Construction Mechanism And Application Of Plant Cellulose/carbon Quantum Dots Functional Composites

Cellulose is the most abundant and renewable biomass resource in nature.It possesses the advantages of rich source,degradability and simple preparation,which has attracted increasing attention in the field of replacing traditional petroleum compound materials.Due to the complexity of preparation and action mechanism of cellulose matrix composites,limite its high-value applications,the application fields are mainly concentrated in conventional areas of pulping and papermaking,biomass refining,etc.Carbon quantum dots（CQDs）are a kind of carbon nanomaterials with the size less than 10 nm.Besides the nano-effects,the excellent physical and chemical properties of good biocompatibility,low toxicity,easy modification,etc.also enable CQDs to show broad application prospects in many fields,such as anti-counterfeiting,sensing,photocatalysis,biological imaging,etc.However,the aggregation-quenching-effect leads to the luminescence quenching of CQDs in solid state,which limits its application range.There are lots of high-reactive sites on the surface of plant cellulose,which can provide sufficient loading space for CQDs,which makes the cellulose/CQDs functional composites derived from the skeleton of cellulose have great development potential and application prospects.Given this,plant cellulose/CQDs functional composites were prepared by the method of physical adsorption and chemical bonding,taking plant cellulose and CQDs as the skeleton and functional additives,respectively.The high-value utilization and mechanism of plant cellulose/CQDs materials are investigated,which provide a scientific basis and research ideas for the in-depth development and comprehensive utilization of plant cellulose/CQDs functional complex.Mainly include the following aspects:（1）The softwood pulp fibers were used as the raw materials,nanofibrillated cellulose（NFC）was prepared by two-stage enzymatic hydrolysis coupling with mechanical grinding treatment.NFC was then treated by periodate to obtain NFC with dialdehyde structure（DANFC）.Using citric acid as a carbon source,rare-earth-doped CQDs（Yb/Er-CQDs）were obtained by doping citric acid with modifiers including ethylenediamine,ytterbium chloride（YbCl₃）and Erbium Chloride（ErCl₃）.The cellulose based Yb/Er-CQDs composite fluorescent material（Yb/Er-CQDs-DANFC）was successfully prepared by reductive amination reaction between DANFC and Yb/Er-CQDs,where the surfaces contain-CHO and-NH₂,respectively.The results show that Yb/Er-CQDs-DANFC emits blue light（450 nm）and green light（550 nm）under excitation of 370 nm ultraviolet light and 980 nm near-infrared light,respectively,showing excellent photoluminescence（PL）and upper conversion photoluminescence（UCPL）.After Yb/Er-CQDs-DANFC was added to water-based ink,its shear-thinning performance and good network structure forming ability in the shearing process improved the thixotropy of the ink,which exhibited good fluorescence anti-counterfeiting performance on different substrates.（2）Using softwood pulp fibers as the raw materials,the carboxylated nanofibrillated cellulose（TNFC）was obtained by the TEMPO oxidation system coupling with ultraparticle grinding technology.Using citric acid and urea as the carbon source and modifier,respectively,four fluorescent CQDs（blue,green,yellow,and red emissions）were fabricated by adjusting the ratio and the reaction temperature of citric acid and urea.The luminescence mechanism of CQDs was studied by density functional theory（DFT）.It was confirmed that the tunable luminescence of CQDs could be achieved by controlling the size of CQDs and the degree of surface oxidation.Subsequently,using TNFC as a sustainable precursor and skeleton,TNFC/CQDs composite materials with four solid-state luminescence properties were prepared by the amidation reaction between TNFC and CQDs,which effectively solved the phenomenon of solid-state quenching.Light-emitting diodes（LEDs）were designed by coating TNFC/CQDs on the inner wall of the lampshade,resulting the pure white LEDs（WLEDs）with chromaticity coordinates（0.34,0.35）and a color rendering index of 89.4.The energy transfer process inside the composites was explained by the fluorescence resonance energy transfer mechanism（FRET）,and the white light emission mechanism of WLEDs was also further revealed.（3）Using softwood pulp as raw material,NFC was prepared by mechanical method.Then,cellulose nanocrystal（CNC）was pretreated by hydrothermal carbonization under alkaline conditions and refined using the H₂O₂/NaOH oxidation system to obtain CQDs.The results showed that the CQDs have excellent PL luminescence and UCPL luminescence properties.Using commercial titanium dioxide P₂₅ as the raw material,the titanic acid nanowires were prepared by hydrothermal reaction under alkaline conditions and then subjected to high-temperature carbonization to obtain Titanium dioxide nanobelts（Ti O₂-NBs）,which were 100-130 nm in width,15-20 nm in thickness,and 5-10μm in length.Under visible light,the catalytic degradation yield and degradation rate of Ti O₂-NBs for sulfadiazine（SDZ）were 1.3 and 1.8 times than that of P₂₅,respectively.Using NFC as the framework,dopamine（PDA）as the binder,CQDs composited Ti O₂-NBs nanomaterials（Ti O₂-NBs-CQDs）as the primary catalyst,the PNFC-Ti O₂-NBs-CQDs composite aerogel with a three-dimensional network structure was prepared by the directional freezing technology.Due to its large specific surface area and excellent adsorption performance,the degradation rate of aerogel materials to SDZ has achieved 95.3%within 120 min,which was 1.8 times that of P₂₅,and the degradation rate was 3.8 times that of P₂₅.Density functional theory was used to construct a model to explore the catalytic mechanism of PNFC-Ti O₂-NBs-CQDs,which provided a theoretical basis for the development of cellulose/CQDs functional materials.（4）Using citric acid and urea as carbon source and modifier,respectively,nitrogen-doped CQDs（NCQDs）were obtained,which could emit 447 nm blue light under the excitation of 365 nm ultraviolet light.Using NCQDs and bovine serum albumin（BSA）as the as the reducing agent and stabilizer,respectively,the silver nano clusters（AgNCs）emittng533 nm green light under 365 nm ultraviolet light excitation were prepared.Due to the strong specific binding ability between AgNCs and cysteine（Cys）,NCQDs/AgNCs ratio nanoprobes were prepared by compounding NCQDs and AgNCs according to the mass ratio 5:4,which were used for specific recognition and content detection of Cys.The results show that the optimal p H value and reaction time of the probe were 7.3 and 35 min,respectively.It offers an excellent detection effect on Cys in the response range of 10-140μM,resulting a minimum detection limit（LOD）of 0.52μM.Finally,using cellulose filter paper as the skeleton,the NCQDs/AgNCs ratio nanoprobe test paper was prepared through the"dipping-drying"method,which realized the visual semi-quantitative detection of Cys.