Preparation Of Xylan Based Composites And Their Applications

Xylan is the main type of hemicelluloses,which is always used as the mold of hemicelluloses.It possesses the character of wide source,low price,renewability,biodegradability and good biocompatibility.To date,the researches about xylan are concentrated on preparation of film,hydrogel,but the xylan is branched heteropolysaccharide with low molecular weight and complex structure which is composited of various sugar unit,the film and hydrogel prepared by xylan are poor in mechanical properties,thus the advantages of xylan have not been fully exploited.Compared with cellulose and lignin,xylan is easy to dissolve,has high accessibility,and high yield of carbon materials after hydrothermal reaction.Therefore,in the ways of direct application,modification and carbonization of xylan,it was tried to make full use of these properties of xylan.Blended with inorganic materials,xylan composite materials and its derivatives with enhanced or new functions were synthesized,which were applied as retention and drainage-aid agent,SERS substrate,electrode of supercapacitor or fluorescence probe.These researches would broaden the application field of xylan and realize the high value utilization of xylan.The main contents are as follows:1?Xylan passivated GQDs for the detection of Fe³⁺GQDs were prepared by graphite in N-methyl pyrrolidone with the assistance of Na OH by ultrasound.The GQDs contained rare oxygen functional groups and surface defect,which was close to the structure of pristine graphene.However,the solubility of GQDs in water is low and GQDs were easy to aggregate and form white precipitate.After the passivation,the xylan passivated GQDs possessed a quantum yield of 36.63%and a fluorescence life of 7.47 nm,and their solubility in water were improved.The fluorescence intensity of GQDs@xylan was not sensitive to the p H and maintained stable in p H=6?10.As a fluorescence probe,GQDs@xylan could selectively detect Fe³⁺based on inner filter effect with a line detection range of 0?75 u M and the limit of detection was 92.8 n M.This work took full advantage of the macromolecular chain structure of xylan and used xylan to passivated the surface of graphene quantum dots for the first time,xylan/graphene quantum dots composite was prepared in this way,which could be an efficient fluorescent probe for selective trace detection of Fe³⁺.2.Green synthesis of Au-Ag bimetal nanoparticles by xylan and their SERS performanceWith xylan as reducing and stabilizing agent,chloroauric acid as the precursor of Au,Tollens reagent as the precursor of Ag,Au@Ag core-shell and Au-Ag hollow alloy were green prepared.The employ of xylan in preparation process refrain from toxic chemical agents and simplified the preparing procedure.The xylan capped on the surface of nanoparticles not only stabilized them to disperse in aqueous solution without aggregation,but also improved its antioxidant activity.When,Au@Ag core-shell nanoparticles were applied as Raman enhanced substrate,0.126 ppm Sudan?could be detected.Therefore,by making use of the reducing end group of xylan,this work not only offers a new method for high-value utilization of xylan but also presents a simple,green,and ultrasensitive surface detection technique for the assessment of food/environmental safety3.The preparation of xylan-g-quaternized chitosan/montmorillonite and its performance as retention and drainage-aid agentIn order to combine the advantages of xylan,QCS and MMT,xylan-g-quaternized chitosan was prepared by click-chemistry reaction and intercalation reaction,which was applied in papermaking industry as retention and drainage-aid agent.Firstly,quaternized chitosan was intercalated into the layer space of montmorillonite to enlarge the layer distance of MMT.Next,the molecular chain of xylan and quaternized chitosan was linked through click chemistry in the gallery of montmorillonite,in this process,the layer distance was furtherly enlarged and even exfoliated.Compared with the three kinds of raw materials,the retention and drainage-aid performance of xylan-g-quaternized chitosan nanocomposites was greatly improved.The maximum flocculation efficiency towards Ca CO₃ was 37.41%,and when the dosage was 0.01mg/g,beating degree was the lowest.Furthermore,the charge accumulation of quaternized chitosan in closed system was released by introducing negative charged xylan.In this work,the grafting modification of xylan was carried out by click chemistry.With the advantages of chitosan and montmorillonite,the retention and drainage properties of xylan were enhanced,and a new papermaking aid was developed.4.The preparation of xylan derivate carbon spheres/carbon dots/graphene flexible supercapacitor and its electrochemical performanceDissolved by Na OH/urea,xylan was carbonized to synthesize carbon spheres by hydrothermal reaction,followed by activated by KOH.The activated carbon spheres and ascorbic acid were added to graphene oxide solution.After filtration,activated carbon sphere/graphene oxide-ascorbic acid composite film was prepared,followed by reducing with L-ascorbic acid.Though engaging carbon sphere as the nanogap of graphene nanosheet,the charge transfer rate between sheets of graphene increased and the ascorbic acid in the composite film could in-suit reduce oxide graphene.As the electrode of supercapacitor,the composite film had an areal capacity of 755 m F/cm²at 1 A·g^-1in two electrode system,the power density was from 22.5 m W/cm²to 2250 m W/cm²,the energy density was from 11.88 m W/cm²to 25.2m W/cm²and the capacity remained 108.7%under 10000 times of circulation.Due to the high hydrothermal carbon yield of xylan,this work prepared xylan carbon sphere,and combined with graphene to prepare electrode materials for supercapacitor,which broadened the application of xylan.5.Xylan self-passivated single-layed graphene quantum dots for the detection of Cr(?)in a microfluidic chipSingle-layered graphene quantum dots(GQDs)are commonly prepared through bottom-up strategy by aromatic molecules or other carbon precursors with complicated procedure.Herein,single-layered GQDs were first fabricated by nonaromatic xylan only with the assistance of Na OH/urea under hydrothermal condition.In this route,xylan completely dissolve and form complex with Na OH/urea.And when the complex is carbonized under hydrothermal condition,ammonia and carbon oxide are released by decomposition of urea,which facilitates the generation of single-layered GQDs and hinders them from interaction and aggregation.The synthesized GQDs are doped with 1.38%nitrogen element and show a fluorescent quantum yield of 23.8%,which are self-passivated with incompletely carbonized xylan.Developed for detection of Cr(VI)as a fluorescence probe,the GQDs present good selectivity and sensitivity.The passivated layer avoids the interference caused by other ions in water,meanwhile,it only can be destroyed by strong oxidants like those containing Cr(VI).The detecting range is from15?M to 150?M and limit of detection is only 0.43?M.By embedding the GQDs into hydrogel and then integrating into microfluidic chip,a visual detection of Cr(VI)with portable and cost-effective equipment is realized.In this work,xylan was transformed into n-doped single-layer graphene quantum dots by hydrothermal reaction with the assistance of Na OH/urea,providing a new way to prepare single-layer graphene quantum dots from non-aromatic molecules and demonstrating a simple method to monitor water environment.