Through Bond Energy Transfer: A Convenient and Universal Strategy toward Efficient Ratiometric Fluorescent Probe for Bioimaging Applications

Fluorescence resonance energy transfer (FRET) strategy has been widely applied in designing ratiometric probes for bioimaging applications. Unfortunately, for FRET systems, sufficiently large spectral overlap is necessary between the donor emission and the acceptor absorption, which would limit the resolution of double-channel images. The through-bond energy transfer (TBET) system does not need spectral overlap between donor and acceptor and could afford large wavelength difference between the two emissions with improved imaging resolution and higher energy transfer efficiency than that of the classical FRET system. It seems to be more favorable for designing ratiometric probes for bioimaging applications. In this paper, we have designed and synthesized a coumarin–rhodamine (CR) TBET system and demonstrated that TBET is a convenient strategy to design an efficient ratiometric fluorescent bioimaging probe for metal ions. Such TBET strategy is also universal, since no spectral overlap between the donor and the acceptor is necessary, and many more dye pairs than that of FRET could be chosen for probe design. As a proof-of-concept, Hg<sup>2+</sup> was chosen as a model metal ion. By combining TBET strategy with dual-switch design, the proposed sensing platform shows two well-separated emission peaks with a wavelength difference of 110 nm, high energy transfer efficiency, and a large signal-to-background ratio, which affords a high sensitivity for the probe with a detection limit of 7 nM for Hg<sup>2+</sup>. Moreover, by employing an Hg<sup>2+</sup>-promoted desulfurization reaction as recognition unit, the probe also shows a high selectivity to Hg<sup>2+</sup>. All these unique features make it particularly favorable for ratiometric Hg<sup>2+</sup> sensing and bioimaging applications. It has been preliminarily used for a ratiometric image of Hg<sup>2+</sup> in living cells and practical detection of Hg<sup>2+</sup> in river water samples with satisfying results.