Figure 2 Fluorescence intensity of CDs with varied irradiation time (A); concentrations of sodium chloride (B); concentrations of hydrogen peroxide (C); temperature (D); organic solvents (E) and metal ions (F).
3.4. Exploration of CDs sensing pH and the corresponding mechanism
During investigating the fluorescence stability of CDs towards pH, we found that their fluorescence intensity showed different response on various pH. To be specific, the fluorescence of CDs continuously and regularly decreased in the range of pH from 2 to 7 (Figure 3A), proving the potential of the CDs prepared here sensing pH in the acidic environment. Accordingly, the linear relationship between the fluorescence intensity of CDs and varying pH was established, while a linear equation was fitted as Y= 718.37X + 164.76 with R2 of 0.984 (Figure 3B). Meanwhile, the fluorescence intensity of CDs solution could well recover when their pH was repeatedly adjusted from 2 to 7 (Figure 3C), indicating their satisfactory acid resistance.
To clarify the mechanism of CDs responding to pH, the further experiments were designed and performed. The zeta potentials of CDs gradually decreased with pH increasing from 7 to 2 (Figure 3D), which revealed that there occurred the interaction between the increased H+ and the surface groups of CDs, thus changing the surface states of CDs and further decreasing their fluorescence. Moreover, the UV spectra of CDs with varied pH were recorded, revealing the blue-shift with pH increasing (Figure 3E), and the infrared spectra of CDs also showed that the stretching vibration peaks of -OH expanded with pH increasing (Figure 3F). Therefore, these findings demonstrated that the decrease of pH led to the protonation of CDs, which altered their surface states, thus resulting in the quenched fluorescence[32,33].