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Sensitive and selective detection of nitro-explosives, particularly 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrophenol (TNP/PA) have received paramount importance owing to their resistance to degradation, high toxicity and national security threats . Hence, finding an efficient detection technique for TNT/TNP in an aqueous environment is a pressing priority to protect public health and environment. In order to resolve this problem, recently, cost-effective colorimetric and fluorescent based methods have been conveniently employed for the selective detection of TNT/TNP in solution and solid phases due to their fast response to sample analysis, high sensitivity, excellent selectivity and easy visualization . Consequently, various fluorescent probes have been reported for the selective recognition of either TNT/TNP by fluorescence enhancement/quenching response in organic medium or binary system [3-4]. Majority of them lose their selectivity associated with single-emission quenching mechanism, lack of applicability in aqueous medium and unavoidable interferences from other nitroaromatics. For this endeavor, in recent years, extensive research has been devoted to developing bi/multifunctional fluorescent probes for the interference-free discriminative detection of TNT and TNP in a complex environment . However, for the aqueous medium detection of TNT and TNP explosives, the progress is still unsatisfactory. Hence, there is an urgent need to develop novel efficient water-soluble fluorescent probes with better properties such as broad range fluorescence spectrum, high selectivity and sensitivity for the discrimination and determination of both TNT and TNP in aqueous medium. Ionic liquids (ILs) are a versatile class of organic compounds with various physicochemical properties and they have found widespread applications in optical and electrochemical sensing . And also, ILs offer inherent advantages in terms of low-cost, large scale production, facile synthetic procedures, high fluorescence efficiency and tuneability which makes them promising water-soluble fluorescent probes for the sensitive detection of explosives at very low concentrations in aqueous medium. In this work, a highly water-soluble ionic liquid based on salophen has been developed. Using Schiff base condensation salophen ((3,3’-((((1E,1’E)-(phenazine-2,3-diylbis(azanylylidene))bis(methanylylidene))bis(4-hydroxy-3,1-phenylene))bis(methylene))bis (1-ethyl-1H-benzimidazol-3-ium) bromide comprising benzimidazolium and phenazine moieties as backbone (PS-IL)) has been synthesized and explored as fluorescent probe for the discriminative detection of TNT and TNP in aqueous environment. The fluorescence emission behaviour of PS-IL towards TNT and TNP shows distinguishable response in solution state and electrospun nanofiber strips, respectively. Furthermore, the distinctive fluorescence response of PS-IL to closely related nitro explosives exhibited the selective discrimination of the structurally related TNT and TNP species. While sensing TNT, the fluorescence of PS-IL could be remarkably enhanced by TNT (Fig. 1A) and the fluorescence of PS-IL was quenched by TNP (Fig. 1B) via the formation of hydrogen bonding interactions. This dual response of enhancement and quenching of PS-IL towards TNT and TNP was further facilitated via restriction of photoinduced electron transfer (PET) and PET process. The binding mechanism based on the off-on PET effect was established using 1H NMR, fluorescence lifetime and fluorescence spectroscopic studies and the proposed interaction mechanism was further confirmed by density functional theory (DFT) studies. The detection limit of PS-IL towards TNT and TNP was found to be 0.95 nM and 7.75 nM, respectively. To the best of our knowledge, for the first time, PS-IL probe has been utilized as a discriminative sensing platform for TNT and TNP detection by colorimetric and fluorimetric methods in a pure aqueous environment as well as solid phase. The sensing ability of PS-IL probe in solution and nanofibers strips confirm the trace level quantification of TNT and TNP and have great potential for real-time detection of TNT and TNP in seawater, groundwater and various environmental contaminants.
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