Coated Sol-Gel Thin Films of some pH- Indicators, Preparation and Characterization

Abstract

Transparent sol-gel thin films immobilized with bromocresol purple (BCP) and bromocresol green (BCG) pH-sensitive indicators were made via the acid catalyzed sol-gel reaction of tetraethylorthosilicate in presence of (BCP) or (BCG) indicators. Different surfactants include; cationic cetyl trimethyl ammonium bromide (CTAB), anionic sodium dodecyl sulfate (SDS) and nonionic Triton X-100 (TX-100) were used to improve the mesostructure of the host material and to increase its porosity. With the exception of BCP in presence of SDS; the BCP and BCG thin films show similar behavior in presence of surfactants as their free counterparts in aqueous solution with very small shifts in the wavelength's of absorption. The color change behavior of the immobilized bromocrersol purple indicator affected significantly and show three absorption bands in presence of SDS at( lmax=435 nm, , lmax=535 nm and lmax=595 nm) comparing with its free counterpart in aqueous solution, while only two absorption bands were observed in case of other surfactants for both BCP and BCG thin films. Both BCP and BCG retain their structures during the sol-gel reactions in terms of response to pH. Different parameters include concentration of indicators and surfactants, temperature, number of layers, response time, life time and number of measurements were investigated. The pKa values of the different prepared BCP and BCG immobilized thin films were determined. The BCP and BCG thin films sensor showed stability, reversibility, repeatability, reproducibility, fast response and long life time behavior. Lowest leaching of BCP and BCG indicators was showed when thin films were dried at 80 0C. Stability of the indicators thin films was observed up to ten month life time. The indicator thin films show very fast response. Construction of multi-layer sol-gel film enabled higher indicator lading and improve signal sensitivity. The electrostatic attraction between indicators molecules, surfactants and silica host matrix enhanced the accommodation of indicators molecules and improve their stability and preserve sensitivity. The polarized light microscopy indicated that the bromocresol purple and bromocresol green molecules were distributed uniformly within the host silica network especially in presence of surfactants.