Organically modified silicate films for stable pH sensors

Abstract

Organically modified silicate films prepared from tetraethoxysilane (TEOS) and either methyltrimethoxysilane, phenyltrimethoxysilane or isobutyltrimethoxysilane have been investigated as pH sensors. The hybrid sol, prepared by hydrolyzing and co-condensing TEOS with the organosilicon monomer, was doped with the pH indicator dyes, bromocresol green or cresol red, and spin cast on a glass cover slip to form a thin film. The pH sensing range, rate of response, stability, and long-term leaching was evaluated as a function of the amount and type of organosilicon precursor introduced into the sol. Films prepared with a 1:1 or 2:1 TEOS:organosilane mole ratio were found to be the best in terms of transparency, uniformity, stability, and response. Response times to solutions of varying pH were less than 60 s. The pH vs. absorbance response curve for both gel-immobilized bromocresol green or cresol red were broad and the mid-point shifted by approximately 2–3 pH units more basic than their reported p K a's in water. When the dye-doped hybrid films were soaked in pH 7.0, 0.1 M phosphate buffer for extended periods of time (1–2 weeks), no significant leaching of the dye was observed. The improved stability of the immobilized dye can be attributed to the organosilicon functional group incorporated into the film.