Simple inkjet-printed, UV-activated oxygen indicator

The preparation and characterisation of a novel, simple, UV-activated, water-based, colorimetric indicator for oxygen is described, comprising a redox dye (methylene blue, MB), a colloidal semiconductor photocatalyst (SC = TiO2), and a sacrificial electron donor (SED = tartaric acid). The colloidal anatase TiO2 particles, with diameter ∼5 nm, are made via the hydrothermal treatment of titanium isopropoxide, with tartaric acid as the peptizing agent, and SED. This ink is inkjet-printed on polyester film, resulting in a physically robust, colorimetric oxygen-indicating film with a thickness of ∼63 nm. Upon exposure to 3 min UVA light (4 mW cm−2), the indicator is readily photobleached when MB is photoreduced to leuco-methylene blue, LMB. The photobleached film recovers its colour in ∼12 h under ambient conditions (∼21 °C, ∼60%RH, 21% O2), but did not recover its original colour in the absence of O2. The rate of recovery in air (from cylinders) exhibits some dependence upon relative humidity at 21 °C, but little at 4 °C and rate of recovery is found to be linearly dependent upon the %O2 present. The rate of colour recovery is reduced upon reducing the temperature from 21 to 4 °C, and has an estimated activation energy for the reaction of the LMB photoreduced dye with O2, Ea = 41 ± 6 kJ mol−1. Potential uses of this simple UV light activated indicator are discussed briefly.