Development of fluorescence sensing mechanism for cell functional analysis

Fluorescence probes are now widely used as indispensable tools in many cell functional analyses. At present, the design of fluorescent probes largely depends on the limited numbers of established sensing mechanisms such as photo-induced electron transfer (PET), photo-induced charge transfer (PCT), and fluorescence resonance energy transfer (FRET). Although these mechanisms are versatile in metal ion sensing, introduction of a new sensing mechanism is highly desirable not only to design a more sophisticated probe with high selectivity and sensitivity but also to expand the diversity of the sensing targets to unveil biological phenomena. In this article, we report the design of dual emission fluorescent probes for metal ions based on a unique fluorescence-sensing mechanism. The fluorescent probes incorporating this sensing mechanism displayed a large emission red-shift on complexation with metal ions such as Cd^II and Ag^I. Xray crystallography and theoretical computational calculations of the Cd^II and Ag^I complexes revealed that the emission shift was induced by non-coordination contact between the aromatic ring of fluorophore and the metal ion (arene-metal ion contact; AM-contact), which modulates the energy levels of molecular orbitals. The fluorescent probe was successfully applied to in cell ratiometric bioimaging of bioactive hydrogen sulfide (H₂S). These successful applications highlight the usefulness of this sensing mechanism in biological fluorescence analysis.