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Endogenous hypochlorous acid (HOCl), an ROS of particular interest, is like a double-edged sword, which has dual influence on both the maintenance of normal physiological functions and the occurrence of many diseases. Methods to track minimal changes of biological HOCl in both multi-modes and real time are essential for precisely tracking this biomolecule with high oxidation activity. However, the methods are still limited due to the difficulty in combining the two functions into one. To resolve this issue, we herein introduced a small-sized fluorescence probe KSQT with two sites which are sensitive to HOCl at nanomolar and micromolar concentrations. With this novel design, our probe achieved real-time and distinct detection of both physiological and pathogenic concentrations of HOCl via turn-on and ratiometric responses in turn, which enables it to effectively avoid the fluorescence cross-talk through simultaneous use of multiple probes and to solve the problem of transmembrane trafficking of previous dual-mode HOCl probes with large size or high positive charges. Together with its marvelous response to HOCl, high biomembrane permeability and biocompatibility, the real-time and dual-mode fluorescence imaging of endogenous HOCl in living HepG2 cells and living zebrafish were accomplished.

Biological hypochlorous acid (HOCl), a reactive oxygen species (ROS) with high activity, plays a crucial role in normal biochemical functions and abnormal pathological processes related to oxidative stress. However, “eco-friendly” tools to unambiguously monitor subtle changes of mitochondrial HOCl in vivo are limited. Herein, a novel mitochondria-specific and “eco-friendly” fluorescent probe (NM1) using triphenylphosphonium as the targeting group is designed for the detection of biological HOCl by a HOCl-promoted triazolopyridine formation strategy. This HOCl-promoted triazolopyridine formation reaction is fast and does not release additional substances except H2O. Therefore, NM1 is not only an “eco-friendly” HOCl probe, but also can target mitochondria to monitor biological HOCl in situ and in real time. Better still, our probe NM1 is capable to image endogenous HOCl in living HepG2 cells and zebrafish.