The development of a fluorescent optical ammonia gas sensor based on FRET mechanisms embedded in Xerogel Matrices

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This thesis contains the development of optical-based planar and fibre waveguide ammonia gas sensor prototypes. The sensing mechanism is based on the change of the fluorescence emission intensities of selected dye pairs caused by ammonia which is due to the Forster resonance energy transfer (FRET) between the dyes. Chapter 1 introduces the reasons for investigating optical ammonia sensors. It highlights the advantage of fluorescence-based optical sensors and discusses the theoretical basics and sensor platform technologies connected to this topic. Chapter 2 focuses on the development of an optical planar waveguide ammonia gas sensor, the sensing mechanism of which is based on FRET between coumarin and fluorescein. The dyes were immobilized into an organically modified silicate matrix during polymerizing methyltriethoxysilane with trifluoropropyl-trimethoxysilane on a PMMA substrate. The resulting dye-doped xerogel films were exposed to different gaseous ammonia concentrations. A logarithmic decrease in coumarin fluorescence emission intensity was observed with increasing ammonia concentration. The coumarin/fluorescein composition was optimized in order to obtain the best ammonia sensitivity. Experiments in the gas sensor setup demonstrated a sensitive and reversible response of the xerogel films to gaseous ammonia. Chapter 3 reports on the development of silica particle impregnated xerogel sensor coatings on PMMA substrates. Fluorescein and rhodamine B labelled mesoporous silica particles were synthesized by post-grafting and co-condensation approaches. The resulting materials exhibited different pore size distributions, particle shapes and sizes. The Forster resonance energy transfer between this dye pair was explored for the different materials by exposure to various concentrations of gaseous ammonia. A logarithmic increase in rhodamine B emission intensity with increasing ammonia concentration was observed for both post-grafted and co-condensed materials. The dye accessibility by ammonia gas in the silica framework was evaluated by the same gas sensor setup reported in Chapter 2. The response to ammonia gas and the recovery with nitrogen gas is explained by comparing the structure properties and dye loading of the materials. Chapter 4 contains the development an optical fibre waveguide ammonia gas sensor. The sensor performance of a PMMA fibre clad with the FITC and RBITC doped xerogel reported in Chapter 3 was investigated. The results of the preliminary fibre sensor measurements and the suitability of this system for wearable applications are discussed. Chapter 5 concludes the thesis by highlighting the most important results and discussing possible experiments or procedures for the improvement of the ammonia gas sensor performance.