Chemical synthesis of monodisperse γ-Fe–Ni magnetic nanoparticles with tunable Curie temperatures for self-regulated hyperthermia

Magnetic nanoparticles (MNPs) possessing low Curie temperatures (TC’s) offer the possibility for self-regulated heating of cancer cells, where the TC acts as an upper limit to heating to prevent damage to neighboring healthy tissue. We report the synthesis of monodisperse metastable fcc γ-phase Fe–Ni MNPs possessing tunable TC’s, whose stoichiometries have been predicted from metastable extensions to an equilibrium Fe–Ni phase diagram. Metastable alloys have been predicted within the constructs of the T0 construction in the Fe–Ni eutectoid phase diagram. Fe–Ni MNPs have been synthesized via chemical reduction in Fe- and Ni-precursors with stoichiometries ranging from Fe90Ni10 to Fe70Ni30. Mn-precursors have been added to further reduce the alloy’s TC. MNP morphology and structure have been confirmed by x-ray diffraction and transmission electron microscopy while magnetic properties were investigated using vibrating sample magnetometry. Fe70Ni30 MNPs were found to have a TC of 82 °C and specific magnetization of 66 emu/g. Addition of 1 wt % Mn to Fe75Ni25 reduced the TC to 78 °C, which is the lowest reported for fcc Fe–Ni alloys. We also report a model for radio frequency self-regulated heating, in which the maximum achievable temperature of water-MNP suspensions (TC<100 °C) or octyl ether-MNP suspensions (TC>100 °C) is approximately the TC of the MNPs suspended in the solution.