Synthesis and Characterization of Gold−Silica Nanoparticles Incorporating a Mercaptosilane Core-Shell Interface

Abstract

The first synthesis and characterization of gold−silica core−shell nanoparticles with a mercaptosilane interface between the core and shell is reported. These nanoparticles exploit the strong interaction between thiols and gold to create a well-defined interface of functional group organization between core and shell. This requires the synthesis of a mercaptosilane monolayer on the colloidal gold surface, which is accomplished indirectly using 1 as a protected precursor to 3-mercaptopropyltriethoxysilane. 1 binds to gold, undergoes sol−gel hydrolysis and condensation, and subsequently deprotects to a thiol via gold-catalyzed thioester hydrolysis. This process results in a monolayer of condensed mercaptosilane on the nanoparticle surface without inducing colloidal instability, which was observed upon direct mercaptosilane addition at the same surface coverage. Similar results were obtained for the previously reported thioester 2, which also binds to gold and deprotects in the bound state, as does 1, but lacks sol−gel active functional groups. Binding experiments with 1 show that its surface-bound thioester has a significantly higher affinity to gold compared with that of 2, which is consistent with polysiloxane formation upon binding 1. Titration of 1 with colloidal gold shows a similar packing density at saturation coverage for 1 as that observed for 2, comprising a footprint of approximately 25 Ų per molecule, and strongly suggests formation of a two-dimensional polysiloxane network on the nanoparticle surface at saturation coverage of 1. Thioester hydrolysis of surface-bound 1 is catalyzed by gold with an apparent activation energy of 7.8 kcal/mol, which is approximately half of the value observed for 2, and a preexponential factor of 967 s^-1, which is more than four decades smaller than the value for 2. These differences in the kinetic parameters are consistent with a significant mass transport limitation for thioester hydrolysis in surface-bound 1, resulting from the cross-linked polysiloxane network, which is not present in the case of 2. Growth of a silica shell after mercaptosilane monolayer synthesis can be observed via transmission electron microscopy. The resulting gold−silica core−shell nanoparticles are robust in that they can withstand organic solvent environments, as well as short-chain thiols, and maintain colloidal stability.