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NANOSCIENCE
Powered only by a burst of light, the contraction of a single molecule has enough muscle to bend the cantilever of an atomic force microscope (AFM), scientists in Germany report. This is the first demonstration of an artificial, single-molecule machine that converts light energy to physical work. Taking advantage of the azobenzene polymer's well-known ability to switch between its stretched-out trans and contracted cis forms when exposed to light, Hermann E. Gaub, professor of biophysics at the University of Munich, and colleagues show that a load attached to the molecule can be moved as the molecule shortens [Science, 296, 1103 (2002)]. Molecular machines are an increasingly fruitful research pursuit, yielding biologically based motors as well as systems such as molecular propellers fashioned from self-assembled complexes. Gaub's group covalently attaches one end of the azobenzene polymer to the cantilever of an AFM and the other to a piece of flint glass. Exposing the molecule to pulses of 420-nm light drives it into its lengthened trans state. Pulses of 365-nm light then cause the molecule's backbone to contract, pulling the cantilever down with a measurable force of several hundred piconewtons. Other molecules, such as stilbenes, could act as single-molecule motors as well, a possibility Gaub's group is exploring. And although the AFM cantilever provides a convenient way to measure the force exerted by the motors, Gaub says, real-world devices could exert force on other nanomechanical elements to make valves or pumps. The motors might even be interconnected into networks, Gaub notes, "but speculations on their potential applications are science fiction yet."
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