Science —

Four-wheeling on the nanoscale: single-molecule vehicles go for a spin

Researchers build a molecule that takes electrons for fuel and drives across a …


Utopian visions of the nanotechnology revolution suggest that one day we'll be able to put tiny machines inside our body to perform routine screening and maintenance. But we're a long way off from that future, as most of the nanoscale "machinery" we've created requires extensive intervention or carefully prepared conditions in order to do anything. But a report in today's Nature describes an impressive feat of molecule-scale engineering: a four-wheel drive "car" that can run across any conductive surface, powered by electrons.

The whole thing is a single molecule. Its core is formed by two hubs that have a five-ringed structure at their core. The hubs are connected by a rigid rod formed from carbon atoms, held together by triple bonds. Each hub is flanked by two "wheels," each consisting of a three-ringed structure. The bulk of the molecule is a carbon backbone, with a small number of nitrogen and sulfur molecules thrown in.

The key to the system is the bond between the wheel and its hub, which is a double bond formed between two carbon atoms. Electrons can cause this double bond to rotate, which places part of the wheel in close proximity to a bulky side-molecule attached to the hub. This bulky piece acts a bit like a ratchet; the wheel requires some vibrational energy to get past it. Once it does, it's positioned so that another dose of electrons can cause it to rotate again.

By repeating this cycle, the wheel will turn indefinitely in a single direction relative to the rest of the molecule. It's worth noting that the wheel analogy is pretty inexact. The part of the molecule that rotates is actually much closer to a large, flat plate. If you could actually go for a ride with wheels like this one, it would be an extremely bumpy one, as the plate would lift the vehicle and then hurtle it forward as it went flat again.

Still, it's so small that the only thing it could take for a ride is another molecule, so the authors are unlikely to hear any complaints.

The car doesn't carry its own fuel supply, but it's relatively easy to provide one. Provided that the temperature is kept at is 7K, there's enough energy in the system to provide the vibrational boost. That leaves the matter of the electrons. The authors fed these to the molecule using an scanning tunneling microscope tip. Placing it on a metal surface (in this case, copper) provided the electrons with some place to go afterwards.

Remarkably, it all worked. The authors gave one of the molecules 10 pulses of electrons, and watched it relocate after each one, moving a total of six nanometers by the time the last was delivered. It didn't move in a straight line, however, as it appears that there are some instances where one or more of the wheels doesn't actually turn. That can cause the molecule to move a shorter distance or even veer off to the side.

Not all of these nanovehicles performed that well due to an unfortunate feature of the chemistry involved in constructing them: it's not possible to precisely control the placement of the side chain that acts as a ratchet to force the wheel to go in a single direction. As a result, it's possible to have the front and back wheels oriented so that they try to push the molecule in opposite directions. Alternately, you can have one side of the molecule moving in one direction and the other side pushing in the opposite one. An accompanying perspective referred to this as "akin to having a car factory in which half the fully assembled vehicles are immobilized when they drop off the production line, because they land on their roofs or sides."

Still, this sort of thing was predictable, and the authors found examples of it: molecules that ended up jittering around in place over the course of 10 pulses of electrons.

We're still a long way off from useful molecule-size machinery, but the work is an impressive demonstration of what some carefully designed chemistry can do. No word yet, however, on how well 900 mV/nanometer works out in terms of fuel economy.

Nature, 2011. DOI: 10.1038/nature10587  (About DOIs).

You must to comment.

Channel Ars Technica