Sustainable Strategy for Upcycling Organic Waste into Highly Luminescent Carbon Dots

Carbon dots (CDs) emerged as a new class of carbon nanomaterials recognized for their exceptional water solubility, (photo)chemical stability, biocompatibility, and low toxicity. The versatility of fluorescent carbon-based nanomaterials underpins their deployment in light-emitting devices, photocatalysis, bioimaging, and sensing. In these applications, a high fluorescence quantum yield (QY_FL) is critical for achieving optimal performance. Notably, CDs can be produced from a wide variety of carbon sources, where converting organic waste into CDs offers a true circular-economy route. However, waste-based CDs typically exhibit QY_FL values below 20%, which severely limits their practical utility. Thus, a robust and reproducible synthesis procedure is required to deliver consistently high QY_FL, regardless of the waste precursor.

This study aimed to establish a simple upcycling strategy for transforming diverse organic waste into brightly fluorescent CDs via hydrothermal treatment. This procedure considers waste materials and citric acid as carbon precursors and ethylenediamine (EDA) as a nitrogen dopant. Optimization at 200 °C for 8 h using corn stover (CS-CDs_8h) set the core parameters. We then validated the procedure on spent coffee grounds, cork powder, and sawdust, which showed CDs with appreciable QY_FL, reaching up to ~40%. CDs were thoroughly characterized by AFM, XPS, FTIR, XRD, fluorescence, and UV–Vis spectroscopy. It is worth mentioning that these CDs displayed compatibility with human cell lines. Finally, a Life Cycle Assessment (LCA) demonstrated that these waste-based CDs are associated with lower environmental impacts when compared with CDs from commercial reagents. Thus, this study provides an efficient, environmentally responsible framework for upcycling a wide range of organic wastes into high-performance carbon dots without sacrificing fluorescence efficiency.

Acknowledgments

We thank the “Fundação para a Ciência e Tecnologia” (FCT, Portugal) for the following funding: UIDB/00081/2020 (CIQUP), LA/P/0056/2020 (IMS), project 2023.13127.PEX, and 2021.05479.BD (Sónia Fernandes PhD grant).