Organically modified silicate thin films doped with colourimetric pH indicators methyl red and bromocresol green as pH responsive sol–gel hybrid materials

The colourimetric pH indicators methyl red and bromocresol green have been immobilised in organic–inorganic silica hybrid matrices prepared by a sol–gel method. The optically transparent sol–gel thin films containing encapsualated dye molecules were deposited onto glass substrates using spin-coating and dip-coating techniques. The organic–inorganic hybrid materials were obtained by co-polymerisation of tetraethoxysilane together with glycidoxypropyltrimethoxysilane as an organically modified sol–gel precursor. Matrix polarity and morphology were varied by using solutions containing different ratios of sol–gel precursors and surfactants. Spectral characterisation of the films was performed using ultra-violet and visible absorption spectroscopy, and the spectral changes in different matrices were identified. The effects of the sol–gel mixtures composition on the mechanical stability and homogeneity of the films, and the colourimetric response to solutions of differing pH were studied. The apparent pKapp values of the immobilised pH indicators were also determined in the different matrices.