The quenching mechanism of the SPN-CDs probe toward ClO
– was examined by UV–Vis spectroscopy,
1H NMR, FTIR, and XPS (Fig.4). As shown in Fig.4(a), the UV–Vis characteristic peaks of the SPN-CDs at 450 nm increase significantly with the addition of low concentrations of ClO
– (0.2 mmol∙L
–1), resulting in a color change from colorless to dark brown under visible light (the insert of Fig.4(a)). This is due to the oxidation of the phenolic groups to quinone groups (Fig. S7, cf. ESM). Furthermore, with the addition of high concentrations of ClO
– (0.5 mmol∙L
–1), the characteristic peak of the SPN-CDs at 450 nm began to decrease until it completely disappeared. This results in a color change from dark brown to colorless under visible light owing to further oxidation of the quinone group. In the FTIR spectrum of the SPN-CDs + ClO
– system (Fig.4(b)), the peak at 1160 cm
–1 was significantly reduced. This indicates that the phenolic group was oxidized, causing a change in the color of the solution (the insert of Fig.4(a)). As shown in Fig.4(c), the
1H NMR peaks at 6.80–7.80 ppm disappeared owing to the change in the benzene ring structure due to the formation of quinone groups [
41]. In addition, the XPS spectrum showed that the O and Cl contents in SPN-CDs + ClO
– substantially increased with the addition of ClO
–, indicating that the C=O bond content increased significantly (Figs. S8–S13, cf. ESM). These results indicate that the oxidation of hydroxybenzene in SPN-CDs to quinone groups results in changes in both color and fluorescence intensity [
27,
42]. At high concentrations of hypochlorous acid, the skeletal structure of the chromophore was destroyed, causing the color to change to colorless (Fig. S14, cf. ESM). Furthermore, the fluorescence lifetimes of the SPN-CDs before and after the addition of ClO
– were determined (Fig.4(d)). Compared to pure SPN-CDs (2.377 ns), the lifetimes of the SPN-CDs after the addition of ClO
– did not change significantly (2.407 ns), suggesting that the quenching mechanism follows a simple static quenching mechanism [
43].