2.15. In situ detection of endogenous SO2 levels in cells by fluorescent probes

SS-1 is a chemoselective fluorescent probe for in situ detection of SO₂, developed and generously given by Prof. Xiaoqi Yu and Prof. Kun Li from Sichuan University [28]. The threshold of detecting SO₂ derivatives in PBS buffer by the SS-1 probe reached 92 nM. Simultaneously, the SS-1 probe was reported to selectively react with SO₂ and exhibit low cytotoxicity [28]. Cells were cultured in chambers, incubated with the fluorescent probe of SO₂ for 0.5 h, rinsed with PBS 3 times, and then fixed with 4% paraformaldehyde for at least 10 min. Cells were rinsed as described above and added an appropriate concentration of DAPI to visualize nuclear DNA. Immunofluorescence results were obtained using a confocal microscope (TCS SP5, Leica Microsystems, Germany), with blue fluorescence representing the nucleus and green fluorescence representing the SO₂ level. The signal intensity of fluorescence was measured using Image J (NIH, USA) software.