A "four-wheel drive car" less than one billionth the length of an average SUV has been built and operated by researchers in the Netherlands and Switzerland. The molecular machine is about 1 nm long and uses electrons as fuel as it navigates across a copper surface. The tiny device could find use in nanometre-sized robotics or as tiny transporters that shift molecules around.

Molecular machines are common in nature. Motor proteins, for example, can move along a surface to transport molecular-sized cargo and are often used to build structures within living cells. Scientists would like to make their own versions of motor proteins, and indeed they have already designed and demonstrated single molecules that can move across surfaces. But these have been mostly passive: to ensure that they travel in a certain direction, they have had to be pulled or pushed.

Now, Ben Feringa of the University of Groningen and colleagues have demonstrated a truly active single-molecule vehicle. Constructed around an organic, carbon-based frame, it has four "wheels" or rotor parts, connected to the body via carbon–carbon double bonds. When the tip of a nearby scanning tunnelling microscope fires electrons at these bonds, they break and re-form the other way round. This process is known as isomerization and causes the wheels to turn, and the vehicle to move forward.

Steering by symmetry

Feringa and colleagues could make their molecular vehicle move in two ways, by adjusting the symmetry or "chirality" of the rotor parts. In one, the vehicle moves along a random path, something that has been performed before with active molecular machines. However, the researchers could also make the vehicle drive in a nearly straight line.

"The important step taken, in my opinion, is that we have shown that we can propel a single molecule along a surface and control directionality," said Feringa. "This is exactly what happens with protein nanomotors that 'walk' along filaments with control of directionality," he added.

'Milestone' reached

Ludwig Bartels at the University of California at Riverside, US, agrees that the ability to control direction is a major step forward. "This work is a milestone towards controlled transport of molecular species across surfaces," he says. "But much work remains – most importantly, the replacement of the energy source away from the tip of a tunnelling microscope (which could in the first place just drag any molecule along, irrespective of its nature), and the achievement of concerted motion of the substrate linkers so that the motion becomes really straight."

James Tour of Rice University in Texas, US, thinks the demonstration brings scientists closer to the goal of using synthetic molecular machines to assemble structures, rather like enzymes do inside the body. "This is an important and fundamental milestone in the quest for nanomachines that will one day do useful work," he says.

The research is described in Nature 479 208.