Constructing near-infrared dyes with D–A–D-type Stokes shifts based on asymmetric electron-donating properties and their applications

An excessive amount of sulfur dioxide (SO₂) poses a threat to the microenvironment of living organisms and, in severe cases, can cause inflammation, cardiovascular diseases, and respiratory damage. As one of the important atmospheric pollutants, SO₂ and its derivatives are widely used in the production of additives and chemicals, and are important factors affecting physiological activities. To achieve the goal of detection, we developed a new near-infrared (NIR) fluorescent probe, Cou-Oxo, which can be used to obtain a ratiometric response to SO₂ derivatives in cells, zebrafish, and plant rhizomes. This novel NIR dye with a large π-system was created by combining two small π-dyes. Its structural features led to a markedly expanded Stokes shift of 321 nm. The NIR ratiometric fluorescence properties of the probe were achieved through the addition reaction of SO₂ derivatives (HSO₃^-) to the CC double bond on the Cou-Oxo probe, which possesses a detection limit of 0.942 μM, a short response time (within 30 s), satisfactory biocompatibility, excellent anti-interference, and pH stability. The probe was successfully used to detect changes in exogenous and endogenous SO₂ levels in cancer cells and living zebrafish. More interestingly, we successfully visualized SO₂ in normal and SO₂ poisoned tobacco seedlings as a model.