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Self-assembled lignin nanospheres with solid and hollow tunable structures
Industrial Crops & Products 144 (2020) 112063
Contents lists available at ScienceDirect
Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop
Short communication
Self-assembled lignin nanospheres with solid and hollow tunable structures
T
Fuquan Xiong, Hang Wang, Han Xu, Yan Qing, Zhiping Wu, Yiqiang Wu* College of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha, 410004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lignin Self-assembly Nanospheres Tunable structures
The fabrication of nanospheres with tailored structure has attracted much attention in material applications. Herein we found that the food additive, butylated hydroxytoluene (BHT), was able to adjust the solid and hollow structure of lignin nanospheres. At BHT concentrations greater than 0.3 mg/mL in tetrahydrofuran (THF), the obtained lignin nanospheres displayed a hollow structure due to phase separation between THF and water caused by BHT molecules, while the lignin nanospheres obtained without adding BHT exhibited a solid structure. BHT and lignin molecules containing a high content of the carboxy group did not participate in the formation of the nanospheres. Increases in the water stirring and dropping speeds reduced the diameter of the nanospheres, while increasing the initial concentration of lignin and BHT resulted in an increase in nanospheres diameters. Moreover, the shell wall thickness and single hole size could be controlled by employing the concentration of BHT and lignin. This work provides novel insights into the preparation of lignin nanospheres with tunable structures.
1. Introduction Lignin is the second most abundant renewable biopolymer in the world after cellulose. It is generally burnt in a recovery boiler to obtain energy due to its complex structure (Xiong et al., 2017a). A small portion of the available lignin can be used for agricultural purposes and other industries such as binders, adhesives, surfactants and dispersants (Lievonen et al., 2016). The development of lignin nanospheres is considered as an efficient use of lignin (Liu et al., 2019b). Lignin nanospheres with a solid structure are mainly used in the fields of ultraviolet (UV) protection (Qian et al., 2017), aggregation-induced emission nanomaterials (Ma et al., 2018), nano-fillers (Xiong et al., 2018), adsorbents (Liu et al., 2019a) and biocatalysts (Sipponen et al., 2018a). For example, cationic lignin nanospheres can act as activating anchors of hydrolases, enabling aqueous ester synthesis (Sipponen et al., 2018a). Hollow structured lignin nanospheres are generally applied in the fields of drug carrier (Figueiredo et al., 2017) and adsorption materials (Li et al., 2016b). For example, hollow lignin azo colloids have been employed for the encapsulation of photosensitive pesticide avermectin, exhibiting excellent controlled release and UV-blocking performances (Deng et al., 2016). However, the self-assembly of lignin molecules with controllable dimensions and morphologies through the controlling of the preparation conditions still remains a great challenge. Currently, much attention has been focused on the fabrication of nanospheres with a tailored structure. Lintinen et al. (2016) produced
⁎
lignin nanoparticles with a structural diversity by adjusting the condensation and hydrolysis reaction parameters. Yiamsawas et al. (2017) obtained lignin nanocarriers with varying morphologies, including solid nanoparticles, core-shell structures and porous nanoparticles, via a combination of miniemulsion polymerization and a solvent evaporation process. However, the morphology-controlled nanospheres presented in the aforementioned studies require complex chemical reactions and rigorous conditions. In order to overcome the hurdles faced in the previous literature, the development of a simple and convenient self-assembly system for the adjustment of the structure of lignin nanospheres is required. In our previous work, it was found that analytical-grade purity tetrahydrofuran containing some impurities would lead to the formation of hollow structured lignin nanospheres (Xiong et al., 2017a). However, the structure of the lignin nanospheres was regulated with undefined impurities, thus sustainability of the preparation of the nanospheres could not be guaranteed. If a chemical, which can adjust the structure of the nanospheres by means of whether it is added, could be found, it should be possible to expand the utilization field of lignin in industry. Butylated hydroxytoluene (BHT), a food additive, may potentially fulfil this role. Thus, this study investigated the formation processes and mechanism of lignin nanospheres with tunable structures.
Corresponding author. E-mail address: [email protected] (Y. Wu).
https://doi.org/10.1016/j.indcrop.2019.112063 Received 10 July 2019; Received in revised form 13 October 2019; Accepted 17 December 2019 0926-6690/ © 2019 Published by Elsevier B.V.
Industrial Crops & Products 144 (2020) 112063
F. Xiong, et al.
2. Materials and methods
3. Results and discussion
2.1. Materials
3.1. Morphology and size of lignin nanospheres
Enzymatic hydrolysis lignin (EHL) was supplied by Hong Kong Laihe Biotechnology Co., Ltd. Butylated hydroxytoluene (BHT) was obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd, while chromatographic grade tetrahydrofuran (THF) was got from Sinopharm Chemical Reagent Co., Ltd.
Typical TEM and SEM images of lignin nanospheres obtained at varying BHT concentrations are showed in Fig. 1. Fig. 1a and e demonstrate that the lignin nanospheres obtained without adding BHT exhibited a solid structure. Furthermore, a few lignin nanospheres existed gaps at the surface as the residual THF in the lignin nanospheres quickly evaporated at room temperature (Xiong et al., 2017b). When BHT was added at a concentration of 0.1 mg/mL in THF, the morphology of the nanospheres did not change significantly apart from many gaps at the surface, suggesting that the nanospheres formation was not altered at lower BHT concentrations (Fig. 1b and f). Further increases in BHT concentration resulted in a clear contrast between the shell and the center, which exhibited a cavity (Fig. 1c, d, g and h). This may be attributed that the addition of BHT will cause phase separation between THF and water, forming a nanoemulsion soft template that brings about the formation of a hollow structure (Xiong et al., 2017a). Higher initial BHT concentrations denote more BHT molecules available for the formation of nanoemulsion, which determines the final size of the hollow nanospheres. This resulted in an increase in the diameter of the nanospheres with BHT concentrations. In addition, the initial concentration of EHL did not change, the participation of lignin molecules in the formation of each nanosphere did not increase significantly. This reduced the thickness of the shell wall and increased the size of the single hole with increasing of BHT concentrations. These results indicate that the structure of the nanospheres, the thickness of the shell wall and the size of the single hole of the hollow nanospheres can be controlled by applying different initial concentrations of BHT. The diameter of the nanospheres in deionized water was tracked using DLS. As shown in Fig. 2a, in comparison with the nanospheres obtained without the addition of BHT in the preparation process, the diameter of the nanospheres obtained at a BHT concentration of 0.1 mg/mL exhibited a slight decrease. This is inconsistent with the results of the TEM and SEM images, possibly due to the higher average polydispersity indexes (PDI) of the latter. When the BHT concentration increased from 0.1 to 0.5 mg/mL, the nanospheres size increased from 410 nm to 583 nm. This can be attributed to the greater amount of BHT molecules forming a nanoemulsion soft template at higher BHT concentrations, leading to an increase in the size of the hollow lignin nanospheres. The variation in the diameter of the lignin nanospheres is consistent with the results of the TEM and SEM images. Moreover, the average PDI of the nanospheres demonstrated an increasing trend with BHT concentrations, yet remained at a relatively low level.
