Homogeneous assay based on low quantum yield Sm(III)-donor and anti-Stokes’ shift time-resolved fluorescence resonance energy-transfer measurement

Distance and temperature dependency in nonoverlapping and conventional Förster resonance energy-transfer

Förster resonance energy-transfer (FRET) is a powerful and widely applied bioanalytical tool. According to the definition of FRET by Förster, for energy-transfer to take place, a substantial spectral overlap between the donor emission and acceptor excitation spectra is required. Recently also a phenomenon termed nonoverlapping FRET (nFRET) has been reported. The nFRET phenomenon is based on energy-transfer between a lanthanide chelate donor and a spectrally nonoverlapping acceptor and thus obviously differs from the conventional FRET, but the mechanism of nFRET and resulting implications to assay design have not been thoroughly examined. In this work, a homogeneous DNA-hybridization assay was constructed to study the distance and temperature dependency of both nFRET and conventional FRET. Capture oligonucleotides were labeled at the 5'-end with a Eu(III)-chelate, and these conjugates hybridized to complementary tracer oligonucleotides labeled with an organic fluorophore at various distances from the 3'-end. The distance dependency was studied with a fluorometer utilizing time-resolution, and the temperature dependency was studied using a frequency-domain (FD) luminometer. Results demonstrated a difference in both the distance and temperature dependency between conventional FRET and nFRET. On the basis of our measurements, we propose that in nFRET thermal excitation occurs from the lowest radiative state of the ion to a higher excited state that is either ionic or associated with a ligand-to-metal charge-transfer state.

Time-resolved detection probe for homogeneous nucleic acid analyses in one-step format

We report here an extension of homogeneous assays based on fluorescence intensity and lifetime measuring on DNA hybridization. A novel decay probe that allows simple one-step nucleic acid detection with subnanomolar sensitivity, and is suitable for closed-tube applications, is introduced. The decay probe uses fluorescence resonance energy transfer (FRET) between a europium chelate donor and an organic fluorophore acceptor. The substantial change in the acceptor emission decay time on hybridization with the target sequence allows the direct separation of the hybridized and unhybridized probe populations in a time-resolved measurement. No additional sample manipulation or self-hybridization of the probes is required. The wavelength and decay time of a decay probe can be adjusted according to the selection of probe length and acceptor fluorophore, thereby making the probes applicable to multiplexed assays. Here we demonstrate the decay probe principle and decay probe-based, one-step, dual DNA assay using celiac disease-related target oligonucleotides (single-nucleotide polymorphisms [SNPs]) as model analytes. Decay probes showed specific response for their complementary DNA target and allowed good signal deconvolution based on simultaneous optical and temporal filtering. This technique potentially could be used to further increase the number of simultaneously detected DNA targets in a simple one-step homogeneous assay.