## 3 Results and Discussion
The N-O moieties could presumably be attributed to nitro functional groups (NO\({}_{2}\)) linked to the aromatic structure, originating from nitric acid treatment. This was in perfect agreement with previous reports [37; 38]. As a matter of fact, nitration of aromatic rings can occur during electrophilic aromatic substitutions without the need for sulfuric acid as catalyst [39]. In addition, the O1s signal at 531.6 eV demonstrated the presence of C=O bonds. Finally, the S2p signal at 166.9 clearly demonstrated the presence of oxidized forms of sulfur-based functional groups, very likely sulfones and sulfoxides [40; 41]. Furthermore, the elemental molar ratio of C, N, S, and O for the three GQD batches A\({}_{2}\)-C\({}_{2}\) was calculated from the XPS analyses and the results are depicted in Table 1.  
First, the elemental content was similar in the three samples investigated in spite of the different biomass waste sources. While the oxygen content was hardly modified during the second step, sulfur was incorporated within the structures in the range of 0.3-1.8%. Additionally, we observed that nitrogen content increased to reach 4-5.4% atomic content, thereby confirming the presence of nitrogen-based functional groups, very likely nitro groups as supposed on the basis of XPS data. FT-IR spectroscopy was used to further \begin{table}
\begin{tabular}{c c c c} \hline  & **Sample A2** & **Sample B2** & **Sample C2** \\ \hline C content (\%) & 58.1 \(\pm\) 2.1 & 64.7 \(\pm\) 2.2 & 65.2 \(\pm\) 2.4 \\ N content (\%) & 5.4 \(\pm\) 0.2 & 4.0 \(\pm\) 0.1 & 4.9 \(\pm\) 0.2 \\ O content (\%) & 34.7 \(\pm\) 1.2 & 30.5 \(\pm\) 1.0 & 29.6 \(\pm\) 1.1 \\ S content (\%) & 1.8 \(\pm\) 0.1 & 0.8 \(\pm\) 0.1 & 0.3 \(\pm\) 0.1 \\ \hline \end{tabular} \({}^{*}\)The elemental analysis results are expressed as the mean of three independent measurements \(\pm\) the standard deviation.  
\end{table} 
Table 1: Percentage of C, N, O, and S atoms in GDQs A\({}_{2}\) (synthesized from orange peel), B2 (synthesized from date stones), and C\({}_{2}\) (synthesized from oak acorms) determined by XPS measurements \({}^{*}\). characterize the nature of functional groups in A\({}_{2}\)-C\({}_{2}\) (Figures 3d, S1b and S2b). All samples displayed identical IR profiles with four main bands, but with some slight differences based on respective intensities. The IR band located at 3300-3500 cm\({}^{-1}\) could be assigned to carboxylic acid functional groups, the intense band at 1600-1610 cm\({}^{-1}\) was assigned to C=C bonds as the elementary units of sp\({}^{2}\) conjugated graphene structure, the band at 1340-1350 cm\({}^{-1}\) was assigned to NO\({}_{2}\) stretching, and the one at 1110 cm\({}^{-1}\) reflected the stretching of C-O bonds which could have originated from epoxy groups. These IR results fully support the existence of various oxygen-based functional groups on the surface of GQDs, which is fully consistent with XPS data. Furthermore, the identification of the presence of various functional groups on GQDs was consistent with their excellent solubility properties in many solvents such as water, ethanol, methanol, acetone, and dimethylformamide. For instance, GQD solubility in water is \(\sim\)2 mg.mL\({}^{-1}\), while it is \(\sim\)500 mg\(\cdot\)mL\({}^{-1}\) in pure ethanol. The size of the final graphene quantum dots was determined by DLS from aqueous suspensions (Figures 4a and S3). The results indicated that all GQDs displayed a narrow size distribution centered at approximately 1.6-1.7 nm. This highlights the success of our protocol to form nanoparticles of identical size and chemical composition regardless of the waste source investigated. Also, these observations attest to the high dispersibility of our GQDs in solution. The high-resolution transmission electron microscopy (HR-TEM) images (Figures 4b,c and S4) totally corroborate the observations made by DLS, depicting individual nanoparticles of less than 2 nm. Figure 3: (a) XPS survey of A2 GQDs (originating from orange peel). (b) Deconvoluted C1s spectrum of A2 GQDs.