1. The N-GQDs were synthesized by c

1. The N-GQDs were synthesized by carbonization of CA (99%,Sigma-Aldrich) with ammonia through hydrothermal treatment.

2. In brief, 40 ml of a CA aqueous solution (1 mg/ml) and an ammonia aqueous solution (2, 5, and 8 ml, 25 wt%) were transferred into a Teflon-lined autoclave and were heated at 200 C for 3 h.

3 After cooling to room temperature, the pH of the light yellow N-GQDs
solution was tuned to 8 with adding NaOHsolution (1 mg/ml) dropwise.

4. The light yellow N-GQDs solution was dialyzed in dialysis tubing (3000 Da, Spectrum Lab. Inc.) against DI water for 4 h to remove impurities and excess ammonia.

5. After dialysis, an aqueous suspension of N-GQDs was obtained.

6. For the synthesis of GQDs, asimilar process was carried out without adding an ammonia solution,and a light yellow solution of GQDs was obtained.
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