Photocatalytic antibacterial and biofilm-preventive activity in liquid of heavy-metal-free coatings based on a phosphorus (P) and fluorine (F) -modified TiO2 photocatalyst has been investigated.They reveal significantly higher immediate and longer-term (biofilm-preventive) inactivation capacity than a reference coating made of the commercial photocatalyst TiO2 P25 on three bacterial species differing in cell wall type and ability to resist oxidative stress (Escherichia coli, Staphylococcus epidermidis, Pseudomonas fluorescens) (up to more than 99% reduction of colonization on P/F-modified TiO2 coating compared to about 50% on P25 TiO2 coating for 10 min UV-A illumination). This results from the P-and F-induced improvement of photocatalyst properties and from the smoother surface topography, which shortens reactive oxygen species (ROSs) diffusion to the outer membrane of the targeted adhered bacteria. Decrease in ROSsrelated impairment of cell wall, respiratory and enzymatic activities confirms the loss of ROSs throughout the bacterial cell degradation. Staphylococcus epidermidis and Pseudomonas fluorescens are less sensitive than Escherichia coli, with a probable relation to the bacterial oxygen stress defense mechanism. The coating antibacterial efficacy was highly affected by phosphate ions and the richness in dissolved oxygen of the reaction medium.

Virtually transparent photocatalytic multilayer films composed of TiO2 nanoparticles and polyelectrolytes were built on model surfaces using layer-by-layer assembly and investigated as photocatalytic nanoporous coatings. Formic acid (HCOOH) and Escherichia coli were used as models for the degradation of gaseous pollutants and for studying antibacterial properties. Positively charged TiO2 nanoparticles were coassembled with negatively charged poly­(sodium 4-styrenesulfonate) (NaPSS) which leads to highly transparent nanoscale coatings in which the content of TiO2 particles is controlled mainly by the number of deposition cycles and the enhanced translucency with respect to titania powders is likely due to the presence of the polyelectrolytes in the interstitial space between the particles. Build-up and structural properties of the films were determined by ellipsometry, quartz crystal microbalance (QCM-D, with dissipation monitoring), and UV–vis spectrophotometry in transmission and scanning electron microscopy. Complementary photophysical and activity tests of (PSS/TiO2) n multilayer films were performed in the gas-phase under UV-A light and revealed a peculiar dependence on the number of layer pairs (LPs), corresponding to a clear deviation from the usual observations in photocatalysis with increasing TiO2 amounts. Most notably, a single LP film showed a strongly enhanced HCOOH mineralization and outperformed films with a higher number of LPs, with respect to the quantity of TiO2 catalyst present in the films. It is believed that the high quantum yield (8.1%) of a coating consisting of a single TiO2 layer which is 6–7 times higher than that of a 6–10 LP film could be due to the optimum accessibility of the TiO2 crystallites toward both HCOOH and water molecules. In thicker films, while no detrimental light screening was observed with increasing the number of LPs, diffusion phenomena could cap the efficiency of the access of the pollutant and water to the catalytic surface. Unlike for HCOOH mineralization, three PSS/TiO2 LPs were required for observing a maximum antibacterial activity of the nanocomposite coatings. This is likely due to the fact that micrometer-sized E. coli bacteria do not enter into the interstitial space between the TiO2 particles and require a different surface morphology with respect to the number of active contact points for optimum degradation.

A UV photoinduced cross-linking of non-modified commercial poly(disulfide) resins (Thioplast) is reported via the air oxidative photocoupling of terminal thiol functions. Catalyzed by a photogenerated guanidine base (TBD), this step-growth photopolymerization is useful to maximize disulfide functions content. The mechanism proceeds through thiol deprotonation into thiolate anions, further oxidized into thiyl radicals, eventually dimerizing into disulfide cross-links. Starting with a detailed structural characterization of the thiol-terminated resin, photooxidative kinetics are studied under exposure to a polychromatic medium-pressure Hg arc using Raman and infrared spectroscopy. The effects of irradiance, film thickness, photobase concentration, resin molar mass, and content of an additional polythiol monomer (reactive diluent) have been investigated. In an effort of upscaling, irradiation under a 365 nm LED panel has enabled the fast preparation of 1.5 µm thick cross-linked poly(disulfide) coatings in a matter of minutes. Capitalizing on the ability of residual thiol groups to react with silver cations, a post-functionalization has been successfully performed, leading to films exhibiting at their surface stable thiolate-silver bonds as proved by X-ray photoelectron spectroscopy. Despite the well-established biocide action of silver ions, no antibacterial action has been evidenced by confocal fluorescence microscopy because of insufficient release.