What is the most effective percentage of Rose Bengal on polyamide fabrics for the visible-light photoinactivation of Gram-positive bacteria?

[EN] Heterogeneous photocatalysts based on fabric materials have achieved great relevance for water disinfection. However, studies have yet to be performed looking for the best percentage of a photosensitizer on fabric, considering the 1O2 generation efficiency as well as the charge distribution of the dye fabrics surface for bacterial inactivation. Therefore, polyamide fabrics (PAF) dyed at different Rose Bengal (RB) percentages (RB-PAF) as an anionic photosensitizer were prepared to determine the best value for photodynamic inactivation of Gram-positive bacteria (Enterococcus faecalis). Time-resolved and steady-state emission measurements, as well as laser flash photolysis experiments, were also performed. RB-PAF at a percentage of 1% (on-weigth-of-fiber, o.w. f.) showed 100% inactivation efficiency against E. faecalis (reduction of more than 6 log10 units in the viable count) in only 15 min. Contrarily, fabrics with RB values of 0.5 or 3% (o.w.f.) showed no bacterial inactivation or only a small activity (1 log10 unit reduction), respectively. Results revealed that the effects observed by RB-PAF are mediated by 3RF* and the subsequent generation of 1O2. The insignificant photooxygenation of 9,10-dimethyl anthracene observed for RB-PAF at 0.5% explains the lack of bacterial inactivation, while the low effect observed for RB-PAF at 3% is produced by electrostatic repulsions between the anionic RB and the bacteria surface. These results reveal that in order to design newly dyed fabrics for disinfection, it is important to optimize the percentage of dye to avoid aggregation of photosensitizers and to obtain an adequate net negative charge distribution in the antimicrobial fabrics.