The synthesis of carbon quantum dots (CQDs) from waste materials has emerged as a strategy for addressing global waste management challenges, achieving synthesis yields reaching approximately 97% while upcycling low-value precursors. This review summarizes recent advancements in the green synthesis of CQDs from a wide range of precursors, including biomass, plastic, and bio-based waste. A primary insight from this review is that the functionality of waste-derived CQDs is directly dictated by the precursor's chemical composition: aromatic-rich waste (e.g., lignin) facilitates core-dominated emission, while polysaccharide-heavy waste (e.g., fruit peels) favors surface-state-mediated photoluminescence. Furthermore, while biomass-derived CQDs often excel in bioimaging and sensing due to intrinsic heteroatoms (N, S, P) and surface functional groups, plastic-derived CQDs offer higher carbon purity and structural uniformity, making them favorable for electrochemical energy storage applications. The paper compares primary synthesis methods, specifically hydrothermal and pyrolysis, based on critical metrics such as quantum yield, which spans from 5% to over 60% in waste-derived systems, and particle size distributions typically maintained within the 1–10 nm range. The reviewed literature demonstrates that waste-derived CQDs can achieve performance metrics comparable to traditional semiconductor quantum dots, exhibiting fluorescent sensing limits at the picomolar level (∼500 pM), > 99% dye degradation efficiency, and over 280% enhancement in composite tensile strength. Moreover, these waste-derived dots are achieving quantum yields (QY) upwards of 60%, fully comparable to, and in some cases exceeding, those achieved using traditional chemical precursors like citric acid. Key innovations discussed include microwave-assisted synthesis for reducing reaction times to under 60 s and the strategic doping of heteroatoms, primarily Nitrogen (N), Sulfur (S), and Phosphorus (P), to tune optoelectronic properties, such as shifting emission to the red region for bioimaging or increasing quantum yield via mid-gap state engineering. Despite this progress, the review identifies specific technical barriers to industrial maturity, notably the batch-to-batch variability stemming from the intrinsic heterogeneity of waste feedstocks, feedstock variability, and the lack of standardized, high-throughput purification protocols. Based on the surveyed literature, high-sensitivity fluorescent sensing and environmental remediation (photocatalysis) show the highest immediate commercial promise due to lower purity requirements compared to energy storage or biomedical applications. Future research must prioritize high-throughput reproducibility and life-cycle cost analysis to transition from laboratory success to a viable circular economy. • Waste-to-CQD synthesis yields reach approximately 97% from various precursors. • Upcycling biomass and plastics produces CQDs with quantum yields exceeding 60%. • Fluorescent sensors achieve picomolar-level detection limits for diverse analytes. • Waste-derived CQDs provide over 99% efficiency in photocatalytic dye degradation. • Composite tensile strength improves by over 280% through CQD incorporation.
Mehmet Melikoglu (Sun,) studied this question.