2.2. Preparation of lignin nanospheres with tunable structures The BHT-THF solutions were obtained by dissolving BHT in THF at concentrations of 0, 0.1, 0.3 and 0.5 mg/mL, respectively. EHL was then dissolved in the BHT-THF solution at a concentration of 0.5 mg/ mL, and the mixture solution was vigorously stirred (600 rpm) at room temperature. Subsequently, deionized water with four times the volume of THF was progressively dropped into the mixture solution at the speed of 2 mL/min, and lignin nanospheres with tunable structures were gradually formed. For the evaporation of THF, the suspension liquid containing lignin nanospheres was continuously stirred in a fume hood for 5 h. Following these steps, the suspension liquid was introduced into a dialysis bag (MWCO: 3500), which was immersed in deionized water (periodically replaced) for the removal of residual THF. The BHT concentrations in THF of 0, 0.1, 0.3 and 0.5 mg/mL, corresponded to lignin nanospheres denoted as 0LNS, 1LNS, 3LNS and 5LNS, respectively.
2.3. Characterization The microstructure of the samples was analyzed using scanning electron microscope (SEM, Tescan) and transmission electron microscopy (TEM, Hitachi HT-7700). The diameter of the nanospheres were analyzed by dynamic light scattering (DLS, Malvern Zetasizer NanoZS90). The functional groups of the obtained samples after freeze dying were tested on a Bruker Fourier Transform Infrared spectrometer (FTIR) in the range of 500−4000 cm−1 using a KBr disc. The chemical characteristics of the surface of the lignin nanospheres after freeze dying were analyzed using X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos, England) with a monochromatic Al Kα as the source. The binding energy scale was corrected by the C 1s peak (284.8 eV).
Fig. 1. TEM (a–d) and SEM (e–h) images of lignin nanospheres obtained at varying BHT concentrations. The initial concentration of EHL was 0.5 mg/mL. The concentration of BHT in THF: (a, e) without adding BHT, (b, f) 0.1 mg/mL, (c, g) 0.3 mg/mL, (d, h) 0.5 mg/mL. 2
Industrial Crops & Products 144 (2020) 112063
F. Xiong, et al.
Fig. 2. Particle size analysis (a) and the FTIR spectra (b) of lignin nanospheres obtained at varying BHT concentrations.
mechanism of solid and hollow lignin nanospheres. Solid lignin nanospheres exhibited a hydrophilic outside surface and hydrophobic core, whereas the hollow lignin nanospheres demonstrated a hydrophobic outside surface and hydrophilic internal surface. This suggests that the carboxy group content of the lignin molecules had a significant effect on its hydrophilic and hydrophobic properties.
3.2. Structural characterization of lignin nanospheres The FTIR spectra in Fig. 2b illustrate the EHL, BHT and lignin nanospheres. According to the literature (Chinna Babu et al., 2011), BHT exhibits peaks at 3625 cm−1 (OH stretching mode of vibration), 2956 cm−1 (CH3 antisymmetric stretching vibration), 1433 cm−1 (CH3 degenerate bending vibrations) and 1150 cm−1 (OH in-plane bending vibration). EHL displayed bands at 2926 cm−1, 1700 cm−1, 1598 cm−1 and 1514 cm−1 due to CH stretching of methyl or methylene groups, C]O stretching, aromatic skeleton vibration and CeC stretching of aromatic skeleton, respectively (Xiong et al., 2017b). Compared with the spectrum of EHL, hollow lignin nanospheres such as 3LNS did not exhibit any new absorption peaks, hinting that BHT molecules did not participate in the formation of hollow lignin nanospheres. The absence of BHT molecules in the hollow lignin nanospheres was further demonstrated using GC–MS (Fig. S1). Moreover, semi-quantitative analysis on the lignin nanospheres produced A1700/A1598 ratios of 0.6489, 0.6523, 0.6522 and 0.6591, corresponding to 0LNS, 1LNS, 3LNS and 5LNS, respectively. Note that these ratios were lower than that of EHL (0.7329). Moreover, the A2935/A1598 ratios of 0LNS, 1LNS, 3LNS and 5LNS were determined as 0.8082, 0.8513, 0.9033 and 0.8772 respectively, all which are higher than that of EHL (0.7469). These results indicate that lignin molecules containing a high content of the carboxy group may not participate in the formation of solid and hollow lignin nanospheres. In order to obtain the chemical structures of the surface of the lignin nanospheres, the XPS spectrum were analyzed. The C 1s spectrum of 0LNS in Fig. 3 show three individual peaks at approximately 284.1 eV, 286.0 eV and 288.5 eV, representing CeC/CH, COC/COH, and OCOH, respectively (Guo et al., 2017; Myint et al., 2016). In comparison to the 0LNS, the C 1s spectrum of 3LNS did not exhibit the OCOH peak at 288.5 eV. The O 1s spectrum of 0LNS and 3LNS further confirmed this result (Fig. S2), which can be attributed to the different formation
3.3. Formation mechanism of lignin nanospheres Lignin nanospheres with tunable structures were able to be prepared, probably attributed to the addition of a specific amount of BHT which in turn affected the miscibility properties between water and THF. THF-water is generally miscible (Fig. S3b). However, the mixture solution of water and THF with the BHT concentration of 0.3 mg/mL has an obvious Tyndall phenomenon (Fig. S3a), indicating phase separation between THF and water caused by BHT molecules. To further investigate the role of BHT in THF, a molecular dynamics (MD) simulation was carried out. More specifically, a rectangular box of 6.3 nm × 6.3 nm × 12.8 nm was established, which was divided into two halves. One half was filled with water molecules, and the other half was filled with THF solution dissolved in a certain amount of BHT molecules (with a BHT concentration of 3 mg/mL in THF). The MD simulation was then performed to observe aggregation behavior of the mixture solution after equilibrium. The snapshot after equilibration in Fig. 4 shows that water and THF were completely immiscible at equilibrium, suggesting that the addition of BHT molecules can cause phase separation in the two solvents, which may lead to the formation of hollow structure lignin nanospheres. BHT distribution analysis revealed that only a very small amount of BHT molecules (e.g. those in the black line box) appeared at the interface between water and THF, whereas a large amount of BHT molecules (e.g. those in the red circle) dispersed in the THF, forming small aggregates. These results indicate that the BHT molecules may not be involved in the formation of the nanospheres at the
Fig. 3. C1 s spectrums of 0LNS and 3LNS. 3
Industrial Crops & Products 144 (2020) 112063
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Fig. 4. The snapshot after equilibration of water and THF after adding in a certain amount of BHT.
