Lignin-derived carbon quantum dot hybrid systems for integrated heavy metal sensing and removal

Heavy-metal (HM) contamination of water resources remains a persistent environmental and public-health challenge, demanding materials that can combine effective removal with reliable detection. In this context, lignin-derived carbon quantum dots (CQDs) have emerged as promising sustainable nanomaterials due to their abundant surface functional groups, tunable optical properties, and strong affinity toward metal ions. Lignin, an underutilized by-product of the pulp, paper, and biorefinery industries, offers a renewable and low-cost precursor for green CQD synthesis, aligning well with circular-economy principles. Therefore, this review critically examines recent advances in the development of hybrid membranes incorporating lignin-based CQDs for simultaneous HM sensing and removal. Further, current review discusses lignin sources, extraction routes, and environmentally benign synthesis strategies for CQDs, with emphasis on how precursor chemistry influences optical behavior and metal-binding performance. Various membrane fabrication approaches, including polymeric and biopolymer-based mixed-matrix systems, are covered in terms of CQD dispersion, mechanical stability, and functional efficiency. Particular attention is given to fluorescence-based sensing mechanisms for toxic metal ions such as lead (Pb 2+ ), cadmium (Cd 2+ ), mercury (Hg 2+ ), and chromium (Cr 6+ ), alongside adsorption-driven removal pathways. Current challenges related to material variability, long-term stability, and real-water applicability are highlighted. Finally, future research directions are proposed to advance hybrid, intelligent membrane systems for sustainable water and wastewater treatment and monitoring.