Impregnation of a pH-sensitive dye into sol–gels for fibre optic chemical sensors

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Abstract
The sol–gel method was used to investigate the preparation of gels which were subsequently impregnated with a pH-sensitive fluorescent dye, fluorescein isothiocyanate (FITC). The work enabled the examination of several aspects of the system, including the leaching of the dye from the gel, a problem which has more severe consequences for fluorescence-based than absorption-based systems, where the use of a silylating agent was found to improve the immobilization of the dye to the substrate. The effect of the loss of dye was investigated in terms of the fluorescent response of the prepared gels, and this was cross-compared with that of the dye in solution, as a function of pH. This showed good agreement. The response time of the gel to pH changes was also observed. Further, aspects of the drying of the gels were investigated in terms of their physical appearance and the characteristics of the substrates thus produced for chemical sensing purposes. Various observations of failure characteristics of these gels under re-immersion in water were considered in light of their potential use, in chemical sensor probes, of sol–gel based systems.