MnO2-protected silver nanoparticles: New electromagnetic nanoresonators for Raman analysis of surfaces in basis environment

The first example of the synthesis of Ag nanoparticles protected by a few nanometers thick layer of MnO2 (Ag@MnO2) has been reported. Synthesized Ag@MnO2 nanoparticles effectively locally enhance the electric field of the incident visible radiation, which allows, for example, for a large enhancement of the efficiency of Raman scattering for species located in the close proximity to such nanostructures. It means that Ag@MnO2 nanoparticles may be used as nanoresonators for shell-isolated nanoparticle-enhanced Raman scattering (SHINERS) measurements. The obtained Ag@MnO2 nanoparticles are almost two orders of magnitude more efficient in enhancing Raman signal than previously used for SHINERS measurements in the alkali environment Au@MnO2 nanostructures. Moreover, in comparison to Ag@SiO2 nanoparticles, which are standard silver nanoresonators for SHINERS experiments, Ag@MnO2 nanoparticles are significantly more stable in the basic conditions. Deposition of the MnO2 layer (by the reduction of KMnO4 with by K-2 C-2 O-4 in an alkaline condition) on hollow silver nanoparticles (h-Ag) has been also analyzed. Hollow silver shells are significantly less stable than the solid Ag nanostructures and are practically entirely destroyed during the process of the MnO2 deposition. However, in this condition, the majority of h-Ag nanoparticles form agglomerates containing about 101 h-Ag items which are connected by MnO2, and after dissolution of the silver auxiliary templates very regular MnO2 sponge nanostructures with the diameter of 150-300 nm are formed. (C) 2016 Elsevier B.V. All rights reserved.