Optical Properties of Silica-Coated Au Nanorods: Correlating Theory and Experiments for Determining the Shell Porosity

Abstract

Gold nanorods (GNRs) coated with mesoporous silica (GNRs@m-SiO2) have proven to be a robust nanostructure with useful applications in biomedical, catalysis, and molecular sensing areas, among others. The m-SiO2 shell improves the nanoparticle stability and grants a concomitant molecular loading capability. One of the factors that determine the specific application of GNRs@m-SiO2 is the porosity degree of the m-SiO2 shell. In the present work, we first studied how the extinction spectra features of GNRs@m-SiO2 in combination with electrodynamics modeling can be used to determine the porosity degree of the m-SiO2 shell produced at two different concentrations of the porogenic surfactant cethyl trimethylammonium bromide (CTAB). The changes on the intensity in the low-frequency region are explained qualitatively in terms of the optical properties of the mesoporous silica spheres formed as byproducts. Varying the CTAB concentration produces a change not only on the porosity but also on the thickness of the m-SiO2 shell. With rigorous discrete dipole approximation (DDA) simulations, together with an effective medium approach (Maxwell-Garnet) for the m-SiO2, it is demonstrated that the peak position of the longitudinal localized surface plasmon resonance (LSPR) plasmon mode depends only on the effective dielectric constant of the m-SiO2 shell (assuming that all the pores are filled by water). The volume fraction of water in the m-SiO2 shell that, in the DDA simulations, fits the peak position of the longitudinal LSPR of the experimental extinction spectra is a measure of the m-SiO2 shell porosity. DDA simulations show that GNRs@m-SiO2 fabricated with the highest CTAB concentration depicts a degree porosity high enough to allow the diffusion of an analyte toward the GNR core. This feature was tested by determining the analytical surface-enhanced Raman spectroscopy (SERS) enhancement factor of Rhodamine 6G as a molecular probe and comparing it with a theoretical SERS enhancement factor in different regions around the GNRs@m-SiO2 structure. Second, we introduce a simple approach, denoted as quasi-static effective medium approach (QSEMA) for determining the resonance condition of the longitudinal LSPR for core-shell prolate spheroids and therefore the shell porosity. This simple approach leads to the same results as those of rigorous DDA simulations using the exact model geometry. Finally, using QSEMA, we determined the uptake through the pores of each component in glycerin-water solvent mixtures of different compositions.