Preparation and size-modulation of silica-coated maghemite nanoparticles

In this study we report on the synthesis and characterization of both silica-coated and aminopropyl-silica-coated maghemite nanoparticles of different particle sizes by one-step reaction of an alkaline aqueous-based dispersion of maghemite nanoparticles (7–10 nm). The nanoparticle surface coating was performed using tetraethoxysilane. The influence of reaction time, amount of maghemite nanoparticles and tetraethoxysilane employed in the synthesis upon the composition and morphology of the as-prepared samples and on the colloidal dispersion properties produced from them was investigated. The weight percentage of silica in the composite samples varied from 5.4 to 94% depending on reaction conditions. When very low amount of maghemite nanoparticles is present in the reaction mixture, large and nearly spherical-shaped multicore maghemite–silica particles (60–80 nm) are predominantly formed. On the other hand, when high amounts of maghemite nanoparticles are employed, smaller core–shell particles and non-spherical aggregates of them were observed. The silica-coated maghemite particles showed negative zeta potential values for their neutral aqueous dispersions while improved colloidal stability (stable magnetic fluid samples) was observed for small-sized suspended particles. In turn, the aminopropyl-silica-coated maghemite particles showed positive zeta potential values greater than +30 mV only for acidic aqueous dispersions and the colloidal stability also varied according to the particle size.