Halochromic resorufin-GPTMS hybrid sol-gel: Chemical-physical properties and use as pH sensor fabric coating

The hybrid GPTMS (3-glycidoxypropyltrimethoxysilane) gel containing the chromophore molecule of the litmus dye, Resorufin (7-Hydroxy-3H-phenoxazin-3-one), was prepared by sol-gel method through epoxy ring opening reaction catalyzed by traces of 1-methylimidazole in acidic ethanol solutions. The wet-gel solution was fully characterized by means of different spectroscopic techniques. Proton and heteronuclear NMR and FT-IR spectroscopy have confirmed the immobilization of the Resorufin into the GPTMS polyethylene oxide polymer, through GPTMS epoxy ring opening and monomer diol-GPTMS condensation/hydrolysis. The halochromic activity of the Resorufin-GPTMS hybrid sol-gel was monitored by UV-vis spectroscopy, in buffered solutions at different pH values, in comparison with the corresponding dye. FT-IR spectroscopy was also used on solid xerogel residue covering glass surface to demonstrate the attempted GPTMS epoxy-ring opening. The morphology of the treated and washed cotton samples, compared to the original one, was investigated by SEM, SEM-EDX and AFM microscopy analysis, in order to assess the aspect of the fibers after the different finishing treatments in the micrometric scale.