| Lignin is a polyaromatic polymer composed of phenylpropanoid units,and it is the second most abundant plant biopolymer on earth after cellulose.However,lignin is not used extensively due to its complexstructures.Lignin micro/nano-particles can provide many opportunities for value-added applications of lignin.Moreover,lignin through chemical modification can be combined with the advantages of lignin and the modified function groups,leading to the application fieldsof lignin more widely.Therefore,the preparation of lignin micro/nano-particles and its chemical modification and preliminary application are explored,which can provide a newapproach for value-added application of ligninproducts.It has important significance for chemical processing of biomass resources.In this paper,enzymatic hydrolysis lignin(EHL)was used as raw material.Lignin micro-nanospheres,lignin hollow micro-nanospheres with a single hole,silicane modified lignin micro-nanospheres and fluorescent lignin nanoparticlescould be obtained.The formation mechanism of lignin micro-nanospheres and lignin hollow micro-nanospheres with a single hole were respectively revealed.Application of lignin micro/nano-particles were explored,including ultraviolet absorption,fluorescence probe and reinforcing filler.The uniform EHLmicro-nanospheres were obtained by a simple self-assembly method.They were formed through a layer-by-layer self-assembly approach from inside to outside based on π-π interactions,which enabled the formation of micro-nanospheres in the sizerange of 190-590 nm.Increasing the pre-dropping EHL concentration resulted in an increase of the diameter and yield of the micro-nanospheres,the yield of the micro-nanosphereswas 72.15% at pre-dropping lignin concentration of 2 mg/mL.The average diameters of the micro-nanospheres had no significant change after 30 days.Compared to the raw lignin,the chemical structural features of the micro-nanospheres had not produced a great change.High preparation temperature brought about the formation of the gaps at the surface of the micro-nanospheres due to the effect of volatile speed of solvent.Moreover,theaverage diameter of the micro-nanospheres decreased with an increase of stirring rate or the dropping speed of water.Compared to the straightforward dialysis method,the micro-nanospheres,which were obtained by the method described in this work,exhibited tunable size and regularity without any gap through initial lignin concentration,stirring rate,the dropping speed of water and the preparation temperature.In the process of lignin hollow micro-nanospheres with a single hole obtained by a straightforward self-assemblymethod,the micro-nanospheres exhibited hollow structure due to the effect of tetrahydrofuran on the self-assembly behavior.Phase separation is existed between AR-THF and water,which forms a nano-emulsion system.Finally,the micro-nanospheres with hollow structureis produced by means of nano-emulsion soft template.Hydrophobic outside surface and hydrophilic internal surface were formed viaa layer-by-layer self-assembly method from outside to inside based on π-π interactions.The chemicalstructureof lignin did not produced a significant changein the preparation process of lignin hollow micro-nanospheres.With increasing of the initial lignin concentration,the diameter of the micro-nanospheres and the thickness of shell wall increased,while the diameter ofthe single hole,the surface areaand the pore volume of the micro-nanospheres decreased.The surface areawas 25.4 m2 g-1at initial lignin concentration of 0.5 mg/mL.Increasing the stirring speed or dropping speed of water resulted in a decrease of the diameter of the hollow micro-nanospheres.Moreover,apparentchangeofthe average diameter of the nanospheres were not observed after 15 days,and the micro-nanosphere dispersions were stable at pH values between 3.5 and 12.When lignin micro-nanospheres were modified with octyltrimethoxysilane(TMOS),Si-OCH3 groups were hydrolyzed into Si-OH groups.A large part of Si-OH groups were grafting modification onto the lignin molecules at the surface of the lignin micro-nanospheres,and the rest of Si-OH groups and the adjacent silane molecules would take place polycondensation reaction for forming Si-O-Si groups.Finally,TMOS would form a thin film at the surface of lignin micro-nanospheres.Increasing TMOS concentration resulted in an increase ofthe grating extent of TMOS onto the lignin micro-nanospheres in certain concentration range.Initial decomposition temperature of lignin micro-nanosphereswas 133.1 °C,while the temperature of silane modified lignin nanospheres increased to 159.8 °C.Fluorescent lignin nanoparticles were prepared by a combination of alkoxysilane chemistry,amidation and sonication.Lignin-PBA(1-pyrenebutyric acid)exhibited obvious signals of UV absorbance and fluorescence using ultraviolet-visible absorbance,fluorescence spectroscopy and confocal microscopy.What is more,the lignin-PBA showed oxygen-responsive behavior that a higher cumulative oxygen time resulted in higher fluorescence quenching of lignin-PBA.Dynamic light scattering showed that the average particles diameter of lignin-PBA was 92.5 nm after sonication.Increasing the 3-aminopropyltriethoxysilane(APTES)concentration brought about the grating extent of APTES increased at the surface of lignin,while the intensities of fluorescence emission of PBA-Lignin increased at first and then decreased due to the self-condensation of APTES onto the lignin surface at higher APTES concentration.Increasing PBA concentration brought about an increase in theintensities of fluorescence emission due to increasing grating extent of PBA.Lignin micro-nanospheres/PVA composite membrane could exhibit a better ultraviolet absorption and visible light transmission performance.Compared with lignin/PVA blend membrane,lignin micro-nanospherescould be evenly dispersed in PVA matrix.Lignin micro-nanospheres/PVA composite membrane did not occurr macroscopic phase separation,while shown the microphase separation.Maximum decomposition temperature and glass transition temperature of pure PVA membrane were 264.7 °C and 104.2 °C,while maximum decomposition temperature and glass transition temperature of the composite membrane containing 1% lignin micro-nanospheres increased to 272.1 °C and 116.8 °C.Moreover,with increasing of lignin micro-nanospheres concentration in PVA matrix,maximum decomposition temperature and glass transition temperature of the composite film gradually increased.Adding a certain amount of ligninmicro-nanospheres in PVA can improve mechanical strength of the film under the condition of elongation at break in a small amount of loss. |
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