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These large magnetite (Fe3O4) magnetic nanoparticles are prepared by decomposing iron oleate precursors at high temperature (290°C – 390 °C). The high reaction temperature encourages the formation of larger nanoparticles. Upon completion, the nanoparticles are precipitated by adding chloroform and acetone, centrifuged, and re-dispersed and stored in hexane.

The resulting ~35 nm iron oxide nanoparticles become water soluble upon deposition of a hydrophilic silica shell. The silica shell is formed on the surface of nanoparticles by decomposing tetraethyl orthosilicate (TEOS) in a cyclohexane solvent in the presence of Igepal CO-520 and ammonium hydroxide. The resulting iron oxide/silica shell nanoparticles have an inorganic diameter of ~50 nm and can be dispersed in water and various aqueous buffers. The nanoparticles are coated by polyethylene glycol derivatives such as methoxy polyethylene glycol silane to improve their long-term stability in aqueous media.