Performance Improvement and Antibacterial Mechanism of BiOI/ZnO Nanocomposites as Antibacterial Agent under Visible Light

Bacterial infections cause various serious diseases including tuberculosis, meningitis, and cellulitis. Moreover, there is an increase in the number of drug-resistant bacterial strains, which has caused a global health issue. Thus, it is highly essential to develop more effective antibacterial agents. Currently, zinc oxide (ZnO) is commonly used as an inorganic antibacterial agent, but with a notable limit in efficiency. In this work, to improve ZnO antibacterial activity under visible light, bismuth oxyiodide (BiOI) with a narrow bandgap of 1.8 eV was used as a suitable refinement to ZnO. Four different BiOI/ZnO nanocomposites were designed and synthesized via a simple mechanical stirring method in an atmospheric environment; these were denoted as BiOI/ZnO-2.5%, BiOI/ZnO-5%, BiOI/ZnO-10%, and BiOI/ZnO-20%. The successful synthesis of the BiOI/ZnO nanocomposites was verified through X-ray powder diffraction, energy-dispersive X-ray analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). A unique BiOI/ZnO heterojunction was also observed for the nanocomposites through high-resolution TEM, XPS, and selected area electron diffraction. Ultraviolet-visible diffuse reflectance spectroscopy revealed that all four BiOI/ZnO nanocomposites exhibited improved visible light absorption and possessed narrower bandgaps than the ZnO nanoparticles (nano-ZnO). Furthermore, the antibacterial activities of all BiOI/ZnO nanocomposites were investigated under visible light against both gram-positive and gram-negative bacteria strains. The results indicated a significant improvement in the antibacterial activities of BiOI/ZnO-10% and BiOI/ZnO-20% against both Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). Strong light exposure was found to be attributable to an increase in the antibacterial activity against S. aureus. In addition, the antibacterial mechanistic investigation was conducted upon visible light activation. The SEM images showed completely broken bacterial cell walls for both bacteria strains after treatment with the BiOI/ZnO nanocomposites. Hydroxyl radicals (& BULL;OH), which are strong reactive oxygen species, generated by the BiOI/ZnO nanocomposites under visible light, were also trapped by 5,5-dimethyl-1-pyrroline-N-oxide. Furthermore, zeta potential analysis revealed the presence of more positively charged BiOI/ZnO nanocomposite surfaces than the surfaces of nanoZnO. The metal ions released from the BiOI/ZnO nanocomposites under visible light were also studied through inductively coupled plasma mass spectrometry. Based on the above results, BiOI/ZnO nanocomposites were found to exhibit antibacterial mechanism similar to that of nano-ZnO. In the dark, E. coli growth was only inhibited by Zn2+ released from both BiOI/ZnO nanocomposites and pure nano-ZnO. After visible light activation, & BULL;OH generated from the BiOI/ZnO nanocomposites mainly contributed to the bacterial cell death of both E. coli and S. aureus. This study proposes an effective strategy to enhance the antibacterial activity of nano-ZnO under visible light upon the formation of nanocomposites with BiOI. Besides, this study indicates that the ZnO-based nanocomposites can be used as a more effective antibacterial agent in clinical applications.