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Progress in Chemistry

Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Preparation and Application of Cellulose Nanocrystalline Chiral Liquid Crystals

  • Siyu Li 1 ,
  • Yifan Liu 1 ,
  • Yuancai Lv 1 ,
  • Xiaoxia Ye 1 ,
  • Chunxiang Lin , 1, * ,
  • Minghua Liu , 1, 2, *
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  • 1 College of Environment and Safety Engineering,Fuzhou University,Fuzhou 350108,China
  • 2 College of Environmental and Biological Engineering,Putian University,Putian 351100,China
* (Chunxiang Lin);
(Minghua Liu)

Received date: 2024-06-28

  Revised date: 2024-11-08

  Online published: 2025-05-08

Supported by

National Natural Science Foundation of China(22208057)

Central Guidance for Local Science and Technology Development Funds Project(2024L3001)

Abstract

Cellulose nanocrystals(CNCs)are rod-like nanomaterials with high crystallinity obtained from natural cellulose. CNCs suspensions can form iridescent films with chiral nematic structure through evaporation-induced self-assembly(EISA),showing unique optical properties and presenting specific structural colors,which has great application potential in the fields of anti-counterfeiting,sensing,optoelectronics and so on. Due to the abundant,green and renewable feedstock,CNCs has become the first choice of the new chiral materials. In this paper,the formation mechanism,structure and optical properties of CNCs chiral liquid crystals are introduced,the preparation methods of CNCs chiral liquid crystals which are typical at home and abroad in recent years are reviewed,and the structural colors and regulation methods of CNCs chiral liquid crystals are discussed. The application progress of CNCs chiral liquid crystals in the fields of anti-counterfeiting materials,template materials,other functional materials and biomedicine is also summarized. Finally,the challenges and research prospects of CNCs chiral liquid crystals are addressed.

Contents

1 Introduction

2 Formation mechanism and structural characteristics of chiral nematic liquid crystals of cellulose nanocrystals

3 Methods for preparing chiral nematic liquid crystals of cellulose nanocrystals

3.1 Sulfuric acid hydrolysis process

3.2 TEMPO oxidation process

3.3 Other oxidation methods

3.4 Organic acid hydrolysis method

4 Structural regulation of chiral nematic liquid crystals in cellulose nanocrystals

4.1 Influence of length-diameter ratio of CNCs

4.2 The influence of external conditions

5 Application of chiral nematic liquid crystals in cellulose nanocrystals

5.1 Anti-counterfeiting material

5.2 Formwork material

5.3 Other functional materials

5.4 Biomedicine

6 Conclusion and outlook

Cite this article

Siyu Li , Yifan Liu , Yuancai Lv , Xiaoxia Ye , Chunxiang Lin , Minghua Liu . Preparation and Application of Cellulose Nanocrystalline Chiral Liquid Crystals[J]. Progress in Chemistry, 2025 , 37(5) : 670 -685 . DOI: 10.7536/PC240616

1 Introduction

Cellulose nanocrystals (CNCs) are rod-like crystals obtained from natural cellulose, with diameters of approximately 5–30 nm and lengths ranging from 50 to 300 nm.[1] Compared with natural cellulose, CNCs possess exceptional physical and chemical properties such as high crystallinity, high mechanical strength, high surface activity, and size effects, leading to their widespread application in fields like composite materials, optoelectronics, medicine, and packaging.[2-4] Among these, rod-like CNCs can form a liquid crystal phase between the liquid and crystalline states through evaporation-induced self-assembly (EISA), known as lyotropic cholesteric liquid crystals, also referred to as nematic chiral liquid crystals. The liquid crystal molecules arrange parallel to each other forming layered structures, with molecules within each layer aligned parallel along their long axes while adjacent layers exhibit small twist angles. The director orientation of each layer continuously and uniformly rotates along the normal direction of the layers, resulting in an overall helical structure. When the molecular long-axis orientation completes a 360 ° rotation, the distance between two layers with identical orientations is defined as the pitch.[5] (Figure 1a). Cholesteric liquid crystals exhibit characteristics such as circular dichroism, selective reflection, optical activity, and iridescent effects. Utilizing the self-assembly property of CNCs, various CNC-based chiral functional materials can be developed, showing great potential for applications in areas such as chiral catalysis, anti-counterfeiting, humidity sensing, and specialized optical devices.[6-8]
图1 a)手性向列液晶示意图[8];b)CNCs的几何扭转的棒状结构示意图[17];c)CNCs棒状颗粒表现为笔直且光滑的结构示意图[17];d)CNCs几何扭转结构示意图[16];e)CNCs呈螺旋状排列示意图[16]

