Optical pH sensors with porphyrin molecules, 5,10,15,20-tetrakis(4-aminophenyl)-21H,23H-porphine (TAPP) and 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP), immobilized on a polystyrene cuvette were fabricated and their sensing properties to the solutions with a pH range of 1.0–12.5 were examined. The characteristic Soret- (∼460 nm) and Q-bands (∼680 nm) of the porphyrin molecules were sensitively changed in its intensity with the change in pH. High sensitivities were obtained for the TAPP sensor both in the Soret- and Q-band regions at pH < 8. The TPyP sensor started to respond in its Soret-band at pH < 6 but significantly high sensitivity was obtained at pH < 3. The TAPP sensor exhibited a fast response and good reproducibility in the pH region. On the other hand, the TPyP sensor was lack of reproducibility in acidic region. Both the sensors exhibited a good stability at pH = 1 owing to the immobilization of the porphyrins.
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