Quenching of Quantum Dot Emission by Fluorescent Gold Clusters: What It Does and Does Not Share with the Forster Formalism

Understanding the interactions that control the energy transfer between dyes, or luminescent quantum dots (QDs), and gold nanoparticles still has several unanswered questions. In this study we probed these interactions using a unique model where CdSe-ZnS QDs were coupled to fluorescent gold nanoclusters (AuNCs). Steady-state and time-resolved fluorescence measurements were used to investigate the effects of spectral overlap and separation distance on the quenching of QD photoemission in these assemblies, using three different size QDs with distinct emission spectra and a variable length polyethylene glycol bridge. We found that the QD photoluminescence quenching efficiency depends on the spectral overlap and separation distance, with larger quenching efficiencies than what would be expected for a QD-dye pair with similar overlap. Moreover, despite the large losses in QD PL, we found no resonance enhancement in the cluster emission for any of the sample configurations used. These results indicate that the mechanism driving the quenching by metal clusters shares an important feature (namely dependence on the spectral overlap) with the Forster dipole-dipole coupling at the heart of fluorescence resonance energy transfer (FRET) and widely validated for dye-dye and QD-dye assemblies. They also prove that the energy losses induced by metal nanostructures are governed by a process that is different from the Forster mechanism.