Superparamagnetic iron oxide nanodiscs for hyperthermia therapy: Does size matter?

For the final realization of magnetic nanoparticles (MNPs) mediated hyperthermia as a viable clinical therapy for cancer treatment, it is necessary to devise novel approaches in order to improve the heating efficiency or Specific Loss Parameter (SLP) of these MNPs. Recently, it has been shown that magnetic nanodiscs with enhanced shape anisotropy, diameters around 200 nm (25 nm thick), and vortex magnetic domain structure exhibit very high SLP values. Despite their high heating efficiency, biomedical applications of these nanodiscs could not be hassle-free due to their relatively big size. Therefore, in this work, we have studied how the heating efficiency of the nanodiscs changes upon size reduction (∼12 nm diameter and ∼3 nm thickness). In addition, we have compared these results with those obtained for more typically studied spherical nanoparticles of similar volume. Transmission Electron microscopy, Atomic Force Microscopy and X-ray Diffraction confirm the disc shape of our MNPs and that they are mostly composed of iron oxide (Fe3O4 or γ-Fe2O3) phase. Magnetometry indicates that the nanodiscs do not exhibit a vortex magnetic domain structure, but still present a superparamagnetic-like behavior, with zero magnetization in the absence of field at room temperature (ideal for biomedical applications) and enhanced effective anisotropy as compared to the spherical nanoparticles. Finally, calorimetric methods based magnetic hyperthermia experiments indicate that the SLP values for these small nanodiscs are much lower than those reported for the bigger disc-shaped nanoparticles, but these superparamagnetic nanodiscs act as better heating mediators than the spherical nanoparticles of similar volume.