Recently, several groups (Anderson, Halas, Zharov, and their co-workers, 2003; El-Sayed
and co-workers, 2006) demonstrated, through pioneering results, the great potential of
photothermal (PT) therapy for the selective treatment of cancer cells, bacteria, viruses, and
DNA targeted with gold nanospheres, nanoshells, nanorods, and nanosphere clusters.
However, the current understanding of the relationship between the nanoparticle/cluster
parameters (size, shape, particle/cluster structure, etc) and the efficiency of PT therapy is
limited. Here, we report theoretical simulations aimed at finding the optimal
single-particle and cluster structures to achieve its maximal absorption, which is
crucial for PT therapeutic effects. To characterize the optical amplification in
laser-induced thermal effects, we introduce relevant parameters such as the ratio of
the absorption cross section to the gold mass of a single-particle structure and
absorption amplification, defined as the ratio of cluster absorption to the total
absorption of non-interacting particles. We consider the absorption efficiency
of single nanoparticles (gold spheres, rods, and silica/gold nanoshells), linear
chains, 2D lattice arrays, 3D random volume clusters, and the random aggregated
N-particle ensembles on the outer surface of a larger dielectric sphere, which mimic
aggregation of nanosphere bioconjugates on or within cancer cells. The cluster particles
are bare or biopolymer-coated gold nanospheres. The light absorption of cluster
structures is studied by using the generalized multiparticle Mie solution and the
T-matrix method. The gold nanoshells with (silica core diameter)/(gold shell
thickness) parameters of (50–100)/(3–8) nm and nanorods with minor/major sizes
of (15–20)/(50–70) nm are shown to be more efficient PT labels and sensitizers
than the equivolume solid single gold spheres. In the case of nanosphere clusters,
the interparticle separations and the short linear-chain fragments are the main
structural parameters determining the absorption efficiency and its spectral shifting
to the red. Although we have not found a noticeable dependence of absorption
amplification on the cluster sphere size, 20–40 nm particles are found to be most
effective, in accordance with our experimental observations. The long-wavelength
absorption efficiency of random clusters increases with the cluster particle number
N at small
N and reveals a
saturation behaviour at N>20.