Carbon monoxide (CO) is a member of the gasotransmitter family that includes NO and H2S and is implicated in a variety of pathological and physiological conditions. Whereas exogenous therapeutic additions of CO to tissues and whole animals have been well studied, the real-time spatial and temporal tracking of CO at the cellular level remains an open challenge. We now report a new type of turn-on fluorescent probe for selective CO detection by exploiting palladium-mediated carbonylation reactivity. The compounds of the invention are capable of detecting CO both in aqueous buffer and in live cells with high selectivity over a range of biologically relevant reactive small molecules, providing a potentially powerful approach for interrogating its chemistry in biological systems.