Optical amplification of photothermal therapy with gold nanoparticles and nanoclusters

Abstract

Recently, several groups (Anderson, Halas, Zharov, and their co-workers, 2003; El-Sayedand co-workers, 2006) demonstrated, through pioneering results, the great potential ofphotothermal (PT) therapy for the selective treatment of cancer cells, bacteria, viruses, andDNA targeted with gold nanospheres, nanoshells, nanorods, and nanosphere clusters.However, the current understanding of the relationship between the nanoparticle/clusterparameters (size, shape, particle/cluster structure, etc) and the efficiency of PT therapy islimited. Here, we report theoretical simulations aimed at finding the optimalsingle-particle and cluster structures to achieve its maximal absorption, which iscrucial for PT therapeutic effects. To characterize the optical amplification inlaser-induced thermal effects, we introduce relevant parameters such as the ratio ofthe absorption cross section to the gold mass of a single-particle structure andabsorption amplification, defined as the ratio of cluster absorption to the totalabsorption of non-interacting particles. We consider the absorption efficiencyof single nanoparticles (gold spheres, rods, and silica/gold nanoshells), linearchains, 2D lattice arrays, 3D random volume clusters, and the random aggregatedN-particle ensembles on the outer surface of a larger dielectric sphere, which mimicaggregation of nanosphere bioconjugates on or within cancer cells. The cluster particlesare bare or biopolymer-coated gold nanospheres. The light absorption of clusterstructures is studied by using the generalized multiparticle Mie solution and theT-matrix method. The gold nanoshells with (silica core diameter)/(gold shellthickness) parameters of (50–100)/(3–8) nm and nanorods with minor/major sizesof (15–20)/(50–70) nm are shown to be more efficient PT labels and sensitizersthan the equivolume solid single gold spheres. In the case of nanosphere clusters,the interparticle separations and the short linear-chain fragments are the mainstructural parameters determining the absorption efficiency and its spectral shiftingto the red. Although we have not found a noticeable dependence of absorptionamplification on the cluster sphere size, 20–40 nm particles are found to be mosteffective, in accordance with our experimental observations. The long-wavelengthabsorption efficiency of random clusters increases with the cluster particle numberN at smallN and reveals asaturation behaviour at N>20.