and lower molecular weight would aggregate on the internal surface of the membrane by π-π interactions (Sipponen et al., 2018b; Xiong et al., 2017a). This increased the pressure gradient between the inside and outside of the membrane such that a single hole appeared at the thinner side (Fig. 5d). Moreover, in the effect of pressure gradient, the BHT-THF solution would gradually remove from the cavity through the single hole. With the evaporation of THF, BHT was deposit on the vessel wall under the action of shear force. Finally, the hollow lignin nanospheres were completely formed (Fig. 5e). Without the addition of BHT to THF, EHL was first dissolved in THF (Fig. 5f). Following this, lignin molecules with a stronger hydrophobic and higher molecular weight formed the cores of the nanospheres at a water content of 50 vol% (Fig. 5g) (Sipponen et al., 2018b; Xiong et al., 2017b). Lignin molecules with a weaker hydrophobic and lower molecular weight then gradually assembled outward by π-π interactions, thus forming the solid lignin nanospheres (Fig. 5h).
preparation process, which is consistent with the FTIR and GC–MS results. The axial density distribution of the three groups was also analyzed (Fig. S4), further proving the above results. In terms of the z-axis density distribution, there was a significant stratification at z = 6.4 nm. Water was mainly distributed in the range of 0 nm–6.4 nm, while THF and BHT were mainly distributed in the range of 6.4 nm–12.8 nm. Analysis of the z-axis density of BHT found that its content was very little at the interface between water and THF and the distribution of density was uneven in THF. This implies that BHT formed small aggregates in THF, such as A and B locations in Fig. S4. The formation process of the nanospheres with tunable structures is presented in Fig. 5, while a possible formation mechanism for the nanospheres is shown schematically in Fig. 6. A certain proportion of BHT and EHL was completely dissolved in THF (TEM image in Fig. 5a). After adding deionized water, phase separation occurred between water (dispersed phase) and the mixture solution (continuous phase), leading to the fact that lignin molecules owned a stronger hydrophobic and higher molecular weight would form a membrane layer at the interface between the mixture solution and water (Sipponen et al., 2018b; Xiong et al., 2017a). To reduce surface energy, lignin molecules tended to form spheres (Fig. 5b). With the further addition of water, a pressure gradient was produced and gradually increased between the inside and outside of the membrane (Ioannou et al., 2005). When the water content reached 50 vol%, phase transition took place between water and the mixture solution (Fig. 5c). This would cause that the mixture solution (dispersed phase) was swathed by the membrane. As the water content further increased, lignin molecules with a weaker hydrophobic
3.4. Size control of lignin nanospheres The influence of the stirring rate, dropping speed of water and the initial concentration of EHL on the morphology and size of the nanospheres obtained at the BHT concentration of 0.3 mg/mL were evaluated. The corresponding TEM and SEM images are shown in Fig. S5, while the particle size analysis is displayed in Fig. S6. The acquiescent the initial concentration of EHL, stirring rate and dropping speed of water were 0.5 mg/mL, 600 rpm and 2 mL/min, respectively. All of the nanospheres were observed to have a hollow structure, which once
Fig. 5. TEM images of the samples obtained from the dispersions at varying water contents on the preparation process of 3LNS (a–e) and 0LNS (f–h). The initial concentration of EHL was 0.5 mg/mL. Water content: (a, f) 0 vol %, (b) 20 vol %, (c, g) 50 vol %, (d) 60 vol %, (e, h) > 80 vol %, after removing THF. 4
Industrial Crops & Products 144 (2020) 112063
F. Xiong, et al.
Fig. 6. Schematic representation of formation process of the lignin nanosphere with tunable structures.
Appendix A. Supplementary data
again proved that the structure of the lignin nanospheres was controlled by BHT concentration in THF. Increasing the stirring rate from 600 rpm to 800 rpm was able to slightly decrease the nanospheres size from 495.8–452 nm (Fig. S6a) due to the increase in shear force (Xiong et al., 2017a). Compared to the nanospheres prepared with dropping speed of water of 2 mL/min (Fig. 1c and g), the diameter of the nanospheres clearly decreased with dropping speed of water of 6 mL/min (Fig. S5b and e). It is attributed that increasing the dropping speed of water may have caused the quick transformation of the lignin molecules into a “frozen” state for the formation of the nanospheres (Li et al., 2016a). With increasing of the initial concentration of EHL, the diameter of the nanospheres increased (Fig. S5c and f). This can be attributed to the greater amount of lignin molecules involved in the formation of each nanospheres when the initial EHL concentration is increased (Xiong et al., 2017a). It is worth to note that the shell wall thickness of the nanospheres had no obvious change when stirring rate and dropping speed of water were changed, whereas an increase in the initial concentration of EHL led to an increase of shell wall thickness of the nanospheres. This suggests that the latter depended on the initial concentration of EHL as well.
