*3.4. Semi-Transparent Coating*

Figure 5 presents the experimental data on the antibacterial activity of semi-transparent coatings formulated with silicate and TiO2 powder (STC-SP). Surprisingly, antibacterial activity was also observed on samples kept in the dark. A quick estimation of the pH of the inoculum with indicator paper showed a pH around 11–12, far too high for *E. coli* survival. It is then reasonable to assume that the antibacterial activity detected on STC-SP samples was not induced by the photocatalytic process, but by the silicates, making the inoculum strongly basic. Results from sample coatings without silicate (STC-P) showed no activity after eight hours' irradiation and no activity after 8 h in the dark. Possible explanations are that photogenerated radicals may have reacted with the binder instead of with bacteria or that the inclusion of nanoparticles within the binder may have prevented UV absorption and physical damage by contact. Moreover, the use of TiO2 powder without a dispersing agent may lead to the formation of aggregates, reducing the surface available for reaction.

**Figure 5.** Antibacterial activity of SCT-SP coatings under UV irradiation (§2.5 W/m2 ) and in the dark. Mean ± SE, *n* = 3.

Figure 6 presents the antibacterial activity obtained with STC-A and STC-A2. The activity reaches 1.49 ± 0.47 log for STC-A and 1.54 ± 0.13 log for STC-A2 after four hours' irradiation. The observed increase of antibacterial activity, compared to SCT-P in which no activity was detected, could be attributed to the use of the TiO2 dispersion. Nanoparticles, stabilized by the dispersing agent, may have provided more active sites for the photocatalytic process. Moreover, the smaller amount of acrylic resin within STC-A and STC-A2 (2%) may have reduced the inclusion of TiO2 nanoparticles compared to SCT-P. The similar activities observed on STC-A (with acrylic resin) and on STC-A2 (without acrylic resin) seem to confirm this hypothesis. Further investigations on the formulation of these coatings along with observations of nanoparticle distribution in the binder will be helpful in the development of antibacterial products for building materials.

**Figure 6.** Antibacterial activity of STC-A and STC-A2 coatings under UV irradiation (§2.5 W/m2 ). Mean ± SE, *n* = 3.