Fig.1 a)Schematic diagram of chiral nematic liquid crystals [8];b)Schematic of the geometrically twisted rod-like structure of CNCs[17];c)Schematic of the rod-like particles of CNCs exhibiting a straight and smooth structure[17];d)Schematic of the geometrically twisted structure of CNCs[16];e)Schematic of the CNCs arranged in a helical shape [16]

As early as 1959, Marchessault et al.[9] found that the suspension of CNCs prepared by sulfuric acid hydrolysis (referred to as sulfuric acid-CNCs) exhibited birefringence; however, it was not until 1992 that Revol et al.[10] pointed out that sulfuric acid-CNC suspensions could spontaneously self-assemble during the drying process, forming a chiral nematic liquid crystal structure, with the chiral structure being fixed in the dried CNC films. Since then, the preparation of nanocellulose liquid crystals via the sulfuric acid method has become a focus of interest for many researchers and has been extensively studied. However, the CNCs produced by this method show poor thermal stability, along with issues such as severe equipment corrosion, high water consumption for neutralization, and environmental pollution[11]. Therefore, to address these problems, a series of new environmentally friendly techniques for preparing chiral nematic liquid crystals with more stable CNC properties have gradually been developed.
This paper mainly introduces the formation mechanism, structure and optical properties of chiral nematic liquid crystals based on CNCs, reviews the preparation methods and regulation approaches of chiral nematic liquid crystals based on CNCs in recent years, and outlines the latest application progress of CNC-based chiral nematic liquid crystals in fields such as anti-counterfeiting materials, template materials, and other functional materials.

