Radiative and nonradiative decay rates of chromophores bound to differently sized gold nanoparticles

Resonant energy transfer (RET) systems consisting of organic dye molecules bound to gold nanoparticles have recently gained considerable interest in materials science as well as in biophotonics. Gold-nanoparticles are chemically inert, so no photobleaching occurs, they have a high extinction coefficient leading to a larger transfer radius and finally gold-nanoparticles do not luminesce, thus crosstalk between donor and acceptor does not exist. The results for various nanoparticle radii show time constants for the energy transfer which turn out to decrease with increasing radii. Supplementary to this nonradiative decay process one has to consider the effect the nanoparticle exerts on the radiative rate of the dye molecule. Changes in the radiative lifetime can be calculated by measuring the signal intensity at t=0. The reduced intensity at t=0 indicates a strong decrease in the molecules´ radiative rate by more than an order of magnitude in the presence of a gold nanoparticle. This drastically decreased emission rate is a consequence of a phase shift between the molecular and the metal dipole leading to a destructive interference effect.