An Australian-led group of researchers has enhanced a suspended core microstructured optical fiber with a conjugated-polymer coating to allow sensitive detection of explosives in a liquid volume within the fiber.1 For example, when testing for 1,4-dinitrobenzene (DNB), which is a nitroaromatic explosive, the setup identifies the substance within a few minutes at concentrations as low as 6.3 parts per million (ppm) in a total sampling volume of 27 nl of acetone.
The technique is based on fluorescence quenching of the surface-attached conjugated polymer, which is poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV); the red fluorescence is stimulated by green laser light. The small size of the fiber's core (which is spatially suspended by three links) enables the strong evanescent-field interactions needed for sensing.
The team consists of scientists from the University of Adelaide (Adelaide, Australia), the Defence Science & Technology Organisation (DTSO; Edinburgh, Australia), and Shanghai University of Electric Power (Shanghai, China).
"Traditionally, explosives detection has involved looking for metals that encase them such as in land mines," says project leader Georgios Tsiminis, from the University of Adelaide's Institute for Photonics and Advanced Sensing. "In today's world, however, homemade improvised explosive devices will often have no metal in them so we need to be able to detect the explosive material itself. This can be difficult as they often don't interact with chemicals and we don't want them near electricity in case they explode."
In addition to sensing the presence of an explosive, the fiber-based device quantifies the amount of explosive by monitoring how the light emission changes over time, says Tsiminis. Because the technique is so sensitive, forensic investigators would simply take swabbed samples from various surfaces, place them in an organic solvent and, within a few minutes, know if there have been explosives present.
1. Fenghong Chu et al., Sensors and Actuators B: Chemical, Vol. 199, p. 22 (2014); dx.doi.org/10.1016/j.snb.2014.03.031