An improved seed-mediated growth method to coat complete silver shells onto silica spheres for surface-enhanced Raman scattering

We synthesized SiO2@Ag core–shell particles with uniform and complete silver shells with an improved seed-mediated growth method. Silver nuclei produced on silica spheres with an electroless plating method served as nucleation sites for the growth of outer silver shells. By adjusting the Ag/SiO2 ratio, the thickness of silver shells was tuned, and the extinction peaks of SiO2@Ag particles shifted from visible to near-infrared (NIR) region. Comparing with previously reported methods, two important strategies were employed in the growth step of silver shells. First, polyvinylpyrrolidone (PVP) was added to control the isotropic growth of silver nuclei and improve the stability of core–shell particles. Second, the growth step of the outer silver shells was completed in a few seconds by increasing the reaction temperature and pH value. Furthermore, the SiO2@Ag core–shell particles were self-assembled into a monolayer, which served as a good SERS substrate.