Joined: 16 Mar 2004
|Posted: Thu Mar 19, 2009 9:44 am Post subject: Nanoshuttle on the Right Track
|Molecular vehicle can carry cargo along a microscopic rail.
The molecular monorail has just left the station. It set off in Barcelona, and travelled about 500 millionths of a millimetre before reaching its destination (in Barcelona).
That might not seem far, but the tiny shuttle devised by Adrian Bachtold, of the National Centre of Microelectronics in Spain, and his colleagues is the first of its kind: a vehicle made essentially from a single molecule that can be reliably and controllably conveyed along a track in a particular direction. They report their work this week in Science.
The achievement is the latest step in efforts to make mechanical devices at the scale of nanometres (millionths of a millimetre) — one of the key objectives of nanotechnology. Nanoscale motors and shuttles that move across surfaces and through space in precisely defined ways might serve as the workhorses of a diminutive mechanical engineering, for example by transporting materials to exact locations to make intricate new structures and materials.
This kind of transport is one of the ways in which living cells achieve their indispensable and impressive molecular organization. For example, protein-based molecular motors ferry packages along tracks in the cell’s scaffolding. Nanotechnological engineers are both envious of and inspired by nature’s ability to make such nanoscale transport networks.
Engineers have harnessed biological molecular machinery to move nanoscale objects along tiny grooves and channels on a surface. But Bachtold’s team has pursued a different strategy based on relatively gigantic molecules called carbon nanotubes.
These tubes are artificially created hollow cylinders of pure carbon in which the atoms are joined in sheets similar to those that stack atop one another in graphite. In carbon nanotubes, the sheets are curled into tubes just a few nanometres wide. They have been hailed as potential building blocks for nanotechnology and have been used as nanoscale pipettes and rails for moving and guiding tiny quantities of material.
This is the first time, however, that nanotubes have been used as both the track and the shuttle. A short segment of a nanotube fitted onto a thinner one, like a sleeve bearing, can slide around or along the other nanotube with very little friction.
Rotor and motor
In 2003, a team at the University of California, Berkeley, used this structure to make a nanoscale rotor6. They attached a miniature slab of material to the nanotube sleeve that could spin like a propeller blade.
Bachtold and colleagues figured that they might also be able to use such a structure to move cargo attached to the sleeve along the track supplied by the inner tube. “Our goal was to make a transporter,” says Bachtold.
Left to itself, this assembly of nanotubes will just rattle around at random, shaken by the vibrations induced by heat energy. But the researchers discovered a way to harness this shaking to drive motion in a particular direction.
They attached each end of the nanotube track, about 300 nanometres long, to metal platforms, so that the tube stretched between them through empty space. Then they fixed a flake of gold to the shuttle tube, which was intended to hold molecular cargo.
When the researchers passed an electrical current through the bridging nanotube, which acts like a ‘wire’ connecting the metal plates, they found that some shuttle tubes moved towards the nearest plate. Others simply revolved at a fixed location.
“At first we thought it was the electrons that were moving the nanotube,” says Bachtold. But the direction of motion didn’t depend on the direction of the current.
Instead, the researchers concluded that the current was simply heating up the device, and that this was what was moving the shuttle. This mechanism “came as a surprise”, says Bachtold.
Because heat is conducted out of the nanotube by the metal plates, the system is hottest in the middle and cooler at the ends. This means that the thermal shaking of the track tube is strongest in the middle — which makes the sleeve tube move towards whichever end is nearer. It is a little like shaking the free end of a rope tied to a tree, with a hoop threaded onto the rope. The waves in the rope will usher the hoop towards the tree.
The researchers’ calculations suggest that these nano-shuttles should be able to move very fast: at about 100 metres a second, around 100 million times faster than those propelled by motor proteins. But the speed seen in the experiments is much more modest, comparable to that in the protein-driven devices, because the nanotubes they used are much bigger than those used in the calculations.
One problem is that the shuttle nanotube gets very hot — so hot, in fact, that the gold plate sometimes melts. “This is a problem,” Bachtold confesses, because the heat would probably destroy any molecular cargo attached to the plate. But he says that the heating would be less if the device were smaller.
“Our very long-term dream is to make nanoscale robots”, Bachtold says. “But we are really just at the beginning.”
Bachtold and his colleagues think that their approach of creating temperature gradients in carbon nanotubes could also be used to drive fluids through the tube, for example to make nano-pipettes and syringes for delivering drugs in precisely controlled amounts