[en] Full text: For biomedical applications of nanoparticles (NPs), factors such as particle size, narrow size distribution, shape and easy surface functionalization with different binders are significantly important. These conditions, together with high saturation magnetization, superparamagnetic behavior, chemical stability and possibility of external magnetic field targeting increase the potential use of spinel ferrites in therapeutic and diagnostic techniques. However, these NPs may induce adverse responses and toxicological studies are a useful approach to evaluate biological safety [1]. In this work, we investigated the biological effect of ultrasmall citrate-coated cobalt (cit-CF) and nickel (cit-NF) ferrite nanoparticles synthetized by a one-step polyol route on normal (MRC5) and tumoral (MCF7) cells. The NPs were characterized by X-ray diffractometry, dynamic light scattering, zeta potential, thermal analysis, infrared spectroscopy, transmission electron microscopy, Mössbauer spectroscopy and vibrating sample magnetometry. The cytotoxicity was evaluated by metabolic viability assays and uptake studies. The results evidenced the formation of spinel phase, mean crystallite size below 4 nm, superparamagnetic behavior at room temperature and near spherical morphology. Colloidal solutions of cit-CF remained stable for 6 months and cit-NF for 4 months. The metabolic viability of normal cells after 24 hours of incubation with 50 μg/mL of NPs was about 99% for cit-CF and 45% for cit-NF. Studies with tumoral cells indicated selectivity factor of 34% and 9% for cit-CF and cit-NF samples, respectively. In all cases, NPs were localized in the intermembrane space. The observed characteristics indicate that cit-CF nanoparticles are good candidates for applications in the biomedical area. References: [1] Amiri, M; Salavati-Niasari, M; Akbari, A. Adv. Colloid Interface Sci., 265, 29-44, (2019). (author)