Xylan is the main type of hemicellulose, which is often used as a model material of hemicellulose. It has the advantages of wide source, low price, renewable, biodegradable, good biocompatibility and so on. At present, the research of xylan based materials mainly focuses on the preparation of membranes, hydrogels and other fields, but xylans are heteropolysaccharides, with complex molecular structure, diverse sugar units, short molecular chains, and multi branching. The prepared membranes and hydrogels have poor mechanical properties, and their advantages have not been brought into full play. Compared with cellulose and lignin, the three major components of plants, xylan molecules have the advantages of easy dissolution, high accessibility and high hydrothermal carbon yield.
This paper makes full use of these advantages of xylan. On the one hand, xylan and inorganic nano materials are compounded at the molecular level, and a variety of new xylan / inorganic nano composites are developed; On the other hand, through hydrothermal carbonization, xylan carbon spheres and graphene quantum dots were efficiently prepared and applied to retention and drainage aids, Raman enhancement, supercapacitors, and ion detection respectively, which broadened the application fields of xylan and realized the high-value utilization of xylan.
The main research contents are as follows:
1. Preparation of xylan passivated graphene quantum dots and its selective trace detection of fe3+:
Graphene quantum dots (gqds) were prepared by ultrasound with graphite as raw material, N-methyl pyrrolidone as solvent and sodium hydroxide as auxiliary reagent. The gqds contain oxygen functional groups and have less surface defects, which are closer to the structure of original graphene, but gqds have low solubility in water and are easy to agglomerate into white precipitates.
In this study, xylan was used to passivate its surface to obtain xylan passivated graphene quantum dots（ GQDs@xylan )。 Compared with before passivation, GQDs@xylan The stability in water is improved, the quantum yield is also increased from 19.12% to 36.63%, and the fluorescence lifetime is increased to 7.47ns. GQDs@xylan The fluorescence intensity of is less affected by pH, and can be kept basically unchanged in the range of ph=6 ~ 10. As a fluorescent probe, the selective detection of fe3+ in solution is realized based on the internal filtering effect, and the linear range of detection is 0 ~ 75 μ M. The detection limit is 92.8nm.
This study makes full use of the characteristics that multi branched xylans are easy to form dense polymer shells. For the first time, xylans are used for the surface passivation modification of graphene quantum dots, xylan / graphene quantum dot composites are prepared, and an efficient fluorescent probe for selective trace detection of fe3+ is obtained.
2. Preparation of gold silver bimetallic nanoparticles by green reduction of xylan and its Raman enhancement effect:
Using xylan as reductant and stabilizer, chloroauric acid as gold precursor and torun reagent as silver precursor, core-shell structures with different shell thickness were prepared in green Au@Ag And Au Ag hollow alloy. The addition of xylan avoids the use of toxic chemical reagents and simplifies the synthesis process. Xylan wrapped on the surface of nanoparticles not only stabilizes nanoparticles and makes them evenly dispersed in aqueous solution, but also improves its ability to resist H2O2 oxidation and corrosion, forms hot spots between interconnected nanoparticles, and enhances the surface Raman enhancement performance of nanoparticles. The core-shell with more uniform shape was prepared by optimizing the amount of xylan Au@Ag Nanoparticles, while avoiding the generation of nano silver clusters. Compared with Au Ag alloy, pure Au and pure Ag nanoparticles, xylan coated Au@Ag The Raman signal of 4-mercaptobenzoic acid has a stronger enhancement effect, and the detection limit reaches 1nm. In addition, xylan wrapped Au@Ag Nanoparticles can detect Sudan I, a food pollutant, with a detection limit as low as 0.126ppm.
Using the reductive end groups and macromolecular chain structure of xylan molecular chain, this study explored the method of preparing gold silver bimetallic nanoparticles with xylan as a green reductant and stabilizer, and provided a simple, green and ultra sensitive surface detection technology for food / environmental safety assessment.
3. Study on Synthesis and performance of xylan-g-chitosan quaternary ammonium salt / montmorillonite retention and drainage aid:
In order to combine the retention and drainage properties of xylan, chitosan quaternary ammonium salt and montmorillonite, the peeled xylan-g-chitosan quaternary ammonium salt / montmorillonite (xylan-g-qcs) nanocomposites were prepared by click chemical reaction and intercalation reaction as a new retention and drainage aid.