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.indcrop.2019.112063. References Chinna Babu, P., Sundaraganesan, N., Dereli, Ö., Türkkan, E., 2011. FT-IR, FT-Raman spectra, density functional computations of the vibrational spectra and molecular geometry of butylated hydroxy toluene. Spectrochim. Acta Part A 79, 562–569. Deng, Y., Zhao, H., Qian, Y., Lü, L., Wang, B., Qiu, X., 2016. Hollow lignin azo colloids encapsulated avermectin with high anti-photolysis and controlled release performance. Ind. Crops Prod. 87, 191–197. Figueiredo, P., Lintinen, K., Kiriazis, A., Hynninen, V., Liu, Z., Ramos, T.B., Rahikkala, A., Correia, A., Kohout, T., Sarmento, B., 2017. In vitro evaluation of biodegradable lignin-based nanoparticles for drug delivery and enhanced antiproliferation effect in cancer cells. Biomaterials 121, 97–108. Guo, N., Li, M., Sun, X., Wang, F., Yang, R., 2017. Enzymatic hydrolysis lignin derived hierarchical porous carbon for supercapacitors in ionic liquids with high power and energy densities. Green Chem. 19, 2595–2602. Ioannou, K., Nydal, O.J., Angeli, P., 2005. Phase inversion in dispersed liquid–liquid flows. Exp. Therm. Fluid Sci. 29, 331–339. Li, H., Deng, Y., Liu, B., Ren, Y., Liang, J., Qian, Y., Qiu, X., Li, C., Zheng, D., 2016a. Preparation of nanocapsules via self-ssembly of kraft Lignin: a totally green process with renewable resources. ACS Sustain. Chem. Eng. 4, 1946–1953. Li, Y., Wu, M., Wang, B., Wu, Y., Ma, M., Zhang, X., 2016b. Synthesis of magnetic ligninbased hollow microspheres: a highly adsorptive and reusable adsorbent derived from renewable resources. ACS Sustain. Chem. Eng. 4 5523-5523. Lievonen, M., Valle-Delgado, J.J., Mattinen, M.L., Hult, E.L., Lintinen, K., Kostiainen, M.A., Paananen, A., Szilvay, G.R., Setälä, H., Österberg, M., 2016. A simple process for lignin nanoparticle preparation. Green Chem. 18, 1416–1422. Lintinen, K., Latikka, M., Sipponen, M.H., Ras, R.H., Österberg, M., Kostiainen, M.A., 2016. Structural diversity in metal–organic nanoparticles based on iron isopropoxide treated lignin. RSC Adv. 6, 31790–31796. Liu, C., Li, Y., Hou, Y., 2019a. Preparation of a novel lignin nanosphere adsorbent for enhancing adsorption of lead. Molecules 24, 2704. Liu, C., Li, Y., Hou, Y., 2019b. A simple environment-friendly process for preparing highconcentration alkali lignin nanospheres. Eur. Polym. J. 112, 15–23. Ma, Z., Liu, C., Niu, N., Chen, Z., Li, S., Liu, S.-X., Li, J., 2018. Seeking brightness from nature: J-Aggregation-Induced emission in cellulolytic enzyme lignin nanoparticles. ACS Sustain. Chem. Eng. 6, 3169–3175. Myint, A.A., Lee, H.W., Seo, B., Son, W.S., Yoon, J., Yoon, T.J., Park, H.J., Yu, J., Yoon, J., Lee, Y.W., 2016. One pot synthesis of environmentally friendly lignin nanoparticles with compressed liquid carbon dioxide as an antisolvent. Green Chem. 18, 2129–2146. Qian, Y., Zhong, X., Li, Y., Qiu, X., 2017. Fabrication of uniform lignin colloidal spheres for developing natural broad-spectrum sunscreens with high sun protection factor. Ind. Crops Prod. 101, 54–60. Sipponen, M.H., Farooq, M., Koivisto, J., Pellis, A., Seitsonen, J., Österberg, M., 2018a. Spatially confined lignin nanospheres for biocatalytic ester synthesis in aqueous media. Nat. Commun. 9, 2300. Sipponen, M.H., Lange, H., Ago, M., Crestini, C., 2018b. Understanding lignin aggregation processes. A case study: budesonide entrapment and stimuli controlled release from lignin nanoparticles. ACS Sustain. Chem. Eng. 6, 9342–9351. Xiong, F., Han, Y., Wang, S., Li, G., Qin, T., Chen, Y., Chu, F., 2017a. Preparation and formation mechanism of renewable lignin hollow nanospheres with a single hole by self-assembly. ACS Sustain. Chem. Eng. 5, 2273–2281. Xiong, F., Han, Y., Wang, S., Li, G., Qin, T., Chu, F., 2017b. Preparation and formation mechanism of size-controlled lignin nanospheres by self-assembly. Ind. Crops Prod. 100, 146–152. Xiong, F., Wu, Y., Li, G., Han, Y., Chu, F., 2018. Transparent nanocomposite films of lignin nanospheres and poly(vinyl alcohol) for UV-Absorbing. Ind. Eng. Chem. Res. 57, 1207–1212. Yiamsawas, D., Beckers, S.J., Lu, H., Landfester, K., Wurm, F.R., 2017. Morphologycontrolled synthesis of lignin nanocarriers for drug delivery and carbon materials. ACS Biomater. Sci. Eng. 3, 2375–2383.
4. Conclusions The structure of lignin nanospheres was controlled with the use of varying BHT concentrations. For BHT concentrations greater than 0.3 mg/mL in THF, lignin nanospheres displayed a hollow structure due to phase separation between THF and water caused by BHT molecules, while the lignin nanospheres obtained without adding BHT exhibited a solid structure. BHT and lignin molecules containing a high content of the carboxy group did not participate in the formation of the nanospheres. An increase in stirring speed and dropping speed of water would bring about a decrease of the diameter of the nanospheres, whereas the diameter of the nanospheres increased with an increase of the initial concentration of EHL and BHT. Moreover, the shell wall thickness and single hole size were able to controlled by varying the concentrations of BHT and EHL. Overall, our results demonstrate that self-assembly system with varying BHT concentrations provide a feasible platform for the adjustment of the structure of lignin nanospheres. Declaration of Competing Interest None. Acknowledgments The authors gratefully acknowledge support from the National Natural Science Foundation of China (31890774, 31890770, 31800491), the Natural Science Foundation of Hunan Province (2019JJ50982) and Scientific Research Project of Education Department of Hunan Province (18B174).
