## 4 Conclusions  
In summary, we have developed a straightforward and reproducible method to prepare graphene quantum dots (GQDs) from biomass waste of different natures by a two-step microwave-assisted synthesis. Preparations were carried out at laboratory scale and could be easily scaled-up, as we proved within our recent patent [36]. To date, we have been delighted to observe that, in spite of the nature of the waste investigated, the resulting GQDs displayed similar properties, regarding their chemical composition and their size and morphology as well as their photophysical properties. DLS and TEM analyses showed that the as-prepared GQDs were monodispersed with similar size distribution in the range of 1-2 nm and with crystal lattices similar to those observed in graphite. In terms of chemical composition, XPS and IR analyses allowed us to identify the presence of some functional groups such as carboxylic acid, and hydroxyl, ether, nitro, and sulfoxide groups that were connected to the graphene-like structure backbone. Regarding the photophysical properties, the nanoparticles exhibited excitation-dependent photoluminescence that was very likely the result of defect emission, i.e., the recombination of an excited electron in the various surface states with a hole in the valence band. Interestingly, our nanoparticles displayed a white-light-emitting feature under UV-light excitation in the solid state. This is a noteworthy point since most CDs and organic molecules are not emissive in the solid state due to multiple FRET occurring between close objects. Furthermore, aggregation-induced luminescence quenching of carbon-based nanomaterials is the main obstacle for their applications, such as in optoelectronic devices and sensors. These encouraging results, combined with the ease of synthesis, make our cheap and easily accessible GQDs very promising nanomaterials as a new phosphor in white-light-emitting diodes and full-color display panels. In this direction, work is currently underway to fabricate GQD-based films and evaluate their electroluminescence features. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14198807/s1](https://www.mdpi.com/article/10.3390/app14198807/s1)]([https://www.mdpi.com/article/10.3390/app14198807/s1](https://www.mdpi.com/article/10.3390/app14198807/s1)). Figure S1. (a) XPS survey scan of the GQDs B\({}_{2}\). (b) ATR-FTIR spectra of GQDs B\({}_{2}\). Figure S2. (a) XPS survey scan of the GQDs C\({}_{2}\). (b) ATR-FTIR spectra of GQDs C\({}_{2}\). Figure S3. DLS size distribution (by number) of (a) B\({}_{2}\) and (b) C\({}_{2}\) dispersed GQDs water suspension at a concentration of 1 mg/mL. Figure S4. HR-TEM images of the as-prepared GQDs (a) B\({}_{2}\) and (b) C\({}_{2}\). Figure S5. Photophysical properties of water-dispersed GQDs B\({}_{2}\). (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 S6. Photophysical properties of water-dispersed GQDs C\({}_{2}\). (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 S7. White light emission obtained from GQDs films deposited on a silicon wafer based on GQDs B\({}_{2}\) and C\({}_{2}\). The spectrum has been truncated at 650 nm for clarity in order to remove the peak corresponding to the second harmonic of excitation. [PERSON] was mainly responsible for writing the manuscript. [PERSON], [PERSON], [PERSON]-[PERSON], [PERSON], [PERSON] and [PERSON] reviewed the manuscript. All authors have read and agreed to the published version of the manuscript. This research was funded by Institut Jean Barriol (Universite de Lorrraine) and SATT Sayens (GREEN-CQDs project). Not applicable. The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.