Backscattering Properties of Gold Nanoshells: Quantitative Analysis and Optimization for Biological Imaging

Abstract

In this article, we investigate the dependence of the backscattering properties of gold nanoshells on the core material, core radius, and shell thickness by using Mie theory of a coated sphere. The size-dependent and dispersive dielectric function of metal nanoparticles permits to consider the localized surface plasmon resonance of gold nanoshells with the classical Mie theory. Our results show that gold nanoshells exhibit resonant backscattering feature due to the resonance of the free electrons in gold shells at a specific incident wavelength. The resonance peak intensity depends on the core material and the dimensions of core and shell, and the resonance wavelength can be tuned to the near-infrared region where biological tissues possess high transmissivity. We obtain the maximum backscattering and the corresponding optimal core radii and shell thicknesses of gold nanoshells at four typical wavelengths (830, 840, 900, and 1310nm) frequently-used in biological imaging. It is found that silica core-gold nanoshells have maximum volume backscattering coefficient (7.01μm-1) at wavelength of 830nm when the core radius is 54.2nm and the shell thickness is 10.1nm. For longer wavelength, the optimal core radius is larger while the optimal shell thickness is thinner, and the maximum volume backscattering coefficient become small. If the core material is changed from silica to vacuum, the gold nanoshells have bigger maximum volume backscattering coefficient with larger core radius and thinner shell thickness. The optimized gold nanoshells can be used as the ideal contrast agents for biological imaging.