Development of a novel europium complex-based luminescent probe for time-gated luminescence imaging of hypochlorous acid in living samples

Luminescent lanthanide complexes are key reagents used in the time-gated luminescence bioassay technique, but functional lanthanide complexes that can act as luminescent probes for specifically responding to analytes are very limited. In this work, we designed and synthesized a novel Eu³⁺ complex-based luminescence probe for hypochlorous acid (HOCl), NPPTTA-Eu³⁺, by using terpyridine polyacid-Eu³⁺, dinitrophenyl, and hydrazine as luminophore, quencher and HOCl-recognizer moieties, respectively. In the absence of HOCl, the probe is non-luminescent due to the strong luminescence quenching of the dinitrophenyl group in the complex. However, upon reaction with HOCl, the dinitrophenyl moiety is rapidly cleaved from the probe, which affords a strongly luminescent Eu³⁺ complex CPTTA-Eu³⁺, accompanied by a ∼900-fold luminescence enhancement with a long luminescence lifetime of 1.41 ms. This unique luminescence response of NPPTTA-Eu³⁺ to HOCl allowed NPPTTA-Eu³⁺ to be conveniently used as a probe for highly selective and sensitive detection of HOCl under the time-gated luminescence mode. In addition, by loading NPPTTA-Eu³⁺ into RAW 264.7 macrophage cells and Daphnia magna , the generation of endogenous HOCl in RAW 264.7 cells and the uptake of exogenous HOCl by Daphnia magna were successfully imaged on a true-color time-gated luminescence microscope. The results demonstrated the practical applicability of NPPTTA-Eu³⁺ as an efficient probe for time-gated luminescence imaging of HOCl in living cells and organisms. (paper)