Unraveling the antimicrobial activity of CuS functionalized titanates under visible LED light irradiation

Titanate nanotubes (TNs) functionalized with CuS nanoparticles using the microwave-assisted hydrothermal method were characterized via XRD, Raman spectroscopy, UV-Vis spectrophotometry, and N₂ physisorption. The as-synthesized CuS/TNs had anatase as the main crystalline phase and the band-gap energy was in the visible region, 2.9 eV. The TNs were recrystallized on titania and functionalized with CuS, forming spherical bundles. SEM showed agglomerates of cauliflower-like semispherical particles. The antimicrobial photoactive assets were evaluated against the bacteria Staphylococcus aureus and Escherichia coli. Inhibition was clearly visible in S. aureus after the first 20 min of exposure to a 6-W LED irradiation lamp. The visible-light catalyzed completely and irreversibly the inactivation of S. aureus after 60 min, however, in the case of E. coli, this material only slightly disturbed its growth, which was recovered after 60 min. The successful result obtained with S. aureus can be explained by the fact that it lacks periplasmic superoxide dismutase (SOD) but has staphyloxanthin for external protection against ROS. However, the CuS/TN particles could release Cu²⁺ ions, which got attached to bacterium structures or entered the cytoplasm; these events together with the generation of ROS under visible LED light helped inactivate quickly staphyloxanthin, thus inflicting permanent damage to the periplasmic membrane.