Figure 1 (A) Fluorescence spectra of CDs, inset: CDs solution under (a) daylight and (b) UV; (B) Fluorescence intensity of CDs solution with different excitations; (C) 3D fluorescence spectrum of CDs solution; (D) HR-TEM image of CDs; (E) Particle size distribution of CDs by DLS; (F) UV-vis spectrum of CDs; (G) XRD pattern of CDs; (H) FTIR spectrum of CDs; (I) XPS survey of CDs.
3.3. Fluorescence stability of CDs
The optical stability of CDs was investigated by altering the UV irradiation time, concentrations of sodium chloride and hydrogen peroxide, temperature, metal ions and organic reagents. As described in Figure 2A, the fluorescence intensity of CDs remained stable with the irradiation of UV 365 nm for 120 minutes. Also, their fluorescence was hardly affected by different concentrations of sodium chloride (Figure 2B), hydrogen peroxide (Figure 2C), and temperature (Figure 2D), demonstrating that CDs exhibited the favorable photobleaching and temperature resistance. In contrast, the fluorescence of CDs obvious deceased in acetic acid rather in other organic solvents (Figure 2E). Again, the metal ions including Na+, K+, Cu2+, Mg2+, Mn2+, Ba2+ and Ca2+ except Fe2+ scarcely affected the fluorescence of CDs (Figure 2F), which may be due to the formed complex between Fe2+ and the functional groups on their surfaces of CDs.