Autoreduction of Metallic Species on the Surface of Silica Nanoparticles by Surface Functionalization

Abstract

AbstractSilica nanoparticles with metallic nanoclusters are of great interest in many applications from bio imaging to optical devices. The nanometric size of metallic particles induces specific absorption properties due to surface plasmon resonance. This absorption mainly depends on the morphology of the nanoparticles. If the encapsulation of metallic nanoparticles into a silica shell is now well developed, there is a great interest on the synthesis of either silica nanoparticles covered by metallic nanoparticles or silica cores with metallic shells. Two main ways are described in the literature to bind metallic nanoparticles onto the silica nanoparticles. The first way consists on the mixing of a metallic colloidal sol with another sol containing silica nanoparticles bringing at their surface the suitable chemical functions, able to properly interact with the metal nanoparticles. The second way consists on the use of a reducing agent to reduce the metallic ions introduced successively into a suspension of silica nanoparticles. Herein is proposed an original third method based on a double surface functionalization of the silica nanoparticles. This method is actually based on an in-situ reduction of metallic ions by two chemical function (amino and thiol) previously grafted onto the surface of the silica nanoparticles. The silica nanoparticles are synthesized by a reverse micro-emulsion sol-gel process. This synthesis gives monodispersed silica nanoparticles of 40 nm diameter. The surface functionalization of the silica nanoparticles is performed by sol-gel reactions within the micro-emulsion, using two silane-coupling agents owning either a thiol function or a diamino function.The functionalized silica shell increases the chemical activity of the surface of the nanoparticles. But the capability of this functionalized surface to reduce metallic ions depends mainly on the chemical function used. Two examples are given in this study: the diamino functions, which reduce the copper ions, and the combination of the diamino and the thiol functions in a silver nitrate solutionwhich induces the growth of small silver nanoparticles (4-5 nm) on the silica nanoparticles’ surface.