Development of organelle-targetable europium complex probes for time-gated luminescence imaging of hypochlorous acid in live cells and animals

Hypochlorous acid (HCIO) plays a vital role in the immune system and is involved in various human diseases. To fully understand its biological functions in cellular signaling pathways, apoptosis and human diseases, effective chemical tools for directly tracing HCIO at subcellular levels are greatly demanded. Herein, two mitochondria-and lysosome-targetable luminescent beta-diketonate-Eu3+ complexes, Mito-BHHBCB-Eu3+ and Lyso-BHHBCB-Eu3+, were developed as probes for the time-gated luminescence detection of HCIO inside mitochondria and lysosomes of living cells, respectively. The probes were designed by incorporating a mitochondria-anchoring (triphenylphosphonium) motif or a lysosome-anchoring (morpholine) motif with a strongly luminescent HOC1-responsive beta-diketonate-Eu3+ complex, BHHECB-Eu3+, to ensure the probe molecules to be driven into mitochondria or lysosomes for responding to HOC1 therein. Upon exposure to HCIO, the probes exhibited a fast luminescence response (within 5 s) towards HCIO with good selectivity and high sensitivity (<15 nM). In live cell experiments, both probes, Mito-BHEIBCB-Eu3+ and Lyso-BHHBCB-Eu3+, were successfully located in the corresponding organelles as expected, which enabled exogenous and endogenous HCIO to be imaged at subcellular levels. Taking advantages of time-gated luminescence bioimaging technique, the uptake of exogenous HC10 by Daphnia magna was also successfully imaged by time-gated luminescence microscopy. The results reveal that Mito-BHHBCB-Eu3+ and Lyso-BHHBCB-Eu3+ could serve as useful tools for real-time imaging of HCIO at subcellular levels and in vivo with high specificity and contrast.