Simulation of Optical Characteristics of a Breast Tumor Incorporated with Silica Coated Gold Nanorods

Silica coated nanoparticles are advantageous in terms of enhanced biocompatibility, colloidal and thermal stability as well as ease of surface functionalization for use in various biomedical applications. Specifically, for plasmonic photothermal therapeutics and photothermally modulated drug delivery, it is always desired to have maximum absorption of the incident EM radiation by the nanoparticles. So, it is required to quantify the absorption cross-section (σ_abs) of silica coated gold nanorods (GNRs) of varying silica coating thickness. Here, the optical properties of silica coated GNRs embeded in breast tumor like medium are computed for 10×41 nm GNRs which are considered to be coated (dense) with silica thickness range of 1-20 nm. Also, periodic and random spatial distributions of these GNRs within the tumor are accounted for calculating the effect of silica thickness on the overall optical properties. For this, finite element method is used wherein the propagation of incident elecromagnetic field is assumed to be perpendicular to the longitudinal axis of GNRs. Results show that for GNRs coated with silica thickness of 1 nm, the plasmonic wavelength is red-shifted by 40 nm as compared to bare GNRs. Furthermore, on increasing the silica thickness from 2-20 nm, plasmonic wavelength is red-shifted by 24 nm.The absorption and scattering cross-section are increased by ~4.5% and ~8% for GNR coated with 1 nm silica as compared to the bare GNR. Further, it is seen that the scattering cross-section of the media is significantly enhanced by ~26% with an increase in silica thickness from 1-20 nm, while there is no significant change in absorption cross-section for higher silica coating thickness up to 20 nm. Considering the spatial distribution of GNRs within the tumor, the σabs values is increased by ~44% for periodically distributed silica coated GNRs as compared to random distribution within the tumor domain. Also, it is observed that the electric field is confined close to the Gold-Silica interface for lower thickness of the silica coating. These discussed results are useful for the selection of silica coating thickness on GNRs for the biomedical applications such as plasmonic photothermal therapy and photothermally modulated drug delivery.