Reversible Surface Engineering of Cellulose Elementary Fibrils: From Ultralong Nanocelluloses to Advanced Cellulosic Materials
纤维素基本纤维的可逆表面工程：从超长纳米纤维素到先进纤维素材料

As the most abundant bioresource on Earth, cellulose is a promising candidate for materials science to move from the fossil fuel era to a more sustainable future. ${}^{[1]}$${}^{[1]}$^([1]){ }^{[1]} Cellulose elementary fibrils are incredibly strong and stiff one-dimensional nanoscale fibrils composed of extended $\beta$$\beta$beta\beta-1,4-glucan chains, which exist as structural components in the cell walls of plants, mantles of tunicates, and biofilms secreted by bacteria. ${}^{[2-4]}$${}^{[2-4]}$^([2-4]){ }^{[2-4]} These unique highly crystalline fibrils exhibit high tensile strengths (2-7.7 GPa), ${}^{[5,6]}$${}^{[5,6]}$^([5,6]){ }^{[5,6]} high elastic modulus ( $\approx 150\mathrm{GPa}$$\approx 150\mathrm{GPa}$~~150GPa\approx 150 \mathrm{GPa} ), ${}^{[6,7]}$${}^{[6,7]}$^([6,7]){ }^{[6,7]} yet low density $(\approx 1.6\mathrm{g}{\mathrm{cm}}^{-3})$$\left(\approx 1.6\mathrm{g}{\mathrm{cm}}^{-3}\right)$(~~1.6(g)cm^(-3))\left(\approx 1.6 \mathrm{~g} \mathrm{~cm}^{-3}\right), so they are seven times stronger but five times lighter than steel. ${}^{[8]}$${}^{[8]}$^([8]){ }^{[8]} Owing to its excellent mechanical performance, low density, biodegradability, and renewability, nanocellulose has recently received extensive attention for fabrication of sustainable, light weight, and robust materials. ${}^{[3,9,10]}$${}^{[3,9,10]}$^([3,9,10]){ }^{[3,9,10]} After isolation from biomass, nanocelluloses have two generic forms, further heterogeneous chemical modification of the surface, the extracted cellulose
作为地球上最丰富的生物资源，纤维素是材料科学从化石燃料时代迈向更可持续发展未来的希望所在。 ${}^{[1]}$${}^{[1]}$^([1]){ }^{[1]} 纤维素基本纤维是由延伸的 $\beta$$\beta$beta\beta -1,4-葡聚糖链组成的一维纳米级纤维，其强度和硬度令人难以置信，是植物细胞壁、鳞藻类外壳和细菌分泌的生物膜的结构成分。 ${}^{[2-4]}$${}^{[2-4]}$^([2-4]){ }^{[2-4]} 这些独特的高结晶纤维具有高拉伸强度（2-7.7 GPa）、 ${}^{[5,6]}$${}^{[5,6]}$^([5,6]){ }^{[5,6]} 高弹性模量（ $\approx 150\mathrm{GPa}$$\approx 150\mathrm{GPa}$~~150GPa\approx 150 \mathrm{GPa} ）、 ${}^{[6,7]}$${}^{[6,7]}$^([6,7]){ }^{[6,7]} 低密度 $(\approx 1.6\mathrm{g}{\mathrm{cm}}^{-3})$$\left(\approx 1.6\mathrm{g}{\mathrm{cm}}^{-3}\right)$(~~1.6(g)cm^(-3))\left(\approx 1.6 \mathrm{~g} \mathrm{~cm}^{-3}\right) ，因此它们的强度是钢的七倍，而重量却是钢的五倍。 ${}^{[8]}$${}^{[8]}$^([8]){ }^{[8]} 由于纳米纤维素具有优异的机械性能、低密度、生物可降解性和可再生性，近来在制造可持续、轻质和坚固材料方面受到广泛关注。 ${}^{[3,9,10]}$${}^{[3,9,10]}$^([3,9,10]){ }^{[3,9,10]} 从生物质中分离出纳米纤维素后，纳米纤维素有两种一般形式，一种是对其表面进行进一步的异质化学修饰，另一种是提取纤维素。
M. Zhou, Q. Chen, C. Chang
M.Zhou, Q. Chen, C. Chang

College of Chemistry and Molecular Sciences
化学与分子科学学院
Engineering Research Center of Natural Polymer-based Medical
天然聚合物医用材料工程研究中心
Materials in Hubei Province, and Laboratory of Biomedical Polymers of Ministry of Education
湖北省材料研究所和教育部生物医用高分子材料实验室
Wuhan University 武汉大学
Wuhan, Hubei 430072, P. R. China
中国湖北武汉 430072
E-mail: changcy@whu.edu.cn
电子邮件： changcy@whu.edu.cn
D. Chen D.陈

State Key Laboratory of New Textile Materials and Advanced Processing Technology
纺织新材料与先进加工技术国家重点实验室
Wuhan Textile University 武汉纺织大学
Wuhan 430073, P. R. China
中国武汉 430073
E-mail: dzchen@wtu.edu.cn
电子邮件：dzchen@wtu.edu.cn

School of Materials Science and Engineering
材料科学与工程学院
Beijing Institute of Technology
北京理工大学
Beijing 100081, P. R. China
中国北京 100081
G. Song G. 宋

Beijing National Laboratory for Molecular Sciences
北京分子科学国家实验室
CAS Key Laboratory of Engineering Plastics
中科院工程塑料重点实验室
Institute of Chemistry 化学研究所
Chinese Academy of Sciences
中国科学院
Beijing 100190, P. R. China
中国北京 100190
elementary fibrils are commonly referred to as nanocelluloses, including crystalline cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs). ${}^{[1]}$${}^{[1]}$^([1]){ }^{[1]} CNCs are obtained by preferentially hydrolyzing the longitudinal periodic amorphous regions of the cellulose with strong acids, resulting in both low yields and aspect ratios. ${}^{[11]}$${}^{[11]}$^([11]){ }^{[11]} CNFs produced by defibrillation have high aspect ratios and exhibit gel-like characteristics in water, which retain the native morphologies of the cellulose elementary fibrils. ${}^{[12,13]}$${}^{[12,13]}$^([12,13]){ }^{[12,13]}
基本纤维通常被称为纳米纤维素，包括结晶纤维素纳米晶体（CNC）和纤维素纳米纤维（CNF）。 ${}^{[1]}$${}^{[1]}$^([1]){ }^{[1]} CNC 是用强酸优先水解纤维素的纵向周期性无定形区域而获得的，因此产量和纵横比都很低。 ${}^{[11]}$${}^{[11]}$^([11]){ }^{[11]} 通过去纤维化生产的 CNF 具有高纵横比，在水中表现出凝胶状特性，保留了纤维素基本纤维的原生形态。 ${}^{[12,13]}$${}^{[12,13]}$^([12,13]){ }^{[12,13]}

