Preparation of Ag@SiO2@GdF3：Er,Yb Core-shell Structure Nanomaterials and Enhanced Up-conversion Luminescence

Abstract

Noble metal nanoparticles such as Ag or Au and rare earth ions doped-up-conversion luminescence materials are all potential in the fields of biological labeling and sensing, diagnosis and biotherapy. When coupling with them together to form core-shell structure composites, noble metal nanostructures will present the strong surface plasmon resonances (SPRs), they can act as the function of "antenna", that is they can transfer the absorption energy to the luminescence particles and enhance their luminescence intensities. Moreover, when positioned in close proximity to metal surfaces, the luminescence materials can exhibit optical property changes (quenching or enhancement of luminescence), likely a result of the changed near-field electro-dynamical environment around the metal that arises from the collective oscillation of conduction electrons. This is a perfect model for studing the effects on luminescence properies of metal and luminescence materials. In this article, Ag nanoparticles were prepared by a glycol reduction method, and SiO2 spacers were coating on the surface of Ag nanoparti- cles by modified Stober method, at last, the core-shell structure Ag@SiO2@GdF3:Er,Yb up-conversion luminescent nanopar- ticles were successfully synthesized by direct precipitation method. The structure and luminescence property of the samples were characterized by XRD, TEM, FTIR, UV-Vis and fluorescence spectra. XRD patterns show that the orthogonal phase GdF3:Er,Yb nanocrystals are coated on the surface of Ag cores. TEM images present that the obtained composites have ob- vious spherical core-shell structure, the diameter of the Ag core is 50 nm, the size of the Ag@SiO2@GdF3:Er,Yb composites is about 80~120 nm, the surface is smooth and the coating is complete, GdF3:Er,Yb nanoparticles are found obviously in the shell. UV-Vis spectra indicate that GdF3:Er,Yb and SiO2 are coated successfully on the surface of Ag, which increased the refractive index of local area around Ag nanoparticles, and the surface plasma absorption peaks of Ag have red shift. The up-conversion luminescent spectra also indicate that the core-shell structure composite particles have the same red and green up-conversion emissions as the pure GdF3:Er,Yb, their strong peaks are all near 655 nm corresponding to the 4 F9/2→ 4 I15/2 transition of Er 3 + . The fluorescent intensity of the core-shell structure composites is stronger than that of the pure GdF3:Er,Yb when the SiO2 as spacers, which indicates that Ag cores realize the up-conversion fluorescent enhancement of GdF3:Er,Yb. The luminescence intensity is enhanced with the increasing of silica thickness and the amount of GdF3:Er,Yb. This core-shell structure has a certain reference value for preparation of highly efficient rare earth up-conversion luminescent nanomaterials. Keywords GdF 3:Yb,Er; core-shell structure; up-conversion luminescence