A ratiometric fluorescent probe for HSO₃⁻ and its application in environmental and food analysis.

Bisulfite (HSO₃⁻) is widely used as a preservative and antioxidant in the food industry and chemical manufacturing, yet its excessive residues pose risks to food quality, environmental safety, and human health. To achieve rapid, selective, and quantitative detection of HSO₃⁻, a ratiometric fluorescent probe based on a benzothiazole-carbazole unit and a benzoindole ion was developed. Upon reaction with HSO₃⁻, the probe exhibits a distinct dual-emission response, fluorescence intensity decreases at 612 nm and increases at 467 nm, enabling robust ratiometric sensing (F₄₆₇/F₆₁₂). Owing to its pronounced fluorescence modulation (emission signal change > 145 nm) and high sensitivity (limit of detection = 0.62 μM), the probe demonstrates reliable performance in aqueous samples and has been successfully integrated into portable test strips and swabs. Furthermore, when coupled with a smartphone-based colorimetric application software, the probe allows convenient and real-time quantification of both gaseous SO₂ and aqueous HSO₃⁻ across environmental and food-related matrices. This work thus delivers a practical, instrument-free platform for sulfite monitoring in complex real-world settings.