## 3 Results and Discussion
The multi-photoluminescence colors of the GQDs could be of high interest for multicolored bioimaging and could be modulated through surface functionalization. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Precursors** & & **Synthetic Methods** & **Purification Procedure** & **Size Range (nm)** & **Yield (wt\%)** & **Ref.** \\ \hline \multirow{4}{*}{Rice husks} & - & Tube furnace 700 \({}^{\circ}\)C for 2 h & & & \\  & - & NaOH 700 \({}^{\circ}\)C for 2 h & & & \\  & - & Ultrasonic with H\({}_{2}\)SO\({}_{4}\) & & & \\  & & and HNO\({}_{3}\) for 10 h & & & \\  & - & Solvothermal heating at & & & \\  & 200 \({}^{\circ}\)C for 10 h & & & & \\ \hline \multirow{4}{*}{Plant leaves} & - & Neon leaves pyrozed at & & & \\  & 1000 \({}^{\circ}\)C for 5 h under Ar & & & & \\  & - & Ball-milling for 1 h & & NaOH and filtration & & \\  & - & H\({}_{2}\)SO\({}_{4}\):HNO\({}_{3}\) (3:1 mixture) & & with 0.22 μm & 5–6 & /\({}^{\star}\) & [45] \\  & & at 90 \({}^{\circ}\)C for 5 h & & & & \\ \hline \multirow{4}{*}{Charcoal} & - & H\({}_{2}\)SO\({}_{4}\):HNO\({}_{3}\) (3:1 mixture) & & & & \\  & & sonication for 2 h & & & & \\  & - & 100 \({}^{\circ}\)C for 24 h & & & & \\ \hline \multirow{4}{*}{Honey} & - & Honey + 1-Butanol at 80 \({}^{\circ}\)C & & & & \\  & & for 1 h & & & & \\  & - & Hexadecylamine at 160 \({}^{\circ}\)C & & & & \\ \hline \multirow{4}{*}{Coffee} & - & Hydrazine and & & & & \\  & ultrasonication for 30 min & & & & \\  & - & Solvothermal heating at & & & & \\  & 200 \({}^{\circ}\)C for 10 h & & & & \\ \hline \multirow{4}{*}{Lignin} & - & Ultrasonication for 12 h & & & & \\  & - & Solvothermal heating at & & & & \\  & 180 \({}^{\circ}\)C for 24 h & & & & & \\ \hline \multirow{4}{*}{Orange peel} & - & H\({}_{2}\)SO\({}_{4}\), 180 \({}^{\circ}\)C for & & & & \\  & & 5 **min**, microwave & & & & \\ \cline{1-1}  & - & HNO\({}_{3}\), 150 \({}^{\circ}\)C for & & & & \\ \cline{1-1}  & & 5 **min**, microwave & & & & \\ \hline \hline \end{tabular} \({}^{\star}\) The yield (weight percentage) has not been given.  
\end{table} 
Table 2: Various bioprecursors used to produce GQDs and relevant information. During the course of a current research project dealing with the synthesis of new organic materials for energy applications in solid devices, we were interested in studying the photoluminescence of our GQDs in the solid state. Aggregation-induced luminescence quenching of carbon-based nanomaterials (CDs and GQDs) is the main obstacle for their application in the solid state. Indeed, solid-state fluorescence is highly desirable in numerous applications such as optoelectronic devices and sensors, which generally require photoluminescence materials emitting in the solid state. Some rare recent reports deal with the emission of CD-based films [51; 52; 53]. We were pleased to observe (Figures 6 and S7) that our GQDs deposited on a silicon wafer displayed a white-light emission without the need for dilution inside a solid dispersant, as has been described in recent studies using agarose [51], polyvinyl alcohol (PVA) [52], or silica matrix [53]. Thus, all A2-C2 GQD samples displayed almost identical white-light emission profiles under 325 nm, with a broad peak centered at 550 nm. It is noteworthy that the emission of our GQDs in the solid state was completely different to the emissions observed in solution and the reason for this is still unclear. Figure 5: Photophysical properties of water-dispersed A\({}_{2}\)QDs. (**a**) UV-vis spectrum and (**b**) normalized emission spectra of GQDs recorded for progressively longer excitation wavelengths ranging from 300 to 500 nm (with a 50 nm increment). Figure 6: White-light emission (\(\lambda_{\text{exc. }}=325\) nm) of A\({}_{2}\)-based films deposited on a silicon wafer. The spectrum has been truncated at 650 nm to remove the peak corresponding to the excitation second harmonic.