Reusable colour-changing sticky labels that act as a cheap and easy way to check the activity of photocatalysis-based self-cleaning glass have been designed by scientists in the UK.

Interest in self-cleaning technologies, including semiconductor photocatalysis (SPC), has been on the increase since the commercialisation of self-cleaning glass by Pilkington in 2001. However, SPC, a process by which light activation of a surface coating, usually titanium dioxide, facilitates the breakdown of organic dirt, is difficult to measure as most coatings are invisible to the eye.

The indicator labels, developed by Andrew Mills and Nathan Wells, from Queen’s University Belfast, should solve this problem. And they overcome disadvantages linked with previous self-cleaning assessment methods, which were time-consuming, laborious and required skilled scientists and expensive equipment.

The labels offer improved versatility over the team’s previous work on photocatalyst activity indicator inks (paii), which only worked on flat surfaces. Furthermore, their use is not limited to a laboratory. If observing the colour change isn’t enough, ‘a simple phone camera app and controllable light source will allow you to detect the coating and assess its activity,’ explains Mills. In fact, ‘the most challenging part of the work has been convincing the photocatalyst community that something this simple could replace the existing, cumbersome tests for photocatalytic materials,’ he adds.

The ink is created from a solution of (poly)vinyl alcohol and methylene blue dye on a polymer support substrate. This film is then cut up and stored on silicone release paper, with an expected shelf life of more than six months.

A colour change arises from photoreduction of methylene blue; the semiconductor film is irradiated on exposure to UV light, generating electron­­­–hole pairs. Photogenerated valence band holes then oxidise a sacrificial electron donor, usually glycerol, enabling reduction of methylene blue to leuco-methylene blue, signified by a change from blue to colourless.

But you don’t need to be a scientist to use these labels and it is this, along with an estimated cost of less than 10p each, that makes them so commercially attractive. Devised for use anywhere and everywhere, and by anyone, a colour change can be observed within minutes by simply wiping clean a photocatalytic surface and sticking on the label in the presence of light. What’s more, when peeled off, leuco-methylene blue is exposed to air and oxidised back to methylene blue, guaranteeing reusability without compromising efficacy.

Photocatalysis experts are impressed by the work. Juan Coronado, from the IMDEA Energy Institute in Spain, comments that ‘self-cleaning surfaces constitute the most successful commercial application of photocatalysis’. And according to Jincai Zhao, from the Chinese Academy of Sciences in Beijing, ‘these end user-friendly labels take the technology to the next level.’