Citrate-Regulated Surface Morphology of SiO2@Au Particles To Control the Surface Plasmonic Properties

In this work, SiO2@Au core–shell particles under ambient conditions were prepared by using 120 nm SiO2 spheres with ca. 4 nm Au nanoparticles decorated on the surfaces as seeds, the aqueous solutions of sodium citrate/HAuCl4 mixtures as growth solutions, and hydroxylamine as reducing agent. The morphology of the Au shells obtained on the SiO2 spheres was readily regulated only by the citrate-to-HAuCl4 molar ratio; no deliberate adjustment of the temperature and pH of the reaction media was needed. When the citrate-to-HAuCl4 molar ratio in the growth solution was below 4:1, the surfaces of the SiO2 spheres were covered with sparsely packed Au nanoparticles with sizes in the range of 20–40 nm, depending on the citrate-to-HAuCl4 molar ratio. When the citrate-to-HAuCl4 molar ratio in the growth solution was above 8:1, the surfaces of the SiO2 spheres were coated by complete, uniform Au nanoshells. Concomitant with this citrate-regulated morphology, the localized surface plasmon resonance peaks of the resulting SiO2@Au particles shifted from 611 nm for the sparse Au nanoparticle coating to 784 nm for the complete Au nanoshell coating. Furthermore, the sparsely packed Au nanoparticle coating showed stronger surface enhancement Raman spectroscopic signals than the uniform Au nanoshell coating, while the latter exhibit higher photothermal efficiency than the former.