Long-Lifetime Luminescent Europium(III) Complex as an Acceptor in an Upconversion Resonance Energy Transfer Based Homogeneous Assay.

Long-lifetime luminescent Eu(III) complexes are widely used as donors in Förster resonance energy transfer to enable time-gated detection of sensitized emission from an intrinsically short-lived acceptor. Here we report a unique energy-transfer system, where the sensitized acceptor emission has prolonged luminescence lifetime compared to the donor and the long lifetime is not cut short upon high energy-transfer efficiency. The infrared-excited, ultraviolet-emitting, Tm(III)-doped upconverting nanoparticles were used as donors, and a luminescent Eu(III)-chelate was used as an acceptor. Upon excitation the sensitized acceptor emission, which is already spectrally resolved from the donor, can be measured even after the donor luminescence has decayed. Because of anti-Stokes characteristics, the time-gated detection is not needed to avoid the autofluorescence. Thus, the long luminescence lifetime can be further modulated and utilized, e.g., in background-free molecular sensing, rendering the system extremely attractive.