2 Formation Mechanism and Structural Characteristics of Chiral Nematic Liquid Crystals from Cellulose Nanocrystals

Cellulose is mainly composed of crystalline and amorphous regions, and the hydroxyl groups on its surface form a complex hydrogen bond network[1]. When subjected to hydrolysis with strong acids, the amorphous regions of cellulose are preferentially hydrolyzed due to their lower resistance to acid attack, while the crystalline regions, which exhibit higher resistance to acid degradation, remain intact, resulting in rod-like CNCs.
It has been reported that the formation of chiral nematic structures in CNCs is mainly attributed to the electrostatic repulsion generated by surface charges and the interaction of other intermolecular forces[12]. During sulfuric acid hydrolysis, some hydroxyl groups on the cellulose surface are converted into sulfonic acid groups, thereby introducing negative charges onto the CNCs surface and forming a stable aqueous dispersion system of CNCs. As water evaporates from the aqueous system, rod-like CNCs undergo self-assembly due to the influence of intermolecular forces such as electrostatic repulsion, transitioning from a disordered (i.e., isotropic phase structure) to a highly ordered anisotropic structure[13]. Therefore, many scholars have believed that the presence of surface charges on CNCs is the fundamental reason for the formation of chiral nematic liquid crystal structures[14]. However, with further research on CNCs, some researchers found[15] that when surface charges of CNCs prepared via sulfuric acid hydrolysis were neutralized using surfactants, their suspensions could still self-assemble into chiral nematic liquid crystals, challenging the traditional view that the formation of chiral nematic phase liquid crystals in CNCs is solely due to surface charges. Subsequent studies revealed that the self-assembly behavior of CNCs is closely related to their geometrically twisted rod-like structure (Figure 1b)[16-17]. Araki et al.[17] successfully prepared CNCs with sulfonic acid groups on their surfaces from bacterial cellulose via sulfuric acid hydrolysis. Due to the electrostatic repulsion caused by surface charges masking the geometric twisting morphology, the rod-like particles appeared straight and smooth (Figure 1c), leading to the formation of a nematic liquid crystal phase through parallel stacking between CNC particles. However, when electrolytes were added to the system to shield some of the surface charges on CNCs, the effective particle size of CNCs decreased, revealing their geometric twisted structure (Figure 1b). Under these conditions, the twisted rod-like CNC particles aligned with each other (Figures 1d and e), forming a chiral nematic structure. Therefore, although surface charges on CNCs are one of the key factors in forming chiral nematic liquid crystal phases, they are not the fundamental cause. The underlying reason lies in the geometric twisted structural characteristics of the CNC particles. These characteristics enable the particles to self-assemble into chiral nematic liquid crystal structures under the combined effects of electrostatic repulsion, van der Waals forces, and hydrogen bonding, which is an entropy-driven process.
CNCs exhibit three stages during their self-assembly process[18] (Fig. 2a, b), which are as follows: the first stage occurs at low concentrations where the suspension exhibits an isotropic phase (1.5%-3.0% in Fig. 2a); the second stage begins when the water evaporates and the suspension concentration reaches a critical concentration for phase separation, denoted as C1, where some CNC particles start to align orderly (anisotropic), causing the suspension to separate into two phases—an isotropic upper layer and an anisotropic lower layer (3.5% and 4.2% in Fig. 2a); the third stage is marked by the full transition of the suspension into an anisotropic phase (6.5% in Fig. 2a), forming one-dimensional photonic crystals with chiral structures that display liquid crystalline properties[19-20]. At this point, the suspension concentration is defined as the critical concentration for complete anisotropy, denoted as C2, and distinct fingerprint textures can be observed under polarized light microscopy (Fig. 2c). The appearance of fingerprint textures serves as a key criterion indicating the presence of a chiral structure. After slow evaporation of the CNC suspension, solid films are formed, preserving the chiral structure within them. Under high-resolution scanning electron microscopy, rod-like CNCs are seen to arrange orderly into multilayers, with each layer parallel to the others. Within adjacent layers, the long axes of the rod-like molecules are slightly rotated relative to one another, creating an overall helical arrangement. The central axis of this helical alignment is called the helical axis, while the distance advanced along this axis after one complete rotation is referred to as the pitch (P). A rotation of 180° corresponds to half a pitch (1/2P)[21], as shown in Fig. 2d. These obtained chiral solid films possess unique optical properties such as optical activity, circular dichroism, and birefringence, manifesting structural coloration at the macroscopic level (Fig. 2e).
图2 a)CNCs悬浮液相分离过程[19];b)CNCs自组装过程的相图和示意图[8];c)CNCs悬浮液的双折射色和指纹织构[22];d)CNCs手性向列液晶排列的示意图(红色棒为纳米纤维素)[23];e)CNCs手性向列液晶分别在左旋圆偏振片和右旋圆偏振片下的POM图[23]

Fig.2 a)Phase separation process of CNC suspension [19];b)Phase diagram and schematic diagram of CNCs self-assembly process[8];c)High concentration CNC suspension showed anisotropic birefringent color and finger texture[22];d)Schematic diagram of chiral nematic liquid crystal arrangement of CNCs(red bars are nanocellulose)[23];e)POM diagram of CNCs chiral nematic liquid crystal under left-handed and right-handed circular polaroids[23]

