Flavonol-based fluorescent probe for bisulfite detection in plant cells and environmental water samples

Bisulfite (HSO₃^-), which exists in equilibrium with sulfite and sulfur dioxide derivatives, plays significant roles in food preservation and biological systems. However, elevated levels of bisulfite are associated with various physiological disorders. Therefore, the development of reliable methods for monitoring bisulfite is of considerable importance. Herein, we report a flavonol-based fluorescent probe, 4-(3-hydroxy-4-oxo-4H-chromen-2-yl) benzaldehyde (BSP), synthesized via a simple two-step reaction using 1-(2-hydroxyphenyl)ethan-1-one and 4-(diethoxy methyl)benzaldehyde. BSP exhibits excellent water solubility and a strong turn-on fluorescence response toward HSO₃^- in PBS buffer (pH 5.0), with green emission at 515 nm and a large Stokes shift of 155 nm. The probe shows high selectivity and sensitivity with a detection limit of 0.19 µM. The sensing mechanism is attributed to nucleophilic addition of HSO₃^- to the aldehyde group, suppressing the n → π* transition and enhancing intramolecular charge transfer (ICT), resulting in significant fluorescence enhancement. BSP was successfully applied for bisulfite detection in onion epidermal cells and environmental water samples.