Nanocrystalline SnO2, ncSnO2, is used as a photosensitiser in a colourimetric O2 indicator that comprises a sacrificial electron donor, glycerol, a redox dye, methylene blue (MB), and an encapsulating polymer, hydroxyethyl cellulose (HEC). Upon exposure to a burst of UVB light the indicator is activated (photobleached) as the MB is photoreduced by the ncSnO2 particles. In the absence of oxygen, the film stays bleached, but recovers its original colour upon exposure to oxygen. Unlike its TiO2-based predecessor, the HEC/glycerol/MB/ncSnO2 O2 indicator is not activated by UVA light from white fluorescent lamps, but is by UVB light. This feature provides much greater control in the activation of the indicator. Other work shows the rate of activation depends upon I0.75, where I is the UVB irradiance, indicating a partial dependence upon the electron-hole recombination process. The half-life of the recovery of the original colour of a UV-activated film, t50, is directly proportional to the ambient level of oxygen. The advantages of using this indicator in modified atmosphere packaging as a possible quality assurance indicator are discussed briefly.
- tin(IV) oxide