| Nitroaromatic explosives are an important group of compounds in both environmental and forensic science. Water and soil can be seriously contaminated by them due to toxicities of them and their degradation products to human being, and ecosystem. Therefore, identification and quantification of traces of nitroaromatics in aqueous phase have attracted great attention during the last few years. Actually, a great variety of analytical methods have been developed for monitoring these compounds both in solution and in air, for example, mass spectrometry, ion mobility spectrometry, electrochemical methods, fluorescence, chemiluminescence, etc. Compared with other analytical techniques, fluorescence is more attractive because of its high sensitivity, high selectivity, and multiple choices in signals or parameters including emission intensity, emission wavelength, profiles of emission spectra, anisotropy, lifetime, and even excimer or exciplex formation. Actually, a number of homogeneous fluorescent sensors and fluorescent film sensors for nitro-aromatics have been designed and prepared. Although the homogeneous sensors are rapid, simple, and show promise for near real-time evaluation of nitro-aromatic contamination in environmental samples, from the viewpoint of practical use, film sensors have a number of advantages like being re-usable, no consumption of reagents and ease to be made into devices. Therefore, design and preparation of fluorescent film sensors for nitroaromatics have become the focus of the research.From the composition of the films, fluorescent film sensors can be fluorescent polymeric films, fluorophore-modified or doped polymeric or oxide films, and fluorescent conjugated polymeric films. It is interesting to note that the performance of these film sensors depends strongly on a number of factors including, at least, the permeability of the analyte in the polymeric films and the strength of the interaction between the analytes and the conjugated polymers. Furthermore, contamination to the analytical system is hard to avoid due to leaking of the chemicals from the polymericfilm. As is well known, polycyclic aromatic hydrocarbons (PAHs) are fluorescent and of high-quantum yield and they are also electron-rich compounds. The strong electron-withdrawing ability of nitroaromatic compounds enables them to form strong electron-transfer complexes with the electron-rich PAHs which could result in a fluorescence quenching and be used to detect nitroaromatics.On the basis of the considerations mentioned above and the previous work conducted in our lab, we thought it might be an even better way to solve the problems by immobilizing a fluorescent electron-rich fluorophore, like dansyl, onto a solid substrate through a self-assembled monolayer (SAMs) terminated with suitable reactive groups. This is because, firstly, the films produced in this way can avoid, at least in theory, the leaking problem which is usually an obstacle in practical use of fluorescent film sensors. Secondly, permeability problem may be solved automatically due to direct exposing to the bulk medium of the fluorophore. And thirdly, the binding of trinitrotoluene (TNT) and other nitroaromatics with the fluorophore is natural due to electron donation and acceptance interaction between them.Three dansyl-modified fluorescent film sensors with different spacer structures have been designed and prepared in this dissertation. These film sensors have been successfully used to detect nitroaromatics in aqueous solution.In the first work, dansyl was immobilized onto glass plate surface through surface reaction with the epoxide-terminated SAM via 1, 3-diaminopropane which turns out to be a new kind of fluorescent sensing film for effectively detecting nitroaromatics in aqueous solution. Fluorescence quenching studies showed that the film is sensitive and selective to the presence of nitroaromatics due to their strong electron-withdrawing ability. The stronger electron-drawing ability of the nitroaromatic is, the stronger the quenching efficiency of the corresponding nitroaromatic is. It has been revealed that the structure and properties of the spacer connecting the sensing fluorophore and the substrate play crucial rule, via the spacer layer screening effect, for the performance of the sensing film. Therefore, it may be expected that even better sensing films can be developed in the future by simple varying the structure, the property, and the density of the spacer.In the second work, a film sensor was fabricated by covalent immobilization of thefluorophore, dansyl, on a glass plate surface via reaction with 1,4-diaminobutane and then with an epoxide-terminated self-assembled monolayer (SAM). As expected, the simple elongation of the spacer by using 1,4-diaminobutane instead of 1,3-diaminopropane improved the sensing performance of the film sensors to nitroaromatics.From the two works mentioned above, we know the importance of the structures and properties of the spacers connecting the sensing fluorophore and the substrate. Therefore, in the third work, 1,6-hexanediamine was adopted as the spacer to get a fluorescent film with better sensing ability. Through fluorescene quenching studies, it was found that the sensibility of the film to nitroaromatics greatly increased. It is interesting to note that the quenching efficiencies of nitrobenzene to the film is greater than that of TNT in aqueous solution which is quite different from our previous work and contrary to the conclusion that the stronger electron-drawing ability of the nitroaromatic is, the stronger the quenching efficiency is. This phenomenon is explained to be related to the conformation of the spacer on the substrate which was thought to be more compact and restrict the interaction between the fluorophore inside the spacer layer and TNT with larger molecular size. On the contrary, it is much easier for nitrobenzene to enter into the spacer layer and quench the fluorescence of the film. This explanation was further confirmed by the solvent effect studies and fluorescent anisotropy studies.
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