Apatite-coated Ag/AgBr/TiO2 nanocomposites: Insights into the antimicrobial mechanism in the dark and under visible-light irradiation

Abstract

For the first time, titania-based photocatalysts with different crystalline compositions (anatase/rutile) and morphology (irregular and faceted nanoparticles) were modified with Ag/AgBr and apatite (HA). The antimicrobial activity was tested for Escherichia coli (E. coli) bacteria and Candida albicans (C. albicans) fungi under visible-light illumination and in the dark for HA-covered and uncovered nanocomposites. It has been found that antimicrobial inactivation mechanism follows two different pathways, depending on the conditions (dark vs. irradiation), i.e., HA loading on the photocatalyst surface affects only the microorganism nourishment from the surrounding media (growth inhibition) in the dark, but significantly induces outer cell wall destruction (leading even to the mineralization) in the presence of light. Furthermore, 2-propanol oxidative decomposition was also investigated. The obtained results suggest that the improved photocatalytic activity of HA-covered samples originates from phosphate anions in HA, hindering the charge carriers’ recombination. In bacteria case, titania's properties are key factor for high bactericidal effects, such as anatase form (electrostatic attraction between microorganisms and photocatalyst surface) and fine crystallites, as opposed to yeast inactivation resembling 2-propanol oxidation, i.e., photocatalytic mechanism, based on reactive oxygen species. Concluding, HA/Ag/AgBr/TiO2 samples are promising “green” materials for decomposition of both microbiological and chemical pollutants.