Recently, mechanical treatments and chemical modification have been employed to prepare CNFs. Mechanical treatment is always conducted with the never-dried condition using a high-pressure homogenizer, grinder, or ultrasonic irradiation to produce the CNFs. ${}^{[14-18]}$${}^{[14-18]}$^([14-18]){ }^{[14-18]} Chemical or enzymatic pretreatments are required to facilitate defibrillation before the mechanical treatment. The resultant CNFs have abundant hydroxyl groups on their surfaces, which improve the material performance by forming hydrogen bonds among the CNFs. TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation is the most popular oxidation method, and it selectively introduces negative charges to the surfaces of the cellulose elementary fibrils by formation of a carboxylate group at the C6 position of glucose. ${}^{[19-21]}$${}^{[19-21]}$^([19-21]){ }^{[19-21]} After the TEMPO treatment followed by mild magnetic homogenization, CNFs with diameters of 3-5 nm are obtained, which can be evenly dispersed in water due to surface electrostatic repulsion. ${}^{[19,22]}$${}^{[19,22]}$^([19,22]){ }^{[19,22]} Although current methods are effective for the preparation of CNFs with high yields, ${}^{[18,23-26]}$${}^{[18,23-26]}$^([18,23-26]){ }^{[18,23-26]} some issues still exist; for instance, both high-energy mechanical treatments and hard oxidation processes severely reduce the lengths of the cellulose elementary fibrils, which limit the construction of all nanocellulose materials. ${}^{[5,16,27,28]}$${}^{[5,16,27,28]}$^([5,16,27,28]){ }^{[5,16,27,28]} On the other hand, CNFs produced by mechanical treatment need surface modification to improve their dispersibility, while the surface charges of TEMPOoxidized CNFs degrade the mechanical and physical properties of the nanocellulose materials due to the presence of electrostatic repulsion among CNFs. ${}^{[8,22]}$${}^{[8,22]}$^([8,22]){ }^{[8,22]} Contrasting with TEMPO-mediated and periodate oxidation, Beaumont et al. demonstrated that regioselective succinylation approach enabled complete isolation of elementary cellulose fibrils from wood pulp fibers. ${}^{[25]}$${}^{[25]}$^([25]){ }^{[25]} Although this impressive method could restore the native supramolecular interaction by removing the succinate groups, the length of resultant CNFs still significantly decreased during the fibrillation of the surface modified cellulose with a high-pressure homogenizer. Therefore, a facile, mild, and energy-efficient method for isolation of the cellulose elementary fibrils must be further developed.
最近，人们采用机械处理和化学改性来制备 CNF。机械处理通常在未干燥的状态下进行，使用高压均质机、研磨机或超声波照射来制备 CNF。 ${}^{[14-18]}$${}^{[14-18]}$^([14-18]){ }^{[14-18]} 在机械处理前需要进行化学或酶预处理以促进去纤维化。生成的 CNF 表面有丰富的羟基，这些羟基可在 CNF 之间形成氢键，从而提高材料的性能。TEMPO（2,2,6,6-四甲基哌啶-1-氧自由基）介导的氧化是最常用的氧化方法，它通过在葡萄糖的 C6 位形成羧酸基，选择性地在纤维素基本纤维表面引入负电荷。 ${}^{[19-21]}$${}^{[19-21]}$^([19-21]){ }^{[19-21]} 经过 TEMPO 处理后，再经过温和的磁性匀浆，可得到直径为 3-5 nm 的 CNFs，由于表面静电排斥作用，这些 CNFs 可均匀地分散在水中。 ${}^{[19,22]}$${}^{[19,22]}$^([19,22]){ }^{[19,22]} 虽然目前的方法可以有效地制备高产率的 CNFs，但 ${}^{[18,23-26]}$${}^{[18,23-26]}$^([18,23-26]){ }^{[18,23-26]} 仍存在一些问题，例如，高能机械处理和硬氧化过程都会严重降低纤维素基本纤维的长度，从而限制了所有纳米纤维素材料的构建。 ${}^{[5,16,27,28]}$${}^{[5,16,27,28]}$^([5,16,27,28]){ }^{[5,16,27,28]} 另一方面，机械处理产生的 CNFs 需要进行表面改性以提高其分散性，而 TEMPO 氧化 CNFs 的表面电荷会降低纳米纤维素材料的机械和物理性能，因为 CNFs 之间存在静电排斥。 ${}^{[8,22]}$${}^{[8,22]}$^([8,22]){ }^{[8,22]} 与 TEMPO 介导的氧化和高碘酸盐氧化不同，Beaumont 等人采用的是 TEMPO 氧化法。 研究表明，区域选择性琥珀酸化方法能够从木浆纤维中完全分离出基本纤维素纤维。 ${}^{[25]}$${}^{[25]}$^([25]){ }^{[25]} 尽管这种令人印象深刻的方法可以通过去除琥珀酸基团来恢复原生超分子相互作用，但在使用高压匀浆器对表面修饰的纤维素进行纤化时，所得到的 CNFs 长度仍会显著下降。因此，必须进一步开发一种简便、温和、节能的纤维素基本纤维分离方法。

