Preparation,Struc Oxide/Microbial Cellulose Composites Ture and Properties of Cuprous

Author:Hu Ying

Supervisor:ke qin fei


Degree Year:2019





Cellulose is the most productive biomaterials as the main compotent of the plants on the earth.However,plants contain hemicellulose,lignin and other sugars,which leads to low purity of cellul lulose and limits its application.In addition to plants,some microorganisms can also synthesize cellulose,called microbial cellulose which has no hemicellulose,lignin and other sugars.Microbial cellulose membrane is made up of the three-dimensional network structure formed by nanofibers.It is an excellent biomedical material because of its high water holding capacity and good biocompatibility and widely used in food,medical materials and other fields.However,the function of microbial cellulose is single,which can not meet the demand of functional medical materials.Cuprous oxides(Cu2O)has high efficiency,broad spectrum and permanent antimicrobial effect,low toxicity and no pollution,which attracts academia and industry.If Cu2O powders could be uniformly compounded on microbial cellulose,it could not only utilize the nano-characteristics and excellent biocompatibility of microbial cellulose,but also exert the functional properties of Cu2O.However,the particle sizes of Cu2O powders differ greatly from the fiber length of microbial cellulose,which makes it difficult to mix directly and uniformly.At present,there is little research on the uniform mixing of microbial cellulose and nanoparticles.It is necessary to explore a suitable preparation method to make microbial cellulose and Cu2O particles uniformly dispersed.In this paper,the methods of homogeneous combination of Cu2O and microbial cellulose were found:in-situ synthesis and electrospinning.In-situ synthesis method was to reduce Cu2+filled in the pores among the nanofibers into Cu2O using glucose and prepare Cu2O/microbial cellulose composite membrane.Electrospinning method was to disperse Cu2O evenly in polyvinyl alcohol(PVA)solution.Cu2O/PVA fibers were prepared by electrospinning.Cu2O/PVA fibers were deposited on microbial cellulose membranes to form(Cu2O/PVA composite fibers)/microbial cellulose composite membranes,so that Cu2O particles were evenly dispersed on microbial cellulose membranes.These two methods could solve the difficulty of uniform dispersion of nanoparticles and cellulose membranes.The following conclusions were obtained through sample preparation,structure and performance analysis.(1)In static culture at 30°C,the strains of G.xylinum ATCC23767 synthesized and secreted cellulose nanofibers with 40100 nm in diameter and 19μm in length,which accumulated into a three-dimensional network structure membrane and its water holding capacity was 99.2%.The effects of natural air-drying,vacuum drying and freeze-drying were compared.The results showed that the pore size,pore volume,specific surface area and air permeability of cellulose membrane were the largest after freeze-drying.The breaking strength and Young’s modulus of microbial cellulose membrane were the highest after vacuum drying.(2)The strains could synthesize the microbial cellulose with different morphology,structure and properties under different conditions.The microbial cellulose membrane was constructed by the nanofibers in static culture at 30°C.The cellulose membrane was produced in static culture at12°C,which consisted of wide and thin flake cellulose.The spherical cellulose was obtained by agitated culture at 12°C,which consisted of strongly twisted band-like cellulose with 1.23.0μm width.Compared with the analysis results through FTIR,XRD,NMR and TGA,the cellulose produced at 30°C had the highest crystallinity and the best heat resistance,while the cellulose produced in agitated medium at 12°C was completely amorphous.The formation mechanism of complete amorphous cellulose was further proposed.In static medium at 30?C,the terminal complexes(TCs)and secreting sites of the cells arranged along the longitudinal axis of the cell and the secreted cellulose assembled into fibrous strcuture;In static medium at 12?C,the TCs and secreting sites of the cells arrayed perpendicularly to the longitudinal axis of the cell and the band-like cellulose pellicles were produced to assembly into thin slice structure.In agitated medium at 12?C,the TCs and secreting sites of the cells were arranged perpendicularly to the longitudinal axis of the cell and the nascent cellulose were band-like near the cell.In agitated medium,the band-like cellulose was strong twisted by the shearing force and the arrangement of intra-and inter-molecular hydrogen bonds among cellulose molecular chains was disrupted,which caused the cellulose molecular chains to array randomly into disordered structure,which could not present the crystalline characteristic peaks in X-ray diffraction pattern.