Fluorescent sensing copolymers: Synthesis, nanofiber fabrication and application in picric acid sensors

Increasing attention has been paid to the detection of explosives due to the occurrence of terrorist attacks around the world. Here, we used free radical polymerization to develop two different types of fluorescent copolymers for use in detecting picric acid. One exhibits aggregation-caused quenching (ACQ) and is called PNNS [poly (<i>N</i>-isopropyl acrylamide-<i>co</i>-<i>N</i>-hydroxymethyl acrylamide -<i>co</i>-styrene-pyrene), poly (NIPAAm-<i>co</i>-NMA-<i>co</i>-St-Py)]. The other possesses aggregation-induced emission (AIE) properties and is called PNNP [poly (<i>N</i>-isopropyl acrylamide-<i>co</i>-<i>N</i>-hydroxymethyl acrylamide-<i>co</i>-2-(1,2,3,4,5-pentaphenyl-1H-silol-1-yloxy) ethyl methacrylate), poly (NIPAAm-<i>co</i>-NMA-<i>co</i>-PPS-HEMA)]. Nanofibrous thin films of these copolymers were obtained by electrospinning. Upon interaction with picric acid, the fluorescence intensity of each copolymer was quenched due to photo-induced electron transfer (PET). The average diameters of PNNS and PNNP nanofibers were 179 ± 28 nm and 235 ± 143 nm, respectively. Sensing performance was evaluated by Stern-Volmer analysis. The Stern-Volmer constant (K_sv) values for PNNS and PNNP nanofibers were 0.012 μΜ^-1 and 0.119 μΜ^-1, respectively. Since the aggregated state of PNNP nanofibrous thin films can increase dramatically, the AIE property of this material provides a large dynamic range. Finally, the reusability of water- and methanol-washed nanofiber thin films was tested, revealing that the nanofiber sensors were reusable for detecting picric acid.