Silica-coated NiPt core-shell nanoparticles stable to Ni Oxidation and aggregation

Magnetic particles with a size of dozens of nanometers or less have excellent operability under external magnetic fields and are required to be chemically stable and resistant to aggregation. Owing to their excellent properties, magnetic fine particles have applications in medical imaging, hyperthermia treatment, solid-phase extraction, and recording media. However, pure metal nanoparticles aggregate or oxidize, necessitating the development of a simple method to suppress these phenomena. In this study, NiPt and SiO₂-coated NiPt (NiPt/SiO₂) nanoparticles were fabricated and characterized. Metallic Ni nanoparticles were synthesized in water exposed to air using Ni(II) acetate tetrahydrate, hexachloroplatinate(IV) hexahydrate, hydrazine, and poly(sodium 4-styrenesulfonate) as the Ni and Pt sources, reducing reagent, and stabilizer, respectively. The simultaneous reduction of Ni and Pt cations afforded NiPt nanoparticles. The NiPt nanoparticles (9.7 ± 3.3 nm) were significantly smaller than the Ni particles synthesized using hydrazine as a reducing reagent (∼300–600 nm). The Pt nuclei were generated faster than the Ni nuclei because of the difference in their standard electrode potentials. The NiPt/SiO₂ nanoparticles were synthesized by adding a tetraethylorthosilicate/(3-aminopropyl)triethoxysilane/ethanol solution to the NiPt nanoparticle colloidal solution. They did not aggregate during 24 h of the silica-coating process, whereas the uncoated NiPt nanoparticles were ionized and significantly aggregated over the reaction time of 24 h. The saturation magnetization values of the uncoated NiPt and NiPt/SiO₂ nanoparticles were 28.0 and 56.9 emu/g-Ni, respectively, which indicated that the silica coating prevented particle aggregation and controlled Ni oxidation on the surface of the metallic NiPt nanoparticles during the synthesis process. The present work contributes to the advancement of methods for synthesizing colloidally stable metal nanoparticles and synthesis environments.