Herein, for the first time, we report an energetically effective surface engineering strategy that enables the liberation of cellulose elementary fibrils from various cellulosic sources with a two-step process involving swelling and esterification (Figure 1a). We used alkali/dimethyl sulfoxide (DMSO) systems as pseudosolvents that selectively broke the hydrogen bonds and van der Waals (vdW) interactions among the less-accessible surfaces of the cellulose elementary fibrils without dissolving or degrading them, and formed highly swollen networks. Then, esterification of the available hydroxyl groups on both the accessible and less accessible surfaces of the cellulose elementary fibrils was realized by adding cyclic anhydrides into the swollen networks with mild stirring; this yielded CNFs bearing carboxyl moieties (Figure 1a) The two processes are accomplished via a one-pot reaction, and
在此，我们首次报告了一种能量有效的表面工程策略，该策略可通过涉及溶胀和酯化的两步过程从各种纤维素来源中释放出纤维素基本纤维（图 1a）。我们使用碱/二甲基亚砜（DMSO）体系作为假溶剂，在不溶解或降解纤维素基本纤维的情况下，选择性地破坏纤维素基本纤维不易接触表面之间的氢键和范德华（vdW）相互作用，形成高度膨胀的网络。然后，通过在轻度搅拌下向膨胀网络中加入环状酸酐，实现了纤维素基本纤维的可触及表面和不太容易触及表面上可用羟基的酯化；这样就得到了带有羧基的 CNF（图 1a）。
the cellulose pulp was converted into a highly stable CNF suspension due to electrostatic repulsions among the CNFs (Figure 1b). Our strategy preserved the morphologies and sizes of the elementary fibrils without mechanical or chemical damage to the potential longitudinal amorphous region and provided CNFs with ultrahigh aspect ratios ( $\approx 1400$$\approx 1400$~~1400\approx 1400 ) (Figure 1c). In comparison with other reported methods, such as TEMPO oxidation, periodate oxidation, and acid hydrolysis, ${}^{[21,29,30]}$${}^{[21,29,30]}$^([21,29,30]){ }^{[21,29,30]} this surface engineering strategy consumed less energy ( $\approx 12.4\mathrm{kJ}/\mathrm{g}$$\approx 12.4\mathrm{kJ}/\mathrm{g}$~~12.4kJ//g\approx 12.4 \mathrm{~kJ} / \mathrm{g} ) and provided higher yields (98%) (Table S1, Supporting Information). Furthermore, the surface charges of the CNFs can be removed easily by saponification, which offers significant opportunities for reconstruction of the hydrogen bonds and vdW interactions among the CNFs and even restoring the surface hydrogen bonding networks of native celluloses. This energy-effective and sustainable method for the isolation of CNFs with ultrahigh aspect ratios provides a new platform for the construction of advanced cellulosic materials by fully utilizing the features of the cellulose elementary fibrils.
由于 CNF 之间存在静电排斥，纤维素浆被转化为高度稳定的 CNF 悬浮液（图 1b）。我们的策略保留了基本纤维的形态和尺寸，而不会对潜在的纵向无定形区造成机械或化学损伤，并提供了具有超高纵横比（ $\approx 1400$$\approx 1400$~~1400\approx 1400 ）的 CNF（图 1c）。与其他已报道的方法（如 TEMPO 氧化、高碘酸盐氧化和酸水解）相比， ${}^{[21,29,30]}$${}^{[21,29,30]}$^([21,29,30]){ }^{[21,29,30]} 这种表面工程策略消耗的能量更少（ $\approx 12.4\mathrm{kJ}/\mathrm{g}$$\approx 12.4\mathrm{kJ}/\mathrm{g}$~~12.4kJ//g\approx 12.4 \mathrm{~kJ} / \mathrm{g} ），产率更高（98%）（表 S1，辅助信息）。此外，CNFs 的表面电荷很容易通过皂化去除，这为重建 CNFs 之间的氢键和 vdW 相互作用，甚至恢复原生纤维素的表面氢键网络提供了重要机会。这种高能效、可持续的超高纵横比 CNFs 分离方法充分利用了纤维素基本纤维的特性，为先进纤维素材料的构建提供了一个新平台。

Native cellulose can be dissolved in lithium chloride/ $N,N^{-}$$N,N^{-}$N,N^(-)N, N^{-} dimethylacetamide ( $\mathrm{LiCl}/\mathrm{DMAc}$$\mathrm{LiCl}/\mathrm{DMAc}$LiCl//DMAc\mathrm{LiCl} / \mathrm{DMAc} ) at high temperatures ( $80{}^{\circ }\mathrm{C}$$80{}^{\circ }\mathrm{C}$80^(@)C80{ }^{\circ} \mathrm{C} ) and lithium hydroxide ( LiOH )/urea aqueous solution at low temperatures $(-5^{\circ }\mathrm{C}).{}^{[31,32]}$$\left(-5^{\circ }\mathrm{C}\right).{}^{[31,32]}$(-5^(@)C).^([31,32])\left(-5^{\circ} \mathrm{C}\right) .{ }^{[31,32]} Based on these solvent systems, we developed pseudosolvents to selectively reduce hydrogen bonding and weaken the vdW interactions among the cellulose elementary fibrils while maintaining the crystallinity and morphology of the cellulose elementary fibrils. Unlike dissolution, in which cellulose is dispersed into the solvent at the single chain level, pseudosolvents are expected to efficiently split the packed elementary fibrils into entangled cellulose networks and allow penetration and grafting of reactive reagents on the surfaces of the cellulose elementary fibrils without penetration into the crystalline core. Based on the dissolution mechanism for cellulose, ${}^{[33-35]}$${}^{[33-35]}$^([33-35]){ }^{[33-35]} alkali/DMSO mixtures were used as pseudosolvents because they can disrupt hydrogen bonding and vdW interactions among cellulose elementary fibrils at room temperature.
原生纤维素可在高温下溶解于氯化锂/ $N,N^{-}$$N,N^{-}$N,N^(-)N, N^{-} 二甲基乙酰胺（ $\mathrm{LiCl}/\mathrm{DMAc}$$\mathrm{LiCl}/\mathrm{DMAc}$LiCl//DMAc\mathrm{LiCl} / \mathrm{DMAc} ）中（ $80{}^{\circ }\mathrm{C}$$80{}^{\circ }\mathrm{C}$80^(@)C80{ }^{\circ} \mathrm{C} ），在低温下溶解于氢氧化锂（LiOH）/尿素水溶液中（ $(-5^{\circ }\mathrm{C}).{}^{[31,32]}$$\left(-5^{\circ }\mathrm{C}\right).{}^{[31,32]}$(-5^(@)C).^([31,32])\left(-5^{\circ} \mathrm{C}\right) .{ }^{[31,32]} ）、 $(-5^{\circ }\mathrm{C}).{}^{[31,32]}$$\left(-5^{\circ }\mathrm{C}\right).{}^{[31,32]}$(-5^(@)C).^([31,32])\left(-5^{\circ} \mathrm{C}\right) .{ }^{[31,32]} 在这些溶剂系统的基础上，我们开发出了假溶剂，可选择性地减少纤维素基本纤维之间的氢键和 vdW 相互作用，同时保持纤维素基本纤维的结晶度和形态。在溶解过程中，纤维素会在单链水平上分散到溶剂中，与此不同的是，预计伪溶剂会有效地将包装好的纤维素基本纤丝分割成纠缠在一起的纤维素网络，并允许反应试剂在纤维素基本纤丝表面渗透和接枝，而不会渗透到结晶核心。根据纤维素的溶解机制， ${}^{[33-35]}$${}^{[33-35]}$^([33-35]){ }^{[33-35]} 碱/二甲基亚砜混合物被用作假溶剂，因为它们在室温下可以破坏纤维素基本纤维之间的氢键和 vdW 相互作用。