5
Contents lists available at ScienceDirect
Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop
Short communication
Self-assembled lignin nanospheres with solid and hollow tunable structures
T
Fuquan Xiong, Hang Wang, Han Xu, Yan Qing, Zhiping Wu, Yiqiang Wu* College of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha, 410004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lignin Self-assembly Nanospheres Tunable structures
The fabrication of nanospheres with tailored structure has attracted much attention in material applications. Herein we found that the food additive, butylated hydroxytoluene (BHT), was able to adjust the solid and hollow structure of lignin nanospheres. At BHT concentrations greater than 0.3 mg/mL in tetrahydrofuran (THF), the obtained lignin nanospheres displayed a hollow structure due to phase separation between THF and water caused by BHT molecules, while the lignin nanospheres obtained without adding BHT exhibited a solid structure. BHT and lignin molecules containing a high content of the carboxy group did not participate in the formation of the nanospheres. Increases in the water stirring and dropping speeds reduced the diameter of the nanospheres, while increasing the initial concentration of lignin and BHT resulted in an increase in nanospheres diameters. Moreover, the shell wall thickness and single hole size could be controlled by employing the concentration of BHT and lignin. This work provides novel insights into the preparation of lignin nanospheres with tunable structures.
1. Introduction Lignin is the second most abundant renewable biopolymer in the world after cellulose. It is generally burnt in a recovery boiler to obtain energy due to its complex structure (Xiong et al., 2017a). A small portion of the available lignin can be used for agricultural purposes and other industries such as binders, adhesives, surfactants and dispersants (Lievonen et al., 2016). The development of lignin nanospheres is considered as an efficient use of lignin (Liu et al., 2019b). Lignin nanospheres with a solid structure are mainly used in the fields of ultraviolet (UV) protection (Qian et al., 2017), aggregation-induced emission nanomaterials (Ma et al., 2018), nano-fillers (Xiong et al., 2018), adsorbents (Liu et al., 2019a) and biocatalysts (Sipponen et al., 2018a). For example, cationic lignin nanospheres can act as activating anchors of hydrolases, enabling aqueous ester synthesis (Sipponen et al., 2018a). Hollow structured lignin nanospheres are generally applied in the fields of drug carrier (Figueiredo et al., 2017) and adsorption materials (Li et al., 2016b). For example, hollow lignin azo colloids have been employed for the encapsulation of photosensitive pesticide avermectin, exhibiting excellent controlled release and UV-blocking performances (Deng et al., 2016). However, the self-assembly of lignin molecules with controllable dimensions and morphologies through the controlling of the preparation conditions still remains a great challenge. Currently, much attention has been focused on the fabrication of nanospheres with a tailored structure. Lintinen et al. (2016) produced
⁎
lignin nanoparticles with a structural diversity by adjusting the condensation and hydrolysis reaction parameters. Yiamsawas et al. (2017) obtained lignin nanocarriers with varying morphologies, including solid nanoparticles, core-shell structures and porous nanoparticles, via a combination of miniemulsion polymerization and a solvent evaporation process. However, the morphology-controlled nanospheres presented in the aforementioned studies require complex chemical reactions and rigorous conditions. In order to overcome the hurdles faced in the previous literature, the development of a simple and convenient self-assembly system for the adjustment of the structure of lignin nanospheres is required. In our previous work, it was found that analytical-grade purity tetrahydrofuran containing some impurities would lead to the formation of hollow structured lignin nanospheres (Xiong et al., 2017a). However, the structure of the lignin nanospheres was regulated with undefined impurities, thus sustainability of the preparation of the nanospheres could not be guaranteed. If a chemical, which can adjust the structure of the nanospheres by means of whether it is added, could be found, it should be possible to expand the utilization field of lignin in industry. Butylated hydroxytoluene (BHT), a food additive, may potentially fulfil this role. Thus, this study investigated the formation processes and mechanism of lignin nanospheres with tunable structures.
Corresponding author. E-mail address: [email protected] (Y. Wu).
https://doi.org/10.1016/j.indcrop.2019.112063 Received 10 July 2019; Received in revised form 13 October 2019; Accepted 17 December 2019 0926-6690/ © 2019 Published by Elsevier B.V.
Industrial Crops & Products 144 (2020) 112063
F. Xiong, et al.
2. Materials and methods
3. Results and discussion
2.1. Materials
3.1. Morphology and size of lignin nanospheres
Enzymatic hydrolysis lignin (EHL) was supplied by Hong Kong Laihe Biotechnology Co., Ltd. Butylated hydroxytoluene (BHT) was obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd, while chromatographic grade tetrahydrofuran (THF) was got from Sinopharm Chemical Reagent Co., Ltd.
Typical TEM and SEM images of lignin nanospheres obtained at varying BHT concentrations are showed in Fig. 1. Fig. 1a and e demonstrate that the lignin nanospheres obtained without adding BHT exhibited a solid structure. Furthermore, a few lignin nanospheres existed gaps at the surface as the residual THF in the lignin nanospheres quickly evaporated at room temperature (Xiong et al., 2017b). When BHT was added at a concentration of 0.1 mg/mL in THF, the morphology of the nanospheres did not change significantly apart from many gaps at the surface, suggesting that the nanospheres formation was not altered at lower BHT concentrations (Fig. 1b and f). Further increases in BHT concentration resulted in a clear contrast between the shell and the center, which exhibited a cavity (Fig. 1c, d, g and h). This may be attributed that the addition of BHT will cause phase separation between THF and water, forming a nanoemulsion soft template that brings about the formation of a hollow structure (Xiong et al., 2017a). Higher initial BHT concentrations denote more BHT molecules available for the formation of nanoemulsion, which determines the final size of the hollow nanospheres. This resulted in an increase in the diameter of the nanospheres with BHT concentrations. In addition, the initial concentration of EHL did not change, the participation of lignin molecules in the formation of each nanosphere did not increase significantly. This reduced the thickness of the shell wall and increased the size of the single hole with increasing of BHT concentrations. These results indicate that the structure of the nanospheres, the thickness of the shell wall and the size of the single hole of the hollow nanospheres can be controlled by applying different initial concentrations of BHT. The diameter of the nanospheres in deionized water was tracked using DLS. As shown in Fig. 2a, in comparison with the nanospheres obtained without the addition of BHT in the preparation process, the diameter of the nanospheres obtained at a BHT concentration of 0.1 mg/mL exhibited a slight decrease. This is inconsistent with the results of the TEM and SEM images, possibly due to the higher average polydispersity indexes (PDI) of the latter. When the BHT concentration increased from 0.1 to 0.5 mg/mL, the nanospheres size increased from 410 nm to 583 nm. This can be attributed to the greater amount of BHT molecules forming a nanoemulsion soft template at higher BHT concentrations, leading to an increase in the size of the hollow lignin nanospheres. The variation in the diameter of the lignin nanospheres is consistent with the results of the TEM and SEM images. Moreover, the average PDI of the nanospheres demonstrated an increasing trend with BHT concentrations, yet remained at a relatively low level.
