One-Pot Synthesis of Oxidation-Resistant Silica-Coated Iron Nanoparticles via Ultrasonically Assisted Citrate Modification

Abstract This study presents a facile, one-pot aqueous coating strategy to synthesize oxidation-resistant silica-coated iron (Fe/SiO2) core–shell nanoparticles under ambient conditions. Citrate-mediated reduction produced metallic Fe cores, with citrate ions and a thin native surface oxide layer acting as interfacial modifiers that promote SiO2 nucleation and eliminate the need for intermediate washing. Ultrasonication-assisted SiO2 encapsulation via a sol–gel process yielded Fe/SiO2 nanoparticles comprising a metallic Fe core (50.4 ± 15.1 nm in diameter) and a SiO2 shell (14.1 nm thick), as confirmed by transmission electron microscopy and energy-dispersive X-ray spectroscopy. Magnetic measurements demonstrated that the saturation magnetization of bare Fe nanoparticles decreased from 105.9 to 39.7 emu g–1 after 72 h in water owing to oxidation, whereas that of Fe/SiO2 nanoparticles remained at 84.6 emu g–1 compared with an initial value of 86.4 emu g–1. X-ray diffraction confirmed the preservation of the metallic α-Fe (body-centered cubic) phase, demonstrating that the SiO2 shell served as an effective diffusion barrier against oxidation. By providing a facile and eco-friendly route to protect metallic nanomaterials robustly, this study opens new avenues for their practical applications in advanced magnetic sensors, biosensors, catalysis, and magnetic separation.