Paper-Based Probes with Visual Response to Vapors from Nitroaromatic Explosives: Polyfluorenes and Tertiary Amines.

Although it is well-known that nitroaromatic compounds quench the fluorescence of different conjugated polymers and form colored Meisenheimer complexes with proper nucleophiles, the potential of paper as a substrate for those macromolecules can be further developed. This work undertakes this task, impregnating paper strips with a fluorene-phenylene copolymer with quaternary ammonium groups, a bisfluorene-based cationic polyelectrolyte, and poly(2-(dimethylamino)ethyl methacrylate) (polyDMAEMA). Cationic groups make the aforementioned polyfluorenes attachable to paper, whose surface possesses a slightly negative charge and avoid interference from cationic quenchers. While conjugated polymers had their fluorescence quenched with nitroaromatic vapors in a non-selective way, polyDMAEMA-coated papers had a visual response that was selective to 2,4,6-trinitrotoluene (TNT), and that could be easily identified, and even quantified, under natural light. Far from implying that polyfluorenes should be ruled out, it must be taken into account that TNT-filled mines emit vapors from 2,4-dinitrotoluene (DNT) and dinitrobenzene isomers, which are more volatile than TNT itself. Atmospheres with only 790 ppbv TNT or 277 ppbv DNT were enough to trigger a distinguishable response, although the requirement for certain exposure times is an important limitation.