Green synthesis of carbon quantum dots from wasted enzymatic hydrolysis lignin catalyzed by organic acids for UV shielding and antioxidant fluorescent flexible film

Biomass wastes have been demonstrated to be sufficient sources for the synthesis of carbon quantum dots (CQDs) using green and sustainable hydrothermal technology, and has been applied in many fields. However, the yield of CQDs from biomass directly hydrothermal is extremely low. This study aimed at prove the feasibility of conducting blue emissive CQD by a hydrothermal technology using enzymatic hydrolysis lignin (EHL) from biomass waste as the precursor and three organic acids such as citric acid (CA), DL-malic acid (DLMA) and oxalic acid (OA) for modification. HR-TEM analysis revealed that CQDs modified by organic acids had superior water solubility and smaller particle size than that of CQDs without organic acids (EHL CQDs). In particular, the citric acid-modified CQDs (CA-EHL CQDs) resulted in an increase in the fluorescence intensity and effective quantum yield (QY) up to 500 % and 4.5 %, respectively, displaying a narrower size distribution and a mean particle size of ca. 3.1 nm. In addition, the CQDs/PVA fluorescent nanocomposite films with UV shielding and antioxidant capacity were also developed in this work, which proved great application prospect in information protection, anti-counterfeiting materials, food packaging and other fields. These results are great contributions towards adopting low-cost, sustainable and green process to produce CQDs with high quantum efficiency and expanding the application range of lignin based CQDs.