Synthesis of monodisperse maghemite nanoparticles by the microemulsion method

The preparation of magnetic nanoparticles has great interest for both fundamental studies and applications. Because the magnetic properties strongly depend on the size and state of aggregation of the particles, synthesis methods to produce nonodisperse and coating stabilized nanoparticles are required. Here, we present a one-pot microemulsion method to produce monodisperse and coated small nanoparticles. The nanoparticles are formed by the coprecipitation reaction of ferrous and ferric salts with two organic bases, cyclohexylamine and oleylamine, into a water-in-oil microemulsion. When oleylamine is used as precipitant agent, a stable colloidal dispersion of maghemite nanoparticles coated with oleylamine is obtained. In contrast, cyclohexylamine does not avoid the aggregation of the particles during the synthesis. It shows that the oleylamine adsorbed at the particle surface can be easily replaced by oleic acid. The spherical shaped particles, capped with a monolayer coating of oleylamine (or oleic acid), show a narrow size distribution of 3.5 ± 0.6 nm, are well crystallized and have high saturation magnetization values.