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A synthesis method is introduced for very small uniform gold particles (diameter less than 5 nm), based on the reduction of hydrogen tetrachloroaurate(III) in ethanol in the presence of (γ-mercaptopropyl)trimethoxysilane (MPS). The surface layer of MPS molecules gives the gold particles a high colloidal stability and allows in principle further reaction with any silane coupling agent. Decrease of the HAuCl4:MPS ratio allows a controlled reduction of gold particle size, resulting in remarkably uniform gold clusters of (sub)nanometer size, observed with high-angle-annular-dark-field scanning transmission electron microscopy. After attachment of (γ-aminopropyl)triethoxysilane (APS) to the MPS surface layer, other molecules may be covalently bound to the gold colloid via the amine group of APS. As an illustrative example we prepared in this manner gold particles labeled with a fluorescent dye. The chemical structure of the surface silanes was studied with Fourier transform infrared spectroscopy.
Buining et al. (Tue,) studied this question.