Synthesis, characterization, and 3D-FDTD simulation of Ag@SiO _2 nanoparticles for shell-isolated nanoparticle-enhanced raman spectroscopy

摘要

Au-seed Ag-growth nanoparticles of controllable diameter (50-100 nm), and having an ultrathin SiO _2 shell of controllable thickness (2-3 nm), were prepared for shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Their morphological, optical, and material properties were characterized; and their potential for use as a versatile Raman signal amplifier was investigated experimentally using pyridine as a probe molecule and theoretically by the three-dimensional finite-difference time-domain (3D-FDTD) method. We show that a SiO _2 shell as thin as 2 nm can be synthesized pinhole-free on the Ag surface of a nanoparticle, which then becomes the core. The dielectric SiO _2 shell serves to isolate the Raman-signal enhancing core and prevent it from interfering with the system under study. The SiO _2 shell also hinders oxidation of the Ag surface and nanoparticle aggregation. It significantly improves the stability and reproducibility of surface-enhanced Raman scattering (SERS) signal intensity, which is essential for SERS applications. Our 3D-FDTD simulations show that Ag-core SHINERS nanoparticles yield at least 2 orders of magnitude greater enhancement than Au-core ones when excited with green light on a smooth Ag surface, and thus add to the versatility of our SHINERS method.