We describe the design and synthesis of a cyanine-based near-infrared ratiometric fluorescent probe, HS-Cy, for H 2 S 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 p-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 H 2 S with a detection limit of 5.0-10 nM, which is the most sensitive among the reported H 2 Ssensing fluorescent probes. HS-Cy was proven to selectively locate into mitochondria with faster trapping kinetics towards H 2 S. Based on this, the endogenously generated H 2 S in human A549 cells was ratiometrically detected and imaged by HS-Cy.

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We propose a novel unsupervised learning framework for activity perception.

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