Biosensing potential of silica/gold nanoshells: Sensitivity of plasmon resonance to the local dielectric environment

The sensitivity of the gold nanoshell localized plasmon resonance (LPR) to the local dielectric environment is studied in terms of a multilayered sphere with a silica core, gold nanoshell, and two dielectric layers modeling the primary functionalization of gold surface and the secondary binding of target biomolecules. The adsorption-induced changes in the extinction and scattering spectra are analyzed by the dipolar Rayleigh approximation and multilayered Mie codes for gold shell diameters of 20-160 nm and (gold shell thickness)/(gold shell diameter) ratios of 0.025-0.75. To calculate the optical polarizability of a multilayered particle, we develop a simple analytical method based on the dipole equivalency principle as applied to a multilayered conjugate and its homogeneous counterpart. This is used to derive simple relations between the local dielectric environment parameters and the gold nanoshell structure. Our analysis predicts greater sensitivity of nanoshell's LPR to the local dielectric environment compared to the solid gold spheres, whereas the absolute changes in spectral maxima are less than those for the equivolume spheres. By contrast, the differential extinction and scattering spectra of gold nanoshells can be used as a sensitive tool for optical monitoring of biomolecular binding onto nanoshell surface. The maximal local-environment sensitivity has been found for the gold (shell/radius) ratios within the range 0.2-0.4.