*3.4. Composition of the Coatings*

EDX results showed that the Ag content in T2 remained fairly constant. T3 showed a high level of Ag leaching possibly caused due to the poor dispersion and segregation of Ag within the coating as was seen from the SEM image of this coating (Figure 3). U2 showed a fairly constant concentration of Mo, except that in the areas where coating had been partially removed, it was not possible to measure the relative concentration of Mo in the coatings due to the weak signal and overlapping of the emission lines from the coating with those from the substrate (Table 3).


**Table 3.** Concentration of dopant as analysed using EDX (error in the measurements was ±10%).

*3.5. Photocatalytic Properties* 

Figure 8 shows the photocatalytic activity of the coatings under fluorescent and UV irradiation.

**Figure 8.** Photoactivity of TiO2 and doped TiO2 coatings under UV (blue bars) and fluorescent light (red bars) irradiation. (**a**) T1; (**b**) T2; (**c**) T3; (**d**) U1; (**e**) U2; (**f**) MC.

A loss of activity for T1–T3 coatings under UV light following the brewery trials was seen to varying degrees. The lower content TiO2-Ag surface (T2) retained the most activity with the exception of samples received from Brewery C. A greater loss of photocatalytic properties of the higher doped Ag coatings was seen, possibly due to the leaching of silver during the process studies. The controls also lost activity following three months storage in the dark compared to the as-deposited samples (UV light). Similar results were seen when photocatalytic activity was assessed under fluorescent light. Comparison of the photocatalytic properties of U1 and U2, showed that the addition of Mo to the heat-treated TiO2 surface increased its photocatalytic activity under UV and fluorescent light and this remained the case following the process studies. Photoactivity was largely retained for Mo-doped surfaces from all breweries with the exception of one of the two samples received from Brewery B. TiO2 alone retained some of its photoactivity to varying degrees when irradiated with UV, although values between the duplicate samples differ. Less activity was shown under fluorescent light exposure, as expected and controls also showed lower photocatalytic activity compared to the as-deposited samples. Compared to the controls stored in the dark, the MC TiO2 surfaces retained much of their photocatalytic activity, with the exception of samples received from Brewery B (under UV), where scratch test and EDX results had shown very little coating had been left on the substrate surface after the trial. As a small area of the substrate remained uncoated during the spray coating process, duplicate samples were not available in the case of MC surfaces.

The differences in photocatalytic activities of the surfaces received from the breweries could be due to the position of the samples and the cleaning regimes used. Work by others has shown that canning machines were markedly less prone to accumulation of microorganisms than bottling machines which use recycled glass bottles [1]. Further, it has been suggested that horizontal surfaces were prone to microbial accumulation and should be avoided in constructions as much as possible. Biofilm formation has also been shown to occur on certain surfaces despite daily cleaning and disinfection [1]. Thus, deposits formed by reaction processes or microbes usually cannot be wholly removed with water from stainless steel [29]. Various cleaners may have different success. In a surface test without soil a hypochlorite-based disinfectant was shown to be effective after an exposure of 10 min against all the microbes tested whereas an isopropanol-based cleaning agent was effective against all the vegetative cells tested [30]. In the presence of soil, hypochlorite was effective against *Listeria monocytogenes* and *Pseudomonas aeruginosa* [30]. The nature of clean may also affect efficacy. At 30 and 50 °C water rinsing at the flow velocities investigated could remove up to 85% of a yeast deposit. At a water rinsing temperature of 70 °C, less yeast deposit could be removed overall [3]. If surfaces were soiled with chemical residue and not cleaned sufficiently, it is possible that this may have an effect on photocatalytic activity. Conversely over aggressive cleaners might damage the surface, as noted previously.
