A motor the size of a molecule
“If you’re trying to make a microscopic motor,” says Professor Richard Jones, “the laws of physics that work for petrol engines fall apart. Instead you have to look at nature.”
Nature’s answer is to produce molecular movement at constant temperatures without producing heat. Professor Jones, from Sheffield University’s Physics Department, has worked with Tony Ryan in the Chemistry Department to do the same thing in the laboratory. “We use a polymer that reacts to changes in the pH and changes shape. When it is in an alkaline environment it is soluble and spreads out, but in acid it contracts. We can tether the polymer down and measure the force the contraction produces.”
A single contraction is not enough for a motor, however. Ideally it should expand and contract repetitively, just as the pistons of an engine continually move up and down. “We use an oscillating chemical reaction called a mixed Landolt reaction,” explains Professor Jones. “The pH keeps changing and the polymer responds accordingly. You end up with a throbbing gel – constant movement but at a microscopic scale.
“This motor would be for a wet environment,” says Professor Jones. “I imagine it in a device about the size of bacteria that swims around. It’s all a long way in the future, but perhaps we may be able to build a microsubmarine that can swim in the bloodstream or something like a synthetic sperm cell, a device that swims towards a ‘smell’.”
Before any of these applications, however, the Sheffield team has to reduce the throbbing mass of gel to a single molecule. The researchers are trying to grow single polymer chains on surfaces. “It is important to get down to a single molecule,” explains Professor Jones. “This will allow us to understand the mini motor much better, and develop these applications in a more controlled way.
“We are also mimicking nature by trying to get the polymer to participate in the pH reaction, not just sit in a reaction bath. If we want to build a machine that swims, the pH changes will have to take place locally around the polymer – you can’t expect the whole pH of the environment to oscillate for you!”
While he speculates on future applications, Professor Jones enjoys the immediate challenge: proving the principles of molecular motors. “At this level there is a tremendous resistance to any motion because the viscosity and random movement of water. It’s certainly an interesting world to design for.”