2.2. Preparation of lignin nanospheres with tunable structures The BHT-THF solutions were obtained by dissolving BHT in THF at concentrations of 0, 0.1, 0.3 and 0.5 mg/mL, respectively. EHL was then dissolved in the BHT-THF solution at a concentration of 0.5 mg/ mL, and the mixture solution was vigorously stirred (600 rpm) at room temperature. Subsequently, deionized water with four times the volume of THF was progressively dropped into the mixture solution at the speed of 2 mL/min, and lignin nanospheres with tunable structures were gradually formed. For the evaporation of THF, the suspension liquid containing lignin nanospheres was continuously stirred in a fume hood for 5 h. Following these steps, the suspension liquid was introduced into a dialysis bag (MWCO: 3500), which was immersed in deionized water (periodically replaced) for the removal of residual THF. The BHT concentrations in THF of 0, 0.1, 0.3 and 0.5 mg/mL, corresponded to lignin nanospheres denoted as 0LNS, 1LNS, 3LNS and 5LNS, respectively.
2.3. Characterization The microstructure of the samples was analyzed using scanning electron microscope (SEM, Tescan) and transmission electron microscopy (TEM, Hitachi HT-7700). The diameter of the nanospheres were analyzed by dynamic light scattering (DLS, Malvern Zetasizer NanoZS90). The functional groups of the obtained samples after freeze dying were tested on a Bruker Fourier Transform Infrared spectrometer (FTIR) in the range of 500−4000 cm−1 using a KBr disc. The chemical characteristics of the surface of the lignin nanospheres after freeze dying were analyzed using X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos, England) with a monochromatic Al Kα as the source. The binding energy scale was corrected by the C 1s peak (284.8 eV).
Fig. 1. TEM (a–d) and SEM (e–h) images of lignin nanospheres obtained at varying BHT concentrations. The initial concentration of EHL was 0.5 mg/mL. The concentration of BHT in THF: (a, e) without adding BHT, (b, f) 0.1 mg/mL, (c, g) 0.3 mg/mL, (d, h) 0.5 mg/mL. 2
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Fig. 2. Particle size analysis (a) and the FTIR spectra (b) of lignin nanospheres obtained at varying BHT concentrations.
mechanism of solid and hollow lignin nanospheres. Solid lignin nanospheres exhibited a hydrophilic outside surface and hydrophobic core, whereas the hollow lignin nanospheres demonstrated a hydrophobic outside surface and hydrophilic internal surface. This suggests that the carboxy group content of the lignin molecules had a significant effect on its hydrophilic and hydrophobic properties.
3.2. Structural characterization of lignin nanospheres The FTIR spectra in Fig. 2b illustrate the EHL, BHT and lignin nanospheres. According to the literature (Chinna Babu et al., 2011), BHT exhibits peaks at 3625 cm−1 (OH stretching mode of vibration), 2956 cm−1 (CH3 antisymmetric stretching vibration), 1433 cm−1 (CH3 degenerate bending vibrations) and 1150 cm−1 (OH in-plane bending vibration). EHL displayed bands at 2926 cm−1, 1700 cm−1, 1598 cm−1 and 1514 cm−1 due to CH stretching of methyl or methylene groups, C]O stretching, aromatic skeleton vibration and CeC stretching of aromatic skeleton, respectively (Xiong et al., 2017b). Compared with the spectrum of EHL, hollow lignin nanospheres such as 3LNS did not exhibit any new absorption peaks, hinting that BHT molecules did not participate in the formation of hollow lignin nanospheres. The absence of BHT molecules in the hollow lignin nanospheres was further demonstrated using GC–MS (Fig. S1). Moreover, semi-quantitative analysis on the lignin nanospheres produced A1700/A1598 ratios of 0.6489, 0.6523, 0.6522 and 0.6591, corresponding to 0LNS, 1LNS, 3LNS and 5LNS, respectively. Note that these ratios were lower than that of EHL (0.7329). Moreover, the A2935/A1598 ratios of 0LNS, 1LNS, 3LNS and 5LNS were determined as 0.8082, 0.8513, 0.9033 and 0.8772 respectively, all which are higher than that of EHL (0.7469). These results indicate that lignin molecules containing a high content of the carboxy group may not participate in the formation of solid and hollow lignin nanospheres. In order to obtain the chemical structures of the surface of the lignin nanospheres, the XPS spectrum were analyzed. The C 1s spectrum of 0LNS in Fig. 3 show three individual peaks at approximately 284.1 eV, 286.0 eV and 288.5 eV, representing CeC/CH, COC/COH, and OCOH, respectively (Guo et al., 2017; Myint et al., 2016). In comparison to the 0LNS, the C 1s spectrum of 3LNS did not exhibit the OCOH peak at 288.5 eV. The O 1s spectrum of 0LNS and 3LNS further confirmed this result (Fig. S2), which can be attributed to the different formation
3.3. Formation mechanism of lignin nanospheres Lignin nanospheres with tunable structures were able to be prepared, probably attributed to the addition of a specific amount of BHT which in turn affected the miscibility properties between water and THF. THF-water is generally miscible (Fig. S3b). However, the mixture solution of water and THF with the BHT concentration of 0.3 mg/mL has an obvious Tyndall phenomenon (Fig. S3a), indicating phase separation between THF and water caused by BHT molecules. To further investigate the role of BHT in THF, a molecular dynamics (MD) simulation was carried out. More specifically, a rectangular box of 6.3 nm × 6.3 nm × 12.8 nm was established, which was divided into two halves. One half was filled with water molecules, and the other half was filled with THF solution dissolved in a certain amount of BHT molecules (with a BHT concentration of 3 mg/mL in THF). The MD simulation was then performed to observe aggregation behavior of the mixture solution after equilibrium. The snapshot after equilibration in Fig. 4 shows that water and THF were completely immiscible at equilibrium, suggesting that the addition of BHT molecules can cause phase separation in the two solvents, which may lead to the formation of hollow structure lignin nanospheres. BHT distribution analysis revealed that only a very small amount of BHT molecules (e.g. those in the black line box) appeared at the interface between water and THF, whereas a large amount of BHT molecules (e.g. those in the red circle) dispersed in the THF, forming small aggregates. These results indicate that the BHT molecules may not be involved in the formation of the nanospheres at the
Fig. 3. C1 s spectrums of 0LNS and 3LNS. 3
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Fig. 4. The snapshot after equilibration of water and THF after adding in a certain amount of BHT.
