We report the design and development of a fluorescent Cd^II ion complex that is capable of the ratiometric detection of H₂S in living cells. This probe exploits the metal-ion-induced emission red shift resulting from direct contact between the aromatic ring of a fluorophore and a metal ion (i.e., arene–metal-ion or “AM” contact). The Cd^II complex displays a large emission blue shift upon interaction with H₂S as the Cd^II-free ligand is released by the formation of cadmium sulfide. Screening of potential ligands and fluorophores led to the discovery of a pyronine-type probe, 6⋅Cd^II, that generated a sensitive and rapid ratio value change upon interaction with H₂S, without interference from the glutathione that is abundant in the cell. The membrane-impermeable 6⋅Cd^II was successfully translocated into live cells by using an oligo-arginine peptide and pyrenebutylate as carriers. As such, 6⋅Cd^II was successfully applied to the ratiometric detection of both exogenous and endogenous H₂S produced by the enzymes in living cells, thus demonstrating the utility of 6⋅Cd^II in biological fluorescence analysis.

The concomitant detection of two biological events facilitates the highly selective and sensitive analysis of specific biological functions. In this article, we report an AND logic-gate-type fluorescent probe that can concurrently sense two biological events in living cells: H₂O₂ accumulation and acidification. The probe exhibits a unique fluorescence sensing mechanism, in which a xanthene fluorophore is oxidatively transformed to a xanthone derivative by H₂O₂, thereby resulting in a clear dual-emission change. This transformation is significantly accelerated under weak acidic conditions, which enables the selective and sensitive detection of H₂O₂ production in an acidic cellular compartment. This unique sensing property was successfully applied to the ratiometric fluorescence imaging of autolysosome formation in selective mitochondrial autophagy (mitophagy), which highlights the utility of this novel probe in autophagy research.

Non-coordinative interactions between a metal ion and the aromatic ring of a fluorophore can act as a versatile sensing mechanism for the detection of metal ions with a large emission change of fluorophores. We report the design of fluorescent probes based on arene–metal-ion interactions and their biological applications. This study found that various probes having different fluorophores and metal binding units displayed significant emission redshift upon complexation with metal ions, such as Ag^I, Cd^II, Hg^II, and Pb^II. X-ray crystallography of the complexes confirmed that the metal ions were held in close proximity to the fluorophore to form an arene–metal-ion interaction. Electronic structure calculations based on TDDFT offered a theoretical basis for the sensing mechanism, thus showing that metal ions electrostatically modulate the energy levels of the molecular orbitals of the fluorophore. A fluorescent probe was successfully applied to the ratiometric detection of the uptake of Cd^II ions and hydrogen sulfide (H₂S) in living cells. These results highlight the utility of interactions between arene groups and metal ions in biological analyses.