Formation of pyrene excimers in mesoporous organically modified silica thin films for visual detection of nitroaromatic explosives
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Pyrene is a polycyclic aromatic hydrocarbon compound. Pyrene has been extensively applied as probing and sensing molecule because of excimer fluorescence which is formed upon interaction of two pyrene molecules in close proximity. In this thesis, we prepared porous thin films with bright pyrene excimer fluorescence and demonstrated their application in visual and rapid detection of nitroaromatic explosive vapors. The fluorescent films were obtained by physically encapsulating the pyrene molecules in the mesoporous organically modified silica (ormosil) networks which were synthesized via a facile template-free sol-gel method. Formation and stability of pyrene excimers were investigated in both porous and nonporous ormosil thin films. Excimer emission was found to be significantly brighter and more stable in porous films compared to nonporous films. The excellent stability of the pyrene excimers in the porous films is due to the nanoscale confinement of pyrene molecules in the porous ormosil network. We studied the nitroaromatic explosive sensing performances of the pyrene doped porous films. Films exhibited a rapid and visible fluorescence quenching when they were exposed to TNT vapor. Fluorescence quenching efficiency of an approximately 100 nm thick porous film was calculated to be 55.6% after exposure to TNT vapor for 30 seconds revealing a rapid sensing behavior. Fluorescence quenching of the films can be easily observed under UV light enabling naked-eye detection of nitroaromatic explosives. A selective quenching was observed in the excimer emission against vapors of nitroaromatic molecules; trinitrotoluene (TNT), dinitrotoluene (DNT) and nitrobenzene (NB) among various aromatic and nonaromatic compounds. Furthermore, quenched excimer emission of the films can be recovered by simply washing the films with water. It is shown that the films can be reused for at least five times after washing. To this respect, pyrene doped ormosil thin films can be presented as facile materials for nitroaromatic explosive sensing applications.