Water as the Solvent in the Stober Process for Forming Ultrafine Silica Shells on Magnetite Nanoparticles

Silica coating of magnetite nanoparticles (MNPs) is of great importance because it offers stability to MNPs against oxidation, water dispersity, and a tailorable surface for functionalities, allowing a wide range of applications in areas such as water pollutant removal and targeted drug delivery. In this work, a simple and green procedure has been developed using water, instead of traditional alcohols, as the solvent in the Stober method to produce well-dispersed MNPs coated with ultrathin (<5 nm) silica outer shells. The resultant core-shell structures possess superparamagnetic properties, high magnetization value of 59 emu/g, and excellent resistance to oxidation when exposed to ultrasonic-accelerated oxidation. The oxidation stability of the coated MNPs is shown to extend to their functionalized product. All syntheses are carried out under ambient conditions using commonly available chemicals and equipment. The Fe3O4@SiO2 core shell structures are characterized using Fourier transform infrared attenuated total reflection spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and a vibrating sample magnetometer.