Superparamagnetic Iron Oxide Nanoparticles (SPIONs) are more and more used in biomedical applications such as therapy (treatment for certain cancers, hyperthermia), diagnostic (contrast agent for Magnetic Resonance Imaging) or both. For these applications, SPIONs must be stable in an aqueous solution, monodisperse, with a narrow size distribution and without aggregation. To obtain these nanoparticles, a microwave process is carried out in this study as an easy, fast and reproducible synthesis method. Currently, in the literature, most synthesis of SPIONs are in ultra-pure water or another solvent. To consider the use of SPIONs in biomedical applications, it is essential to ensure the preservation of the physico-chemical parameters of the nanoparticles in the physiological medium to validate a synthesis process. With this objective, this study reports a comparison between the SPIONs synthesis in ultra-pure water and the SPIONs direct synthesis in a physiological serum (containing NaCl). To complete this comparison, the dispersion of SPIONs in physiological serum after an elaboration in ultra-pure water is reported. Characterizations of these different SPIONs samples are carried out to determine the physico-chemical parameters and magnetic properties. SPIONs are characterized by Transmission Electronic Microscopy, Dynamic Light Scattering, X-Ray Diffraction, Raman spectroscopy and magnetic measurements. Finally, to check if SPIONs can be used as contrast agent for MRI, a relaxometry measurement is performed.