As shown in Figure 2a, large cellulose fiber bundles (several micrometers in diameter) were observed in native tunicate cellulose which were converted into microfibrils with diameters of tens to hundreds of nanometers after swelling for 8 h in the pseudosolvent comprising DMSO saturated with $\mathrm{LiOH}\left(2\mathrm{mg}{\mathrm{mL}}^{-1}\right)$$\mathrm{LiOH}\left(2\mathrm{mg}{\mathrm{mL}}^{-1}\right)$LiOH(2mgmL^(-1))\mathrm{LiOH}\left(2 \mathrm{mg} \mathrm{mL}^{-1}\right). When the swelling time was extended to 24 h , the cross-sections of cellulose microfibrils gradually split at the nanoscale, whereas the structures of the cellulose elementary fibrils (several nanometers in diameter) were well preserved in the nanonetworks. The resultant swollen cellulose exhibited gel-like behavior and high viscosity, so we studied the viscosity changes during the swelling process (Figure 2b and Figure S1, Supporting Information) with rheological technology. As the swelling time was increased from 1 to 8 h , the apparent viscosity of the suspension rapidly increased from 19.1 to 450.2 Pa s at a shear rate of $0.1{\mathrm{s}}^{-1}$$0.1{\mathrm{s}}^{-1}$0.1s^(-1)0.1 \mathrm{~s}^{-1} (Figure S1a, Supporting Information), indicating disassembly of the cellulose fiber bundles into microfibrils. The pseudosolvent penetrated the microfibrils and even into the spaces between adjacent elementary fibrils, which formed translucent, gel-like, and highly
如图 2a 所示，在含有 $\mathrm{LiOH}\left(2\mathrm{mg}{\mathrm{mL}}^{-1}\right)$$\mathrm{LiOH}\left(2\mathrm{mg}{\mathrm{mL}}^{-1}\right)$LiOH(2mgmL^(-1))\mathrm{LiOH}\left(2 \mathrm{mg} \mathrm{mL}^{-1}\right) 的饱和二甲基亚砜组成的伪溶剂中溶胀 8 小时后，原生unicate 纤维素中出现了大纤维素纤维束（直径为几微米），并转化为直径为几十到几百纳米的微纤维。当溶胀时间延长到 24 小时时，纤维素微纤维的横截面逐渐在纳米尺度上分裂，而纤维素基本纤维（直径为几纳米）的结构在纳米网络中得到了很好的保留。因此，我们利用流变学技术研究了溶胀过程中的粘度变化（图 2b 和图 S1，佐证资料）。随着溶胀时间从 1 小时增加到 8 小时，在 $0.1{\mathrm{s}}^{-1}$$0.1{\mathrm{s}}^{-1}$0.1s^(-1)0.1 \mathrm{~s}^{-1} 的剪切速率下，悬浮液的表观粘度从 19.1 Pa s 迅速增加到 450.2 Pa s（图 S1a，佐证资料），表明纤维素纤维束分解成了微纤维。伪溶剂渗入微纤维，甚至渗入相邻基本纤维之间的空隙，形成半透明、凝胶状和高度透明的纤维素纤维束。

Figure 1. Surface engineering strategy for the cellulose nanofibrils (CNFs). a) Exfoliation of cellulose elementary fibrils via a two-step approach and advanced cellulose material assembly via surface deionization. The cyclic anhydrides used for esterification of cellulose include: succinic anhydride (SA), maleic anhydride (MA), phthalic anhydride (PA), octenyl succinic anhydride (OSA), and dodecenylsuccinic anhydride (DSA). b) Photographs of tunicate cellulose pulp, swollen tunicate cellulose, and phthalic anhydride esterified tunicate CNF suspensions. c) TEM image of tunicate CNFs with ultrahigh aspect ratio.
图 1：纤维素纳米纤维（CNFs）的表面工程策略。a) 通过两步法剥离纤维素基本纤维，并通过表面去离子法组装高级纤维素材料。用于酯化纤维素的环状酸酐包括：琥珀酸酐（SA）、马来酸酐（MA）、邻苯二甲酸酐（PA）、辛烯基琥珀酸酐（OSA）和十二烯基丁二酸酐（DSA）。b) 鳞片纤维素浆、膨胀的鳞片纤维素和邻苯二甲酸酐酯化的鳞片状 CNF 悬浮液的照片。
entangled fibrils with high viscosity. As the swelling time was increased further to $8-24\mathrm{h}$$8-24\mathrm{h}$8-24h8-24 \mathrm{~h}, the viscosity of the system gradually decreased (Figure S1b, Supporting Information), which was attributed to disruption of the hydrogen bonds among the elementary fibrils. Because the viscosity hardly changed after swelling for 20 h , we speculated that the cellulose was fully swollen, and the elementary fibrils had more individual characteristics rather than being entangled or cross-linked into bundles. Additionally, the pseudosolvent was allowed to freely diffuse among the elementary fibrils, which enabled the transfer and contact of reagents between the elementary fibrils.
缠结的纤维具有高粘度。随着溶胀时间进一步延长至 $8-24\mathrm{h}$$8-24\mathrm{h}$8-24h8-24 \mathrm{~h} ，体系的粘度逐渐下降（图 S1b，佐证资料），这是因为基本纤维之间的氢键被破坏了。由于溶胀 20 h 后粘度几乎没有变化，我们推测纤维素已完全溶胀，基本纤维具有更多的个体特征，而不是纠缠或交联成束。此外，假溶剂可以在基本纤维之间自由扩散，这使得试剂可以在基本纤维之间转移和接触。

Small-angle X-ray scattering (SAXS) measurements were used to characterize the structural changes occurring in the cellulose during swelling in LiOH/DMSO (Figure 2c). Before swelling, a nearly straight double logarithmic SAXS curve for the tunicate
小角 X 射线散射 (SAXS) 测量用于描述纤维素在 LiOH/DMSO 中溶胀过程中发生的结构变化（图 2c）。溶胀前，单胞纤维素的 SAXS 曲线几乎是笔直的双对数曲线。
cellulose decayed with a power law of -3.33 , which was related to the smooth surface of the bundled cellulose consisting of elementary fibrils packed along the axis. Because the intimate proximity between cellulose elementary fibrils decreased the electron density contrast, SAXS measurements may not distinguish the widths of the individual fibrils. ${}^{[36]}$${}^{[36]}$^([36]){ }^{[36]} Therefore, the scattering intensity of the sample was attributed to the pore structure rather than the interfaces of the elementary fibrils. After swelling for 8 h or 24 h , knee-shaped lines were observed in scattering patterns because of increases in the spacing and electron density fluctuations at the interfaces between the elementary fibrils. The data were fitted and analyzed with the generalized Guinier-Porod model. ${}^{[37]}$${}^{[37]}$^([37]){ }^{[37]} The Porod exponent ( $n$$n$nn ) describes the surface morphologies of the scattering particles, e.g., $n=4$$n=4$n=4n=4 for particles with smooth surfaces, and $n=3$$n=3$n=3n=3 indicates a rough surface or
纤维素的衰减幂律为-3.33，这与纤维素束的表面光滑有关，纤维素束由沿轴线排列的基本纤维组成。由于纤维素基本纤维之间的紧密性降低了电子密度对比度，SAXS 测量可能无法区分单个纤维的宽度。 ${}^{[36]}$${}^{[36]}$^([36]){ }^{[36]} 因此，样品的散射强度归因于孔隙结构，而不是基本纤维的界面。膨胀 8 小时或 24 小时后，散射图案中出现了膝形线条，这是因为基本纤维之间的间距和界面处的电子密度波动增加了。数据采用广义 Guinier-Porod 模型进行拟合和分析。 ${}^{[37]}$${}^{[37]}$^([37]){ }^{[37]} 波罗指数（ $n$$n$nn ）描述了散射粒子的表面形态，例如， $n=4$$n=4$n=4n=4 表示表面光滑的粒子， $n=3$$n=3$n=3n=3 表示表面粗糙或粗糙的粒子。

