Scheme 1 probe C23
The probe itself does not fluoresce due to intramolecular rotation. In the presence of HSA, the probe molecules specifically bind to the hydrophobic cavity of HSA, causing the probe molecules to be unable to rotate, resulting in strong red fluorescence. When the probe coexists with SO2 alone, a specific Michael addition reaction occurs (as shown in Scheme 2), which reduces the π-conjugated structure of the probe and produces obvious blue fluorescence.