Shaping Up Zn-Doped Magnetite Nanoparticles from Mono- and Bimetallic Oleates: The Impact of Zn Content, Fe Vacancies, and Morphology on Magnetic Hyperthermia Performance

The currently existing magnetic hyperthermia treatments usually need to employ very large doses of magnetic nanoparticles (MNPs) and/or excessively high excitation conditions (H × f > 10¹⁰ A/m s) to reach the therapeutic temperature range that triggers cancer cell death. To make this anticancer therapy truly minimally invasive, it is crucial the development of improved chemical routes that give rise to monodisperse MNPs with high saturation magnetization and negligible dipolar interactions. Herein, we present an innovative chemical route to synthesize Zn-doped magnetite NPs based on the thermolysis of two kinds of organometallic precursors: (i) a mixture of two monometallic oleates (FeOl + ZnOl), and (ii) a bimetallic iron-zinc oleate (Fe_3-y Zn _y Ol). These approaches have allowed tailoring the size (10-50 nm), morphology (spherical, cubic, and cuboctahedral), and zinc content (Zn _x Fe_3-x O₄, 0.05 < x < 0.25) of MNPs with high saturation magnetization (≥90 Am²/kg at RT). The oxidation state and the local symmetry of Zn²⁺ and Fe^2+/3+ cations have been investigated by means of X-ray absorption near-edge structure (XANES) spectroscopy, while the Fe center distribution and vacancies within the ferrite lattice have been examined in detail through Mössbauer spectroscopy, which has led to an accurate determination of the stoichiometry in each sample. To achieve good biocompatibility and colloidal stability in physiological conditions, the Zn _x Fe_3-x O₄ NPs have been coated with high-molecular-weight poly(ethylene glycol) (PEG). The magnetothermal efficiency of Zn _x Fe_3-x O₄@PEG samples has been systematically analyzed in terms of composition, size, and morphology, making use of the latest-generation AC magnetometer that is able to reach 90 mT. The heating capacity of Zn_0.06Fe_{2.9 4}O₄ cuboctahedrons of 25 nm reaches a maximum value of 3652 W/g (at 40 kA/m and 605 kHz), but most importantly, they reach a highly satisfactory value (600 W/g) under strict safety excitation conditions (at 36 kA/m and 125 kHz). Additionally, the excellent heating power of the system is kept identical both immobilized in agar and in the cellular environment, proving the great potential and reliability of this platform for magnetic hyperthermia therapies.