Size effect of gold on Ag-coated Au nanoparticle-embedded silica nanospheres

Abstract

Ag-coated Au nanoparticle (NP)-embedded silica nanospheres (SiO₂@Au@Ag NSs) were prepared using three different Au NPs of 2.5, 7 and 15 nm diameter to investigate their optical properties. The UV-vis spectra of the SiO₂@Au@Ag NSs prepared from 2.5 nm Au NPs exhibited a longitudinal spectrum at 50 μM of AgNO₃ with the maximum peak at 410 nm. However, it was red-shifted to 430 nm and the wavelength was broadened from 400 to 800 nm when the AgNO₃ concentration was increased from 50 to 300 μM. This absorption increase was effected by the formation and growth of the Ag shell on the Au NPs on the surface of SiO₂ NSs. Hence, the SiO₂@Au@Ag NSs prepared from 7 nm Au NPs was broadened from 400 to 800 nm. The SERS intensity increased with increasing size of the Au NPs used to prepare SiO₂@Au@Ag NSs. In contrast, the reproducibility of the SERS intensity of the SiO₂@Au@Ag NSs was higher when 2.5 nm Au NPs were used.