Optical pH sensor based on spectral response of newly synthesized Schiff bases

Abstract In most of the common designs, pH optodes rely on weak acidic dyes whose dissociated and undissociated forms have different absorption or emission maximums. In this work the newly synthesized Schiff bases; N , N ′-bis(4-diaminobenzyliden)-1,2-cyclohexandiamine [I], N , N ′-bis(4-diaminobenzyliden)-1,2-ethanediamine [II] and 2,6-bis[(4-dimethylaminophenylimino)ethyl]pyridine [III], have been used for pH sensing in four different plasticized PVC matrices. The Schiff bases exhibited absorption and emission based optical responses to protons in the pH range of 3.0–7.8, and, therefore, can be used as an optical pH sensor for near neutral region of pH scale. The absorption and emission spectra were recorded by preparing thin films on a Mylar polyester support. Responses of the sensor slides were fully reversible within the dynamic range and the response time was approximately 3 min under batch conditions. All of the compositions exhibited a significant absorption based response to pH in immobilized form. Throughout spectroscopic studies, relative signal change of 79% has been achieved for sensor dye (I). In contrast to the weak emission peaks of (I) and (II), the sensor dye (III) exhibited a significant emission based response and is evaluated in a different category.

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