A Roadmap of Lignin-Derived Carbon Dots: A Systematic Review of Synthesis Parameters to Obtain Desired Optical Properties

As a relatively new member of the nanocarbon family, carbon dots (CDs) have attracted significant scientific interest over the past two decades due to their unique attributes, including tunable luminescence, facile functionalization, and excellent biocompatibility. The synthesis of CDs from biomass has emerged as an appealing strategy, not only for its sustainability but also for the inherent heteroatom doping and surface functionalities imparted by the precursor materials. However, raw biomass is often considered chemically impure and highly variable, which can limit reproducibility and control over the physicochemical properties of resulting CDs. Refinement of biomass into well-defined constituent molecules, such as lignin, which is rich in aromatic structures, offers a promising pathway toward the production of high-quality CDs. For successful commercial translation of CDs in areas such as device fabrication, sensing, and biomedical applications, reproducible and well-defined synthetic protocols that enable precise control over CD properties are essential. With this objective, the present review summarizes lignin-derived CD synthesis methodologies and systematically examines their influence on optical properties, considering their diverse origins and processing methods. This review aims to serve as a practical roadmap for researchers, facilitating the rational selection of biomass sources and synthesis strategies for designing high-performance CD-based materials with tailored properties while maintaining a strong emphasis on sustainability.