Green and gram-scale synthesis of uniform graphene quantum dots from biomass waste: A highly selective probe for nanomolar Hg²⁺ sensing

Graphene quantum dots (GQDs) have attracted considerable attention in the scientific community due to their unique quantum confinement effects, tunable bandgap, and size-dependent optoelectronic properties. However, fundamental challenges such as low product yield, limited scalability, poor quality, and high cost remain major bottlenecks to their commercialization. In this study, we introduce a novel, green, gram-scale, cost-effective, and renewable approach to produce size-controlled GQDs using biomass waste as a green precursor and only water as a green solvent. The results demonstrate a product yield of over 1g of high-quality uniform GQDs achieved in a single-batch single-step sustainable process—representing a significant advancement in GQDs synthesis. Notably, these GQDs exhibit a narrow size distribution with an average size of 1.6 ± 0.4 nm and showcase remarkable optical properties, including a high quantum yield of up to 22 %. More importantly, these fluorescent GQDs are employed to design a selective and sensitive optical sensor for detecting mercury ions (Hg²⁺), addressing the growing concern of environmental contamination and severe negative health effects associated with mercury exposure. The sensor shows exceptional selectivity and a detection limit as low as 4 ± 0.05 nM. This work simultaneously addresses the issues of yield, cost, quality, and sustainability, thereby opening avenues for the practical application of GQDs in various fields.