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Optical physics

Optical physics

Molecular machines get light battery

10 May 2002

Light has been used to obtain mechanical work from a single molecule for the first time. In a technique that could be used to power molecular machines of the future, Hermann Gaub of Ludwig-Maximilians University in Munich and colleagues made a polymer molecule stretch and contract by shining light on it. A cantilever attached to the molecule measured the forces exerted during this cycle, which was repeated several times before the molecule broke (T Hugel et al 2002 Science 296 1103).

Nano-scale devices based on molecular machines could play a major role in the future of electronics, medicine and communications. But a major challenge in the development of such devices is to find a suitable power source.

Now Gaub’s group has shown that light can be used to power mechanical transitions in an azobenzene polymer. This polymer is a chain-like molecule with a dangling side group, which can have one of two orientations – known as “trans” and “cis” – with respect to the main molecule. The polymer molecule is longer when the side group is in the trans position and shorter when it is in the cis position.

Chemists have long known that this molecule can be switched between its long and short states by illuminating it with light of certain wavelengths, and several chemical processes are based on this effect in bulk samples of the polymer. But Gaub and colleagues have shown that the effect can also be useful at the single molecule level.

The researchers made a sample of the polymer containing a mixture of long and short molecules, and attached a cantilever – in the form of an atomic force microscope – to one of the cis molecules. After illuminating the sample with visible violet light, the cis molecules flipped into the trans state, and the cantilever registered the force produced by the molecule as it stretched by 1.4 nm. When Gaub’s team subsequently shone higher-frequency ultraviolet light onto the polymer, a compression force was detected as the molecule contracted.

To establish whether the molecule could do mechanical work, the researchers repeated the experiment after applying loads – up to 500 pN – to the molecule using the cantilever. When they illuminated the molecule as before, they found that it continued to stretch and contract against the applied force. This allowed them to calculate that the molecule had done mechanical work as its length changed in response to light.

Although the process is currently rather inefficient, Gaub’s team is optimistic that it can be improved so that the system could operate in a real device.

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