Novel photocatalyst-based colourimetric indicator for oxygen

The preparation and characterisation of a novel, UV-activated solvent-based, colourimetric indicator for O2 is described, comprising a redox dye (methylene blue, MB), semiconductor photocatalyst (TiO2), and a sacrificial electron donor (SED), all dispersed/dissolved in a polymer medium (sulfonated polystyrene, SPS). Upon exposure, the indicator is readily photobleached as the MB is converted into its oxygen-sensitive, leuco form, LMB. Unlike its water-based counterpart, the recovery of the original colour is very slow (ca. 5 days cf. 6 min), probably due to the largely hydrophobic nature of the polymer encapsulation medium. The kinetics of film photobleaching appear to fit very well, in terms of: irradiance, [TiO2] and [MB], to the usual Langmuir-Hinshelwood type equation associated with a photocatalytic process. The glycerol appears not only to function as a SED, but also a plasticizer and medium for dye dissolution. The kinetics of colour recovery of the photobleached film appear directly dependent upon the ambient level of O2 but shows a more complex dependence upon the relative humidity, RH. The photobleached film does not recover any of its colour over a 24 h period if the RH < 20% at 21 °C but does recover at an increasing rate with increasing RH above 20%. The dye appears to form a very stable, water-insoluble ion-pair moiety with the SPS, so much so that the dye cannot be leached out by immersing the indicator in water. Potential uses of this UV light activated indicator are discussed briefly.