Figure 2. Swelling of cellulose in pseudosolvents. a) Schematic diagram and scanning electron microscope (SEM) images of tunicate cellulose in $\mathrm{LiOH}/$$\mathrm{LiOH}/$LiOH//\mathrm{LiOH} / dimethyl sulfoxide (DMSO). b) Apparent viscosity of tunicate cellulose in LiOH/DMSO under a shear rate of $0.1{\mathrm{s}}^{-1}$$0.1{\mathrm{s}}^{-1}$0.1s^(-1)0.1 \mathrm{~s}^{-1}.c) Small-angle X-ray scattering (SAXS) profiles of tunicate cellulose in LiOH/DMSO with different swelling times. d) Raman spectra of DMSO, tunicate cellulose, and the tunicate cellulose/LiOH/DMSO suspension. e) Wide-angle X-ray scattering (WAXS) profiles of tunicate cellulose before and after swelling.
图 2：纤维素在伪溶剂中的膨胀a) $\mathrm{LiOH}/$$\mathrm{LiOH}/$LiOH//\mathrm{LiOH} / 二甲基亚砜（DMSO）中单纤维素的示意图和扫描电子显微镜（SEM）图像。 b) $0.1{\mathrm{s}}^{-1}$$0.1{\mathrm{s}}^{-1}$0.1s^(-1)0.1 \mathrm{~s}^{-1} 剪切速率下 LiOH/DMSO 中单纤维素的表观粘度。c) 不同溶胀时间下单列纤维素在 LiOH/DMSO 中的小角 X 射线散射（SAXS）图谱。 d) DMSO、单列纤维素和单列纤维素/LiOH/DMSO 悬浮液的拉曼光谱。
“collapsed” polymer chains. In our SAXS measurements, the $n$$n$nn values of the samples after swelling for 8 h and 24 h were 3.82 and 3.97, respectively, because the highly crystalline elementary fibrils had smooth surfaces and sharp interfaces after swelling. ${}^{[38]}$${}^{[38]}$^([38]){ }^{[38]} Furthermore, the $s$$s$ss parameter depended on the shapes and dimensions of the particles (rods: $s=1$$s=1$s=1s=1 and lamellae: $s=2$$s=2$s=2s=2 ). The $s$$s$ss values of tunicate cellulose were 1.54 and 1.57 after swelling for 8 h and 24 h , respectively, which was related to the ribbon shape of the CNFs. ${}^{[38,39]}$${}^{[38,39]}$^([38,39]){ }^{[38,39]} Moreover, the calculated diameters of the scattering objects ( $D$$D$DD ) for the swollen cellulose were 9.36 and 8.82 after 8 h and 24 h , respectively, which reflected the sizes of the elementary fibrils (Table S2, Supporting Information). These results suggested that the interfaces between elementary fibrils were formed after swelling for 8 h and that the networks composed of cellulose elementary fibrils became looser after swelling for 24 h . Based on the rheological and SAXS results, a swelling time of 24 h was used for further investigation.
"塌缩 "聚合物链。在我们的 SAXS 测量中，样品在溶胀 8 小时和 24 小时后的 $n$$n$nn 值分别为 3.82 和 3.97，这是因为高结晶的基本纤维在溶胀后表面光滑，界面锐利。 ${}^{[38]}$${}^{[38]}$^([38]){ }^{[38]} 此外， $s$$s$ss 参数取决于颗粒的形状和尺寸（棒状： $s=1$$s=1$s=1s=1 和片状： $s=2$$s=2$s=2s=2 ）。膨化 8 小时和 24 小时后，簇状纤维素的 $s$$s$ss 值分别为 1.54 和 1.57，这与 CNF 的带状形状有关。 ${}^{[38,39]}$${}^{[38,39]}$^([38,39]){ }^{[38,39]} 此外，膨胀纤维素的散射物体（ $D$$D$DD ）的计算直径在 8 小时和 24 小时后分别为 9.36 和 8.82，这反映了基本纤维的尺寸（表 S2，佐证资料）。这些结果表明，纤维素基本纤维之间的界面在溶胀 8 小时后形成，而纤维素基本纤维组成的网络在溶胀 24 小时后变得更加松散。根据流变学和 SAXS 的结果，进一步的研究采用了 24 小时的溶胀时间。

Although LiOH was incompletely soluble in DMSO (Figure S2a,b, Supporting Information), the precipitate disappeared after adding cellulose into the LiOH/DMSO mixture. The Raman peak for DMSO $(\mathrm{S}=\mathrm{O})$$(\mathrm{S}=\mathrm{O})$(S=O)(\mathrm{S}=\mathrm{O}) shifted from 1048 to $1020{\mathrm{cm}}^{-1}$$1020{\mathrm{cm}}^{-1}$1020cm^(-1)1020 \mathrm{~cm}^{-1} in the spectrum of cellulose/LiOH/DMSO (Figure 2d), indicating that
虽然 LiOH 不完全溶于二甲基亚砜（图 S2a、b，佐证资料），但在 LiOH/DMSO 混合物中加入纤维素后，沉淀消失了。在纤维素/LiOH/DMSO 的光谱中，DMSO 的 $(\mathrm{S}=\mathrm{O})$$(\mathrm{S}=\mathrm{O})$(S=O)(\mathrm{S}=\mathrm{O}) 拉曼峰从 1048 转移到 $1020{\mathrm{cm}}^{-1}$$1020{\mathrm{cm}}^{-1}$1020cm^(-1)1020 \mathrm{~cm}^{-1} （图 2d），这表明
the hydroxyl groups of the cellulose also participated in polar coordination with the LiOH. Therefore, we speculated that stable cellulose/LiOH/DMSO complexes were formed through hydrogen bonding, electrostatic interaction, vdW interaction (Figure S2c, Supporting Information), ${}^{[40]}$${}^{[40]}$^([40]){ }^{[40]} and the hydrophobic interaction (weakly polar methyl group of DMSO) that also played a significant role in the intercalation between the stacked cellulose 200 crystal planes. Based on the viscosity (Figure S3a, Supporting Information), swelling of the cellulose in the pseudosolvent was affected by the ion diameters, alkalinity, and solvent polarity, and LiOH/DMSO had the best swelling effect on cellulose (Figure S3b, Supporting Information). The crystallinity of cellulose remained almost unchanged after swelling in the pseudosolvent (Figure 2e and Figure S4, Supporting Information). Besides, we measured the molar mass of cellulose before and after the swelling by using size exclusion chromatography (SEC), where the molecular weight of cellulose changed hardly (Figure S5, Supporting Information). These results indicated that the LiOH/DMSO selectively disassembled the hydrogen bonding and vdW interactions among the cellulose elementary fibrils and maintained the original crystal structures and the degree of polymerization of the cellulose. Compared to other solvents
纤维素的羟基也参与了与 LiOH 的极性配位。因此，我们推测稳定的纤维素/LiOH/DMSO 复合物是通过氢键、静电作用、vdW 作用（图 S2c，佐证资料）、 ${}^{[40]}$${}^{[40]}$^([40]){ }^{[40]} 和疏水作用（DMSO 的弱极性甲基）形成的，这些作用在堆叠的纤维素 200 晶面之间的插层中也发挥了重要作用。根据粘度（图 S3a，佐证资料），纤维素在假溶剂中的溶胀受离子直径、碱度和溶剂极性的影响，其中 LiOH/DMSO 对纤维素的溶胀效果最好（图 S3b，佐证资料）。在假溶剂中溶胀后，纤维素的结晶度几乎保持不变（图 2e 和图 S4，佐证资料）。此外，我们还利用尺寸排阻色谱法（SEC）测量了溶胀前后纤维素的摩尔质量，发现纤维素的分子量几乎没有变化（图 S5，佐证资料）。这些结果表明，LiOH/DMSO 选择性地分解了纤维素基本纤维之间的氢键和 vdW 相互作用，保持了纤维素原有的晶体结构和聚合度。与其他溶剂相比

