A bright, water-soluble far-red fluorescent probe for detecting thiols in both mitochondria and lysosomes in living cells

Biological thiols such as glutathione (GSH), cysteine (Cys), and homocysteine (Hcy) are important molecules that help to keep the redox balance inside cells. Their subcellular distribution varies across organelles such as mitochondria and lysosomes, and dysregulated thiol levels are implicated in various pathological conditions. Therefore, the development of effective biothiol sensors for subcellular imaging is of significant interest. In this study, we have reported a water-soluble, far-red fluorescent probe (QNB-DNBS) based on a boron dipyrromethene (BODIPY) fluorophore modified with a quaternary ammonium group and conjugated with a DNBS quencher. The incorporation of a quaternary ammonium group on BODIPY enhances aqueous solubility and also acts as a mitochondria and lysosome-targeting moiety. QNB-DNBS remains weakly fluorescent upon excitation at 600 nm but exhibits a significant fluorescence enhancement at 630 nm (<i>Φ</i> = 0.61 in the presence of GSH) upon reaction with thiols. The probe demonstrates exceptional sensitivity, with detection limits of 1.2 nM for GSH, 1.9 nM for Cys, and 4.7 nM for Hcy. QNB-DNBS enables real-time fluorescence imaging of thiols in human blood serum samples and cancer cells and effectively monitors thiol depletion during ferroptosis. These findings highlight the potential of QNB-DNBS as a powerful tool for selective and real-time thiol detection in biological systems.