The tiniest Lego nanomachines: A tale of nanoscale motors, rotors, switches and pumps...
Adam Rifkin stashed this in Nanotech!
In their quest to forge molecular machines that can actually do something useful, researchers are starting to integrate several different components into a single device.
In May this year, Stoddart unveiled17 an artificial molecular pump that pulls two ring molecules out of solution onto a storage chain. Each ring slips over a stopper at one end of the chain, attracted to a switchable binding point. Flipping that switch pushes the ring over a second barrier farther along the chain, where it reaches a holding area (see 'Nano machines').
The system is not able to pump any other type of molecule, and it took a lot of trial and error to build. “It's been a long road,” sighs Stoddart. But it proves that molecular machines can be used to concentrate molecules, pushing a chemical system out of equilibrium in the same way that biology can build up a store of potential energy by forcing ions or molecules up a concentration gradient. “We're learning how to design an energy ratchet,” he says.
Stoddart says that such developments could enable the field to progress in two major directions: stay nano, giving the machines molecular-scale jobs that cannot be achieved in any other way; or go macro, using trillions of them together to reshape materials or move substantial cargoes, like an army of ants.
Perhaps the prime example of the nano approach is Leigh's molecular assembly line 18. Inspired by the ribosome, it is based on a rotaxane system that picks up amino acids from its axle and adds them to a growing peptide chain. But the devices could have macro applications. Over 36 hours, 1018 of them working together can produce a few milligrams of peptide. “It doesn't do anything that you can't do in the lab in half an hour,” says Leigh. “Yet it shows that you can have a machine that moves down a track and picks up molecular building blocks and puts them together.” Leigh is now working on other versions of the machine to make sequenced polymers, with tailored material properties.
Conversely, trillions of molecular machines working together could change the properties of materials in the macroscopic world. Gels that expand or contract in response to light or chemicals, for example, could act as adjustable lenses or sensors. “In the next five years, I bet you'll get the first smart materials where you have switches incorporated,” says Feringa.
Rotaxane-like molecules are already starting to see commercial applications. The Nissan Scratch Shield iPhone case, launched in 2012 and based on work by Kohzo Ito at the University of Tokyo, is made of polymer strands threaded through pairs of barrel-shaped cyclodextrin molecules connected in a figure-of-eight shape. Pressure on a normal polymer coating would break the connections between the chains, leaving a scratch. But the cyclodextrin rings act like the wheels of a pulley system, allowing the polymer strands to slip through without breaking 19. The films can even protect a brittle screen from a sustained beating with a hammer.
For Stoddart, this shows that the components developed by molecular architects are already ripe for application. “This field has come a long way,” says Stoddart. “Now we have to start showing it's useful.”