Organic-inorganic hybrid sol–gel membranes for pH sensing in highly alkaline environment

This work reports the synthesis of organic-inorganic hybrid (OIH) membranes for potential optical fibre sensors (OFS) functionalisation for pH monitoring of concrete and reinforced concrete structures (RCS). As precursors, 3-glycidyloxypropyltrimethoxysilane and Jeffamine (R) THF-170 were used. The OIH sol-gel materials were doped with phenolphthalein (phph) for pH sensing proposes and cetyl trimethyl ammonium bromide as surfactant. OIH membranes with pH sensing ability properties were successfully obtained. The dielectric properties of the resultant membranes were determined by electrochemical impedance spectroscopy. The logarithm of resistance obtained was 10.19 Omega cm(2) for the undoped matrix and, 7.06 Omega cm(2) for the doped one with phph. The results obtained in simulated concrete pore solutions, for the OIH produced, showed to be promising for application in fresh concrete. The transmittance of the synthesized OIH materials decreased after the doping with phph. The matrix showed intrinsic fluorescence and doping it only led to small deviations in the wavelength of the peak of maximum emission. Regarding thermal analysis, no significant changes were obtained on the thermal profile of the doped and undoped OIH membranes. The pH variation was detected by the OIH membrane colour change. Stability studies regarding the release of phph from the OIH matrix led to a mean dissolution time equal to 75.4 (+/- 0.02) hours, indicating that the proposed system reveals long-term stability.