Aqueous extract phyto-mediated synthesis of ZnO nanoparticles and ZnO-bentonite nanocomposites: Physical analysis and investigation of antibacterial activity

Multidrug-resistant (MDR) and highly drug-resistant (HDR) bacteria are responsible for life-threatening infections, prompting interest in phyto-mediated synthesis and the application of inorganic nanoparticles (NPs) and their nanocomposites (NCs) for antimicrobial therapy. This study investigated the synthesis of zinc oxide nanoparticles (ZnO NPs) and ZnO-activated bentonite nanocomposites (ZnO/A-bentonite NCs) using the aqueous leaf extract of Hagenia abyssinica and optimal calcination temperatures of 500 °C and 650 °C, respectively. Characterization by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) confirmed the synthesis and structure of the NPs and NCs, revealing average crystallite sizes of 13.5–28.7 and 3.41–22.3 nm, respectively. The NPs exhibited absorption in the UV-A region, while the NCs showed absorption in the UV-B region. Compositing ZnO NPs with A-bentonite reduced their band gap from 3.24 to 3.07 eV. High-Resolution Transmission Electron Microscopy (HRTEM) of selected ZnO 100 NPs and ZnO/A-bentonite 100 NC showed particles with average sizes of 17.9 and 15.7 nm, respectively. Elemental analysis of purified and A-bentonite revealed an increase in Na₂O content and a decrease in CaO content, indicating Na⁺ ions replaced Ca²⁺ ions. Significant antimicrobial activity was demonstrated against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) bacteria by the leaf extract, ZnO NPs, A-bentonite, and ZnO/A-bentonite NCs at various concentrations, with NPs and NCs synthesized with excess leaf extract exhibiting superior activity. A selected ZnO/A-bentonite 300 NC demonstrated minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of 312.5 and 625 μg/mL against E. coli and 156.25 and 312.5 μg/mL against S. aureus, respectively.