An optical pH sensor with extended detection range based on fluoresceinamine covalently bound to sol–gel support

An optical pH sensor was developed based on the fluorophor, fluoresceinamine isomer II (FA), covalently immobilized in a sol–gel matrix. This sol–gel matrix was created by the copolymerization of two precursors, methyltriethoxysilane (MTES) and 3-glycidoxypropyltrimethoxysilane (GPTMS), in an ethanolic solution. Fluoresceinamine was covalently bound to the glycidoxypropyl chain group of GPTMS, thereby preventing it from leaching. Moreover, the immobilization of the fluoresceinamine also extended its linear detection range. The sensor showed good repeatability, a short response time of less than 8 s, high long-term stability and no temperature effect in the biologically relevant range. In the pH range of 4–10, the sensor was very sensitive and its linear range was found to be between pH 6 and 9 (R² = 0.995).