Emission based fiber optic pH sensing with Schiff bases bearing dimethylamino groups

In most of the common designs, pH probes rely on weak acidic dyes whose dissociated and undissociated forms have different absorption or emission maxima. In this work dimethylamino bearing fluorescent Schiff bases SB-I and SB-II have been characterized and used for pH sensing. Quantum yield and acidity constant (pKa) calculations of the Schiff bases were performed in the conventional solvents, polyvinyl chloride (PVC) and ethyl cellulose (EC). The immobilized Schiff bases SB-I (pKa = 4.25 in PVC) and SB-II (pKa = 10.68 in PVC) exhibited absorption and emission based optical response to proton in the pH range of 2.0–7.0 and 8.0–12.0, respectively. Immobilization of the dye effectively enhanced the fluorescence of these materials compared to solution. Responses of the sensor slides were fully reversible within the dynamic working range and the response times were less than 1 min. Relative signal changes of 63% and 95% have been achieved for sensor dyes of SB-I and SB-II in the PVC matrix, respectively. The cross-sensitivity of the dyes to different metal cations was also investigated. Presence of metal cations in test medium did not restrict the proton sensing ability of the dyes.