Urea nh2 2co

Ferric (III) chloride hexahydrate (FeCl₃-6H₂O, Sigma-Aldrich, Steinheim, Germany), ferric (II) sulfate heptahydrate (FeSO₄-7H₂O, Sigma-Aldrich, Steinheim, Germany), 1,1,1,3,3,3-hexafluoroisopropanol (HFIP, Saint Louis, MO, USA), urea ((NH₂)₂CO, Sigma-Aldrich, Steinheim, Germany), Poly(3-hydroxybutyrate) (PHB, natural origin, MW 500,000) and gelatin (from porcine skin, type A) were purchased from Merck (Sigma-Aldrich, Steinheim, Germany). Deionized water obtained by Millipore Milli-Q system (Darmstadt, Germany) was used in all experiments.

The graphene quantum dots (GQDs) were synthesized using a hydrothermal route with citric acid and urea as the precursors. The details of the synthesis have been explained elsewhere11 . In brief, citric acid (C₆H₈O₇, Sigma-Aldrich) and urea ((NH₂)₂CO, Sigma-Aldrich) were used as carbon and nitrogen sources, respectively. A precursor solution was prepared by dissolving 75 mg urea and 40 mg citric acid in 100 mL pure water at room temperature under stirring for 5 min. The molarities of C and N in the solution were fixed at 0.0042 mol/L and 0.25 mol/L, respectively. The precursor solution was heated in a stainless steel autoclave with an inner Teflon liner for 180 min at 0.8–1.0 MPa under a specific temperature profile (see Supplementary Fig. S1). The hydrothermal system was modified to allow extraction of the sample solution from the autoclave after a certain elapsed reaction time. To investigate the transformation of the C–N configurations in the GQDs, samples were collected from the autoclave at elapsed reaction times of 50, 90, and 180 min during the hydrothermal synthesis.