Optical Properties of Silver Nanoparticles Coated with Silica

Silver nanoparticles (AgNp) play an important role in various domains, including nanomedicine, water disinfection, biosensing, biomedical engineering, photonics and optoelectronics. This is mainly due to their unique optical characteristics which result from the localized surface plasmon resonance (LSPR) and which are comparable to those of gold and copper nanoparticles. Coating Ag with silica (SiO2) improves the thermal stability of nanoparticles, prevents their agglomeration and oxidation and offers adjustable solubility in various solvents, but may eventually affect their optical properties.

In this study, we use the theory of effective media to calculate optical spectra of spherical Ag nanoparticles coated with SiO2 and monodispersed in aqueous solution. The theory of the homogenization gives the absorption and diffusion spectra in terms of the filling factor of nanoparticles and the dielectric function of each component (i.e., AgNp core, SiO2 shell, and surrounding medium). To describe the dielectric function of AgNp, we used the Drude critical point model (DCP), which adjusts the experimental data of metal permittivity in the 200-1400 nm band and describes electron oscillations more appropriately than the classical and commonly used Drude model. The effects of silica layer thickness and AgNp filling factor on optical spectra are reported in this work.