3 Preparation Method of Cholesteric Nematic Liquid Crystal from Cellulose Nanocrystals

At present, the main preparation methods for CNCs include acid hydrolysis, enzymatic hydrolysis, oxidation methods, biosynthesis, ionic liquid dissolution, deep eutectic solvent (DES) method, and the American high-value-added pulping method (AVAP). However, not all CNCs prepared by these methods can undergo self-assembly behavior; CNCs prepared by different methods possess distinct surface chemical properties, thereby exhibiting varying liquid crystalline properties. CNCs prepared by sulfuric acid hydrolysis easily form ordered liquid crystalline phases, whereas CNCs prepared by hydrochloric acid hydrolysis have difficulty in forming ordered liquid crystalline phases[24]. As previously mentioned, the formation of chiral nematic structures of CNCs is highly dependent on electrostatic repulsion generated by surface charges[14,16]. During sulfuric acid hydrolysis, some hydroxyl groups on the cellulose surface are converted into sulfonic acid groups, thereby introducing negative charges onto the CNC surfaces and subsequently forming stable aqueous dispersions of CNCs. With the evaporation of water from the aqueous system, influenced by intermolecular forces such as electrostatic repulsion, rod-like CNCs arrange themselves via self-assembly, transforming from a disordered (i.e., isotropic phase structure) arrangement into a highly ordered anisotropic structure (Figure 3). Currently, reported methods for preparing chiral liquid crystals of CNCs include sulfuric acid hydrolysis[25], oxidation methods (including TEMPO oxidation[26], potassium permanganate oxidation[27], persulfate oxidation[28], etc.), and organic acid hydrolysis[29], among others.
图3 纤维素形成手性向列型液晶相结构示意[30]

Fig.3 Schematic diagram of chiral nematic liquid crystal phase structure formed by cellulose[30]

3.1 Sulfuric Acid Hydrolysis Method

Sulfuric acid hydrolysis is the earliest and most effective method for preparing chiral nematic liquid crystals of CNCs, and it is also one of the most studied methods[25]. While hydrolyzing cellulose, sulfuric acid also introduces sulfonic acid negative charge groups on its surface, thereby allowing CNC particles to be stably dispersed in water due to electrostatic repulsion from the negative charges on their surfaces[13]. As the suspension concentration increases, rod-like molecules undergo self-assembly under a combined effect of electrostatic repulsion and intermolecular forces to form chiral nematic phase liquid crystals.
During the preparation of CNCs chiral liquid crystals by sulfuric acid hydrolysis method, the hydrolysis conditions significantly affect the yield and morphology of CNCs. Numerous studies have shown that the optimal sulfuric acid concentration is approximately 64 wt%, with a hydrolysis temperature ranging from 45 to 60 ℃. The hydrolysis time generally varies between 30 and 120 minutes depending on the type of cellulose raw materials[31]. When the sulfuric acid concentration is lower than 60 wt%, the acid hydrolysis reaction is incomplete, making it impossible to obtain a stable CNCs suspension; while when the concentration exceeds 70 wt%, the crystalline regions tend to swell or even hydrolyze into glucose, thereby reducing the yield of CNCs[32]. Qi et al.[33] investigated the relationship between sulfuric acid hydrolysis conditions and the morphology of CNCs (Fig. 4). Experiments showed that using 64 wt% sulfuric acid under different hydrolysis conditions could produce nanocellulose with varying morphologies. When the hydrolysis temperature was 45 ℃ and the time was 45 min, the resulting rod-like CNCs exhibited a type I crystal structure. Fingerprint textures were successfully observed at a CNCs concentration of 3 wt%. However, under hydrolysis conditions of 35 ℃ and 75 min, only cellulose nanospheres (CNS) could be obtained. Furthermore, Dai Linlin et al.[34] used bleached conifer pulp as the raw material and conducted single-factor experiments to study the influence of reaction conditions during sulfuric acid hydrolysis on the characteristics of CNCs. The results indicated that excessively high hydrolysis temperatures or prolonged hydrolysis times would lead to varying degrees of cellulose carbonization. When the sulfuric acid concentration was 64 wt%, the temperature was 45 ℃, and the hydrolysis time was 30 min, the prepared CNCs had lengths of approximately 200–400 nm and diameters around 5–15 nm. Self-assembly behavior began to occur when the CNCs suspension concentration reached 2.2 wt%, and clear fingerprint textures were observable when the concentration increased further to 2.83 wt%.
图4 不同形貌纳米纤维素的自组装示意图[33]

Fig.4 Schematic illustrations of different morphologies of obtained NC [33]

The sulfuric acid hydrolysis method is currently the most commonly used approach for industrial CNCs production due to its mature and stable process. However, this method suffers from high production costs, significant equipment corrosion, and difficulties in sulfuric acid recovery. Moreover, the CNCs obtained via sulfuric acid hydrolysis exhibit poor thermal stability. Therefore, from the perspective of current industrial practice, the large-scale preparation of CNC cholesteric liquid crystals using concentrated sulfuric acid hydrolysis remains unsustainable in terms of environmental impact, economic feasibility, or product performance.

