Novel photocatalyst-based colourimetric indicator for oxygen: Use of a platinum catalyst for controlling response times

The preparation and characterisation of a novel, UV-activated, solvent-based, colourimetric indicator for oxygen is described, comprising a redox dye (methylene blue, MB), semiconductor photocatalyst (Pt-TiO₂), and a sacrificial electron donor (SED = glycerol), all dispersed/dissolved in a polymer medium (sulfonated polystyrene, SPS). Upon exposure to UVA light, the Pt-TiO₂/MB/glycerol/SPS oxygen indicator is readily photobleached as the MB is converted into its oxygen-sensitive, leuco form, LMB. In contrast to its non-platinised TiO₂ counterpart (TiO₂/MB/glycerol/SPS oxygen indicator), the recovery of the original colour is faster (ca. 1.5 days cf. 5 days at 21 °C). This is due to the catalytic action of the 0.38 wt% platinum loaded onto the semiconductor photocatalyst, TiO₂, on the oxidation of the photogenerated LMB by ambient O₂. Furthermore, by increasing the level of platinum loading, recovery times can be decreased further; e.g. a Pt-TiO₂/MB/glycerol/SPS oxygen indicator with platinum level of 1.52 wt% recovers fully within 12 h. A study of the kinetics of recovery as a function of film thickness revealed the recovery step is not controlled by the diffusion of O₂ through the film, but instead dependent upon the slow rate of oxidation of LMB to MB by O₂ in the low dielectric polymer encapsulation medium. Other work showed this recovery is only moderately dependant upon temperatures above −10 °C and very sensitive to relative humidity above 30% RH. Potential uses of this UV light activated indicator are discussed briefly.