Each of the single-molecule, 244-atom submersibles built at Rice University has a motor powered by ultraviolet light. With each full revolution, the motor’s tail-like propeller moves the sub forward 18 nanometers. And with the motors running at more than a million RPM, that translates into speed. Though the sub’s top speed amounts to less than 1 inch per second, that’s a breakneck pace on the molecular scale.
While they can’t be steered yet, the molecular motors are powerful enough to drive the sub-10-nanometer subs through solutions of moving molecules of about the same size. That’s akin to a person walking across a basketball court with 1,000 people throwing basketballs at him.
The motors complete each revolution in four steps. When excited by light, the double bond that holds the rotor to the body becomes a single bond, allowing it to rotate a quarter step. As the motor seeks to return to a lower energy state, it jumps adjacent atoms for another quarter turn. The process repeats as long as the light is on.