Figure 3. Structure and morphology of cellulose and cellulose nanofibrils (CNFs). a) Fourier transform infrared spectrum (FTIR) and b) solid-state ${}^{13}\mathrm{C}$${}^{13}\mathrm{C}$^(13)C{ }^{13} \mathrm{C} NMR spectra of tunicate cellulose and the phthalic anhydride esterified CNFs. c) Wide-angle X-ray scattering (WAXS) profile of the tunicate CNFs. d) Morphology of CNFs exfoliated from different cellulose sources. e) Diameter and length distribution of the tunicate CNFs. f) The aspect ratios of the CNFs in this work are higher than those reported in the literature. g) A radar chart illustrates the advantages of our strategy presented in this work. ${}^{[21,29,30].}$${}^{[21,29,30].}$^([21,29,30].){ }^{[21,29,30] .}
图 3：纤维素和纤维素纳米纤维（CNFs）的结构和形态。a) 单胞纤维素和邻苯二甲酸酐酯化 CNFs 的傅立叶变换红外光谱 (FTIR) 和 b) 固态 ${}^{13}\mathrm{C}$${}^{13}\mathrm{C}$^(13)C{ }^{13} \mathrm{C} NMR 光谱。 c) 单胞 CNFs 的广角 X 射线散射 (WAXS) 图谱。d) 从不同纤维素来源剥离的 CNFs 的形态。 e) 鳞片状 CNFs 的直径和长度分布。 f) 本研究中 CNFs 的纵横比高于文献报道的纵横比。 g) 雷达图说明了我们在本研究中提出的策略的优势。 ${}^{[21,29,30].}$${}^{[21,29,30].}$^([21,29,30].){ }^{[21,29,30] .}
(Table S3, Supporting Information), the LiOH/DMSO system avoided the negative effects of dissolution, degradation, amorphization, heating, and mechanical treatments (e.g., highpressure homogenization or ultrasonication) on cellulose, resulting in the formation of highly accessible nanoscale networks composed of cellulose elementary fibrils.
(表 S3，佐证资料），LiOH/DMSO 系统避免了溶解、降解、变质、加热和机械处理（如高压均质或超声）对纤维素的负面影响，从而形成了由纤维素基本纤维组成的高度可访问的纳米级网络。

