Small (d ∼ 8 nm) magnetite nanoparticles, FeONP, are prepared and coated with mercaptopropyl trimethoxysilane (MPTS) to form FeONP@MPTS. In the coating step controlled MPTS/FeONP molar ratios are used, ranging from 1 to 7.8 × 10. The total quantity of MPTS per FeONP is determined by SEM-EDS analysis and the average number of free, reactive -SH groups per FeONP is calculated by a colorimetric method. At very low molar ratios MPTS forms a submonolayer on the FeONP surface with all -SH free to react, while on increasing the MPTS/FeONP molar ratio the (CHO)Si- groups of MPTS polymerize, forming a progressively thicker shell, in which only a small fraction of the -SH groups, positioned on the shell surface, is available for further reaction. The MPTS shell reduces the magnetic interactions occurring between the magnetite cores, lowering the occurrence and strength of collective magnetic states, with FeONP@MPTS showing the typical behaviour expected for a sample with a mono-modal size distribution of superparamagnetic nanoparticles. Interaction of FeONP@MPTS with gold nanostars (GNS) was tested, using both FeONP@MPTS with a MPTS submonolayer and with increasing shell thickness. Provided that a good balance is used between the number of available -SH and the overall size of FeONP@MPTS, the free thiols of such nanoparticles bind GNS decorating their surface, as shown by UV-Vis spectroscopy and TEM imaging.