As long as no light hits the device, the ring is held in place by electrostatic forces at the first station. When exposed to light, one end of the molecule releases an electron, which travels to the first station and cancels the force holding the ring in place. This frees the ring to move to the second station. When the device is removed from the light, the first station regains its attraction and the ring slips back.

Stoddart says the new device could lead to low-power computer processor or memory elements that are optically controlled.

Another application could be delivering drugs to the site of cancerous tumors. The devices might be used as mechanical stoppers to plug holes in nano-scale spheres. Once the spheres travel through the body and attach to cancer cells, light shined on the site of a tumor would trigger the ring to move, unplugging the hole and releasing drugs stored inside. This highly targeted drug delivery could reduce side effects and make drugs more effective.

Before any such system could work, though, it would need to be shown that the nanodevice itself won’t cause problems in the body.

In its current state, such a molecular machine is still limited. “So far there’s no way of connecting it to the outside world,” says Dean Astumian, physics professor at the University of Maine. It simply floats around in a solution of organic solvents. “To be applied, at some level you would have to be able to harness the motion to do some nanoscale physical task,” he says. “At present, no one has been able to actually hook this up.”

Still, Astumian says the recently published work shows promise over earlier attempts at such machines. “The big advantage here is you don’t need very much experimental manipulation to keep it running,” he says. “As long as you keep the light, it should work.”