Conjugated Polymers-Based Fluorescent Sensing Films And Creation Of Its Related Instrument

The design and fabrication of chemical sensors with high sensitivity and selectivity has attracted extensive attention for several decades since they play a great role in environmental monitoring, medical diagnosis, forensic analysis, especially in anti-terrorism. Film sensors offer advantages in terms of reversibility and reproducibility, which are the two crucial parameters for practical applications of fluorescent sensors and their actual device implementation. As a novel kind of sensing element, conjugated polymers (CPs) have recently emerged as one important family of sensing units and are used extensively in chemical sensors. CPs provide several advantageous features:1) high molar extinction coefficient (106 M-1 cm-1),2) the backbone of CPs enabling the rapid propagation of an exciton throughout the individual polymer chain, which is the so-called "molecular wire effect",3) super-fast photo induced electron transfer or energy transfer between CPs and analytes (in several hundred femtoseconds), which leads to "super-quenching effect". More recently, most of the scientists have paid their attention to developing sensing materials, such as design and synthesis of novel CPs. The results showed that this strategy can greatly enhance the sensing properties. However, this is not the only way to fabricate high performance film sensors. It was proven that the sensing performance of film sensor can also be improved through changing the preparation methods of film sensors.On the basis of the above discussion and the research progress in our lab, the objective of the present dissertation is to fabricate a series of CPs-based film sensors with novel sensing mechanisms. After a brief review on polymer sensors for nitro-aromatic explosive detection (the first Chapter), the design strategies and fabrication of several CPs-based film sensors and their sensing abilities were described in the following chapters (Chapter 2 to 6). These film sensors were proved to be highly sensitive and selective to the target analytes, such as nitro-aromatic compounds and vapor of organic compounds. In addition, the related film sensor-based portable detector for nitro-aromatic explosives has been developed in our lab.In Chapter 3, a novel fluorescent film was fabricated by doping the aggregates of hexaphenylsilole (HPS) into a chitosan film. It was demonstrated that the fluorescence emission of the film is sensitive and highly selective to the presence of picric acid (PA). The detecting limit for PA is about 2.1×10-8 mol/L. Introduction of 2,4,6-trinitrotoluene (TNT),2,4-dinitrotoluene (DNT), nitrobenzene (NB), phenol, benzene, toluene, methanol, ethanol, and zinc nitrate (Zn(NO3)2) had little effect upon the fluorescence emission of the film. The selectivity of the film was attributed to the specific electrostatic association effect of the protonated substrate film to picrate anion and the screening effect of the film to the interferents. The network structure of the substrate film is also favorable for the stabilization of the fluorescence emission of the hybrid film through preventing the further aggregation of silole aggregates. Fluorescence lifetime measurements revealed that the quenching is static in nature. Furthermore, the quenching process is fully reversible. Considering the simplicity of the preparation and the outstanding performances of the hybrid film, it is expected that the film may be developed into a real-life PA sensor.In Chapter 4, two poly(pyrene-co-phenyleneethynylene)s of different compositions (PyPE-1 and PyPE-2) were synthesized and characterized. The two polymers had been casted onto glass plate surfaces to fabricate films (Film 1, Film 2) for investingating their sensing performances, separately. It has been demonstrated that the fluorescence emissions of the two films are sensitive to the presence of 2,4,6-trinitrotoluene (TNT) in aqueous phase. Interestingly, TNT shows little effect upon the emission of the parent polymer, poly(phenyleneethynylene) (PPE). The difference was explained by 1) theπ-πinteraction between pyrene moieties contained in the co-polymers and the analyte, TNT, molecules, and 2) more suitable matching of the LUMOs (lowest unoccupied molecular orbital) of the pyrene-containing conjugated polymers with that of TNT molecules. Further experiments demonstrated that the sensing is reversible, and rarely encounters interference from commonly found compounds, including other NACs. Fluorescence lifetime measurements revealed that the quenching is static in nature. The smart performance of the films and the easiness of their preparation guarantee that the films may be developed into sensor devices for super-sensitive detection of TNT in groundwater or seawater.In Chapter 5, the self-assembly monolayers (SAMs) techniques was used to fabricate sensing films. A fluorescent film sensor was prepared by chemical assembly of oligo(diphenylsilane)s on a glass plate surface, and was used for the detection of nitroaromatic compounds (NACs) in vapor phase. This design combines the advantages of fluorescent films based on single-layer chemistry and the signal amplification effect of conjugated polymers, and provides an effective way to create novel fluorescent sensing films for NACs explosives. The advantages have been demonstrated experimentally by the super sensitive response of the film mentioned above to the presence of trace amounts of NACs in vapor phase. Further experiments showed that the sensing process is reversible, and the common interferents have no interference to the process. Fluorescence lifetime measurement revealed that the quenching is static in nature. The super sensitive response, the reversibility and free interference of the sensing process make the film a promising NACs sensor.In Chapter 6, a fluorescence behavior controllable conjugated polymer (CPs)-based fluorescent film was developed by chemically attaching poly(2,5-dihexadecyloxy-phenyleneethynylene) (M-PPEs) onto a glass plate surface. It was revealed that the profile of the fluorescence emission spectrum of the film depended upon the polarity of its medium. This dependence has been attributed to the alteration of the conformation of the side chains of the polymer in immobilized state. In "poor" solvents or vapors, the side chains may adopt a compact coil conformation, resulting in aggregation of the immobilized polymers and thereby fluorescence emission of the film is reduced because of the so called aggregation induced fluorescence quenching effect. Whereas in "good" solvents or vapors, the side chains tend to be swollen and adopt extended or loose coil structure, and thereby preventing the aggregation of the polymers, coupled with increasing the fluorescence emission. Interestingly, this alteration process is fully reversible, and the retention time for each equilibration is less than 1 min. The film is also responsible for the changes in the compositions of mixture solvents, such as THF/methanol. In particular, a two-input INH and OR logic gates were presented on the basis of the film. No doubt, this finding can be taken as a new strategy for the design of CPs and self-assembled monolayer (SAM) based fluorescent sensing films, and will definitely expand their applications.In Chapter 7, a fluorescent conjugated polymer (CPs)-based amino acid chemsensor has been built up successfully by using "turn-on" strategy. The sensing performances were determined by the "proper interaction" between functional groups of polymers and copper (Ⅱ) ions. Obviously, the imidazole-functionalized CPs, P2, is a perfect sensing material to fabricate the amino acid chemsensors.In the appendix, a highly sensitive and portable explosive detector was created, which was based on the sensing films prepared in Part 2 and 3. The basic structure was provided in this chapter. No doubt, the economical, stable and portable detector with high-quality performance would gain extensive applications in the fields of environmental monitoring, anti-terrorism and nonmetal landmine detection.The main contributions of the thesis are described as follows:(1) Expending the design strategies for fabrication of sensing films. It is found that the substrate played the key role in sensing process, which gave a new way to fabricate the similar sensing films.(2) Designing and synthesizing a class of new conjugated polymers, which have been proved that it is an effective method to enhance the sensing performances of sensing films.(3) It was demonstrated clearly that the combination of the advantages of fluorescent films based on mono-molecular-layer chemistry and those based on conjugated polymers is an effective and feasible way to create novel fluorescent film sensors. Further research work showed that altering the side chains can affect the fluorescence properties, which may be a new strategy to make high performance sensing films.(4) On the basis of the above-mentioned fluorescent sensing films, an economical, stable and portable explosive detector with high performance has been created in our lab.