Eco-friendly synthesis of fluorescent cobalt-doped manganese ferrites for thermo-therapeutic applications

The synthesis of high-quality nanoparticles for biomedical applications through environmentally friendly methods is a major challenging task. In this work, oxidative precipitation and a novel method, oxidative hydrothermal synthesis, are explored as one-pot eco-friendly synthesis routes to obtain single phase multicore-like and single-core citrate-stabilized cobalt and/or manganese-doped ferrite (CoxMn1-xFe2O4, 0 ≤ x ≤ 1) nanoparticles, respectively. The Co2+/Mn2+ ratio and synthesis method were found to influence the nanoparticles' morphology and structure, with oxidative precipitation producing multicore-like structures and hydrothermal synthesis yielding single-core particles with crystalline sizes in the range of 2 − 10 nm and 4 − 11 nm, respectively. Inductively coupled plasma optical emission spectrometry (ICP-OES) indicated a composition near the expected values. Mixed ferrites displayed improved saturation magnetization up to 60 A.m2/kg, exhibiting superparamagnetism at room temperature. The particles’ stability was evaluated in physiological pH, and the mixed ferrites displayed intrinsic fluorescence emission in the violet-green range with a maximum wavelength near 425 nm. The cobalt and/or manganese-doped ferrites were evaluated for magnetic hyperthermia and photothermia, displaying high heating efficiency for several alternating magnetic field conditions (up to ∼2.5 nHm2/kg) compatible with biological applications, and high light-to-heat conversion efficiency (up to ∼53%) with near infrared (NIR) laser irradiation at 808 nm. Hereby, the oxidative precipitation and oxidative hydrothermal synthesis are promising eco-friendly methods for the development of crystalline citrate-stabilized cobalt and/or manganese-doped ferrites with multicore-like and single-core spherical morphology, respectively, without requiring additional post-synthesis treatments and with suitable properties for biomedical applications, such as magnetic hyperthermia, photothermia and as prospective fluorescent probes for bio-imaging.