Physicochemical Insights into the Antibacterial Performance of Green-Synthesized CeO₂@ Se Nanocomposites against Gram-Positive and Gram-Negative Bacteria

<title>Abstract</title> <p> The development of multifunctional antibacterial nanomaterials through low-energy and environmentally compatible routes is increasingly important for addressing microbial contamination and antimicrobial resistance. In this study, a cerium oxide–selenium (CeO₂–Se) nanocomposite was prepared using an aqueous <italic>Eucalyptus globulus</italic> leaf extract as a reducing and stabilizing medium. Equal volumes of cerium- and selenium-precursor solutions were combined with 15 mL of the extract and reacted at 40°C for 10 min at pH 6.9, without centrifugation, washing, calcination, or other post-synthesis thermal treatment. UV–visible spectroscopy showed intense ultraviolet absorption with a broad absorption contribution extending toward the near-visible region. X-ray diffraction revealed a low-crystallinity multiphase pattern containing reflections attributable to cubic fluorite CeO₂ and selenium-related crystalline domains, supporting formation of a coupled CeO₂–Se system. Field-emission scanning electron microscopy demonstrated strongly interconnected and agglomerated, predominantly quasi-spherical nanostructures, with representative particle diameters ranging from 39.83 to 48.87 nm. The aqueous dispersion exhibited a negative zeta potential of − 28.06 ± 5.16 mV and an electrophoretic mobility of − 2.18 ± 0.40 µm·cm·V⁻¹·s⁻¹, indicating moderate electrostatic stabilization. Antibacterial activity was concentration dependent and markedly greater against Gram-positive <italic>Staphylococcus aureus</italic> than against Gram-negative <italic>Escherichia coli</italic> . Across the tested concentration range, inhibition zones decreased from 20 to 16 mm for <italic>S. aureus</italic> and from 8 to 5 mm for <italic>E. coli</italic> . The greater susceptibility of <italic>S. aureus</italic> suggests that bacterial-envelope characteristics influence nanocomposite–cell interactions. Overall, the findings identify green-synthesized CeO₂–Se nanocomposites as promising antibacterial materials and demonstrate that their phase composition, surface charge, nanoscale morphology, and colloidal behavior collectively govern their biological performance. </p>