A method of purifying a plurality of metal oxide particles produced from a synthesis process comprising the step of washing a plurality of metal oxide particles in a first solvent composition comprising of at least one aliphatic ether, and at least one flocculant. In one embodiment, the plurality of metal oxide particles are iron oxide particles produced from a thermal decomposition synthesis process between an iron-oleate complex and oleic acid in 1-octadecene, wherein the first solvent composition comprises a 1:1 (vol/vol) ratio of an aliphatic ether in the form of diethyl ether and a flocculant in the form of methanol. The washed iron oxide particles are further washed in a second solvent composition comprising a 1:1 (vol/vol) ratio of hexane and ethanol, and then finally dispersed in hexane. The resulting iron oxide particles find use as a contrast agent for magnetic resonance imaging (MRI) or as magnetic particles in magnetic separation, magnetism-directed targeting or magnetism-induced heating.