Superparamagnetic iron oxide nanoparticles as radiosensitizer via enhanced reactive oxygen species formation

► Ultrasmall citrate-coated SPIONs with γFe2O3 and Fe3O4 structure were prepared. ► SPIONs uptaken by MCF-7 cells increase the ROS production for about 240%. ► The SPION induced ROS production is due to released iron ions and catalytically active surfaces. ► Released iron ions and SPION surfaces initiate the Fenton and Haber-Weiss reaction. ► X-ray irradiation of internalized SPIONs leads to an increase of catalytically active surfaces.