Reporter-embedded TiO<inf>2</inf> core-mixed metal shell nanoparticles with enormous average surface-enhanced raman scattering enhancement factors

We report the development of Raman reporter-embedded TiO2 nanoparticles coated with mixed Ag and Au shells, showing enormous ensemble surface-enhanced Raman scattering (SERS) enhancement factors (up to 10 10). Effects of shell composition on the enhancement are investigated both experimentally and theoretically. Colloidal TiO2 nanoparticles are first tagged with meso-tetra(4-carboxyphenyl)porphine or tris(2,2′-bipyridyl)ruthenium(II) chloride, used as reporter molecules. They are subsequently coated with either a Ag shell or a mixed Au-Ag shell with different compositions. The resulting nanostructures are characterized by UV-visible spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, particle size analyzer, and Raman spectroscopy. These Raman reporter-embedded TiO2 core-metal shell nanoparticles exhibit reproducible SERS signals of the reporter molecules with very high average enhancement factors. Interestingly, depending on the excitation wavelength, bimetallic Au-Ag shell nanostructures with proper Au/Ag ratios display higher enhancement factors than Ag-only shell. Simulation results based on equivalent dielectric functions show a very good match with the experimental observations. © 2011 American Chemical Society.