Nanosurface Energy Transfer from Long-Lifetime Terbium Donors to Gold Nanoparticles

Photoluminescence (PL) quenching by gold nanoparticles (AuNPs) is a frequently applied principle in nanobiosensing. The quenching is most often explained in terms of the Fö rster resonance energy-transfer (FRET) mechanism, and more rarely in terms of the nanosurface energy-transfer (NSET) mechanism. Although both consider nonradiative resonance energy transfer, there are significant differences in predictions of the strength and the distance-dependence of the quenching. Here, we investigate the energy transfer to AuNPs from a terbium(III)-complex (Tb) with a long (millisecond) PL decay time with the aim to provide a better understanding of the underlying energy-transfer process. The binding of Tb-labeled streptavidin (Tb-sAv) to biotinylated AuNPs (biot-AuNPs) was studied using light-scattering spectroscopy. Quenching of the PL of Tb-sAv upon binding to biot-AuNPs of different diameters (5, 30, 50, 80 nm) was studied by time-resolved PL spectroscopy. Energy-transfer efficiencies were found to be practically independent of the AuNP size. Analysis according to FRET theory yielded donor−acceptor distances that were inconsistent and far beyond the expected Tb−AuNP distance. In contrast, the NSET model yielded a good agreement between the Tb-to-AuNP surface distance estimated from the geometry of the Tb-sAv/biotin-AuNP assembly (4.5 nm) and those calculated from PL lifetime analysis, which range from 4.0 to 6.3 nm. Our findings strongly suggest that NSET (and not FRET) is the operational mechanism in PL quenching by AuNPs, which is important information for the development, characterization, and application of nanobiosensors based on PL quenching by AuNPs.