Novel pH-based photocatalyst activity indicator hydrogel film (Hpaii)

•A new type of photocatalyst activity indicator is reported based on a photocatalysed pH change.•The indicator is a hydrogel (agar) film, containing a photoacid generator and a pH indicator (yellow to red).•The indicator works on lab-based TiO2-solgel films and commercial samples of self-cleaning glass, paint and tiles.•Easy to make and use with a long shelf-life, when stored in a fridge.

A novel, hydrogel-supported photocatalyst activity indicator, Hpaii, is presented, based on an agar gel containing the pH indicator dye propyl red (PR), and the photoacid generator, diphenyliodonium nitrate (DPIN). The latter is broken down when the film is on an active photocatalytic film, and the acid released protonates the PR, turning the gel from a yellow colour to a red. Initially tested on a TiO2 sol-gel film, the Hpaii is also shown to be effective in indicating the activity of several different commercially available photocatalytic surfaces, including: glass, tiles and paint. This is the first example of a water-based photocatalyst activity indicator ink based on monitoring a photocatalytic process via the change in pH induced in the indicator films, as revealed by the pH-indicator, propyl red.

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