SiO 2 shell formation mechanism and enlargement on hydrophobized nanoparticles via a reverse microemulsion process

The formation mechanism of SiO2 shells on oleate-modified inorganic nanoparticles by a reverse microemulsion method was investigated by dynamic light scattering measurements. Changes in the hydrodynamic diameter upon addition of the nonionic surfactant, Igepal CO-520, and ammonia water revealed that the surfactant-exchange and aqueous layer formation took place at the surface of the inorganic nanoparticles. The aqueous layer functioned as a reaction site for SiO2 shell formation. The proposed mechanism explains how core-shell particles having a single core are obtained. Additionally, the limitation of a maximum core-shell particle size obtained by this process can also be explained by the proposed mechanism. SiO2 shell growth was further examined by consideration of this mechanism. Incremental addition of Igepal CO-520 and ammonia was observed to facilitate the expansion of a reverse microemulsion layer surrounding the Fe3O4 nanoparticles and the continuing growth of the SiO2 shell. Further growth of the SiO2 shell can also be achieved by the Stöber process. The core-shell particles can grow to diameters in excess of 100 nm while maintaining narrow particle size distributions and a single-core structure is obtained throughout. The approach presented here offers a way to fabricate various core-shell particles comprising hydrophobized inorganic nanoparticles and SiO2 shells, which have potential for biomedical practical applications.