I'm going to take a break this evening from the med-chem side of my science. There's a paper in the preprint section of the ACS journal Nano Letters that's one of the neatest things I've seen in a while. Jim Tour's group at Rice has been working in this area for quite a while now, and they now report something they call a "nanocar."
It's a single large molecule, built from standard organic chemistry reactions. There are two straight axles, made out of acetylene compounds (which are rod-shaped), and another connecter between running between them. On both ends of each axle is a fullerene (a buckyball), and getting those attached was the apparently one of the trickiest parts of the whole synthesis, which took several years. The other tough part seems to have been hanging enough greasy chains off the various structural parts of the thing so that it could be dissolved in an organic solvent. Here's the synthetic scheme and a drawing of the molecule. (That link currently seems to work for non-subscribers - the full article is here.)
Those fullerenes are wheels. They can turn independently, because the bond between them and the next acetylene is freely rotatable, and that seems to be just what they're doing. By finally making one of these that could be taken up into a solvent, Tour's group managed to get some of these things onto a gold metal surface, which is a perfect background to use for Scanning Tunneling Microscope (STM) imaging. And here they are. (The fullerenes show up very well in STM imaging, and they're pretty much all you can see.) Buckyballs are already known to stick very well to gold, so Tour's people had to heat up the metal to get things moving. Once they got up to about 170 C, though, the molecules - the nanocars - began to roll around.
Now, molecules sitting on metal surfaces move around all the time, but they mostly just slide and hop by thermal wiggling. There are several lines of evidence to show that these are really rolling, though. For one thing, a three-wheeled symmetrical variety was made, and it just spins in place. (That link also has a nifty rendered version of both types of molecule, but those are rather idealized portraits. For one thing, they don't show all the long side chains decorating the frame, which would make the whole car look rather Rastafarian.) The cars also appear to only move along their long axis, with slight pivots as one set of wheels breaks free before the other side does. (The nano-differential has yet to be invented). Finally, the team used the STM tip to drag a nanocar along, and showed that it couldn't be towed sideways - the wheels dug in rather than rolling.
It's easy to dismiss this work as a stunt, which is what I once did with one of Tour's other ideas. But this is the beginning of the real thing. A larger, more functionalized version of the nanocar might carry other molecules along and dump them at will, which is what this group seems to be working on now. These are small steps toward controlled nanoscale delivery, which is a small step toward a nanotech assembler.
We're a long way from that. But for now, there are any number of interesting experiments waiting to be run. You have to wonder how these things will behave on other surfaces, for one thing. If they drive better on some than others, you could imagine directing them around on small roads which have been fabricated by chip-building techniques. There are other molecular forms that could be used as wheels, and other potential ways to move them around rather than just heating them up. Just looking at these structures gave me an idea of my own: how about making the axle part of the molecule by incorporating a structure that would absorb at particular infrared wavelengths? That would show up as motion in the chemical bonds, and might provide a means to make a motor to drive these things. Eventually we're going to have grad students standing around an STM rig, betting on which of their designs will make it across an atomic landscape first. . .