Superparamagnetic iron oxides (SPIO) nanoparticles (NPs) are used as negative contrast agents in magnetic resonance imaging (MRI). However, controlled synthesis of highly stable, biocompatible, reproducible SPIO NPs in mild conditions is still very rare which is demonstrated by the fact that many commercial contrast agents have been withdrawn from the market due to the problems with effective and reproducible synthesis. Here, we introduce a facile and controlled synthesis of Gd-doped SPIO NPs. We have found that different amount of Gadolinium (5, 10 and 20%) replacing the Fe mols in the beginning of the synthesis influence the final physico-chemical properties of the later products. With increasing amount of Gd ions, the higher amount of organic versus inorganic content is calculated (from TGA analysis) which is mirrored in decreasing of saturation magnetization (from 78 Am2/kg for SPIO without Gd to 15 Am2/Kg for 20% Gd-doped SPIO). All prepared samples are stable in PBS and till 10% of Gd doping biocompatible at high concentrations used (500 g/ml). Moreover, 10% Gd-SPIO (cMNPs_Gd10) shows the most stable properties also in a high ionic strength (1 M of NaCl). This sample is promising for mesenchymal stem cells (MSC) labeling. After 24 hours, robust internalization into the MSC was confirmed by fluorescence optical microscopy and by strong negative signal of SPIO labeled MSC measured by clinical 1.5 T MRI.Keywords: Controlled synthesis, superparamagnetic iron oxides (SPIO), in vitro characterization, mesenchymal stem cells (MSC), magnetic resonance imaging (MRI)
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