A water-soluble fluorescence resonance energy transfer (FRET) probe for hypochlorous acid (HOCl), dansyl rhodamine B piperazinoacetohydrazide, was designed, synthesized and characterized. The dansyl moiety in the probe acted as a FRET donor and the rhodamine moiety acted as a FRET acceptor. The two moieties were connected by a HOCl-cleavable active bond, and cleavage of this linker decreased the FRET efficiency and increased the fluorescence intensity of the donor at 501 nm. The water solubility of the probe was improved compared with other probes by introduction of the cationic rhodamine fluorophore. As a result, the probe could be used to detect HOCl in aqueous biosystems with a linear range of 2-10 mol/L and a detection limit of 80 nmol/L (signal- to-noise = 3). The probe was successfully applied to fluorescence imaging of HOCl in HeLa cells.
