Magnetic nanocrystals are used in many fields, such as biomedicine, separation technology and material science. For many applications, the nanocrystals need to be encapsulated into a polymeric matrix, thus generating composite nanoparticles. These particles are composed of a core of magnetite nanocrystals and a shell of polymer. It is of utmost importance to control both the amount of nanocrystals encapsulated and the final particle size and size distribution.

In this work, we have prepared novel magnetic colloids having small size (<100nm) and uniform size distribution trough miniemulsion polymerization and have characterized their size distribution and the extent of magnetite incorporation via light scattering and TEM analysis. Magnetite nanocrystals have been prepared through coprecipitation reaction of mixtures of iron salts in presence of ammonium hydroxide and a ligand. Solubility of the nanocrystals in various organic solvents can be tuned by changing the ligands that are anchored on their surface. It was found that the optimization of magnetite encapsulation is strongly dependent upon the chemical nature of ligands, and this motivated us to test different ligands. Our results indicate that, if the ligand is not compatible with the polymer matrix, phase separation occurs and the magnetite accumulates at the surface of the polymer leading to colloidal instability of the dispersion. Ligands carrying reactive polymerizable moieties turned out be particularly effective to achieve a uniform dispersion of magnetite into the polymer matrix.

In order to increase the amount of magnetite incorporated in the magnetic colloid we have added a low boiling point solvent to the monomer before the emulsification. The solvent facilitates the emulsification by reducing the fraction of magnetic material in the droplets, and is subsequently evaporated during and/or after the polymerization reaction. In this manner, it is possible to fine tune the particle size of the colloids by varying the amount of the solvent added.