3.2 TEMPO Oxidation Method

The TEMPO oxidation method employs the TEMPO oxidation reagent (2,2,6,6-tetramethylpiperidin-1-yl)oxyl to selectively oxidize the primary hydroxyl groups at the C6 position in the cellulose molecular structure into carboxyl groups under relatively mild conditions, thereby achieving carboxylation of cellulose. This method can also disrupt the hydrogen bond network in the amorphous and paracrystalline regions of cellulose, softening its rigid structure, which makes cellulose fibers easier to process without altering their original crystalline structure. The oxidized cellulose can be readily dispersed into CNCs and CNFs through mechanical treatment. Compared with traditional sulfuric acid hydrolysis, the TEMPO oxidation method offers milder processing conditions, lower equipment requirements, and higher operability. The resulting nanocellulose contains a large number of carboxyl groups, exhibits excellent dispersion properties, and possesses a larger aspect ratio and higher specific surface area; therefore, it has attracted considerable attention from researchers.
TEMPO reagent was first used to oxidize the primary alcohol hydroxyl groups at the C6 position of dextran into carboxylic acid groups[38]. Subsequently, this system has gradually been applied to cellulose oxidation and has become a common preparation method for oxidizing cellulose. Combining this method with mechanical or acid hydrolysis methods can yield nanocellulose with surface carboxylation and strong negative charge. Due to the high stability of carboxylated CNCs suspensions, they can also self-assemble into cholesteric liquid crystals. He et al.[39] used cotton cellulose as raw material, first hydrolyzing it with hydrochloric acid at 60 °C for 7 h, followed by TEMPO-mediated oxidation under pH 10-11 for 10 h, obtaining a well-dispersed CNCs suspension. The study found that when the CNCs suspension concentration reached 4.1 wt%, an anisotropic phase began to form along with obvious optical birefringence (Fig. 5a); as the concentration increased to 9.0 wt%, a fingerprint texture could be clearly observed (Fig. 5b), indicating a cholesteric liquid crystal pitch of approximately 6 μm (Fig. 5c). On this basis, Xu et al.[40] successfully prepared carboxylated CNCs by performing TEMPO oxidation after hydrochloric acid hydrolysis on bleached cotton pulp, obtaining films with left-handed chiral nematic structures through EISA, with POM and cross-sectional SEM results confirming the chiral structure (Fig. 5d-f).
图5 a~c)羧基化CNCs悬浮液的POM照片[39];d)TEMPO氧化法制备羧基化CNCs手性薄膜[40];e)TEMPO氧化法制备羧基化CNCs的POM图[40];f)TEMPO氧化法制备羧基化CNCs手性薄膜的SEM图[40]

Fig.5 a~c)POM photos of carboxylated CNCs suspension[39]; d)Preparation of carboxylated CNCs chiral films by TEMPO oxidation method[40];e)POM diagram of carboxylated CNCs prepared by TEMPO oxidation method[40]; f)SEM image of carboxylated CNCs chiral films prepared by TEMPO oxidation method[40]

TEMPO oxidation is the most commonly used method for preparing carboxylated CNCs. This method offers mild preparation conditions and simple operation, and the resulting carboxylated CNCs possess advantages such as easy modification and good thermal stability. However, TEMPO reagent is relatively expensive and has certain toxicity. The preparation process requires strict control of the reaction system's pH, and the reaction time is long, which is not conducive to industrial production.

