Monodisperse magnetite nanoparticles (MNPs) with controlled sizes and shapes were prepared. The synthesis was carried out by traditional and inverted co-precipitation method with some modifications such as presence of visible light, room temperature and absence of inert atmosphere. Sodium dodecyl sulphate (SDS) was employed as stabilizer. The mentioned experimental parameters were conveniently adjusted to obtain suitable MNPs, to be efficiently employed in biomedical applications. In particular the size, shape, surface charge and magnetic properties were evaluated. MNPs were thoroughly characterized. From characterization data, it emerged that the inverted co-precipitation in presence of visible light rendered mainly nanorods. Modifying the SDS concentration the shape was tuned from nanorods to nanospheres and finally, at higher concentrations, nanocubes were obtained. All prepared formulations resulted hydrophilic. This property was justified in terms of the stabilization mechanism of SDS. Stability of MNPs aqueous dispersions was evaluated regarding to the variation of hydrodynamic diameter as a function of the time. In this regard, all the formulations resulted stable during, at least, 30 days. The prepared nanosystems exhibited satisfactory magnetic properties with saturation magnetization slightly lower than raw magnetite. The combination of size, shape, surface charge, hidrophilicity and magnetic behaviour make the magnetic nanosystems here obtained highly suitable and promising to diagnostic and therapeutic applications.

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Doxorubicin (DOXO) is a chemotherapeutic agent widely used for the treatment of solid tumors and hematologic malignancies in both adults and children. However, DOXO causes short- and long-term cardiotoxicity and others undesirable side effects, such as nephrotoxicity and neurotoxicity. Magnetic nanoparticles (MNPs) allow the delivery of drugs specifically to target place, employing an external magnet. Moreover, they may act as contrast agents in MRI providing information on the diagnostic of diverse pathologies. In this way, two functions may be combined in a unique nanosystem known as theranostic. Also, the MNPs can be modified with folic acid (MNPs@FA) to increase the uptake by cancer cells that overexpress the FA receptors. In previous works, our collaborators obtained and characterized MNPs, MNPs@FA, and MNPs@FA@DOXO. It is essential to study the biosafety of nanotheranostic, and there is no published study of FeO nanoparticles developmental toxicity. Because of that, this work aimed to study the in vivo toxicity and biocompatibility of DOXO, MNPs@FA, and MNPs@FA@DOXO using zebrafish embryo and larvae as an animal model. Viability, developmental toxicity, changes in spontaneous movement (neurotoxicity), changes in cardiac rhythm (cardiotoxicity), and efficiency of DOXO-uptake were studied. While the 48-h treatment with 50 μg/mL of DOXO resulted in a 30% larvae death and the development of significant morphological abnormalities, the treatment with MNPs@FA@DOXO and MNPs@FA did not reduce the viability and did not cause developmental abnormalities. Besides, the MNPs@FA@DOXO reduced the cardiotoxicity and promoted a more rapid and significant uptake of DOXO by zebrafish larvae.