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

Abstract

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, 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 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.