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Captura de Tela 2015-09-17 às 08.45.15The ability of TiO2, and other semiconductor sensitisers, to promote reductive photocatalysis, RP, especially of dyes, is significant and, although less well-known than oxidative photocatalysis, is of growing importance, according to the researchers Andrew Mills and Nathan Wells [1]. A significant example of a RP process is with respect to photocatalyst activity indicator inks. paiis, which provide a measure of the activity of a photocatalytic film under test via the rate of change of colour of the dye in the ink coating due to irreversible RP. In contrast, by incorporating the semiconductor sensitiser in the ink, rather than outside it, it is possible to create an effective UV dosimeter, based on RP, which can be used as a sun-burn warning indicator.

In the above examples the dye is reduced irreversibly, but when the photocatalyst in an ink is used to reversibly photoreduce a dye, a novel, colourimetric oxygen-sensitive indicator ink can be created, which has commercial potential in the food packaging industry [2]. Finally, if no dye is present in the ink, and the semiconductor photocatalyst-loaded ink film coats an easily reduced substrate, such as a metal oxide film, then it can be used to reduce the latter and so, for example, clean up tarnished steel. The above are examples of smart inks, i.e. inks that are active and provide either dynamic information (such as UV dose or O2 level) or a useful function (such as tarnish removal), and all work via a RP process and are discussed in a recent review reported in Chemical Society Reviews.

[1] Andrew Mills and Nathan Wells, Reductive photocatalysis and smart inks, Chem. Soc. Rev., 44, 2849-2864, 2015.

[2] S. Lee, Andrew Mills and A. Lepre, Chem. Commun.,1912–1913, 2004.

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