We report a transformation of hydrophobic iron oxide nanoparticles to hydrophilic and biocompatible maghemite nanocrystals by controlled thermal treatment followed by dextran coating. Sodium sulfate salt was used as matrix for preventing aggregation during the thermal treatment at high temperature. Through the thermal treatment and subsequent isolation process, highly crystalline bare maghemite nanocrystals with high magnetization were produced. Subsequent coating with dextran derivatives produced hydrophilic and biocompatible iron oxide nanocrystals. Among various kinds of dextran derivatives, polyanionic carboxymethyl dextran (CM-dextran) was most efficient for stabilizing the nanocrystals in aqueous media. CM-dextran coated nanocrystals exhibited high relaxivity originating from the high magnetization and assembled structure.
