*2.4. Resazurin Intelligent Ink for Photocatalytic Testing*

Intelligent ink based on the dye Resazurin (Rz) was used to determine the photocatalytic activity of the TiO2 thin films. This dye is commonly used as an indicator ink for the determination of photocatalytic activity due to its novel photo-reductive mechanism that can be monitored visually through a colour change from blue to pink upon degradation, and also quantitatively by following the degradation of the resazurin peak over time using UV-Vis spectroscopy (Perkin Elmer, Waltham, MA, USA) [21]. The ink was sprayed onto the TiO2 surface and the degradation of the dye monitored before and after UV irradiation at 365 nm. The intelligent ink was made from deionized water (40 mL), hydroxyethyl cellulose polymer (3 g, 1.5 wt.% aqueous solution, Aldrich, Poole, UK), glycerol (0.3 g, Aldrich) and Resazurin (4 mg, Aldrich). The samples were sprayed with the ink solution using an aerosol spray gun and subsequently irradiated at a distance of 15 cm with a 365 nm UV lamp (2 × 8 W-365 nm Tube, Power: 32 W) at incremented times so that the photocatalytic degradation could be monitored closely. For spray coating the glass substrates were mounted on a sheet of paper and placed vertically on a wall in a fume hood as previously reported by Kafizas *et al.* [21], the thin film surface was coated in stages by spraying the substrate horizontally from left to right in short even steps, to ensure coverage of the entire substrate. This process was repeated several times until a pale blue coating was clearly visible. The resulting UV-Vis spectroscopy data was normalised utilising additional data collected using a blank glass substrate coated with the intelligent ink without a TiO2 thin film, and a clean glass substrate without a TiO2 thin film. The half-life could then be found from the normalised data to give the values quoted in Table 2.
