9 November 2011—Inside our bodies, a remarkable set of molecular machines speeds along our internal processes, ferrying goods where needed, dividing cells, and contracting muscles. The challenge for nanoengineers, says chemist Ben Feringa, "is to design a completely artificial system that can do the same thing as these biological molecules."
In a step toward that goal, Feringa and colleagues at the University of Groningen, in the Netherlands, have constructed a molecule that looks and seems to move like a four-wheeled car. Feringa’s team was responsible for the first light-driven molecular motor, but their latest creation is propelled forward by energetic electrons shot from a scanning tunneling microscope (STM).
Other groups have created molecular rotors fixed to a surface—and even nanocars built on fullerene wheels—but none have achieved the complexity of this molecule, with its four rotorlike lobes, say the scientists. The mechanism of its movement is not fully understood, but scientists believe that all four wheels rotate in unison following a single pulse of electrons. If that’s the case, it’s an entirely new system, says chemist Petr Král, an associate professor of chemistry at the University of Illinois at Chicago. Král first suggested the possibility of driving molecular rotors with tunneling electrons but was not involved in the Netherlands research.
The Feringa group’s early light-driven motors worked only in a liquid, but building a motor that worked on a dry surface, as the new car does, required some careful tuning: A molecular machine needs enough flexibility to move freely across a dry surface but not so much that it detaches completely, explains Tibor Kudernac, who was a postdoctoral researcher in Feringa’s lab when the car was being built.
Over the course of five years and three different designs, the molecular car was built with just such flexibility. According to the group’s theory, a single 500-millivolt pulse causes a change in the molecule’s structure on the outer side of each wheel. That new shape catalyzes another slight twist in the structure, and two more identical steps complete a revolution. The molecule’s four appendages look more like paddle wheels than rounded tires, and as they flop end over end, the molecule trudges forward 0.7 nanometers per rotation.
Biological molecules that carry cargo around inside cells have the benefit of a molecular rail to guide them. Feringa’s molecules have no such roadways, and yet they roll forward in a relatively straight line. That movement requires simultaneous excitation of each of the four wheels with a single pulse of electrons at the center of the molecule (the car’s body). Sometimes that pulse doesn’t get to each of the wheels at the same time, so the molecule occasionally drifts a bit off course, like a car in need of realignment.
"Our system is very primitive," says Feringa, "but it shows that you can use a molecular motor to propel something directionally along a surface."
It is not yet clear if the proposed mechanism for the car’s movement is accurate, emphasizes Král. The forward motion is clearly being powered by electric pulses, but Král is not convinced that a single pulse is driving the concerted motion of all four motors. "If it was true that all the motors are working in tandem, it would be quite interesting," says Král. "But we can’t tell from the description [Feringa has] given."
Still, Feringa’s work serves as proof of the concept that a molecular machine can be driven, without a track, by an external source of power, in a single direction. The hope is that synthetic molecules like these can eventually be developed into functional nanorobots.
Next, the group plans to make its molecular machine a bit hardier. Currently, the molecule’s mechanism works only at cryogenic temperatures, on conductive surfaces, and inside a vacuum. In that environment, Kudernac explains, "there is no movement at all unless the molecule is excited," which makes it easier to control its motion. They hope to redesign the molecule to help it respond to electrical pulses while remaining unaffected by the forces that molecules experience at room temperature. The researchers also hope to power the rotors with light, a feature that would allow them to drive the molecule’s movement on nonconductive surfaces.