| Due to the stubbornness and complexity of lignin structure,it is extremely difficult to separate a large amount of lignin while maintaining its original structure as much as possible,thus hindering the exploration of the physicochemical properties and molecular structure of lignin,and limiting the high-value utilization of lignin.As an important active functional group of lignin,phenol hydroxyl group,especially catechol,plays a key role in the high-value utilization of lignin in the biomedical field.Therefore,bagasse,the characteristic agricultural by-product in Guangxi,was applied as the material to extract lignin with methanolic hydrochloric acid treatment at room temperature.In addition,a differential ultraviolet method was built for rapid quantification of phenolic hydroxyl groups and then the construction and performance exploration of bagasse lignin hydrogel with specific structures was completed so as to provide theoretical and technical support for the separation,extraction and high value utilization of protolignin.Alcohol acid can effectively separate lignin with complete structure under mild conditions.At room temperature,the bagasse was extracted with methanol hydrochloride for 24 h to obtain bagasse methanolic hydrochloric acid lignin(BHL).The residue after treated with methanolic hydrochloric acid was successively extracted by acetone/water,0.1 wt.%sodium hydroxide solution and 1 wt.%sodium hydroxide solution,and the residual lignin(BAL,BSL0.1 and BSL1,respectively)was extracted.In order to better study the physical and chemical properties and molecular structure of lignin,bagasse milled wood lignin(BMWL)was considered as the control group.The micro morphology,thermal degradation behavior and structural differences of the above separated lignin were investigated by using field emission scanning electron microscopy(FE-SEM),Fourier infrared spectroscopy(FT-IR),three-dimensional infrared spectroscopy(3D FT-IR)of pyrolysis gas products,quantitative carbon spectroscopy nuclear magnetic resonance(13C NMR),and heteronuclear single quantum relationship(1H-13C HSQC NMR).The results indicated that BHL was a typical S/G/H-type lignin,and its yield was 24.2 wt.%,which was more than 12 times that of BMWL.BHL contained moreβ-aryl ether structures,fewer condensation structures(β-5,β-β,etc.)and Cα=O structures,which inferred that methanolic hydrochloric acid holds smaller negative impacts on the lignin structure.Differential ultraviolet spectroscopy has the advantages of simplicity,rapidity,economy and high efficiency in the determination of phenolic hydroxyl of lignin.Four G-type phenolic lignin model compounds,for example,vanillyl alcohol,dimethoxybiphenyl,G-stilbene,and vanillin,were used to determine a suitable acid base solvent system and to establish a set of equations for calculating the content of G-type phenolic hydroxyl groups by the differential ultraviolet absorption coefficient(Δε)at the largest characteristic peak;then,using vanillyl alcohol(Dominant G-type),4-hydroxy-3,5-dimethoxybenzyl alcohol(S-type)and 4-hydroxycinnamate methyl ester(H-type)phenolic lignin model compounds,a set of equations for calculating the content of S/G/H-type phenolic hydroxyl groups were built for the first time;Considering the contribution of S/G/H type phenolic lignin,the calculation equations of calculating the content of catechol in S/G/H type lignin were established for the first time by using 4-methyl catechol in this work.The results showed that the accuracy of the new differential ultraviolet method for the determination of G type phenolic hydroxyl groups was improved by 60%when comparing with the traditional differential ultraviolet method;Nuclear magnetic resonance spectroscopy on phosphorus element(31P NMR)was applied to verify the accuracy of the new differential ultraviolet method for the determination of phenolic hydroxyl groups.Catechol in BHL provides the reactive site,which is conducive to the formation of high strength hydrogel.Functional module BHL@Fe with different molar ratios of Fe3+to BHL were constructed by BHL containing with catechol and Fe3+.The module BHL@Fe was mixed with monomers of N,Ndimethylacrylamide(DMAA)and α-methacrylic acid(MAAc)to self-assemble into hydrogels at room temperature.The effects of different molar ratios of Fe3+to BHL and BHL@Fe additions on the microscopic morphology,mechanical properties,antibacterial properties and biocompatibility of the hydrogels were investigated.BHL@Fe played as the role of nano fillers,which acted as crosslinking points of polymerization to form lamellar stacked porous hydrogels.Catechol-Fe3+coordination bond and hydrogen bond were both cross-linked bonds and "sacrificial bonds",providing theoretical support for the good mechanical properties of hydrogels.When the molar ratio of Fe3+:BHL is 1:3 and the addition amount of BHL@Fe is 30%,the bactericidal rate of the hydrogel for Escherichia coli(E.coli)and Staphylococcus aureus(S.aureus)was more than 95%.However,the hydrogel was non-toxic to normal human keratinocytes.It indicated that BHL can be used as wound dressings in the biomedical field. |
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