Study On Lignin-based Carbon Quantum Dots Fluorescent Packaging Material

Carbon quantum dots（CQDs）,an emerging"zero-dimensional"carbon material,can be a promising alternative due to its good water solubility,photostability,tunable photoluminescence,and low cytotoxicity.It has received widespread attention.However,CQDs obtained from biomass have poor optical properties,which limit the application of CQDs.In order to realize the low-cost preparation of CQDs with excellent optical properties.In this paper,lignin-based CQDs with excellent optical properties were obtained through process optimization and organic acid catalysis by using waste enzymatically hydrolyzed lignin as carbon source,and the functional application of lignin-based CQDs was effectively attempted.The effects of hydrothermal conditions,types of organic acids and the concentration of organic acids on the surface morphology,structure and optical properties of CQDs were studied by transmission electron microscopy,Fourier transform infrared spectroscopy,scanning electron microscopy,X-ray diffraction,X-ray photoelectron spectroscopy,UV-vis spectrophotometer and fluorescence spectrophotometer.The main conclusions are as follows:（1）In this paper,lignin-based CQDs emitting blue fluorescence were synthesized by a green and convenient one-step hydrothermal technology using waste EHL as carbon precursor.Single factor variable method was used to study the effects of EHL（carbon source）concentration,hydrothermal temperature and hydrothermal time on the fluorescence intensity of CQDs.The optimal synthesis process of CQDs was determined:the concentration of carbon source was 50 mg/m L,the hydrothermal temperature was220℃,and the hydrothermal time was 12 h.The morphology,particle size,structural composition of CQDs,as well as the fluorescence changes at different p H values and the specific selectivity to Fe³⁺were studied.The results showed that the particle size of lignin-based CQDs is about 7 nm,the size distribution is uniform,and there is no obvious agglomeration.The surface of CQDs is rich in oxygen-containing functional groups hydroxyl and carboxyl groups,and has good water solubility and photoluminescence properties.CQDs have certain p H-responsive properties and selectivity for Fe³⁺.Lignin-based CQDs can be used as fluorescent probes and fluorescent sensors,providing ideas for a wide range of applications in food packaging detection and other fields.（2）On the basis of the first chapter,citric acid（CA）,DL-malic acid（DLMA）and oxalicacid（OA）were added to catalyze lignin,respectively.HR-TEM analysis showed that the organic acid-modified CQDs had superior water solubility and smaller particle size than CQDs without organic acid（EHL CQDs）.In particular,citric acid-modified CQDs（CA-EHL CQDs）,with an average particle size of 3.1 nm,had the smallest particle size and the most uniform size distribution.The fluorescence intensity of CA-EHL CQDs increased to 500%and the fluorescence quantum yield（QY）increased from 0.8%to 4.5%.In addition,this study developed a CQDs/PVA nanocomposite with blue fluorescence,UV shielding and oxidation resistance,which has a broad application prospect in information protection,anti-counterfeiting materials,food packaging and other fields.（3）On the basis of the previous two chapters,the effects of citric acid addition level on CQDs fluorescence intensity and quantum yield were further studied to find the optimal addition level of citric acid.The results showed that the 0.8CA-EHL CQDs had excellent fluorescence properties when the mass ratio of EHL（carbon source）and citric acid was 5:4.Compared with EHL CQDs,0.8CA-EHL CQDs with an average particle size of 2.4 nm,had the smallest particle size and narrow size distribution.The fluorescence intensity was increased to 610%and the fluorescence quantum yield was increased from 0.8%to 5%.In addition,the 0.8CA-EHL CQDs/PVA fluorescent nanocomposites also had excellent visibility,UV shielding properties,and antioxidant properties,and could be used in information protection,anti-counterfeiting materials,food packaging and other fields.