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.