After full swelling in the pseudosolvent, phthalic anhydride was used for surface functionalization of the cellulose elementary fibrils. Esterification was carried out without an added catalyst due to the alkalinity of the pseudosolvent, which activated the hydroxyl groups. The yield and carboxylate content of the tunicate CNFs were tailored by varying the reaction conditions (Figure S6, Note S1, Supporting Information). Besides, four other cyclic anhydrides, including succinic anhydride, maleic anhydride, octenylsuccinic anhydride, and dodecenylsuccinic anhydride, were employed to modify tunicate cellulose (Figure S7a, Note S2, Supporting Information). The yields and carboxylate contents of the resultant CNFs were 65.6-97.6% and 0.565$1.179\mathrm{mmol}\mathrm{g}{}^{-1}$$1.179\mathrm{mmol}\mathrm{g}{}^{-1}$1.179mmolg^(-1)1.179 \mathrm{mmol} \mathrm{g}{ }^{-1}, respectively (Figure S7b, Supporting Information), and the carboxyl contents of the CNFs were determined by conductometric titration (Figure S7c, Supporting Information). After they were added to the cellulose/LiOH/DMSO sus-
在假溶剂中充分溶胀后，使用邻苯二甲酸酐对纤维素基本纤维进行表面功能化。由于伪溶剂的碱性可激活羟基，因此酯化过程无需添加催化剂。通过改变反应条件，可定制单宁酸盐 CNF 的产率和羧酸盐含量（图 S6，注 S1，佐证资料）。此外，还采用了其他四种环酸酐，包括琥珀酸酐、马来酸酐、辛烯基丁二酸酐和十二烯基丁二酸酐来改性单宁酸纤维素（图 S7a，注 S2，证明资料）。CNFs的产率和羧基含量分别为65.6-97.6%和0.565 $1.179\mathrm{mmol}\mathrm{g}{}^{-1}$$1.179\mathrm{mmol}\mathrm{g}{}^{-1}$1.179mmolg^(-1)1.179 \mathrm{mmol} \mathrm{g}{ }^{-1} （图S7b，佐证资料），CNFs的羧基含量通过电导滴定法测定（图S7c，佐证资料）。将 CNFs 加入到纤维素/LiOH/DMSO 的悬浮液中后，用电导滴定法测定其羧基含量（图 S7c，支持信息）。
pension, the cyclic anhydrides preferentially formed ester bonds with hydroxyl groups through ring opening, which introduced negative charges on the surfaces of the CNFs and prohibited reformulation of the hydrogen bonds between the CNFs. Successful esterification was indicated by the Fourier transform infrared spectrum (FTIR), in which the C=O stretching peaks of the ester groups and carboxylate groups appeared at 1724 and $1562{\mathrm{cm}}^{-1}$$1562{\mathrm{cm}}^{-1}$1562cm^(-1)1562 \mathrm{~cm}^{-1}, respectively (Figures 3a and S8, Supporting Information). ${}^{[25]}$${}^{[25]}$^([25]){ }^{[25]} Moreover, esterification of the tunicate CNFs with phthalic anhydride was verified by the NMR signals for the carbonyl ester groups (C7) at 178.3 ppm and the aromatic rings (C8-C13) centered at 125.8 ppm in the solid-state ${}^{13}\mathrm{C}$${}^{13}\mathrm{C}$^(13)C{ }^{13} \mathrm{C} NMR spectrum (Figure 3b). ${}^{[41]}$${}^{[41]}$^([41]){ }^{[41]} After deconvolution of the C4 peak, peaks at 90 and 85 ppm were observed, which were assigned to the inner crystalline core and externally accessible/less accessible surfaces of the CNFs, respectively. ${}^{[42]}$${}^{[42]}$^([42]){ }^{[42]} Compared to the native tunicate cellulose, the crystallinities of the CNFs modified by phthalic anhydride decreased slightly from $85.7\mathrm{\%}$$85.7\mathrm{\%}$85.7%85.7 \% to $84.4\mathrm{\%}$$84.4\mathrm{\%}$84.4%84.4 \% (as shown by NMR), while XRD indicated a lower crystallinity of $70.3\mathrm{\%}$$70.3\mathrm{\%}$70.3%70.3 \% for the CNFs; this was attributed to preferential orientation of the CNFs along the X-ray beam during the X-ray diffraction measurements since the peak intensity increased for the 1-10 planes and decreased for the 200 planes (Figure 3c). These signs indicated that the modifications occurred on the fibrillar surfaces and that the cellulose I crystals remained intact in the nanofibrils. Similarly, the crystallinities of the tunicate CNFs determined by XRD
养恤金方面，环状酸酐通过开环优先与羟基形成酯键，这在 CNF 表面引入了负电荷，阻碍了 CNF 之间氢键的重塑。傅立叶变换红外光谱（FTIR）显示酯化成功，其中酯基和羧基的 C=O 伸展峰分别出现在 1724 和 $1562{\mathrm{cm}}^{-1}$$1562{\mathrm{cm}}^{-1}$1562cm^(-1)1562 \mathrm{~cm}^{-1} 处（图 3a 和 S8，佐证资料）。 ${}^{[25]}$${}^{[25]}$^([25]){ }^{[25]} 此外，在固态 ${}^{13}\mathrm{C}$${}^{13}\mathrm{C}$^(13)C{ }^{13} \mathrm{C} NMR 光谱中，以 125.8 ppm 为中心的羰基酯基（C7）和芳香环（C8-C13）的 NMR 信号验证了邻苯二甲酸酐对鳞片状 CNF 的酯化作用（图 3b）。 ${}^{[41]}$${}^{[41]}$^([41]){ }^{[41]} 对 C4 峰进行解卷积后，观察到 90 和 85 ppm 的峰，这两个峰分别归属于 CNF 的内部晶核和外部可触及/不可触及表面。 ${}^{[42]}$${}^{[42]}$^([42]){ }^{[42]} 与原生单胞纤维素相比，邻苯二甲酸酐修饰的 CNF 结晶度略有下降，从 $85.7\mathrm{\%}$$85.7\mathrm{\%}$85.7%85.7 \% 降至 $84.4\mathrm{\%}$$84.4\mathrm{\%}$84.4%84.4 \% （如 NMR 所示），而 XRD 显示 CNF 的结晶度较低，为 $70.3\mathrm{\%}$$70.3\mathrm{\%}$70.3%70.3 \% ；这归因于在 X 射线衍射测量过程中 CNF 沿着 X 射线束的优先取向，因为 1-10 平面的峰值强度增加了，而 200 平面的峰值强度降低了（图 3c）。这些迹象表明，改性发生在纤维表面，而纤维素 I 晶体在纳米纤维中保持完整。同样，通过 XRD 测定的单胞氯化纤维素的结晶度为
decreased to 51.1-67.6% after modification with other cyclic anhydrides (Figure S9, Supporting Information). The yields and chemical and crystal structures of CNFs from other bioresources, such as softwood, cotton, bagasse, and bacteria, were also characterized (Figures S10 and S11 and Table S4, Supporting Information), revealing that the cyclic anhydrides were homogeneously grafted onto the surface hydroxyls of the CNFs, and the crystalline structures of the elementary fibrils were well preserved.
用其他环酐改性后，CNF 的产率降至 51.1-67.6%（图 S9，佐证资料）。对其他生物资源（如软木、棉花、甘蔗渣和细菌）的 CNF 产量、化学结构和晶体结构也进行了表征（图 S10 和 S11 以及表 S4，"支持信息"），结果表明环酐均匀地接枝到 CNF 的表面羟基上，基本纤维的晶体结构保存完好。

