Mild Acidolysis-Assisted Hydrothermal Carbonization of Lignin for Simultaneous Preparation of Green and Blue Fluorescent Carbon Quantum Dots

Biomass waste is a renewable, cost-effective, and eco-friendly feedstock for carbon quantum dot (CQD) preparation, but its high-efficiency utilization is still a challenge. Here, we report a mild acidolysis-assisted hydrothermal strategy toward the simultaneous preparation of green and blue fluorescent CQDs from lignin. The synthesis pathway includes first the acidolysis of lignin and then hydrothermal carbonization of its supernatant and solid residue, separately. The fluorescence mechanism of the two CQDs is elucidated through the investigation of their structure, chemical composition, and optical properties combined with quantum chemistry calculations. Their formation mechanism is also explored; nuclear magnetic resonance (NMR) data demonstrate that green CQDs are derived from the side-chain scission of lignin by the bottom-up method, while blue CQDs inherit the aromatic bulk of lignin via the top-down method. The optimal yield of two CQDs can be achieved as 30.6% for green CQDs and 15.2% for blue CQDs by adjusting the acidolysis temperatures from 30 to 110 °C. This comprehensive strategy promotes the high-efficiency conversion of biomass into CQDs and provides a theoretical basis for exploring the fluorescence mechanism and formation mechanism of CQDs.