*2.2. Methods*

2.2.1. Hydrothermal Carbonization and Separation of CQDs from HTC Process Liquid

HTC experiments of different precursors were performed in a glass-lined 100 mL Parr batch reactor (reactor series 4590, Moline, IL, USA). A Parr proportional-integral-derivative (PID) controller (model 4590) with an accuracy of ±2 ◦C was used to control the reaction temperature. The pressure was not controlled but was monitored during the experiment with a pressure transducer and a gauge. HTC experiments were conducted at 220 ◦C for a residence time of 30 min. The reactor was loaded with 45 mL solution contained 10% (*w*/*v*) of precursor and the rest was deionized (DI) water. The reactor was closed and heated at a constant rate of 10 ◦C/min until it reached the set temperature and then it was maintained at isothermal conditions for 30 min. At the end of the residence time, the heater was turned off, heating elements were removed from the reactor, and the reactor was rapidly cooled to room temperature (~30 ◦C) by placing it in an ice-water bath. Once the reactor reached room temperature, the gaseous products produced during the reaction were vented in a fume hood by opening the vent valve. Finally, the lid of the reactor was opened and HTC process liquid was filtered by Whatman 41 filter paper. Dark brown HTC process liquid samples were collected in centrifuge tubes and stored in the refrigerator for further synthesis. As the major goal of this study was to evaluate the variation of optical, morphological, and chemical properties of CQDs with precursors, the hydrochar and gaseous products were not further characterized. However, the mass balance and physico-chemical characterization of hydrochars can be found elsewhere [23]. All the experiments were completed in duplicate to check reproducibility.

The dark brown HTC process liquid was centrifuged at 10,000 rpm for 15 min by a Sorvall BIOS 16-series centrifuge from Thermo Fisher Scientific (Waltham, MA, USA) to separate the larger particles from the liquid phase. The fluorescent CQD containing liquid phase was then filtered with a standard syringe filter (0.22 μm). The filtered CQD containing solution was then evaporated under reduced pressure by vacuum distillation. The concentrated product from vacuum distillation was kept in the high vacuum freeze dryer for 24 h to obtain powdered CQDs. The powdered CQDs were collected and refrigerated in glass vials for further investigations. A schematic of the CQD synthesis and characterization strategy is shown in Figure 1.

**Figure 1.** A schematic of the CQD synthesis and characterization strategy.
