Unlocking the dual roles of carbon monoxide by a rapid fluorescent probe: from monitoring pneumonia therapy in animals to cadmium resistance in plants.

Carbon monoxide (CO) has long been viewed as an environmental pollutant, but recent work points to its role as an endogenous signaling molecule in infection and stress. Here, we present RDM-CO, a near-infrared fluorescent probe built by attaching an allyl formate recognition unit to a rhodamine scaffold. When CO and Pd2+ are both present, the probe undergoes a Tsuji-Trost deallylation that turns on fluorescence at 653 nm within 13 min. RDM-CO shows a 63 nm Stokes shift, good selectivity over other biologically relevant species, a detection limit of 1.35 µM, and no obvious toxicity to cells. Using this probe, we visualized endogenous CO production in LPS-stimulated macrophages and found it localized to mitochondria. We also used RDM-CO to follow CO dynamics in a mouse model of bacterial pneumonia-allowing us to monitor anti-inflammatory treatment effects-and in plant roots under cadmium stress. These experiments demonstrated that RDM-CO can be used to study CO signaling across different biological systems.