Competition of Charge and Energy Transfer Processes in Donor-Acceptor Fluorescence Pairs: Calibrating the Spectroscopic Ruler.

Sensing strategies utilizing Förster resonance energy transfer (FRET) are widely used for probing biological phenomena. FRET sensitivity to the donor-acceptor distance makes it ideal for measuring the concentration of a known analyte or determining the spatial separation between fluorescent labels in a macromolecular assembly. The difficulty lies in extracting the FRET efficiency from the acceptor-induced quenching of the donor emission, which may contain a significant non-FRET contribution. Here, we demonstrate a general spectroscopic approach for differentiating between charge transfer and energy transfer (ET) processes in donor-acceptor assemblies and apply the developed method for unravelling the FRET/non-FRET contributions in cyanine dye-semiconductor quantum dot (QD) constructs. The present method relies on correlating the amplitude of the acceptor emission to specific changes in the donor excitation profile in order to extract ET-only transfer efficiencies. Quenching of the donor emission is then utilized to determine the non-ET component, tentatively attributed to the charge transfer. We observe that the latter accounts for 50-99% of donor emission quenching in QD-Cy5 and QD-Cy7 systems, stressing the importance of determining the non-FRET efficiency in a spectroscopic ruler and other FRET-based sensing applications.