3.3 Other Oxidation Methods

Compared with CNCs prepared by the sulfuric acid method, carboxylated CNCs exhibit higher surface activity and suspension stability, as well as excellent self-assembly capability, making them more widely applicable in the field of chiral nematic liquid crystals. However, due to the limitations of the TEMPO oxidation method, other oxidation systems such as potassium permanganate[41] and ammonium persulfate[42] have been gradually developed and applied in the preparation of carboxylated CNC-based chiral nematic liquid crystals.
Zhou et al.[43] prepared carboxylated CNCs by hydrolyzing pulp under relatively mild conditions (50 ·C) using potassium permanganate and oxalic acid in an H2SO4 (1.0 wt%) medium, obtaining rod-like CNCs with average diameters of 10-22 nm and lengths of approximately 150-300 nm. The reaction process and mechanism are shown in Fig. 6a and 6b. Experiments revealed that when the concentration of the carboxylated CNC suspension reached 7.0 wt%, a chiral nematic liquid crystal self-assembly behavior similar to that of CNCs prepared via traditional H2SO4 hydrolysis could also be observed (Fig. 6c). This report provides an effective and low-cost method for preparing carboxylated CNCs; however, due to the difficulty in removing Mn2+ introduced into the reaction solution, which demands high operational requirements, its practical application is somewhat limited.
图6 a)高锰酸钾法制备CNCs工艺流程[43];b)高锰酸钾氧化法制备CNCs机理[43];c)高锰酸钾法CNCs悬浮液的POM图[43]

Fig.6 a)Process flow of preparation of CNCs by potassium permanganate oxidation method[43];b)Mechanism of preparation of CNCs by potassium permanganate oxidation[43];c)Finger texture formed in 7.0 wt%~10.0 wt% concentration of CNC suspension prepared by potassium permanganate oxidation under polarizing microscope[43]

Ammonium persulfate (APS) tends to decompose into sulfate radicals with stronger oxidizing properties when heated, thereby increasing the acidity of the system. As a result, APS can disrupt the amorphous regions of cellulose while oxidizing it, enabling the one-step preparation of carboxylated CNCs and significantly shortening the preparation cycle, which offers unique advantages over traditional TEMPO oxidation and other oxidation methods[44]. This method was first discovered by Leung et al.[28], followed by Gray et al.[45], who reported that carboxylated CNC suspensions prepared using this method could form cholesteric liquid crystals at a concentration of 5 wt%. However, this report did not investigate parameters such as critical concentration or pitch variation. On this basis, He et al.[18] used tunicate cellulose as raw material and synthesized carboxylated CNCs with high surface charge density through APS oxidation and ultrasonication (Fig. 7a). Their study found that phase separation occurred in the CNC suspension at a concentration of 3.5 wt%, and a distinct fingerprint texture became observable when the concentration increased to 4.1 wt% (Fig. 7b), confirming the formation of chiral nematic liquid crystals from carboxylated CNCs. Although APS is inexpensive, its consumption during preparation is extremely high (228.01 g of ammonium persulfate is required for every 10 g of pulp), leading to excessively high preparation costs and limiting the large-scale application and promotion of this method.
图7 a)硫酸铵氧化海鞘纤维素制备羧基化CNCs流程图[18];b)CNCs的POM图和SEM图[18]

Fig.7 a)Flow chart of carboxylated CNCs prepared by oxidation of ascidium cellulose with ammonium sulfate[18]b)POM and SEM diagrams of CNCs suspension at different concentrations[18]

