Research On Novel Fluorescent Chemosensors For The Ultra-Sensitive And Highly Selective Detection Of Explosives Aqueous Solution Based On Tetraphenylethene Derivatives

High selective and sensitive detection of nitroaromatic compounds(NACs) such as 2,4,6-trinitrotoluene(TNT), 2,4-dinitrotoluene(DNT) and picric acid(PA) is very important to both national security and environmental protection because they are not only recognized as toxic pollutants but also nitro-explosives, which is attracted much attention. NACs are commonly detected by fluorescent film sensors through photoinduced electron transfer(PET) in a quenching process owing to their strong electron accepting capability. Among a variety of fluorescent sensing materials, candidates with aggregation-induced emission(AIE), as good electron donors, are one of the most promising materials for explosives detection in both aqueous solution and vapor phase. AIE active tetraphenylethylene-based(TPE) derivatives have been widely investigated as film sensory materials and electron-donating groups for their extremely high sensitivity towards NACs because of a superamplification quenching effect, which can be believed to enhance the interaction between the electron-rich materials and the electron-deficient explosives.For constructing fluorescent thin film sensor, spin-casting is the most common approach, while the low permeability of the analyte and slow response in the coated film restrict the sensing performance. Therefore, a number of new alternative methods have been proposed, such as freeze drying, self-assembled monolayer, molecular imprinting, and so on. However, most of these methods still have complicated preparation steps and insufficient sensitivity problems. In recent years, electrospinning is a convenient and effective technique for generating various nanofibrous membranes with specific porous structure. The electrospun nanofibers with high porosity and a large surface area per unit mass are conducive for the interaction between the quencher and the fluorescent sensor, which has the great potential to provide fast response ang high sensitivity in sensing applications.Based on the above considerations, the novel fluorescent film sensors based on poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)] simply doped with TPE-2p TPA and TPE-2ptol were developed for the ultra-sensitive and highly selective detection of nitroaromatic explosive aqueous solution, for example PA solution in this paper. Specifically, the main contents are as follows:(1) Various factors, such as different mixed organic solvent ratios, high voltage, receiving distance, electrospun rate and other external environment reasons, were explored towards P(VDF-HFP) nanofiber morphology. The optimal conditions are given: mixed solvents of Act and DMAC volume ratio of 7: 3, electrospun voltage of 15 k V, the syringe speed of 1.2 m L/h, receiving distance of 15 cm, the optimum temperature is 20-23 ℃, the suitable humidity of 25%-30%. Under above optimum electrospun conditions, a series of TPE-2p TPA/P(VDF-HFP) nanofibrous film sensors were fabrieated by doping 5%, 10% and 15% contents of TPE-2p TPA. The sensing performance of fluorescent nanofibrous membranes towards PA aqueous solution was conducted, which possessed surplus sensitivity, high selectivity, good reversibility and satisfactory reproducibility. The detection limit reached 10-17 g/m L to PA solution, and the quenching constants were 1.972 × 106 M-1 for 5%, 3.116 × 106 M-1 for 10% and 7.065 × 106 M-1 for 15%, respectively. Moreover, In addition, it was demonstrated that the static fluorescence quenching process was possibly dominant in the TPE-2p TPA/P(VDF-HFP) nanofibrous film sensors, and the sensing mechanism for PA detection might be F?rster resonance energy transfer(FRET) and PET.(2) The novel TPE-2p TPA/P(VDF-HFP) fluorescent film sensors with different structures of "alligator-like" nanofibers, "pills-like" spin-coated films, "scaly-like" nanofibrous film, ect., were designed and prepared through a mixed solution of volatile solvents in a variety of different characteristics. The problems of the low permeability of quenchers, low sensitivity and slow response were successfully solved in these newly fluorescent film sensors to PA aqueous solution with ultra-sensitive detection and short quenching balance time. In addition, the chloroform solvent influence on the morphology of the film sensors(fluorescent nanofibrous film as an example in this paper) with same fluorescent molecule content was developed. It was found that the higher concentration of TPE-2p TPA chloroform solution in the mixed organic solutions, namely the smaller proportion of chloroform solvent in the mixed solvents, the easier construction of novel sensors with new structures and more excellent sensing performance. It was worth noting that the the quenched maximum equilibration time was 600 s in the "ball-like" TPE-2p TPA/P(VDF-HFP) fluorescent nanofibers to all concentrations of PA solution in this thesis, but the value was still far less than the values reported in the literature. The nanofibrous film possessed excellent reversibility and satisfactory reproducibility towards a range of concentrations of PA solutions after twenty-six quenching-regeneration cycles.(3) The fluorescence sensing material in the chemosensor was changed by TPE-2ptol instead of TPE-2p TPA. TPE-2ptol fraction in TPE-2ptol/P(VDF-HFP) fluorescent nanofibrous film was 20%, and the rough surface and "scaly-like" nanofibers were prepared. The detection limit of fluorescent nanofibrous sensors towards PA aqueous solution was 1.000 × 10-17 g/m L, and the quenching constant was 2.353 × 105 M-1. It was illustrated that the quenching process was mainly static and fluorescence quenching mechanisms might be PET and FRET. Futhermore, the "scaly-like" TPE-2ptol/P(VDF-HFP) fluorescent nanofibrous film in air and water possed good photochemical stability, high selectivity and satisfactory reversibility to PA aqueous solution.