Phenomenology of optical scattering from plasmonic aggregates for application to biological imaging and clinical therapeutics

Near-field coupling between plasmonic resonant nanoparticles and the associated shifts in scattering spectra enables the accomplishment of unprecedented observation of the co-localization dynamics of in-situ biomolecules on nanometer length-scales. We have recently shown that resonant nanoparticles conjugated to antibodies for cell-surface receptors provide a sensitive probe allowing the unambiguous resolution of not only the time sequence, but also the details of the intracellular pathway, for receptor-mediated endocytosis in live cells. In terms of general principles, the classical electrodynamics determining the scattering cross-section for nanoparticle aggregates is straightforward. However, the specifics of the angular dependence of the differential cross-section at a single wavelength, the wavelength dependence of this cross-section, and the correct implementation and interpretation of statistical averages of cross-section properties over an ensemble of aggregate morphologies are generally quite complicated, and in fact are often misinterpreted in the literature. Despite this complexity, we have constructed a set of few-parameter formulae describing optical scattering from nanoparticle aggregates by judicious combination of experimental results with extensive, near-exact simulation using the T-matrix technique. These phenomenological results facilitate the practical use of nanoparticle aggregates for biological measurement and clinical therapeutic applications.