Leaching in sol–gel-derived silica films for optical pH sensing

Abstract

The effect of different processing parameters on the efficiency of entrapment of pH indicators in a sol–gel-derived silica film is reported. An evanescent wave absorption approach was used to monitor dopant-leaching from these films. The issues investigated include the effect on the rate of leaching of the water:alkoxide ratio, the pH of the leachant solution and the choice of silicon alkoxide. Results indicate that the most suitable sol–gel films in terms of leaching and response time are those prepared using tetraethoxysilane as precursor, acid catalysis and a water:alkoxide ratio of at least 4. Leaching is more pronounced for films used directly after preparation and varies for different pH environments. Implications of leaching for the long-term use of sol–gel based pH sensors are discussed.