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

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 spectrumat 50 mu M 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 mM. 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.