Silane-coated magnetic nanoparticles with surface thiol functions for conjugation with gold nanostars

Small (d ∼ 8 nm) magnetite nanoparticles, Fe3O4NP, are prepared and coated with mercaptopropyl trimethoxysilane (MPTS) to form Fe3O4NP@MPTS. In the coating step controlled MPTS/Fe3O4NP molar ratios are used, ranging from 1 to 7.8 × 104. The total quantity of MPTS per Fe3O4NP is determined by SEM-EDS analysis and the average number of free, reactive –SH groups per Fe3O4NP is calculated by a colorimetric method. At very low molar ratios MPTS forms a submonolayer on the Fe3O4NP surface with all –SH free to react, while on increasing the MPTS/Fe3O4NP molar ratio the (CH3O)3Si– groups of MPTS polymerize, forming a progressively thicker shell, in which only a small fraction of the –SH groups, posi- tioned on the shell surface, is available for further reaction. The MPTS shell reduces the magnetic inter- actions occurring between the magnetite cores, lowering the occurrence and strength of collective magnetic states, with Fe3O4NP@MPTS showing the typical behaviour expected for a sample with a mono- modal size distribution of superparamagnetic nanoparticles. Interaction of Fe3O4NP@MPTS with gold nanostars (GNS) was tested, using both Fe3O4NP@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 Fe3O4NP@MPTS, the free thiols of such nanoparticles bind GNS decorating their surface, as shown by UV-Vis spectroscopy and TEM imaging.