Mitochondria-Targetable Ratiometric Time-Gated Luminescence Probe for Carbon Monoxide Based on Lanthanide Complexes

As a critical gasotransmitter, carbon monoxide (CO) has been demonstrated to be related with mitochondrial respiration, but the monitoring of CO in mitochondria remains a great challenge. In this work, a unique ratiometric time-gated luminescence (TGL) probe, Mito-NBTTA-Tb³⁺/Eu³⁺, that can specifically respond to mitochondrial CO has been developed. The probe was designed by incorporating a mitochondria-targeting moiety, triphenylphosphonium, into a CO-activatable terpyridine polyacid derivative, 4'-(4-nitrobenzyloxy-2,2':6',2''-terpyridine-6,6''-diyl) bis(methylenenitrilo) tetrakis(acetic acid), for coordinating to Eu³⁺ and Tb³⁺ ions to construct lanthanide complex-based probe for ratiometric TGL detection of CO. Upon reaction with CO, accompanied by the conversion of nitro group to amino group, a 1,6-rearrangement-elimination reaction occurs, which leads to the cleavage of 4-nitrobenzyl group from Mito-NBTTA-Tb³⁺/Eu³⁺, resulting in the significant increase of Tb³⁺ emission at 540 nm and moderate decrease of Eu³⁺ emission at 610 nm. After the reaction, the I₅₄₀/ I₆₁₀ ratio was found to be 48-fold enhanced. This feature allowed Mito-NBTTA-Tb³⁺/Eu³⁺ to be employed as a ratiometric TGL probe for CO detection with the I₅₄₀/ I₆₁₀ ratio as a signal. In addition, the probe showed outstanding mitochondria-localization characteristic, which enabled the probe to be successfully applied to imaging CO within mitochondria of living cells under TGL and ratiometric modes. The application of Mito-NBTTA-Tb³⁺/Eu³⁺ was demonstrated by the visualization and quantitative detection of exogenous and endogenous CO in living cells and mouse liver tissue slices, as well as in living Daphnia magna and mice. All of the results suggested the potential of Mito-NBTTA-Tb³⁺/Eu³⁺ for the quantitative monitoring of CO in vitro and in vivo.