A near-infrared ratiometric fluorescent probe for rapid and highly sensitive imaging of endogenous hydrogen sulfide in living cells

We describe the design and synthesis of a cyanine-based near-infrared ratiometric fluorescent probe, HS-Cy, for H2S detection, which features rapid response, high sensitivity, and mitochondria targeting. After a rapid quenching at 780 nm by initial nucleophilic addition on the aldehyde group, HS-Cy experienced a polymethine pi-electron conjugation modulation triggered by a second nucleophilic addition on the ester, releasing the cyanine fluorophore which underwent tautomerism from enol form to ketone form. Therefore, gradual emergence of a new peak at 625 nm was observed, constructing a ratiometric signal for H2S with a detection limit of 5.0-10 nM, which is the most sensitive among the reported H2S-sensing fluorescent probes. HS-Cy was proven to selectively locate into mitochondria with faster trapping kinetics towards H2S. Based on this, the endogenously generated H2S in human A549 cells was ratiometrically detected and imaged by HS-Cy.