Urea

The reagents used for the synthesis were aluminum sulfate 14–18 H₂O (51.0–57.5%, SAMCHUN, Pyeongtaek-si, Republic of Korea), sodium aluminate (38.0–42.0% Na₂O, 51.0–55.0% Al₂O₃, JUNSEI, Tokyo, Japan), isophthalic acid (99.0%, SAMCHUN, Seoul, Republic of Korea), sodium hydroxide (98.0%, SAMCHUN, Republic of Korea), fumaric acid (99.0%, SAMCHUN, Republic of Korea), urea (99.0%, SAMCHUN, Republic of Korea), ethanol (94.0%, DUKSAN, Seoul, Republic of Korea), and deionized water.

Typically, N-GQDs are synthesized by dissolving 0.21 g of citric acid (99%, Sigma-Aldrich, St. Louis, MO, USA) and 0.18 g of urea (99%, Samchun, Seoul, Republic of Korea) into 5 mL of deionized (DI) water and stirring to form a clear mixture solution. The resultant solution is then transferred into a 50 mL Teflon-lined stainless autoclave. The sealed autoclave is heated to 160 °C in an oven, and then a hydrothermal reaction occurs for 4 h. After the reaction, the autoclave reactor is cooled down to room temperature. The obtained aqueous mixture solution is centrifuged at 5000 rpm for 5 min to collect the upper layer. Furthermore, the obtained upper aqueous solution is dialyzed with a dialysis membrane (3500 Da, Spectrum Lab. Inc., Piscataway, NJ, USA) overnight, and the dialyzed aqueous solution is collected to obtain a pristine N-GQDs solution (2 mg·mL⁻¹).

All the chemicals were used without further purification. Zinc acetate dehydrate (Zn(CH₃CO₂)₂∙2H₂O, >99.0%), zinc nitrate hexahydrate (Zn(NO₃)₂∙6H₂O, >99.0%), ethanolamine (C₂H₇NO, >99.0%), 2-methoxyethanol (C₃H₈O₂, >99.3%), and potassium stannate trihydrate (K₂SnO₃∙3H₂O, 99.9%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Hexamethylenetetramine (HMTA, (CH₂)₆N₄, 99.0%) and urea (CO(NH₂)₂, 99.0%) were purchased from Samchun Chemicals (Seoul, Korea) and hydrochloric acid (HCl, 35%) was purchased from Daejung Chemicals & Metals (Siheung, Korea).