Angew. Chem. Int. Ed.http://doi.org/f2dqfz (2013)

Stimuli-responsive materials have many advantages for constructing mechanically active machines at the micro- and nanoscales. However, although there are many examples of materials that respond to an external stimulus, it is far more difficult to design one that 'remembers' being activated and continues to provide a mechanical function once the stimuli have been removed. Now, Ayusman Sen, Scott Phillips and co-workers at Pennsylvania State University have fabricated a polymer microsphere that acts as a pump once it has been activated by UV light. The chemical reaction that causes the pumping of the fluid surrounding the microsphere continues even after removal of the light.

This polymer sphere is functionalized with two different difluoromethylaryl reagents, one of which breaks down when activated by UV light — releasing several products including fluoride. This fluoride then reacts with the second reagent bound to the polymer, with additional fluoride being released. The two reactions combine to set up a self-propagating autoinductive reaction that amplifies the concentration of fluoride. Once the second reaction is underway, switching off the UV light will terminate the first reaction, but not the second, which continues until all of the secondary reactant is consumed.

The pumping action arises from the formation by the two reactions of a concentration gradient of small-molecule products. The products — a mixture of fluoride ions, protons, carbon dioxide, an aminobenzaldehyde derivative and an organosilyl — diffuse away from the polymer into the fluid. This induces movement of the surrounding fluid towards the microsphere, which the team observed by monitoring the motion of small polystyrene tracer particles. In addition to the continuous pumping behaviour shown by the bifunctionalized microsphere, the team also developed a pump formed from just the UV reactive reagent. This pump could be repeatedly turned on and off by controlling the UV light source.