Scientists in Japan and Ireland have synthesised a fluorescent organic molecule that detects explosive vapours. Tuning the self assembled structure enhances its sensing efficiency by nine times. 

Masayuki Takeuchi at the National Institute for Materials Science in Japan and co-workers prepared a charge transfer molecule of binaphthyl functionalised with donor acceptor substituted stilbene. The stilbene has donor and acceptor functional groups and exhibits charge transfer fluorescence, but when in the presence of species with stronger electron acceptor properties, such as the nitro groups on the explosives trinitrotoluene (TNT) the fluorescence switches off. 

The combination of a chiral group, the binaphthyl, and the stilbene chromophore induces chirality in the stilbene in the self assembled film state, explains Takeuchi. This stabilises the molecule, preventing fluorescence quenching in isolation. 

Self assembly into nanofibres in solution enhances the compund's sensor activity further says Takeuchi. 'Morphology tuning was achieved by varying the volume of toluene in the chloroform-toluene mixture which is used as the medium for the self-assembly process,' he adds. This tuning enhances the detection efficiency of explosive vapour by up to nine times. 

Aiping Zhu, an expert in organic photochromic compounds and mechanism studies at the University of Michigan, US, says the work is very interesting, but thinks the future of explosives detection lies in another direction. 'I think fluorescence turn-on strategy is the future for explosive detection. In turn-on mode, a non-fluorescent precursor is converted into fluorescent indicator only in the presence of molecules of interest,' he comments. 

Takeuchi intends to apply this work in the detection of other environmentally important species such as other explosives and pollutants in mud and water. 

Anna Roffey 

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