pH optical sensors based on sol–gels: Chemical doping versus covalent immobilization Original Research Article

pH sensitive fluorescent sol–gels were obtained by both covalent immobilization of aminofluorescein (AF) via isocyanate or epoxy groups, and by co-condensation of tetramethoxysilane (TMOS) and phenyltrimethoxysilane (Ph-TriMOS) in the presence of AF. The gel precursors were deposited on glass supports, cured, and characterized in terms of response to pH, pKa values, effects of ionic strength, response time, leaching, long-term stability, and photostability. The addition of Ph-TriMOS is found to exhibit a pronounced effect on the performance of the materials. Sensor layers based on TMOS doped with AF were found to be most appropriate for purposes of sensing pH in giving large relative signal changes and displaying rapid response times over the pH range 4–9.