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Magnetic iron oxide nanoparticles (MIONs) can generate heat under an alternating magnetic field, enabling a wide range of applications from water treatment to cancer hyperthermia therapy. For most magnetic heating applications, it is crucial to generate a high level of heat with a low dose of MIONs. Current methods to increase the specific absorption rate (SAR) of MIONs include increasing their size and doping iron oxide nanocrystals with other metal elements. Here, we demonstrate that controlled oxidation and surface modification can significantly increase SAR of MIONs. We synthesized MIONs of different core sizes and with different coatings, including phospholipid-PEG and triethylenetetramine (TETA). We oxidized PEG-coated MIONs in a controlled fashion and measured the SAR values of the MIONs under different oxidation states. We found that, with controlled oxidation, the SAR values of 15-nm and 18-nm MIONs increased by ∼1.87 fold after two weeks of oxidation. A similar fold-increase in SAR was achieved for 15-nm MIONs with TETA coating compared with PEG coating. We systematically characterized the physical properties of MIONs and showed that oxidation caused MIONs to transition from magnetite to maghemite, resulting in increased anisotropy constant and SAR values. Our results demonstrate new approaches to significantly increase the heating capacity of MIONs by controlled nanocrystal oxidation and surface modification.

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