3.4 Organic Acid Hydrolysis Method

In recent years, researchers have found that organic acids (such as p-toluenesulfonic acid, oxalic acid, and maleic acid) can be used to hydrolyze cellulosic materials for the preparation of CNCs[46-48]. Compared with other methods, the organic acid hydrolysis method involves milder reaction conditions, causes less corrosion to equipment, and allows the hydrolyzed organic acids to be recovered through simple recrystallization at low temperature or room temperature. This effectively addresses the issue of difficulty in recovering acids in traditional acid hydrolysis methods, opening up a green, economical, and sustainable pathway for the preparation of CNCs[49].
Existing studies have shown that when certain dicarboxylic acids are used, the hydroxyl groups on cellulose molecules can undergo a Fischer-Speier esterification reaction with organic acids to obtain carboxylated CNCs[50], making possible the self-assembly of chiral nematic liquid crystals from their suspensions. Researchers have reported a simple and efficient method for preparing functionalized cellulose nanocrystals with high yield, in which hardwood dissolving pulp is subjected to one-step esterification and hydrolysis with molten oxalic acid dihydrate at 110°C, yielding carboxylated CNC suspensions exhibiting obvious birefringence under polarizers[51] (Figure 8a). To gain a clearer understanding of the liquid crystalline behavior of CNCs prepared using the oxalic acid method, Jia et al.[52] systematically compared the properties of carboxylated CNCs prepared by the oxalic acid method and sulfuric acid method using filter paper as the raw material. The study revealed that unlike the liquid crystalline behavior of sulfuric acid-prepared CNCs, the carboxylated CNCs obtained via the oxalic acid method could not self-assemble into highly ordered structures independently. However, when cationic solutions were externally added for regulation, the hydrogel exhibited significant birefringence (Figure 8b). Nevertheless, the report did not further evaluate its self-assembly behavior, leaving it still unclear whether carboxylated CNCs prepared using organic acid methods possess the ability to self-assemble into chiral nematic liquid crystal phases. In response, our research group previously explored the self-assembly behavior of carboxylated CNCs prepared by organic acid hydrolysis of defatted cotton[53]. It was found that rod-like carboxylated CNCs obtained via maleic acid hydrolysis of defatted cotton displayed birefringence at a mass concentration of 4.6%, but no fingerprint texture was observed upon further concentration. When catalysts and esterifying agents were introduced during the hydrolysis process, the resulting CNC suspension exhibited an obvious fingerprint texture under polarized light microscopy at a mass concentration of 9%, confirming the formation of a chiral nematic liquid crystalline phase. Furthermore, cross-sectional SEM images of the solidified films confirmed that the CNCs self-assembled into highly ordered layered structures (Figure 8c). This demonstrates that through property modulation, carboxylated CNCs prepared via organic acid hydrolysis can also undergo self-assembly to form chiral nematic liquid crystals. However, the addition of catalysts and esterifying agents during hydrolysis increases production costs and complicates the composition of recoverable organic acid components in subsequent processing.
图98 a)草酸水解法制备羧基化CNCs悬浮液的双折射[51];b)硫酸和草酸分别制备CNCs悬浮液的SEM图[52];c)有机酸水解脱脂棉制备羧基化CNCs及其液晶行为研究[53]

Fig.8 a)Birefringence of carboxylated CNCs oxalate suspension[51];b)SEM preparation of CNCs suspension by sulfuric acid and oxalic acid respectively[52];c)Preparation of carboxylated CNCs from absorbent cotton by succinic acid hydrolysis and its liquid crys-tal behavior[53]

To address this issue, our research group subsequently used defatted cotton as a raw material to prepare carboxylated cellulose nanocrystals (OA-CNCs) via hydrolysis and esterification with recyclable oxalic acid under conditions without any catalysts or esterifying agents. By optimizing the hydrolysis process and conditions, rod-like nanocrystals with aspect ratios of approximately 1.9–5.1 were obtained, with surface COOH contents of about 0.11–0.17 mmol/g. The results showed that OA-CNC suspensions begin to undergo phase separation at a concentration of about 1.0 wt% and completely transition from isotropic to anisotropic phases at around 3.0 wt% (Fig. 9a, b). The clear fingerprint texture observed under polarized light microscopy is shown in Fig. 9c. The used oxalic acid can still be utilized for the preparation of CNC chiral liquid crystals after five cycles (Fig. 9d).
图9 a)OA-CNCs悬浮液不同样品浓度的相分离图[54];b)OA-CNCs悬浮液不同浓度下的相变图[54];c)OA-CNCs悬浮液不同浓度的POM照片[54];d)循环草酸制备的CNCs悬浮液的POM照片[54]

Fig.9 a)Phase separation plots of OA-CNCs suspensions at different sample concentrations[54];b)Phase transition plots of OA-CNCs suspensions at different concentrations[54];c)Photographs of POM of OA-CNCs suspensions at different concentrations[54];d)POM photos of CNCs suspension prepared by circulating oxalic acid[54]

In summary, it can be seen that through process control during preparation, recyclable organic acid hydrolysis can also yield carboxylated CNCs capable of self-assembling into chiral liquid crystals, providing a sustainable route for the large-scale production of CNC-based chiral liquid crystals.

