C&EN: TODAY'S HEADLINES - A BETTER SENSOR FOR NERVE GAS

RON DAGANI

Timothy M. Swager thinks he has the best detection system so far devised for sniffing out chemical warfare agents such as tabun (GA), sarin (GB), and soman (GD).

Swager

Other sensor systems rely on detecting the molecular shape or a specific spectroscopic signature of a chemical warfare agent. But an enemy could change the shape of the molecule slightly so the detectors don’t respond to it, says Swager, a chemistry professor at Massachusetts Institute of Technology. As he’s fond of pointing out, “It isn’t the shape of the molecules that kills you—it’s their reactivity.”

Swager’s new chemosensor, developed with postdoc Shi-Wei Zhang, detects nerve agents by how they react with an indicator molecule [J. Am. Chem. Soc., published online March 4, http://dx.doi.org/10.1021/ja029265z]. One of their indicator molecules is a naphthalene derivative bearing a pyridyl and a hydroxy (or siloxy) substituent.

Nerve agents such as sarin are volatile organophosphorus compounds that react with a hydroxy group in biological tissue to form a phosphate ester. When this reaction occurs at the catalytic site of acetylcholinesterase—an enzyme critical to nerve function—the enzyme’s activity is inhibited, leading to convulsions and death.

In the detection system, organophosphates, which are used as model compounds for the nerve agents, react with the hydroxy or siloxy group of the indicator to form a phosphate ester. Because this ester is a good leaving group, the indicator molecule spontaneously undergoes an intramolecular cyclization reaction, yielding a tetracyclic product that fluoresces at a different wavelength and intensity than the starting compound.

Swager and Zhang have shown that the sensor, in the form of a thin film, responds in seconds to 10 ppm of diisopropylfluorophosphate vapor, which is a less reactive relative of sarin gas. Visually, the sensor signals the presence of a toxic compound by shifting its fluorescence emission from blue to green.

“It would be better visually if it was green to red,” Swager says, noting the dye could be modified to optimize the response. He believes that the sensor has the potential to detect nerve agents in the sub-parts-per-billion range. The system is not designed to identify the agents it detects.

This is “an important new approach to sensing nerve gases,” comments Eric V. Anslyn, a chemistry professor at the University of Texas, Austin.

Chemistry professor Michael J. Sailor of the University of California, San Diego, agrees, noting that Swager’s system is designed to “turn on when it sees analyte. Fluorescence sensors that turn on in response to analyte are fundamentally more sensitive than those that turn off.”

Swager is collaborating with Nomadics, a technology company based in Stillwater, Okla., to further develop the detection system for use in defense and homeland security. “We believe it’s a significant advance,” says Nomadics chemist Lawrence F. Hancock. When used with the firm’s vapor-phase fluorescence sensor platform, he says, Swager’s indicator compounds provide rapid detection of nerve agents.

Swager and Zhang note that their indicator also responds to similarly reactive toxic industrial chemicals, such as thionyl chloride (SOCl₂), that could also pose a threat to homeland security.