Unlocking HClO detection via an AIE "off-on-off" fluorescent switch

Hypochlorous acid (HClO) is a key reactive oxygen species (ROS) involved in both normal physiology and disease pathogenesis. Imbalances in its intracellular levels are strongly associated with inflammatory disorders and cancer. In this work, we introduce TPE-ML, a novel aggregation-induced emission (AIE) probe built on a thiomorpholine scaffold. This probe features a distinctive "off-on-off" fluorescence switching behavior. It exhibits a Stokes shift of approximately 170 nm and enables sensitive detection of HClO with a limit of detection as low as 1.37 μM. Solvent-dependent assays confirm its AIE characteristics: the fluorescence intensity increases 15-fold in solvent mixtures with 99% water fraction. Upon selective oxidation by HClO, TPE-ML converts from an AIE-active aggregated state to a dispersed state, accompanied by marked fluorescence quenching. Theoretical calculations further reveal an increased dipole moment and hydrophobicity (log P) further altered electronic properties after oxidation. Using the standard addition method, TPE-ML achieved quantitative analysis of HClO in strawberries, apples, baby cabbage, and lettuce, with spiked recoveries ranging from 98.3% to 107.8%. It also permitted on site visual monitoring, as the fluorescence turned from yellow to dark under 365 nm UV illumination. Furthermore, this probe allowed real time imaging of both externally added and endogenously produced HClO in living 4 T1 cells, underscoring its biomedical promise. Collectively, this work introduces a fresh AIE based sensing mechanism for HClO, thereby enriching the methodological toolbox for ROS detection in biological specimens.