Photocatalyst activity indicator ink

They are applied to the coated product (usually by a pen, brush, or drawdown bar) and show a color change or disappearance when exposed to ultraviolet radiation.

A heterogeneous photocatalyst is a material that uses absorbed light energy (usually UV) to drive desired reactions that would not otherwise proceed under ambient conditions.

Paiis address the industry need for a rapid, simple, inexpensive method to demonstrate and assess the activities of the usually thin, invisible to the eye, photocatalytic coatings present on self-cleaning products.

[20] One way to achieve a visual demonstration of photocatalysis is to use a dyestuff, like methylene blue, dissolved in water, as the organic species to be mineralised, since, as the photocatalytic process proceeds, the colour of the dye disappears as it is oxidised.

[15][25] They are inexpensive, easy to use and provide a very quick route to demonstrating the presence of a photocatalytic film, even under low levels of UV light.

The ink is applied to the photocatalyst coating, usually using either a felt-tipped pen, air-brush, rubber stamp, paint brush, or a drawdown bar, and then exposed it to sunlight or an alternative, appropriate light source.

The ink identifies the presence of the photocatalyst coating by changing colour upon irradiation of the latter at a rate (usually < 10 min[15]) which provides a measure of the film's activity.

[32] The rate of the rapid colour change associated with photocatalyst activity indicator inks has also been directly correlated with the photocatalytic oxidation of methylene blue [33][34] and NOx.

A commercial paii ink sold in a felt tip pen. It can be easily applied on a construction material (e.g. glass) to reveal the presence of a photocatalytic coating: in this case by changing its colour from blue to pink
Figure 1. Schematic of the overall reaction, for the photocatalytic mineralisation of an organic pollutant on the surface of a titania photocatalyst film. (1) Ultra-band gap light generates electron-hole pairs. (2) Photogenerated holes migrate to the surface and can react with surface hydroxyl groups to generate hydroxyl radicals. (3) Organic pollutants are oxidised to their mineral form via these photogenerated hydroxyl radicals. (4) Photogenerated electrons can react with adsorbed oxygen to generate superoxide and subsequent other reactive species which can also oxidise organic pollutants.
Figure 2. Photocatalyst Activity Indicator Ink ( paii ). Upon irradiation with UV light, photogenerated electrons (e − ) and holes (h + ) are produced on the surface of the self-cleaning coating. The sacrificial electron donor (SED) present in the paii ink effectively 'mops-up' the holes (h + ), allowing the electrons (e − ) to reduce the dye (D ox ) to another (usually colourless) form (D red ).