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 (MnFe ₂ O ₄ NPs) with a hard phase (CoFe ₂ O ₄ 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 MnFe ₂ O ₄ and CoFe ₂ O ₄ 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 MnFe ₂ O ₄ NPs and CoFe ₂ O ₄ 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 (r ₂ ) 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 r ₂ /r ₁ value renders the system suitable for T ₂ -weighted MRI imaging.
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