Thesis

Development of colorimetric indicators based on Methylene Blue

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Awarding institution
  • University of Strathclyde
Date of award
  • 2012
Thesis identifier
  • T13030
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Abstract
  • This study largely focuses on the development of an oxygen indicating label for possible use in modified atmosphere packaging. The work presented here builds on a system developed by Mills et al., i.e. a UVA-light activated reversible oxygen indicator ink comprised of a nanoparticulate semiconducting photocatalyst (usually TiO₂), redox dye (usually methylene blue), sacrificial electron donor and an encapsulating polymer. This ink could be cast as a thin film activated by UV light which, upon its absorption by the photocatalyst, creates an electron-hole pair. The photogenerated holes oxidise the concomitant sacrificial electron donor while the photogenerated electrons reduce the redox dye. This results in a dramatic colour change (usually blue to colourless). A photobleached film thus remains colourless in the absence of oxygen but quickly regains its original blue colour if oxygen is reintroduced. However, this ink is water-soluble and not suitable for printing on common packaging materials; this problem is addressed in Chapters 3 and 5 where solvent-based alternative O2-sensitive inks are described. The development of a more photo-stable ink, activated by UVB rather then UVA light, is the subject of Chapter 4. Interesting observations made while working on the above indicators lead to development of a series of humidity and temperature indicators based on phenothiazine dyes (Chapter 6). Methylene Blue and urea within a supporting polymer makes an ink which casts as a pink opaque film which turns into transparent blue film after reaching a humidity threshold (>80% RH). In addition, a combination of a polymer (hydroxypropyl cellulose) and a phenothiazine dye cast as a film was found to change colour upon heating, which could then be reversed by exposing the film to a humid air stream. Potential uses of such indicators are discussed. Chapter 7 reports on the red coloured forms of Methylene Blue observed upon treatment with alkali and helps correct the literature associated with this effect. Publications resulting from this work are attached at the end of the thesis.
Resource Type
DOI
Date Created
  • 2012
Former identifier
  • 941851

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