Role of interface quality in iron oxide core/shell nanoparticles on heating efficiency and transverse relaxivity

Core/shell Fe₃O₄/γ-Fe₂O₃ nanoparticles have been designed as a promising agent for magnetic hyperthermia and contrast agents for MRI imaging applications, because of their high magnetic moment and biocompatibility. Three sets of core/shell nanoparticles with the same core diameter of 8 nm and shell thicknesses of 1, 3 and 5 nm were synthesized by chemical coprecipitation. The nanoparticles were coated with PEG and the specific absorption rate for the nanoparticles was determined by applying 3.6 kA/m alternating magnetic field with 236 kHz frequency. The biocompatibility of the nanoparticles was confirmed by MTT assay. Three sets of nanoparticles presented in the study have shown the ability to kill 80-85% HeLa cells upon exposure to alternating magnetic field for 10 minutes in the presence of the nanoparticles. The transverse relaxivity of water protons for the three sets of the nanoparticles were obtained using 400 MHz Bruker NMR. The core/shell interface structure is found to be of poor quality because of interface defects which results in interface spin glass structures. The existence of the spin glass clusters suppresses the interface exchange coupling which leads to weak interface magnetic anisotropy. The insignificant role of core/shell structure on the heating efficiency is attributed to the insignificant contribution of the interface structure towards the effective anisotropy.