A laser-based nerve gas sensor under development illustrates that we should not underestimate the power of the commonplace. First cobbled together for $24 from spare parts from CD players, the device outperforms nerve gas sensors that cost thousands more. Detectors and alarms identify and warn of possible exposure to nerve agents, but the most effective -- portable ion-mobility spectrometers -- can cost $7500, putting them out of reach of all but the military. Moreover, the systems tend to register false positives when confronted with common substances such as pesticides, petroleum products and antifreeze. A civilian hazardous materials team suspecting chemical warfare cannot afford the public hysteria that may accompany a false alarm. The new device, developed at the University of California by postdoctoral researcher Jun Gao and a team led by Michael J. Sailor, is composed of a red laser pointer, a beamsplitter, two photodiodes and a porous silicon chip coated with a catalyst that reacts to phosphorous-fluorine bonds. The catalyst hydrolyzes P-F bonds to yield hydrogen fluoride, which etches the oxidized chip, said Sonia Létant, a postdoctoral researcher at the university and member of the team. The reflected laser beam generates interference fringes from the chip, and the chemical reaction changes its refractive index, producing a nonreversible shift in the fringes. "We had previously showed that HF gas could be selectively detected with an oxidized porous silicon interferometer," Létant said. "The same principle is applied to the detection of fluorophosphonate nerve agents." These include the G-type nonpersistent nerve gases such as sarin, made infamous in the 1995 Tokyo subway attack by the Aum Shinri Kyo religious sect. "All the nerve agent molecules that contain this bond, like sarin, soman, GF and DFP, will trigger the reaction," she said. There is little risk of false positives, however, because few other common chemicals feature the P-F bond, and HF is rather rare in the atmosphere. Létant said that, while the chips cannot be reused, they are cheap and easy to produce in quantities. Moreover, because the sensors are so inexpensive, civilian and military teams could deploy them in arrays that would both detect nerve gas and track its spread over a target area, a form of continuous monitoring that currently is not possible. The handheld sensor, which is being readied for tests at the US Army's Aberdeen Proving Ground in Maryland, has displayed a sensitivity to DFP of 800 ppm for an exposure time of five minutes. Honglae Sohn is developing catalysts in William C. Trogler's lab at the university that will improve the sensitivity of the device and resist deactivation at low pH.