Real-time detection of TNT analogues in water using fluorescent dendrimer films

Abstract

We have studied the use of fluorescent dendrimer thin films for the detection of nitro-aromatic analogues of and 2,4,6-trinitrotoluene (TNT) in water. The fluorescent sensing material used was a dendrimer composed of first-generation biphenyl dendrons, a 9,9,9′,9′-tetra-n-propyl substituted 2,2′-bifluorene core, and 2-ethylhexyloxy surface groups. The dendrimer had a solid state photoluminescence quantum yield (PLQY) of 49 ± 5% and was insoluble in water. Exposing the film to a nitro-aromatic analyte in water (including seawater) led to a rapid decrease in the fluorescence intensity. The decrease in fluorescence intensity arose from photoinduced electron transfer from the fluorescent dendrimer to the nitroaromatic analyte, with the process being reversible. That is, in the presence of the nitroaromatics the fluorescence was quenched, but on removal of the analyte, the fluorescence was restored. We have also developed a simple paper-based test strip that can be used for detection of the nitro-aromatic analytes in water. Dropping an aqueous solution of 2,4-dinitrotoluene or TNT onto the dendrimer-embedded paper strip led to a rapid visual decrease in the fluorescence intensity. The decrease in intensity could be compared to a standard card to determine the concentration, with the limit of detection using a phone camera being around 0.2 ppm. False positives were not observed when the dendrimer-embedded paper strips were exposed to a range of metal cations or different counteranions at concentrations much greater than are typically found in sea or freshwater.