A Novel Route to Toluene-Soluble Magnetic Oxide Nanoparticles: Aqueous Hydrolysis Followed by Surfactant Exchange

The ability of capped nanoparticles of inorganic materials to dissolve in nonpolar solvents such as toluene creates opportunities for size-selective precipitation, surface modification through covalent means, incorporation into polymer composites, and the possibility of digestive ripening. Toluene-soluble nanoparticles are usually prepared in nonpolar solvents, where they are capped by hydrophobic surfactants; such preparations often require specialized high-temperature solvents and expensive precursors. Many transition metal oxides are easily prepared in nanoparticulate form cheaply and in quantity, by hydrolysis in aqueous media. However, capping of nanoparticles in aqueous media to make them nonpolar is usually difficult. We present here a simple process of surfactant exchange that permits 4nm particles of the magnetic spinel oxides MFe2O4 (M = Mn and Zn) to be prepared in water in the presence of a surfactant and to then be transferred to toluene wherein the particles are capped by a second surfactant, a long-chain amine. The amine-capped nanoparticles can be successively precipitated and redissolved from their toluene solutions. Mussbauer spectroscopy and SQUID magnetization have been used to characterize magnetic properties of the nanoparticles.