The morphologies of CNFs exfoliated from different cellulose sources via modification with phthalic anhydride are shown in Figure 3d. All of the raw cellulose materials were converted into 1D nanofibrils, and the diameters and lengths were measured by AFM (Figure S12, Supporting Information) and statistically analyzed with “FiberApp” software (Figure S13, Supporting Information). For the tunicate cellulose, the average diameter and length of the CNFs were approximately 7.1 nm and $9.8\mu \mathrm{m}$$9.8\mu \mathrm{m}$9.8 mum9.8 \mu \mathrm{~m}, respectively, demonstrating an ultrahigh aspect ratio of approximately 1400 (Figure 3e). Similarly, the tunicate CNFs esterified with other anhydrides also exhibited ultrahigh aspect ratios of 1000-1300 (Figures S14 and S15, Supporting Information). It is worth noting that the average length of PA esterified CNFs (tunicate) significantly decreased to $3.4\mu \mathrm{m}$$3.4\mu \mathrm{m}$3.4 mum3.4 \mu \mathrm{~m} after being treated under 500 W ultrasound for 20 min (Figure S16, Supporting Information). We also confirmed that the average length of TEMPO oxidized CNFs (tunicate) was $3.8\mu \mathrm{m}$$3.8\mu \mathrm{m}$3.8 mum3.8 \mu \mathrm{~m} (Figure S17, Supporting Information), due to the high-energy mechanical processing. ${}^{[43]}$${}^{[43]}$^([43]){ }^{[43]} In the case of bacterial cellulose, the average diameter and length of the CNFs were approximately 18.6 nm and $21\mu \mathrm{m}$$21\mu \mathrm{m}$21 mum21 \mu \mathrm{~m}, respectively, which were much higher than those of the CNFs from plant-derived cellulose. For instance, the CNFs from softwood, cotton, and bagasse had smaller diameters ( $2-5\mathrm{nm}$$2-5\mathrm{nm}$2-5nm2-5 \mathrm{~nm} ) and aspect ratios (300-500), which were consistent with the reported cellulose elementary fibrils. ${}^{[1,44]}$${}^{[1,44]}$^([1,44]){ }^{[1,44]} The difference in the dimensions of various CNFs was attributed to the diversity of cellulose samples synthesized in different biological sources, which also corresponded to the results from wide-angle X-ray scattering (WAXS) characterization (Table S4, Supporting Information). Because strong mechanical disintegration (such as high-pressure homogenization, ball milling, or ultrasound) was avoided, the tunicate CNFs produced with our strategy exhibited higher aspect ratios than those prepared by other methods, such as enzymolysis, carboxymethylation, acid hydrolysis, TEMPO oxidation, and periodate oxidization (Figure 3f and Table S5, Supporting Information). Moreover, the pseudosolvent/reactants and ethanol used for the preparation of CNFs could be recycled with a high recovery ratio, while CNFs could be fabricated by using recycled reagents (Figure S18, Supporting Information). Therefore, this work provides a convenient, flexible, energy-efficient ( $12.4\mathrm{kJ}{\mathrm{g}}^{-1}$$12.4\mathrm{kJ}{\mathrm{g}}^{-1}$12.4kJg^(-1)12.4 \mathrm{~kJ} \mathrm{~g}^{-1} ), and sustainable method for producing CNFs with high yields ( $\approx 98\mathrm{\%}$$\approx 98\mathrm{\%}$~~98%\approx 98 \% ) and ultrahigh aspect ratios (Figure 3g and Note S3, Supporting Information).
图 3d 显示了从不同纤维素来源经邻苯二甲酸酐改性剥离的 CNFs 的形态。将所有纤维素原料转化为一维纳米纤维，用原子力显微镜测量其直径和长度（图 S12，佐证资料），并用 "FiberApp "软件进行统计分析（图 S13，佐证资料）。对于unicate纤维素，CNFs的平均直径和长度分别约为7.1 nm和 $9.8\mu \mathrm{m}$$9.8\mu \mathrm{m}$9.8 mum9.8 \mu \mathrm{~m} ，显示出约1400的超高纵横比（图3e）。同样，与其他酸酐酯化的鳞片状 CNF 也表现出 1000-1300 的超高纵横比（图 S14 和 S15，佐证资料）。值得注意的是，在 500 W 超声波下处理 20 分钟后，PA 酯化 CNFs（unicate）的平均长度明显降低至 $3.4\mu \mathrm{m}$$3.4\mu \mathrm{m}$3.4 mum3.4 \mu \mathrm{~m} （图 S16，佐证资料）。我们还证实，由于高能机械加工，TEMPO 氧化 CNFs（unicate）的平均长度为 $3.8\mu \mathrm{m}$$3.8\mu \mathrm{m}$3.8 mum3.8 \mu \mathrm{~m} （图 S17，佐证资料）。 ${}^{[43]}$${}^{[43]}$^([43]){ }^{[43]} 细菌纤维素的 CNFs 平均直径和长度分别约为 18.6 nm 和 $21\mu \mathrm{m}$$21\mu \mathrm{m}$21 mum21 \mu \mathrm{~m} ，远高于植物纤维素的 CNFs。例如，来自软木、棉花和甘蔗渣的 CNFs 的直径（ $2-5\mathrm{nm}$$2-5\mathrm{nm}$2-5nm2-5 \mathrm{~nm} ）和纵横比（300-500）都较小，这与已报道的纤维素基本纤维一致。 ${}^{[1,44]}$${}^{[1,44]}$^([1,44]){ }^{[1,44]} 不同 CNFs 的尺寸差异归因于不同生物来源合成的纤维素样品的多样性，这也与广角 X 射线散射 (WAXS) 表征的结果相符（表 S4，佐证资料）。由于避免了强烈的机械分解（如高压均质、球磨机或超声波），与其他方法（如酶解、羧甲基化、酸水解、TEMPO 氧化和高碘酸盐氧化）相比，用我们的策略制备的鳞片状 CNF 具有更高的纵横比（图 3f 和表 S5，佐证资料）。此外，用于制备 CNFs 的假溶剂/反应物和乙醇可回收利用，回收率高，同时可利用回收试剂制备 CNFs（图 S18，佐证资料）。因此，这项工作为制备高产率（ $\approx 98\mathrm{\%}$$\approx 98\mathrm{\%}$~~98%\approx 98 \% ）和超高纵横比的 CNFs 提供了一种方便、灵活、节能（ $12.4\mathrm{kJ}{\mathrm{g}}^{-1}$$12.4\mathrm{kJ}{\mathrm{g}}^{-1}$12.4kJg^(-1)12.4 \mathrm{~kJ} \mathrm{~g}^{-1} ）和可持续的方法（图 3g 和注 S3，佐证资料）。

Due to the presence of negative charges, the CNFs were homogeneously dispersed in water. For example, the benzoic acid groups provided negative charges in the CNFs, which could be protonated at low $\mathrm{pH}(<6)$$\mathrm{pH}(<6)$pH( < 6)\mathrm{pH}(<6) and saponified at high $\mathrm{pH}(>11)$$\mathrm{pH}(>11)$pH( > 11)\mathrm{pH}(>11) (Figure 4a).
由于存在负电荷，CNF 在水中均匀分散。例如，CNF 中的苯甲酸基团可提供负电荷，在低 $\mathrm{pH}(<6)$$\mathrm{pH}(<6)$pH( < 6)\mathrm{pH}(<6) 时可质子化，在高 $\mathrm{pH}(>11)$$\mathrm{pH}(>11)$pH( > 11)\mathrm{pH}(>11) 时可皂化（图 4a）。

The former process was reversible and rapidly completed within 1 s , while the latter process was irreversible and slow (maintain fluidity within 4 h and complete gelation after 12 h at pH 13 ). For $\mathrm{pH}<6$$\mathrm{pH}<6$pH < 6\mathrm{pH}<6 or $>11$$>11$> 11>11, the CNF suspensions were unstable and gelation even occurred with a low concentration of $0.1\mathrm{wt}\mathrm{\%}$$0.1\mathrm{wt}\mathrm{\%}$0.1wt%0.1 \mathrm{wt} \% because the electrostatic repulsions among the CNFs were eliminated. Under stable conditions, the zeta potential of the CNF suspension decreased from -24.9 to -40.0 mV as the pH was increased from 6 to 11 (Figure 4b). All hydrophilic CNFs ( $0.1\mathrm{wt}\mathrm{\%}$$0.1\mathrm{wt}\mathrm{\%}$0.1wt%0.1 \mathrm{wt} \% ) were highly stable in water, and their optical transmittance changed only slightly after storage for 120 days (Figure S19a,b, Supporting Information). In addition, the CNFs were dispersed in other polar solvents, such as DMSO, N, N-dimethylformamide (DMF), and DMAc (Figure S19c, Supporting Information).
前一个过程是可逆的，并在 1 秒内迅速完成，而后一个过程是不可逆的，且速度较慢（在 pH 值为 13 的条件下，4 小时内保持流动性，12 小时后完全凝胶化）。对于 $\mathrm{pH}<6$$\mathrm{pH}<6$pH < 6\mathrm{pH}<6 或 $>11$$>11$> 11>11 ，CNF 悬浮液不稳定，甚至在 $0.1\mathrm{wt}\mathrm{\%}$$0.1\mathrm{wt}$