Facile control of silica shell layer thickness on hydrophilic iron oxide nanoparticles via reverse micelle method

Facile procedures to control silica thickness and surface morphology on core–shell nanoparticles (Fe3O4@SiO2) are proposed. In this proposal based on reverse micelle method, we used water dispersed iron oxide nanoparticles. The sol–gel reaction was controlled by the change of the pH of dispersion without washing or re-dispersed procedures. When dispersion with pH 9 was used for silica coating, it takes 3 days to reach 15 nm thick of the silica layer, and pH 10 dispersion needs only around 12 h. The required thickness could be simply obtained by a choice of the pH and reaction time. Aggregation of the core–shell nanoparticles was well prevented by surface modification by silane coupling agent containing imidazolium cationic moiety without washing the nanoparticles before treatment. Furthermore, surface roughness of silica layer was also simply controlled by change of the reaction speed and addition of water soluble ionic molecules.