Surfactant-Free Synthesis of Melon Seed–Like CeO₂ and Ho@CeO₂ Nanostructures with Enriched Oxygen Vacancies: Characterization and Their Enhanced Antibacterial Properties

To overcome the poor antibacterial performance of cerium oxide (CeO₂) nanoparticles at low concentrations, melon seed–shaped CeO₂ (MS-CeO₂) and holmium (Ho)-doped CeO₂ (Ho@MS-CeO₂) nanoparticles were synthesized using a simple precipitation method without the addition of any surfactants. The surface morphology, phase structure, crystallinity, Ce³⁺ and Ce⁴⁺ valence, lattice defects, and reactive oxygen species (ROS) production of both synthesized nanostructures were examined using different techniques, i.e., scanning electron microscopy (SEM), energydispersive X-ray (EDX), resolution transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), Raman spectroscopy, ultraviolet (UV) spectra, fluorescence spectra, and zeta potential (ζ). The results show that under certain stirring and aging temperatures, CeO₂ and Ho-doped CeO₂ nanoparticles with a melon seed–like morphology can be prepared in a short period. Both nanoparticles were tested as antiseptic agents against G⁺ and G^– bacteria (E. coli and S. aureus), and the results confirmed that the Ho@MS-CeO₂ nanostructures exhibited remarkable antimicrobial activity at a low concentration (0.5 mg/L) compared with the control group, which is attributed to the reversible conversion of Ce³⁺ and Ce⁴⁺ in the ceria crystal lattice, enriched oxygen vacancy, ROS species production, and positive surface charge.