## 3 Results and Discussion
(c) Deconvoluted N1s spectrum of A2 GQDs. (d) FT-IR spectrum of A2 GQDs. It is noteworthy that nanoparticles had a tendency to aggregate when deposited on the holey carbon-based TEM grids (Figure 4a) which is expected for planar aromatic fragments due to strong van der Waals attractions between graphene-like sheets. Also, GQD TEM images show clear hexagonal honeycomb networks, as expected for graphene quantum dots [42]. Also, the high-resolution TEM (HR-TEM) image (Figure 4b) exhibits crystal structures. Detectable lattices in the selected-area electron-diffraction (SAED, inset of Figure 4b) pattern revealed the crystalline structure of GQDs. Well-resolved lattice fringes with an interplanar spacing of 0.210 nm were observed, which is close to the (101) facet of carbon graphite. The synthesis of graphene quantum dots from biomass wastes has emerged as a credible alternative feedstock owing to their renewable, cheap, and green characteristics, particularly in comparison with glucose and citric acid, which that have been extensively used as precursors for GQD synthesis [43; 44]. In that context, many biomass sources such as rice husks [34], plant leaves [45], charcoal [46], honey [47], coffee [48], and lignin [49] have been used to produce GQDs. In comparison with this investigation, Table 2 depicts the synthetic routes, purification procedures, yields, and range of sizes reported in recent and relevant publications. Among the various synthetic routes reported to produce GQDs from biomass precursors, the procedure we developed is extremely rapid owing to the use of microwave heating which allow each step to proceed in five minutes, while other procedures need some hours to proceed. Considering the purification step, avoiding the use of dialysis is a substantial advantage since that purification technique is expensive and very time-consuming. Also, it is worth considering that the GQDs we produced were of smaller size than most of those reported in the literature. Finally, the weight percentage yield was in the same range as other reported yields. Figure 4: Size characterization of A2 GQDs (originating from orange peel) determined by (**a,b**) HR-TEM and (**c**) DLS with analysis by number. The micrograph (**a**) depicts dispersed GDQs on the carbon support. An individual graphene quantum dot of \(-2\) nm is depicted in (**b**) with inset depicting the diffraction and fringe pattern. The optical properties of the GQDs prepared from different waste sources were further examined in detail. The UV-vis absorption spectrum of A\({}_{2}\) (Figure 5a) was identical to those of B\({}_{2}\) and C\({}_{2}\) (Figures S5a and S6a, respectively). The light-yellow solution showed two absorption peaks in the UV range, including a small and broad one centered at 350 nm ascribed to n - \(\pi^{\bullet}\) transitions in C=O and a peak at 200 nm which was related to \(\pi\) electron transition from \(\pi\) to \(\pi^{\bullet}\) of C=C bonds in the aromatic domains of the graphitic structure. Steady-state fluorescence spectroscopy was performed with diluted water solutions of A\({}_{2}\)-C\({}_{2}\) in order to get insight into the photoluminescence features of the prepared nanoparticles. All samples displayed typical excitation-dependent photoluminescence behavior using different excitation wavelengths ranging from 300 nm to 500 nm (Figures 5b, S5b and S6b). With the increase in the excitation wavelength, GQDs displayed emission peaks, shifting to longer wavelengths up to 600 nm when excited at 500 nm. Although the excitation-dependent PL mechanism is still a controversial topic [50], this tunable emission is of great interest for applications in various domains. The origin of this excitation-dependent photoluminescence is very likely the result of defect emission, i.e., the recombination of an excited electron in the various surface states with holes in the valence band [7]. Indeed, we could eliminate the quantum-confinement effect according to the narrow size distribution of the as-prepared nanoparticles.