and lower molecular weight would aggregate on the internal surface of the membrane by π-π interactions (Sipponen et al., 2018b; Xiong et al., 2017a). This increased the pressure gradient between the inside and outside of the membrane such that a single hole appeared at the thinner side (Fig. 5d). Moreover, in the effect of pressure gradient, the BHT-THF solution would gradually remove from the cavity through the single hole. With the evaporation of THF, BHT was deposit on the vessel wall under the action of shear force. Finally, the hollow lignin nanospheres were completely formed (Fig. 5e). Without the addition of BHT to THF, EHL was first dissolved in THF (Fig. 5f). Following this, lignin molecules with a stronger hydrophobic and higher molecular weight formed the cores of the nanospheres at a water content of 50 vol% (Fig. 5g) (Sipponen et al., 2018b; Xiong et al., 2017b). Lignin molecules with a weaker hydrophobic and lower molecular weight then gradually assembled outward by π-π interactions, thus forming the solid lignin nanospheres (Fig. 5h).
preparation process, which is consistent with the FTIR and GC–MS results. The axial density distribution of the three groups was also analyzed (Fig. S4), further proving the above results. In terms of the z-axis density distribution, there was a significant stratification at z = 6.4 nm. Water was mainly distributed in the range of 0 nm–6.4 nm, while THF and BHT were mainly distributed in the range of 6.4 nm–12.8 nm. Analysis of the z-axis density of BHT found that its content was very little at the interface between water and THF and the distribution of density was uneven in THF. This implies that BHT formed small aggregates in THF, such as A and B locations in Fig. S4. The formation process of the nanospheres with tunable structures is presented in Fig. 5, while a possible formation mechanism for the nanospheres is shown schematically in Fig. 6. A certain proportion of BHT and EHL was completely dissolved in THF (TEM image in Fig. 5a). After adding deionized water, phase separation occurred between water (dispersed phase) and the mixture solution (continuous phase), leading to the fact that lignin molecules owned a stronger hydrophobic and higher molecular weight would form a membrane layer at the interface between the mixture solution and water (Sipponen et al., 2018b; Xiong et al., 2017a). To reduce surface energy, lignin molecules tended to form spheres (Fig. 5b). With the further addition of water, a pressure gradient was produced and gradually increased between the inside and outside of the membrane (Ioannou et al., 2005). When the water content reached 50 vol%, phase transition took place between water and the mixture solution (Fig. 5c). This would cause that the mixture solution (dispersed phase) was swathed by the membrane. As the water content further increased, lignin molecules with a weaker hydrophobic
3.4. Size control of lignin nanospheres The influence of the stirring rate, dropping speed of water and the initial concentration of EHL on the morphology and size of the nanospheres obtained at the BHT concentration of 0.3 mg/mL were evaluated. The corresponding TEM and SEM images are shown in Fig. S5, while the particle size analysis is displayed in Fig. S6. The acquiescent the initial concentration of EHL, stirring rate and dropping speed of water were 0.5 mg/mL, 600 rpm and 2 mL/min, respectively. All of the nanospheres were observed to have a hollow structure, which once
Fig. 5. TEM images of the samples obtained from the dispersions at varying water contents on the preparation process of 3LNS (a–e) and 0LNS (f–h). The initial concentration of EHL was 0.5 mg/mL. Water content: (a, f) 0 vol %, (b) 20 vol %, (c, g) 50 vol %, (d) 60 vol %, (e, h) > 80 vol %, after removing THF. 4
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Fig. 6. Schematic representation of formation process of the lignin nanosphere with tunable structures.
Appendix A. Supplementary data
again proved that the structure of the lignin nanospheres was controlled by BHT concentration in THF. Increasing the stirring rate from 600 rpm to 800 rpm was able to slightly decrease the nanospheres size from 495.8–452 nm (Fig. S6a) due to the increase in shear force (Xiong et al., 2017a). Compared to the nanospheres prepared with dropping speed of water of 2 mL/min (Fig. 1c and g), the diameter of the nanospheres clearly decreased with dropping speed of water of 6 mL/min (Fig. S5b and e). It is attributed that increasing the dropping speed of water may have caused the quick transformation of the lignin molecules into a “frozen” state for the formation of the nanospheres (Li et al., 2016a). With increasing of the initial concentration of EHL, the diameter of the nanospheres increased (Fig. S5c and f). This can be attributed to the greater amount of lignin molecules involved in the formation of each nanospheres when the initial EHL concentration is increased (Xiong et al., 2017a). It is worth to note that the shell wall thickness of the nanospheres had no obvious change when stirring rate and dropping speed of water were changed, whereas an increase in the initial concentration of EHL led to an increase of shell wall thickness of the nanospheres. This suggests that the latter depended on the initial concentration of EHL as well.
