“In the molecular machines we are familiar with in the ‘big world’, the parts, such as cogs, flywheels and pistons, do not move unless a force is applied to them,” explains team leader David Leigh. “At the molecular scale, however, molecules and their parts are constantly moving through Brownian motion and we need to find ways to control the direction of this motion if we are to develop fully-functioning nanomachines.”

Although researchers have already developed molecular motors based on “overcrowded” alkenes that can be driven by light, this mechanism only works for rotary motion and cannot drive linear molecular motors. It cannot be adapted to run off chemical fuels either (like the ones biological motors employ).

Fundamentally different type of mechanism

Last year, Leigh’s team made the first autonomous chemically-fuelled molecular motor that runs as long as a chemical fuel is present. This rotary motor relies on information transfer between the machine components: a blocking group adds as soon as the ring has moved past a certain point in a given direction and that group also prevents the ring moving backwards through Brownian motion.

“Our latest motor operates through a fundamentally different type of mechanism and does not require information transfer between the machine components,” Leigh tells nanotechweb.org. “It works thanks to its environment switching from being acid to base, which simultaneously provides a driving force for ring movement and directional control of that movement.”

Simple and effective driving

The researchers use trichloroacetic acid (Cl3CCOOH) as the fuel in their motor. Cl3CCOOH undergoes base-catalysed decarboxylation, and by adding an excess of this acid to a solution containing the molecular motor and another chemical (triethylamine, or Et3N), they were first able to make the medium acidic and then, as the Cl3CCOOH decomposes, basic.

“This means that a single pulse of the Cl3CCOOH fuel can rotate a [2]catenane motor by 360° in one direction, generating chloroform (CH3Cl3) and then carbon dioxide (CO2) as the only waste products of motor operation,” explains Leigh. “This process can be repeated with each pulse of fuel, allowing us to completely rotate all of the motor components.”

The driving mechanism is simple and effective and can be used to power both rotary and linear molecular motors, he adds. Indeed, the researchers say that they employed it in two different rotary motors and a linear molecular pump.

Inspired by biology

Any kind of future molecular nanotechnology will require such a power source and a motor mechanism, and our driving mechanism is inspired by biology, says Leigh. “Many biomolecular motors catalyse the hydrolysis of chemical fuels such as adenosine triphosphate (ATP) and use this energy to direct motion through information ratchet mechanisms. In our work, we show that trichloroacetic acid and catenanes can play a similar role.”

The team, reporting its work in Science DOI: 10.1126/science.aao1377, says that it is now working out how to use the new motor mechanism to power chemical synthesis. “Our next step will be to employ the motor in some useful tasks,” adds Leigh.