Green synthesis of lignin-based carbon dots via a solvent-free self-foaming strategy

The synthesis of biomass-derived carbon dots (CDs) often suffers from solvent dependency, poor control over reaction pathways, and limited mechanistic understanding of structure formation. Herein, we present a solvent-free self-foaming confined-condensation strategy in which the coordinated thermal reactions of alkali lignin, amino acids, and oxalic acid generate in situ confined microenvironments during thermal treatment. This engineered multiphase system leverages bubble confinement to influence nucleation and carbonization behavior, suppress over‑carbonization, and promote effective nitrogen incorporation from basic amino acids. The process eliminates reaction solvents and avoids pressurized reactors, thereby simplifying the synthetic procedure and reducing solvent-related post-treatment requirements. Rather than claiming a fundamentally new solvent-free concept, this work highlights a self-foaming confined-condensation strategy that integrates low-cost lignin, readily available amino acids, and oxalic acid to regulate lignin carbonization under ambient pressure. The resulting OKLCDs function as a sensitive and selective fluorescent probe for Cu²⁺ detection, achieving a detection limit of 1.35 μM and recoveries exceeding 92% in real water samples. This work provides a practical solvent-free confinement-regulation approach for lignin valorization and the preparation of functional carbon nanomaterials.