(3)The microbial cellulose membranes were impregnated in a copper salt solution and the copper ions adsorbed on the fibers and the pores among the nanofibers.The glucose reduced Cu2+into Cu2O under the heating condition and the cuprous oxide particles were uniformly embedded in the pores among the nanofibers.Only when the reaction temperature was higher than 55?C and the molar ratio of NaOH to CuSO4 was greater than 2.5,the glucose could reduce CuSO4 into Cu2O.When the initial concentration of Cu element in CuSO4 solution was 0.0128%and the content of Cu element in Cu2O/microbial cellulose composite membrane reached 5.18%,indicating that the microbial cellulose membrane could enrich the copper ions and the cuprous oxide particles.(4)Through the regulation of CuSO4,NaOH concentration and reaction temperature,the Cu2O particles with shapes of irregular,spherical,positive hexahedron,positive eight-hedron,twelve-hedron and six needles flower-like could be obtained.When the molar ratio of NaOH to CuSO4 was greater than 2.5,the irregular shape of cuprous oxide could be formed,and the polyhedron structure was synthesized gradually with the increase of the molar ratio of NaOH to CuSO4.The infrared analysis results showed that the in-situ synthesized Cu2O could interact with the cellulose molecules.The smaller the anion radius of copper salt was,the stronger the interaction between Cu2O and cellulose molecule was,and the more obvious the blue shift of FTIR peak of Cu-O bond was.The Cu2O/microbial cellulose composite membrane could reflect the XRD characteristic diffraction peaks of the Cu2O particles when the content of Cu2O was greater than 1%in the composite.Six needle flower like Cu2O could not refltct the XRD characteristic diffraction peaks of Cu2O,even the content of Cu2O was up to 5.8%,which indicated that the six needle flower like Cu2O was amorphous structure.TGA results showed that the higher the Cu2O content in the composite membrane was,the higher the initial decomposition temperature was and the higher the maximum weightlessness temperature of the composite membrane was.(5)NaIO4 can oxidize cellulose to produce aldehyde group.The crystallinity of cellulose decreased significantly after oxidation.The aldehyde groups in the aldehyded cellulose could reduce Cu2+into Cu2O particles with a diameter of 2030 nm.The Cu2O nanoparticles enwrapped the cellulose nanofibers layer-by-layer and formed into brick-like structure.(6)The results of the antibacterial qualitative analysis showed that the cuprous oxide/microbial cellulose composite membrane had some antibacterial effects on Escherichia coli,Staphylococcus aureus,Rhizopus Niger and Aspergillus niger.The composites showed better antibacterial effect on Staphylococcus aureus comparing to on the Escherichia coli;better antibacterial effect on Rhizopus Niger comparing to on the Aspergillus niger.The results of quantitative analysis showed that the antibacterial rate of the cuprous oxide/microbial cellulose composite membrane for Escherichia coli and Staphylococcus aureus reached 99.9%.The minimum inhibitory concentration(MIC)of Staphylococcus aureus was less than the MIC of Escherichia coli,indicating that the effect of cuprous oxide on Gram-positive bacteria was better than that of Gram-negative bacteria.(7)Cuprous oxide not only had excellent antibacterial properties,but also was sensitive to ammonia(NH3).The sensing performances of Cu2O and polyacrylic acid(PAA)were compared.The PAA/PVA composite membrane and(Cu2O/PVA fiber)/microbial cellulose composite membrane were prepared by electrospinning.The adsorption capacity of the composite membrane for NH3 was measured by the change of vibration frequency of quartz crystal microbalance(QCM).The increase of PAA content in PAA/PVA composite membranes resulted in the increase of QCM frequency variation.The frequency variation of pure PAA fiber membranes in 0.5 mg/m3NH3 atmosphere was 4.7 Hz.The increase of Cu2O content in(Cu2O/PVA fiber)/microbial cellulose composite membrane resulted in the increase of QCM frequency variation.When the Cu2O content in composite membrane was 2.79%,the frequency variation in 0.5 mg/m3 NH3atmosphere was 7.3 Hz,which indicated that the Cu2O had better sensing performance for NH3than PAA.