## 3 Results and Discussion  
In this work, GQDs were prepared by a facile two-step MW-assisted synthesis with biomass wastes (orange peel (**A\({}_{0}\)**), date stones (**B\({}_{0}\)**), and oak acorns (**C\({}_{0}\)**)) as starting materials. Firstly, the materials were dried, crushed, and ground to give homogeneous powders as precursors. The microstructures of these powders were evaluated by scanning electron microscopy (SEM). Images (Figure 1a-c) demonstrate that all precursors' powders displayed important micro-porosity. Within the first step, A\({}_{0}\)-C\({}_{0}\) powders were subjected to acidic hydrothermal treatment in monomode microwave reactors. The microwave heating method usually allows for the setting up of green, fast, and economic processes in comparison with conventional refluxing or solvothermal methods. Furthermore, monomode microwave reactors allowed us to set up chemical transformations with an accurate implementation of synthetic parameters (i.e., temperature and pressure) with high reproducibility between batches. Thus, samples A\({}_{0}\)-C\({}_{0}\) were heated at 180 \({}^{\circ}\)C for 5 min in conc. sulfuric acid to afford carbonaceous products A\({}_{1}\)-C\({}_{1}\) as dark powders. During this step, cellulose-based starting materials A\({}_{0}\)-C\({}_{0}\) underwent various successive chemical transformations such as depolymerization, dehydration, and further polymerization, resulting in the formation of materials A\({}_{1}\)-C  
The Fourier-transform infrared (FT-IR) spectra of products A\({}_{1}\)-C\({}_{1}\) were highly similar irrespective of the nature of the biomass waste and clearly showed the presence of oxygen-containing functional groups such as carboxylic acid and hydroxyl groups. The presence of carboxylic acid functional groups was confirmed by the presence of a broad band in the region of 3300-2500 cm\({}^{-1}\). In addition, C=O stretching was observed in the region of 1700 cm\({}^{-1}\) and C-O-C stretching in the region of 1000 to 1200 cm\({}^{-1}\). Furthermore, a strong stretching vibration of C=C could be observed at 1574-1580 cm\({}^{-1}\), indicating the presence of sp\({}^{2}\) units. We could also identify bands of small intensities at around 3000-3100 cm\({}^{-1}\), corresponding to C-H stretching. The SEM images (Figure 1d-f) indicate that the shape of carbonaceous materials generated during the first step was not uniform. While A\({}_{1}\) was composed of angular microstructures, B\({}_{1}\) and C\({}_{1}\) were generally spherical. The particle sizes were a few dozen micrometers. In the second step, carbonaceous materials A\({}_{1}\)-C\({}_{1}\) were treated with conc. nitric acid in microwave reactors at 150 \({}^{\circ}\)C for 5 min, after which the black carbon suspension was completely digested to afford a clear orange solution. After purification, the final A\({}_{2}\)-C\({}_{2}\) GQDs were obtained as yellowish powders with yields ranging from 33 to 40% (wt%). After purification, the prepared GQDs were then systematically characterized in terms of chemical composition (elemental analysis and XPS), size (DLS and TEM), and photophysical properties (absorbance and luminescence spectroscopies). GQDs A\({}_{2}\)-C\({}_{2}\) displayed almost the same chemical composition as that evidenced by X-ray photoelectron spectroscopy (Figures 3a, S1a and S2a, and Table 1). From the full-scan XPS spectrum (Figure 3a) C, N, O, and S were detected with peaks at 284.8 eV (C1s), 405.9 eV (N1s), 532.9 eV (O1s), and 167.6 eV (S2p), respectively. All GQDs displayed almost the same spectra. To determine the C and N configurations in the A\({}_{2}\) GQDs, C1s and N1s spectra were analyzed (Figure 3b,c). The C1s spectra could be decomposed into four main peaks at 284.8 eV, 286.1 eV, 287.5 eV, and 288.9 eV, attributed to sp\({}^{2}\) C=C, C-O/C-N, C=O, and COOH groups, respectively. The N1s spectrum could be deconvoluted into three peaks centered at 400.4 eV, 402.1 eV, and 405.9 eV, corresponding to pyridinic/pyrrolic N, graphitic N, and N-O, respectively.