Excimer-FRET Cascade in Dual DNA Probes: Open Access to Large Stokes Shift, Enhanced Acceptor Light up, and Robust RNA Sensing.

The efficacy of fluorescent hybridization assays is often limited by the low signal-to-background ratio of the probes that can be partially overcome by sophisticated signal amplification methods. Deep understanding of the mechanisms of fluorescence quenching and energy transfer in complex DNA probes and the choice of optimal donor/acceptor pairs along with rational design can significantly enhance the performance of DNA probes. Here, we proposed and studied novel Förster resonance energy transfer (FRET) dual DNA probes with the excimer-forming pyrene pair as a donor and sulfo-Cy3 dye as an acceptor, which demonstrated remarkable 75-fold enhancement of sulfo-Cy3 fluorescence upon target capturing. Stokes shift up to 220 nm minimizes fluorescence crosstalk. Time-correlated single-photon counting revealed two excited states of pyrene excimer wherein only one is directly involved in the resonance energy transfer to sulfo-Cy3. Optimized DNA probes demonstrated high sensitivity with excellent signal-to-background ratio, which were applied for visualization of 18S rRNA by fluorescent in situ hybridization in HEK-293T cells.