Green Synthesis of Tunable Fluorescent Carbon Quantum Dots from Lignin and Their Application in Anti-Counterfeit Printing

Lignin converted to carbon quantum dots (CQDs) attracts tremendous attention for large-scale production of carbon nanomaterials and valueadded disposal of biomass wastes (such as the black liquor from pulping industry and the residue from hydrolysis of biomass). The green synthesis of lignin-derived CQDs is reported via a facile two-step method with the adjustment of acid additives containing N or S. The resulting series of CQDs exhibit bright fluorescence in gradient colors from blue to yellowish green, among which the N, S co-doped CQDs with the addition of 2,4-diaminobenzene sulfonic acid show an optimal fluorescence quantum yield (QY) of 30.5%. The red-shift photoluminescence emission behaviors of these CQDs can be attributed to the increased graphitization degree and reduced optical energy band gaps (2.47 -> 2.17 eV) with regard to the incorporation of various heteroatoms. The improved fluorescence QYs are consistent with the variation trend of the increased N/C content in the CQDs. The yellowish greenemissive CQDs with bright fluorescence, strong water solubility, and excellent chemical stability perform well in anti-counterfeiting printing. The promising and sustainable approach for the synthesis of tunable fluorescent CQDs exhibits the value-added utilization of lignin for the fluorescence ink production.

Automated summary

The green synthesis of lignin-derived CQDs with N, S co-doping shows optimal fluorescence quantum yield of 30.5%. The bright fluorescent CQDs with yellowish green emission, strong water solubility, and excellent chemical stability have potential applications in anti-counterfeiting printing. The improved fluorescence quantum yields are consistent with the increased N/C content in the CQDs.