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

Magnetic nanoparticles (MNPs) possessing low Curie temperatures (T_C’s) offer the possibility for self-regulated heating of cancer cells, where the T_C 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 T_C’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 T_C. 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 T_Cof 82 °C and specific magnetization of 66 emu/g. Addition of 1 wt % Mn to Fe₇₅Ni₂₅ reduced the T_C 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(T_C<100 °C) or octyl ether-MNP suspensions(T_C>100 °C) is approximately the T_C of the MNPs suspended in the solution.