Explosives can now be detected at picogram levels thanks to a polymer developed by scientists in the US.

William Trogler and his team at the University of California, San Diego, made a silafluorene-fluorene copolymer to identify nitrogen-containing explosives. It is the first of its kind to act as a switchable sensor with picogram detection limits.

TNT detection versus nitrate ester detection

Trogler's polymer can detect explosives at much lower levels because, unlike existing systems, it detects particles instead of explosive vapours. Trogler sprayed the polymer solution over the test area, let it dry then shined UV light on it. Spots of explosive quench the fluorescent polymer and turn blue.

The polymer is able to show the difference between nitrate esters, such as trinitroglycerin, and nitroaromatic explosives, such as TNT. Initially, polymer-treated spots of both compounds appear blue under UV light, but after further exposure the trinitroglycerin spot fluoresces green-yellow whilst the TNT spot remains blue. This colour change is thought to be due to photooxidation of the fluorenyl groups of the polymer.

Trogler was surprised to find that adding a spirofluorene co-monomer gave the polymer a 100 per cent efficient conversion of UV light into fluorescence, describing this increase as dramatic. 'From a technology perspective, the most surprising thing was the ability to use photochemistry to attain a reasonably chemospecific turn-on sensor,' he says. The technology is now being commercially produced by RedXDefense, a security company based in the US, and has even been featured in an episode of the television program CSI: Miami.

The team are currently working on a similar system to detect peroxide-based explosives and say they hope to be able to investigate perchlorates and organic nitrates too.

Sylvia Pegg