Photocatalyst activity indicator inks (Paii's)

The use of photocatalyst activity indicator inks, paiis, to provide readily a measure of the activity of photocatalytic films is described. The photocatalyzed bleaching of dyes in aqueous solution either oxidatively or reductively is outlined briefly, followed by a description of the basic components of a typical, water-based paii, namely, a brightly colored dye that is irreversibly reduced, such as resazurin (Rz), a sacrificial electron donor, glycerol, and a polymer, such as hydroxyethyl cellulose (HEC), all soluble in water. It is shown that the Rz paii, when cast as an ink film, upon ultra-bandgap irradiation, changes color at a rate that is proportional to the ability of the photocatalytic test substrate to photo-oxidize a typical organic pollutant, such as stearic acid. Other paiis are described. The result of a study of the kinetics of the photocatalyzed reduction of the dye in a paii is described and discussed, along with the recent use of digital photography and color analysis as an alternative to UV/vis spectrophotometry in order to make the use of the inks easier, especially in the field. The emergence of an ISO based on the Rz international standard is noted, and a brief review of the use of paiis in photocatalysis is given. Finally, the potential limitations of paiis are discussed.