The highly sensitive and reliable detection of explosives such as nitro substituted compounds (DNT, TNT and TNP) is of paramount importance for civilian and military security. NACs are also recognized as poisonous or carcinogenic explosive chemicals to the environment. Currently, explosive detection either relies on canines or highly sophisticated measurement techniques, such as mass spectrometry, gas chromatography, Raman spectroscopy, etc. High cost and/or complexity of these techniques limit their wide availability, especially in the field. Compared to those fluorescent polymers are particularly interesting for fluorescence based rapid detection as they exhibit large signal amplification due to the delocalization and rapid diffusion of excitons throughout the individual polymer chains1. Moreover, the fluorescence polymers having high PL quantum efficiency, favorable redox potential and easy synthesis scheme need to be explored which can act as excellent electron donor for rapid electron transfer to NACs having electron-withdrawing nitro groups on the aromatic ring in order to make them as efficient optical sensor with ultra-high sensitivity2.
Here, we report the synthesis and multimodal sensing applications of a highly emissive and electron-rich alanine based dansyl tagged copolymer P(MMA-co-Dansyl-Ala-HEMA) (DCP) which exhibited high sensitivity and selectivity towards DNT, TNT and TNP in solution at lower range of μM level and also with saturated vapour of NACs. The high quantum yield of the co-polymer (77.3%) makes it an ideal candidate for sensing in solution as well as in vapor phase. In solution, the fluorescence signal from DCP co-polymer gets significantly quenched upon addition of aliquots of DNT, TNT, and TNP caused by photo-induced electron-transfer i.e. quantified by plotting Stern–Volmer plot (KSV = 1.1×103 M-1,1.3×103 M-1 and 1.6×104 M-1 for DNT, TNT and TNP). The quenching mechanism was further established by time-resolved fluorescence and steady state absorption spectroscopy which was found to be predominantly dynamic in nature as lifetime of polymer (14.9 ns) is reduced to 13.9, 13.9 and 13.8 ns for DNT, TNT and TNP. The energetics of sensing process was calculated by Density Functional Theory(DFT) studies this is also in good agreement with the obtained result.
To explore the possibility of using the fluorescent co-polymer as sensor array, a prototype thin film polymer sensor was fabricated using drop-casted thin film of DCP which was able to detect saturated nitroaromatic vapor in real time with high selectivity. The initial fluorescence intensity of the 20 nm thin film of DCP was quenched to 19% for DNT, 13% for TNT and 4% for TNP in just 2 min.
In summary, this work opens up a novel approach for designing light weight and compact prototype sensor for field use as well as for environmental monitoring.
1. Sun, X. et al. Chem. Soc. Rev. 2015,44,8019.
2. Rochat, S. et. al. ACS appl. mater. & interfaces 2013,5 (11),4488.