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Synthetic Shape Shifters

27 January 2005 (All day)
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Turn on. When chloride and carbon monoxide molecules bind to the catalyst above, atomic reshuffling gets things glowing.

Researchers report this week that they have created a synthetic catalyst that works the same way as enzymes do. When the new catalyst encounters its target, it changes shape, causing the solution containing it to glow. The technology is so sensitive it may soon be used to detect trace amounts of a wide variety of compounds, such as explosives and pollutants.

The new work is similar in spirit to the polymerase chain reaction, which amplifies virtually any snippet of DNA and attaches a fluorescent probe to the copies. Chemists have previously lacked such a broadly applicable sensing technique. They have come up with numerous schemes for detecting low concentrations of various compounds, but few work for a wide array of compounds.

Seeking such a strategy, researchers at Northwestern University in Evanston, Illinois, created an organic ring-shaped molecule with four metal atoms: two zincs and two rhodiums. When a pair of chemical targets--n this case, carbon monoxide gas and chloride ions--approach, the rhodium atoms cut their ties to nearby sulfur atoms and latch onto the targets instead. This atomic reshuffling relaxes the molecule's shape, separating the zinc atoms and allowing them to convert acetic anhydride--also present in the solution--into acetic acid. The acetic acid then donates a proton to another compound to create a highly fluorescent tracer and the entire solution lights up.

Northwestern University chemist Chad Mirkin, whose team reports its work online this week in the Journal of the American Chemical Society, notes that detecting chloride isn't anything special. Electrochemical detectors already exist for the job. But unlike those and other detection schemes, Mirkin and colleagues say, the new approach should make it possible to detect many different target compounds simply by altering the sensing molecule.

"This is very novel and interesting work," says Wenbin Lin, a chemist at the University of North Carolina, Chapel Hill. The ability to control a catalyst's activity by changing its shape, he adds, brings synthetic chemists one step closer to matching biology's molecule-making prowess. "Biology has been doing this for millions of years. Now chemists are just starting to get there," Lin says.

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