A highly sensitive ratiometric fluorescent probe for detecting HSO₃^-/SO₃^2- and viscosity change based on FRET/TICT...

Sulfur dioxide (SO₂) is a significant gas signaling molecule in organisms, and viscosity is a crucial parameter of the cellular microenvironment. They are both involved in regulating many physiological processes in the human body. However, abnormalities in SO₂ and viscosity levels are associated with various diseases, such as cardiovascular disease, lung cancer, respiratory diseases, neurological disorders, diabetes and Alzheimer's disease. Hence, it is essential to explore novel and efficient fluorescent probes for simultaneously monitoring SO₂ and viscosity in organisms.

We selected quinolinium salt with good stability, high fluorescence intensity, good solubility and low cytotoxicity as the fluorophore and developed a highly sensitive ratiometric probe QQD to identify SO₂ and viscosity changes based on Förster resonance energy transfer/twisted intramolecular charge transfer (FRET/TICT) mechanism. Excitingly, compared with other probes for SO₂ detection, QQD not only identified HSO₃^-/SO₃^2- with a large Stokes shift (218 nm), low detection limit (1.87 μM), good selectivity, high energy transfer efficiency (92 %) and wide recognition range (1.87-200 μM), but also identified viscosity with a 26-fold fluorescence enhancement and good linearity. Crucially, QQD was applied to detect HSO₃^-/SO₃^2- and viscosity in actual water and food samples. In addition, QQD had low toxicity and good photostability for imaging HSO₃^-/SO₃^2- and viscosity in cells. These results confirmed the feasibility and reliability of QQD for HSO₃^-/SO₃^2- and viscosity imaging and environmental detection.

We reported a unique ratiometric probe QQD for detecting HSO₃^-/SO₃^2- and viscosity based on the quinolinium skeleton. In addition to detecting HSO₃^-/SO₃^2- and viscosity change in actual water and food samples, QQD could also monitor the variations of HSO₃^-/SO₃^2- and viscosity in cells, which provided an experimental basis for further exploration of the role of SO₂ derivatives and viscosity in biological systems.