Silica-Protected Hollow Silver and Gold Nanoparticles: New Material for Raman Analysis of Surfaces

The first example of Raman analysis of various surfaces using hollow-silver (h-Ag) and hollow-gold (h-Au) nanoparticles protected by the silica layer has been reported. Synthesized h-Ag@SiO2 and h-Au@SiO2 nanoparticles are efficient electromagnetic resonators, which locally enhance the electric field of the incident radiation. This effect leads to an increase of the efficiency of Raman scattering for molecules being in the close proximity to h-Ag@SiO2 and h-Au@SiO2 nanoresonators. In contrast to solid spherical Au nanoparticles typically used for construction of nanoresonators for shell-isolated nanoparticle-enhanced Raman scattering (SHINERS) measurements, for which a change of the diameter of the nanoparticle causes only a small change of the position of its plasmon band, the plasmon band for hollow spherical Au nanoparticles can be changed within broad range of the visible electromagnetic radiation. It suggests that silica-covered h-Au nanoparticles can be used in the broader wavelength range of the electromagnetic radiation than previously used “solid” nanoresonators. We also found that the samples of hollow Ag nanoparticles induce, on average, stronger enhancement of the Raman signal than the respective samples of solid nanoresonators synthesized from the same amount of metal. Furthermore, the efficiency of h-Ag@SiO2 and similar h-Au@SiO2 nanoresonators has been compared for the enhancing of Raman spectrum of 4-mercaptobenzoic acid (MBA) chemisorbed on the flat gold surface. For the excitation radiation of the wavelength of 632.8 nm, the increase of the Raman signal from MBA molecules that are in a close proximity to h-Ag@SiO2 nanoparticles is larger by ca. 2 orders of magnitude than when h-Au@SiO2 nanostructures are used.