Fiber opticpH sensor for the acidic range using fluorescent dye-doped sol-gel materials

A plethora of new sensor technologies have emerged for chemical, biological, environmental and security applications. However, their adoption has been constrained by lack of integration of vastly different technical areas. Fiber optic chemical sensing is one such technology; a combination of spectroscopy, materials science and optical engineering. In this work, the example of an acidic pH sensor is used to individually develop these three areas and integrate them into a working device. Acidic pH is an important parameter in chemical processes, with no suitable optical alternatives.; The fluorescent pH indicator 5-(and 6)-carboxydichlorofluorescein (CDCF) was doped in a sol-gel matrix. CDCF has a pKa of 4.53, making it well suited for the acidic range. MTMS and TEOS gels were developed as films with good mechanical and chemical stability. The pH state of the immobilized dye affects the ratio of the absorption peaks. The fluorescence spectrum was red shifted in the organically modified gel. The fluorescence of hybrid spin-coated thin films resembled that of TEOS gels rather than that of hybrid bulk gels. The pH response was reversible and repeatable with a dynamic range of 5 pH units. Changes in ionic strength cause hysteresis in the sensor's response, which disappears when the film surface is saturated with electrolytes. The mechanical strength of the films in air and when immersed in solution was improved by using specific aging routines. Photo-bleaching was reduced by limiting excitation time to the duration of the measurement. Sonication, organic modification and aging in pH 2 buffer reduced leaching of the dye. Solid surface energy of the gels increased with organic modification. Hybrid gels were hydrophobic, resulting in slow response times. The diffusion constant of hydronium ions was found to depend on protonation state of the indicator and ionic strength of the solution. The diffusion constant in 0%MTMS and 10%MTMS films was of the order of 10-9 cm2/s, while in 50%MTMS and 100%MTMS gels, it was of the order of 10-11 cm2/s. Sol-gel films with moderate organic modification and an appropriate aging cycle produced the effective pH sensors.