A ratiometric fluorescent probe based on FRET mechanism for the detection of SO₃^2-/HSO₃

Sulfites/bisulfites (SO₃^2-/HSO₃^-), known for their strong reducing, preservative, and antioxidant properties, are widely used in various fields such as food processing, papermaking, textiles, chemical synthesis, and water treatment for their functions of bleaching, decolorization, preservation, and antioxidation. However, long-term excessive intake may pose potential risks to human health. Moreover, their excessive presence in the environment can lead to accumulation in plant tissues, thereby inhibiting plant growth. In order to conveniently detect their contents, a fluorescent probe based on a 3-hydroxyffavonol moiety and a benzoindole derivative was designed and synthesized. Upon reaction with the target analytes, the fluorescence resonance energy transfer (FRET) photophysical process within the probe is disrupted, exhibiting fluorescence quenching at 613 nm alongside a concomitant enhancement of the emission signal at 464 nm. This change in the ratiometric fluorescence signal (F₄₆₄/F₆₁₃) enables the effective detection of SO₃^2-/HSO₃^-, achieving a low detection limit of 0.55 μM. The probe was successfully applied to the quantitative analysis of residual SO₃^2-/HSO₃^- in sulfur-fumigated food samples, with spike recoveries ranging from 97.0% to 103.0%. It can also be used for quantitative screening of SO₃^2-/HSO₃^- in soil environments based on changes in the chromaticity values of fluorescence color. More importantly, it has been fabricated into more convenient hydrogel-based detection strips. With the aid of smartphone color recognition software, test strips facilitate the rapid detection of SO₃^2-/HSO₃^- in solution samples and the capture and visual identification of SO₂ gas. The design and development of this probe provide an effective and portable potential analytical tool for SO₃^2-/HSO₃^--related detection, holding significant importance in areas such as food safety and environmental monitoring.