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.indcrop.2019.112063. References Chinna Babu, P., Sundaraganesan, N., Dereli, Ö., Türkkan, E., 2011. FT-IR, FT-Raman spectra, density functional computations of the vibrational spectra and molecular geometry of butylated hydroxy toluene. Spectrochim. Acta Part A 79, 562–569. Deng, Y., Zhao, H., Qian, Y., Lü, L., Wang, B., Qiu, X., 2016. Hollow lignin azo colloids encapsulated avermectin with high anti-photolysis and controlled release performance. Ind. Crops Prod. 87, 191–197. Figueiredo, P., Lintinen, K., Kiriazis, A., Hynninen, V., Liu, Z., Ramos, T.B., Rahikkala, A., Correia, A., Kohout, T., Sarmento, B., 2017. In vitro evaluation of biodegradable lignin-based nanoparticles for drug delivery and enhanced antiproliferation effect in cancer cells. Biomaterials 121, 97–108. Guo, N., Li, M., Sun, X., Wang, F., Yang, R., 2017. Enzymatic hydrolysis lignin derived hierarchical porous carbon for supercapacitors in ionic liquids with high power and energy densities. Green Chem. 19, 2595–2602. Ioannou, K., Nydal, O.J., Angeli, P., 2005. Phase inversion in dispersed liquid–liquid flows. Exp. Therm. Fluid Sci. 29, 331–339. Li, H., Deng, Y., Liu, B., Ren, Y., Liang, J., Qian, Y., Qiu, X., Li, C., Zheng, D., 2016a. Preparation of nanocapsules via self-ssembly of kraft Lignin: a totally green process with renewable resources. ACS Sustain. Chem. Eng. 4, 1946–1953. Li, Y., Wu, M., Wang, B., Wu, Y., Ma, M., Zhang, X., 2016b. Synthesis of magnetic ligninbased hollow microspheres: a highly adsorptive and reusable adsorbent derived from renewable resources. ACS Sustain. Chem. Eng. 4 5523-5523. Lievonen, M., Valle-Delgado, J.J., Mattinen, M.L., Hult, E.L., Lintinen, K., Kostiainen, M.A., Paananen, A., Szilvay, G.R., Setälä, H., Österberg, M., 2016. A simple process for lignin nanoparticle preparation. Green Chem. 18, 1416–1422. Lintinen, K., Latikka, M., Sipponen, M.H., Ras, R.H., Österberg, M., Kostiainen, M.A., 2016. Structural diversity in metal–organic nanoparticles based on iron isopropoxide treated lignin. RSC Adv. 6, 31790–31796. Liu, C., Li, Y., Hou, Y., 2019a. Preparation of a novel lignin nanosphere adsorbent for enhancing adsorption of lead. Molecules 24, 2704. Liu, C., Li, Y., Hou, Y., 2019b. A simple environment-friendly process for preparing highconcentration alkali lignin nanospheres. Eur. Polym. J. 112, 15–23. Ma, Z., Liu, C., Niu, N., Chen, Z., Li, S., Liu, S.-X., Li, J., 2018. Seeking brightness from nature: J-Aggregation-Induced emission in cellulolytic enzyme lignin nanoparticles. ACS Sustain. Chem. Eng. 6, 3169–3175. Myint, A.A., Lee, H.W., Seo, B., Son, W.S., Yoon, J., Yoon, T.J., Park, H.J., Yu, J., Yoon, J., Lee, Y.W., 2016. One pot synthesis of environmentally friendly lignin nanoparticles with compressed liquid carbon dioxide as an antisolvent. Green Chem. 18, 2129–2146. Qian, Y., Zhong, X., Li, Y., Qiu, X., 2017. Fabrication of uniform lignin colloidal spheres for developing natural broad-spectrum sunscreens with high sun protection factor. Ind. Crops Prod. 101, 54–60. Sipponen, M.H., Farooq, M., Koivisto, J., Pellis, A., Seitsonen, J., Österberg, M., 2018a. Spatially confined lignin nanospheres for biocatalytic ester synthesis in aqueous media. Nat. Commun. 9, 2300. Sipponen, M.H., Lange, H., Ago, M., Crestini, C., 2018b. Understanding lignin aggregation processes. A case study: budesonide entrapment and stimuli controlled release from lignin nanoparticles. ACS Sustain. Chem. Eng. 6, 9342–9351. Xiong, F., Han, Y., Wang, S., Li, G., Qin, T., Chen, Y., Chu, F., 2017a. Preparation and formation mechanism of renewable lignin hollow nanospheres with a single hole by self-assembly. ACS Sustain. Chem. Eng. 5, 2273–2281. Xiong, F., Han, Y., Wang, S., Li, G., Qin, T., Chu, F., 2017b. Preparation and formation mechanism of size-controlled lignin nanospheres by self-assembly. Ind. Crops Prod. 100, 146–152. Xiong, F., Wu, Y., Li, G., Han, Y., Chu, F., 2018. Transparent nanocomposite films of lignin nanospheres and poly(vinyl alcohol) for UV-Absorbing. Ind. Eng. Chem. Res. 57, 1207–1212. Yiamsawas, D., Beckers, S.J., Lu, H., Landfester, K., Wurm, F.R., 2017. Morphologycontrolled synthesis of lignin nanocarriers for drug delivery and carbon materials. ACS Biomater. Sci. Eng. 3, 2375–2383.
4. Conclusions The structure of lignin nanospheres was controlled with the use of varying BHT concentrations. For BHT concentrations greater than 0.3 mg/mL in THF, lignin nanospheres displayed a hollow structure due to phase separation between THF and water caused by BHT molecules, while the lignin nanospheres obtained without adding BHT exhibited a solid structure. BHT and lignin molecules containing a high content of the carboxy group did not participate in the formation of the nanospheres. An increase in stirring speed and dropping speed of water would bring about a decrease of the diameter of the nanospheres, whereas the diameter of the nanospheres increased with an increase of the initial concentration of EHL and BHT. Moreover, the shell wall thickness and single hole size were able to controlled by varying the concentrations of BHT and EHL. Overall, our results demonstrate that self-assembly system with varying BHT concentrations provide a feasible platform for the adjustment of the structure of lignin nanospheres. Declaration of Competing Interest None. Acknowledgments The authors gratefully acknowledge support from the National Natural Science Foundation of China (31890774, 31890770, 31800491), the Natural Science Foundation of Hunan Province (2019JJ50982) and Scientific Research Project of Education Department of Hunan Province (18B174).
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