Firstly, chitosan quaternary ammonium salt (QCS) is inserted into the layer space of montmorillonite to expand the layer spacing of montmorillonite, and then the molecular chain of xylan and QCS is linked in the layer space of montmorillonite by clicking chemical reaction. In this process, the layer spacing of montmorillonite is further increased until stripping.
Compared with the three raw materials, the retention and drainage performance of xylan-g-qcs nanocomposites has been greatly improved. The maximum flocculation efficiency of calcium carbonate is 37.41%. When the addition amount is 0.01mg/g, the beating degree is the lowest.
In addition, by linking the negatively charged xylan with the positively charged quaternary ammonium salt of chitosan, the charge accumulation caused by QCS in a closed system is avoided. In this study, xylan was grafted and modified by click chemistry. Using xylan as polyanion electrolyte, it has many branches and is easy to hydration and swelling. At the same time, combined with the advantages of chitosan and montmorillonite, the retention and drainage properties of xylan were enhanced, and a new papermaking additive was developed.
4. Preparation and performance study of xylan carbon ball / graphene supercapacitor:
After xylan was dissolved in sodium hydroxide / urea system, nitrogen doped xylan carbon spheres (XCS) were prepared by hydrothermal carbonization, and then activated xylan carbon spheres (axcs) were obtained by KOH high temperature activation. Then, axcs and ascorbic acid were added to graphene oxide solution at the same time, and activated xylan carbon spheres / graphene oxide film (axcs/go) was obtained by suction filtration, and then ascorbic acid was added again to reduce go. In the process of reduction of the composite membrane, the go in the outer layer will become hydrophobic after being reduced to RGO, preventing the reductant from penetrating into the interior of the membrane. At this time, the ascorbic acid in the membrane can reduce go in situ, and the carbon ball as the connector of graphene lamella increases the charge transfer rate between graphene layers, so the specific capacitance of the axcs/rgo composite membrane is improved. In the double electrode system, the current density is 1A? When g-1, it has a specific capacitance of 755mf/cm2, the power density is 22.5 ~ 2250mw/cm2, and the energy density is 11.88 ~ 25.2mwh/cm2. After 10000 cycles, the capacitance retention rate is 108.7%.
In this study, xylan carbon spheres were prepared by taking advantage of the high yield of xylan hydrothermal carbon, and the electrode materials of supercapacitors were prepared by compounding with graphene, which widened the application field of xylan.
5. Study on the detection of Cr (Ⅵ) in water by xylan self passivation monolayer graphene quantum dots combined with microfluidic control:
Monolayer graphene quantum dots (sgqds) are usually prepared from aromatic molecules or other carbon precursors by bottom-up methods.
In this study, nitrogen self doped graphene quantum dots (n-sgqds) were prepared under hydrothermal conditions for the first time with the aid of naoh/ urea and xylan without benzene ring as the precursor. In this process, xylan is completely dissolved and forms a complex with naoh/ urea. When carbonized in hydrothermal reaction, urea decomposes and releases ammonia and carbon dioxide, which promotes the formation of monolayer graphene quantum dots and hinders their interaction and agglomeration. The prepared graphene quantum dots are doped with 1.38% nitrogen, the quantum yield is 23.8%, the fluorescence lifetime is 5.76ns, and the surface is self passivated by incomplete carbonized xylan, which avoids the agglomeration of quantum dots. The xylan self passivated monolayer graphene quantum dots have good selectivity and sensitivity when used as a fluorescent probe to detect Cr (Ⅵ) in water. The passivation layer avoids the interference of other ions in water, and can only be damaged by strong oxidants such as Cr (VI). The linear detection range of Cr (Ⅵ) is 5 ~ 150 μ M. The detection limit is only 4.1 μ M。 By embedding quantum dots into hydrogels and integrating them into microfluidic chips, the visual detection of Cr (VI) is realized.
In this study, xylose was hydrothermal converted into nitrogen doped single-layer graphene quantum dots. With the assistance of naoh/ urea, a new way to prepare single-layer graphene quantum dots with non aromatic molecules was provided, and a simple and easy visualization method was provided for water environment monitoring.