Magnetic hyperthermia efficiency and MRI contrast sensitivity of colloidal soft/hard ferrite nanoclusters

The use of magnetic nanostructures as theranostic agents is a multiplex task as physiochemical and biochemical properties including excellent magneto-responsive properties, low toxicity, colloidal stability and facile surface engineering capability are all required. Nonetheless, much progress has been made in recent years synthesis of "all-in-one" MNPs remain unambiguously challenging. Towards this direction, in this study is presented a facile incorporation of a soft magnetic phase (MnFe₂O₄ NPs) with a hard phase (CoFe₂O₄ NPs) in the presence of the biocompatible polymer sodium dodecyl sulfate (SDS), into spherical and compact bi-magnetic nanoclusters (NCs) with modulated magnetic properties that critically enhance hyperthermic efficiency and MRI contrast effect. Hydrophobic MnFe₂O₄ and CoFe₂O₄ NPs coated with oleylamine of the same size (9 nm) were used as primary building units for the formation of the bi-magnetic NCs through a microemulsion approach where a set of experiments were conducted to identify the optimal concentration of SDS (19.5 mM) for the cluster formation. Additionally, homo-magnetic NCs of MnFe₂O₄ NPs and CoFe₂O₄ NPs, respectively were synthesized for comparative studies. The presence of distinct magnetic phases within the bi-magnetic NCs resulting in synergistic behavior, where the soft phase offers moderate coercivity H_c and the hard one high magnetization M_s. Increased specific loss power (SLP) value was obtained for the bi-magnetic system (525 W/g) when compared with the homo-magnetic NCs (104 W/g for MnNCs and 223 W/g for CoNCs) under field conditions of 25 kA/m and 765 kHz. Relaxivities (r₂) of the bi-magnetic NCs were also higher (81.8 mM^-1 s^-1) than those of the homo-magnetic NCs (47.4 mM^-1 s^-1 for MnNCs and 3.1 mM^-1 s^-1 for CoNCs), while the high r₂/r₁ value renders the system suitable for T₂-weighted MRI imaging.