Synthesis of GQDs

Pristine GQDs was fabricated by pyrolyzing citric acid. In a typical run of GQDs preparation, 2 gr acid citric was put in the 5 mL beaker and heated for 30 min at 200 °C to turn the colorless citric acid powder to the pale orange liquid. Obtaining this temperature is essential in this process, because producing GQDs is not feasible at temperatures below 200 °C. The obtained liquid was mixed with the 100 mL of 0.25 M NaOH, and the solution was continuously stirring for about 5 min. Finally, the resultant solution was neutralized to pH = 7 by adding drops of 2 M HCl and cooling to room temperature. This solution should be stored in a dark place and temperature of about 15 °C (Fig. 1b).