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

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 lone-term use of sol-gel based pH sensors are discussed. (C) 1998 Elsevier Science B.V.