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Tailoring the properties of superparamagnetic nanoparticles (MNPs) is essential for various nano-based biological applications. Having control over the properties of the MNPs permits a maximum flexibility. Starting from monodisperse iron oxide MNPs produced by thermal decomposition, we report on the optimization and characterization of a first and second seed mediated growth step by varying the surfactant amount and by optimizing the heating steps. We demonstrate the ability to gradually increase the size of crystalline MNPs from 6 over 9 to 12 nm with an improving monodispersity as demonstrated by Transmission Electron Microscopy, Dynamic Light Scattering and X-ray diffraction. The magnetic properties of the MNPs, studied by Vibrating Sample Magnetometry, were in concert with their size increase. We also show the functionalization of these particles with polyethylene glycolated silanes, to render the MNPs stable in water. Different characterization techniques, namely Transmission Electron Microscopy, Dynamic Light Scattering, Fourier-transform InfraRed, Thermo gravimetric analysis and X-ray Photoelectron Spectroscopy, confirmed the successful engraftment of the silanes on the MNP's surface. In conclusion, the proposed route of step-wise synthesis in combination with silane functionalization allows fine tuning the physical properties of iron oxide MNPs for applications in an aqueous environment.