No abstract

A chemical core-shell strategy is developed here for the synthesis of ferrofluids based on nanoparticles of different ferrites with different mean sizes. A heterogeneity of chemical composition, associated with a superficial enrichment of iron, allows to obtain chemically stable ionic colloids. We propose here a coreshell model to describe the synthesized nanoparticles, which is tested by chemical and magnetic measurements performed at the various steps of the synthesis. The thickness of the superficial layer, rich in iron, is ranging between 0.4 and 1.3 nm, depending on the nanoparticle size and on the underlying ferrite. Its density is found close to that of maghemite, and its magnetization depends on the core ferrite. It is low with a cobalt ferrite core and larger for the three other ferrites investigated here (NiFe 2 O 4 , CuFe 2 O 4 , and ZnFe 2 O 4 ). Magnetic measurements prove that there is a strong redistribution of Zn 2+ ions inside the core of the synthesized nanoparticles based on ZnFe 2 O 4 .

We report on stable colloidal aqueous suspensions of magnetic nanostructures made of copper, nickel, and zinc ferrites. These magnetic fluids could represent a new alternative for biological applications. The basic steps of the nanoparticles synthesis, their chemical surface treatment, and their peptization in a stable colloidal sol are given. Their chemical composition is carefully checked, and X-ray diffraction patterns provide both their mean size and a structural characterization. Magnetization results obtained at 300 K are presented and discussed.