Preparation of multifunctional mesoporous silica particles: the use of an amphiphilic silica precursor with latent amine functionality in selective functionalization of the inner surface

Abstract

The mesoporous silica particle has three different areas as a functionalization site, including an outer surface, a silica framework and an inner surface. We functionalized the inner surface selectively using an amphiphilic silica precursor with latent amine functionality and prepared a multifunctional mesoporous silica particle showing fluorescence resonance energy transfer (FRET). An organotriethoxysilane consisting of a perylene diimide unit and two triethoxysilyl groups was co-condensed with tetraethoxysilane and the amphiphilic organotriethoxysilane [(4-octylphenyl-3-(triethoxysilyl)propylcarbamate)] in the presence of cetyltrimethylammonium bromide under basic conditions. The resulting periodic mesoporous silica particle contained perylene diimide units in a silica framework as a FRET donor. The hydrophobic part of the amphiphile was tethered to the pore wall through a thermally reversible urethane bond. The surfactant molecules were extracted by reflux in a mixture of 1,4-dioxane, HCl and water. In this process, hydrophobic groups in the pore were also removed to produce amino groups on the inner surface. The rhodamine units were introduced into the pore as a FRET acceptor by the reaction of the isothiocyanate group of Rhodamine B isothiocyanate with the amino group on the inner surface, forming a thiourea linkage. The outer surface of the silica particle could be also functionalized by grafting with 3-(trimethoxysilyl)propyl acrylate before extraction of the surfactant molecules. A silica–polymer thin film was prepared where the silica particles were embedded in a poly(2-hydroxyethyl methacrylate) matrix. Both the suspension of the silica particles in ethanol and their polymer composite film showed the efficient FRET effect.