DNA-linked metal nanosphere materials: Structural basis for the optical properties

The structural basis for the aggregation-induced optical properties of coll oidal gold nanosphere aggregates is examined by means of electrodynamics ca lculations. Recently developed methods for calculating the electrodynamic r esponse of aggregates composed of large numbers of small metal nanospheres in a dielectric medium are used to determine the optical changes associated with the formation of spherical aggregates. The calculations use accurate nanoparticle polarizabilities determined from Mie theory, an iterative conj ugate-gradient solution algorithm, and fast-Fourier transform methods for e fficient solution of the electrodynamic interacting nanoparticle equations. The UV extinction lowering and the shifting and broadening of the visible plasmon peak observed experimentally in solutions of DNA-linked gold nanosp heres are explained as the collective electromagnetic response of thousands of nanoparticles.