3.3. Preparation of CDs

CDs were synthesized through a hydrothermal method [27]. Briefly, 0.84 g of citric acid and 1 mL of ethylenediamine solution (1:4 molar ratio of citric acid to ethylenediamine) were added to 20 mL of deionized water. The mixture was stirred, and the resulting suspension was transferred to a Teflon-lined autoclave and heated at 180 °C for 5 h. After being cooled to room temperature, the water-soluble CDs were purified by dialysis for about 24 h using a dialysis tube (1000 Da molecular weight cutoff). Finally, the CDs powder (20-CDs) was obtained by freeze-drying the sample. Four other types of CDs powders were prepared by controlling the amount of citric acid used in the process. These samples are referred to as 40-CDs, 50-CDs, 60-CDs and 80-CDs, in which the mole fraction of citric acid is 40%, 50%, 60% and 80%, respectively.