Fine-tuning bromide AIE probes for Hg detection in mitochondria with wash-free staining.

Mercury ions (Hg) primarily target mitochondria in the cells. Therefore, the development of novel probes that specifically target mitochondria in the presence of Hg is of immense importance. Most previously reported probes that utilize the softness of S, Te, O, and/or N atoms for Hg binding often face problems such as fluorescence quenching and off-target signals. In this study, bromide-hydrocarbon pyridinium salts were designed to target the mitochondria and chelate Hg via Hg-Br coordination bonds. As a prototype, four aggregation-induced emission (AIE) fluorogens, namely TPP-Br, TPP-Cl, R1, and R2, with a similar D-π-A structure but slight differences in their halogen substituents, were designed. Among them, only TPP-Br achieved the highly selective and sensitive detection of Hg by triggering its AIE properties, resulting in remarkable emission enhancement (80-fold), colorimetry, and the Tyndall effect. TPP-Br exhibited high selectivity and sensitivity to Hg with a detection limit of 0.35 μM, rapid response time (<10 s), and large Stokes shift of 185 nm. Their interaction modes were studied using a combination of H nuclear magnetic resonance spectroscopy, scanning electron microscopy, fluorescent lifetime decay, and theoretical calculations. TPP-Br exhibited a low emission background in cells, whereas in the presence of Hg, mitochondria were lit up with wash-free staining. This study provides a powerful tool for accurately diagnosing mercury poisoning-related diseases in mitochondria.