Mechanical activation and silver supplementation as determinants of the antibacterial activity of titanium dioxide nanoparticles

Metals and metal oxides have subpar antibacterial activities compared to those of small-molecule antibiotics, yet there are hopes that with proper compositional and structural adjustments this gap might be bridged. In this study, titanium dioxide (TiO2) nanoparticles were mechanically activated and combined with particulate silver through simple reduction process elicited by UV irradiation and assisted with the ultrasound. The resulting powders in various combinations (Ag vs. no Ag, activated vs. non-activated) were characterized using a range of experimental techniques and assessed for their antibacterial activities. The preparation procedure presented in this work prevails over the disadvantages of many chemical routes, most critically by avoiding the use of toxic substances. The mechanical activation did not reduce the particle size or crystallinity of TiO2 nor did it consistently alter the bandgap, yet it enabled the doubling of the amount of silver incorporable into the material. Further, while both mechanical activation and the addition of silver in the amount not exceeding 0.5 wt% produced barely detectable structural changes in the material, they both augmented its antibacterial activity. The precursor TiO2 powder produced no inhibition zone against any of the four bacterial species tested, while the mechanical activation of TiO2 led to the formation of distinct inhibition zones against each of the four bacterial species tested. The addition of silver to activated TiO2 further widened the inhibition zones and it also imparted the antibacterial activity to non-activated TiO2. The boost in the antibacterial activity achieved by the short mechanical activation was of a similar magnitude as the boost obtained after the addition of silver. The antibacterial activity was not different for different species when no silver was added to the system. However, with the addition of silver, species selectivity was obtained, as the composites were more effective against the two Gram-negative species ( Escherichia coli and Klebsiella pneumoniae ) than against the two Gram-positive ones ( Staphylococcus aureus and Bacillus subtilis ). The antibacterial activity increased with the addition of silver in the broth assay, but it was mediocre compared to that detected in the agar assay, attesting to the poor dispersability of the powders and their best performance when the bacterial cells migrate to the composite surface than vice versa . The findings of this study give hope that with appropriate microstructural or compositional alterations, the antibacterial activity of metal oxide powders and inorganic materials in general can be made comparable to that of small -molecule antibiotics.