Scientists have designed a single-molecule device capable of converting light into mechanical energy, according to a new report. The findings, published today in the journal Science, could aid efforts to design motors or switches for future molecular machines.

Previous research had shown that some polymers react to light by changing their configuration. This, in turn, can change the length of the material they comprise. In the new work, a team of scientists led by Hermann Gaub of the Center for NanoSciences at the Ludwig-Maximilians University in Munich, Germany, set out to test whether this phenonmenon applies at the smallest scale possible: a single molecule. They attached one end of a polymer molecule to a glass slide and the other to the tip of an atomic force microscope (AFM). By shining light of two different wavelengths on the polymer, the team caused it to switch between two distinct lengths a number of times.

The researchers next tried to determine whether their polymer molecule could convert this optical excitation into mechanical work. After stretching the molecule, they applied a force to the tip of the AFM that pulled on the polymer. Five pulses of light caused the molecule to contract and move the AFM tip. The polymer was then reversibly restored to its original length by pulses of light with a longer wavelength.

The experimental results suggest that photosensitive molecules could be used as tiny switches or actuators in nanoscale devices. The system is by no means optimized, says study co-author Nolan B. Holland, now at Case Western Reserve University, but it does show that these molecules can add mechanical work into a system. Different molecules may have greater ranges of motion or be able to withstand greater forces. The preliminary nature of the work, however, leads Gaub to call speculations on potential applications "science fiction" for the time being.