The gas attack on the Tokyo subway in 1995 demonstrated the need to rapidly detect and identify sarin and similar substances used in chemical warfare. David R. Walt and colleagues have devised a system to do just that (J. Am. Chem. Soc. doi:10.1021/ja057057b; 2006).

The authors attached a fluorescent indicator to polymeric, micrometre-sized beads. The indicator, fluoresceinamine, reacts with nerve agents such as sarin and soman that contain chemical groups known as phosphoryl halides. This causes the beads to fluoresce within seconds of a vapour burst (see lower image; red indicates fluorescence). As the reaction also generates acid, which reduces the intensity of the fluorescence, the authors coated the beads with another polymer that included units of the organic compound pyridine. This basic layer neutralized the acid, maximizing the beads' shine.

The fluoresceinamine reaction is specific to a certain class of chemical, so the microbeads don't respond to other, chemically distinct warfare agents such as mustard gas. Crucially, everyday vapours such as ethanol, toluene and water also fail to stimulate fluorescence, allowing the unambiguous detection of sarin-like chemicals.

Credit: ACS

The beads can be easily integrated into current ‘electronic nose’ technology used to detect and respond to many different vapours. Incorporated into the sensor array of such a nose, the sarin-detecting beads could react to small amounts of toxin in high concentrations of background vapours. Rapid-response nerve-agent detectors for public areas may soon be a reality.