Sulfonate-modified phenothiazine luminophore: Molecular structure, spectroscopic properties and DFT calculation for selective hypochlorous acid sensing and intracellular imaging

Cell apoptosis is a vital physiological process for maintaining organism homeostasis, and abnormal levels of hypochlorous acid (HClO, key reactive oxygen species) act as a biomarker of apoptotic progression. Herein, a water-soluble sulfonate-grafted phenothiazine fluorophore QS-PTZ was rationally designed to study the photophysical response to HClO. The oxidation of the phenothiazine thioether group to sulfoxide by HClO, triggering a pronounced turn-on fluorescence signal centered at 600 nm. Comprehensive photophysical characterizations reveal the fluorophore exhibits high selectivity toward HClO over competing biological interferents, rapid optical response within 20 s, wide pH tolerance (4.0–11.0), and a low detection limit of 30.16 nM with excellent linear correlation (R 2 = 0.9968). HR-MS and DFT calculations verify the oxidation recognition mechanism, Absolute fluorescence quantum yields increase from 0.16% for free QS-PTZ to 7.47% after HClO oxidation, and the average fluorescence lifetime prolongs from 0.60 ns to 7.76 ns, quantitatively confirming the suppression of non-radiative decay pathways. Benefiting from good water solubility and biocompatibility confirmed by CCK-8 cytotoxicity tests, QS-PTZ was successfully applied for confocal fluorescence imaging of endogenous HClO upregulation in H 2 O 2 -stimulated A549 cell apoptosis. This work provides a phenothiazine-based luminophore with well-defined photophysical properties for monitoring apoptosis-related HClO fluctuations.