Optically transparent silica film with pH-sensing properties: Influence of chemical immobilization and presence of β-cyclodextrin on protolytic properties of alizarin yellow

Abstract

A sol-gel synthetic rote was proposed for preparation of optically transparent silica films with covalently bonded azobenzene dye, alizarin yellow. Effect of chemical immobilization on protolytic properties of pH sensor moieties was studied using UV–vis spectroscopy. It was found that indicator dye attached to the silica framework through amide linkage retains the ability to respond on pH change by color transition. The response time of surface azobenzene groups evaluated from the kinetic studies in acidic and neutral medium consists of 10 and 5 min, correspondingly. Acid ionization constants of individual and grafted alizarin yellow were evaluated by spectrophotometric titration in phosphate buffer solutions. It was found that involving of indicator dye carboxylic group in covalent bonding with silica surface leads to the shift of pKa2 value, reflecting the dissociation of phenolic hydroxyl group, from 10.9 to 5.5. Influence of β-cyclodextrin on pH-sensing properties of individual and grafted alizarin yellow was evaluated by spectrophotometric titration in phosphate buffer solutions. Sensor material showed good reversibility, repeatability and long-term stability at immersion in aqueous solutions with pH in the range from 1 to 8.