Abstract
<title>Abstract</title> <p> Multifunctional ZnO-based nanocomposites with enhanced visible-light activity and antibacterial performance have attracted considerable attention for environmental and biomedical applications. In the present work, pristine ZnO, Fe <sub>3</sub> O <sub>4</sub> /ZnO, and Cu-doped Fe <sub>3</sub> O <sub>4</sub> /ZnO nanocomposites were successfully synthesized using a microwave-assisted ultrasonication method. The structural, morphological, optical, and antibacterial properties of the synthesized materials were systematically investigated using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), UV–Visible spectroscopy, scanning electron microscopy (SEM), dynamic light scattering (DLS), and photoluminescence (PL) spectroscopy. XRD analysis confirmed the formation of crystalline hexagonal ZnO and spinel Fe <sub>3</sub> O <sub>4</sub> phases without detectable impurity phases, while FTIR verified the characteristic metal–oxygen bonding. SEM and DLS analyses revealed that Cu doping reduced the particle size and promoted the formation of nanoflake-like structures with improved dispersion. UV–Visible spectroscopy demonstrated progressive band-gap narrowing from 3.17 eV for pristine ZnO to 2.56 eV for Fe <sub>3</sub> O <sub>4</sub> /ZnO and further to 2.50 eV for the Cu-doped Fe <sub>3</sub> O <sub>4</sub> /ZnO nanocomposite, resulting in enhanced visible-light absorption. The pronounced quenching of PL emission confirmed efficient suppression of electron–hole recombination and improved charge separation following Fe <sub>3</sub> O <sub>4</sub> incorporation and Cu doping. Antibacterial studies against <italic>Shigella flexneri</italic> , <italic>Bacillus subtilis</italic> , <italic>Klebsiella pneumoniae</italic> , and <italic>Staphylococcus aureus</italic> demonstrated that the Cu-doped Fe <sub>3</sub> O <sub>4</sub> /ZnO nanocomposite exhibited the highest antibacterial activity, with inhibition zones reaching 15 mm. The enhanced antibacterial performance is attributed to the synergistic effects of band-gap reduction, heterojunction-assisted charge transfer, Cu-induced electron trapping, reduced particle size, and increased generation of reactive oxygen species. The incorporation of Fe <sub>3</sub> O <sub>4</sub> additionally provides magnetic recovery potential, making the nanocomposite attractive for practical environmental applications. These findings demonstrate that Cu-doped Fe <sub>3</sub> O <sub>4</sub> /ZnO nanocomposites are promising multifunctional materials for water disinfection, antimicrobial coatings, and environmental remediation. </p>