Probing the absorption and release of nitroaromatic vapours from fluorescent dendrimer films for the detection of explosives

We present a study into three generations of fluorescent carbazole-based dendrimers that exhibit strong binding with nitroaromatic compounds accompanied by photoluminescence (PL) quenching, making them attractive sensing materials for the detection of explosives such as 2,4,6-trinitrotoluene (TNT). The absorption and release of the (deuterated) TNT analogue, 4-nitrotoluene (pNT), from thin films of the dendrimers were studied with a combination of time-correlated neutron reflectometry and PL spectroscopy. When saturated with pNT the PL of the films was quenched and could only be recovered with a nitrogen gas flow upon heating to 40-80°C. Although the majority of the absorbed pNT was removed by this method the neutron reflectometry showed that the recovered films still contained residual quantities of pNT. However, the proportion of the PL recovered increased strongly with generation with the third generation dendrimer exhibiting full recovery. This result is attributed to a combination of two effects. Firstly, the dendrimer films present a range of binding sites for nitroaromatic molecules, with molecules bound to the higher energy sites remaining after the thermal recovery. Secondly, the exciton diffusion coefficient decreases with dendrimer generation, inhibiting the PL quenching by the bound pNT.