4 Structural Regulation of Cholesteric Liquid Crystal of Cellulose Nanocrystals

The chiral films formed by CNC self-assembly can exhibit bright iridescent colors (also known as structural colors) by selectively reflecting left-handed polarized light. Structural coloration differs from pigment-based coloration and typically exhibits iridescence; it is widely present in natural biological systems such as insect cuticles and bird feathers, and can change under external stimuli. These characteristics make CNC chiral liquid crystals promising for applications in optical anti-counterfeiting, sensing, templating materials, biomedicine, and other fields. The structural color of CNC chiral films is closely related to the helical pitch of their ordered spiral structure. By adjusting the liquid crystal's pitch, control over the wavelength and bandwidth of reflected incident light can be achieved, thereby enabling the preparation of film materials with different optical properties. The regulation of the nematic chiral liquid crystal structure of CNCs can be accomplished through adjustments to factors including their intrinsic properties, ionic strength, ultrasonic parameters, magnetic fields, and environmental temperature and humidity (Fig. 10)[55].
图10 CNCs手性向列液晶结构的调控方法

Fig.10 Regulation methods for chiral nematic liquid crystal structures of CNCs

4.1 Effect of Aspect Ratio of CNCs

The formation of chiral nematic liquid crystals from CNCs highly depends on the characteristics of the CNC particles, including their particle size, aspect ratio, and surface properties[56]. By adjusting hydrolysis conditions such as acid concentration, oxidant dosage, reaction time, and reaction temperature, the degree of reaction can be controlled, thereby modifying the properties of CNC particles to achieve regulation of the chiral liquid crystal structure.
Abidi et al.[57] systematically investigated the influence of CNCs size on their self-assembly behavior. The study found that compared with larger-sized CNCs, smaller-sized CNCs tend to self-assemble into cholesteric nematic phases with larger pitches (Figure 11a). Subsequently, Duan Min[58] experimentally demonstrated that during the preparation of CNCs cholesteric liquid crystals via sulfuric acid hydrolysis, increasing the hydrolysis time or reaction temperature can reduce the particle size of CNCs, thereby increasing the pitch of the CNCs chiral films. In addition to hydrolysis time and temperature, the structure of cholesteric liquid crystals can also be controlled by adjusting the acid-to-material ratio (i.e., the ratio of acid amount to raw material mass). When the acid-to-material ratio increases, the CNCs size decreases, and the critical concentration for phase separation of the cholesteric nematic liquid crystal increases, consequently reducing the pitch of the CNCs chiral films[59]. Moreover, the structure of CNCs cholesteric liquid crystals can be regulated through selection of different cellulose raw materials. Zhang et al.[60] employed sulfuric acid hydrolysis to isolate CCNCs, ACNCs, and TCNCs from cotton pulp, algae, and tunicate cellulose respectively. Experimental results showed that the obtained CNCs exhibited aspect ratios of 15±10, 18±12, and 75±60 respectively, while the critical concentrations for cholesteric liquid crystal phase separation decreased successively, and the pitches of the chiral films increased accordingly (Figure 11b).
图11 a)不同尺寸CNCs形成手性向列液晶螺距变化以及CNCs悬浮液临界浓度和螺距的变化趋势图[57];b)不同长径比CNCs悬浮液的POM图[60]

Fig.11 a)CNCs of different sizes form a diagram of the change of pitch of chiral nematic liquid crystal and a diagram of the change trend of the critical concentration and pitch of CNC suspension[57];b)POM diagrams of CNCs suspension with different aspect ratios[60]

4.2 Influence of External Conditions

4.2.1 Effect of Externally Added Electrolyte

Adding electrolytes to CNC suspensions is also one of the methods for regulating chiral liquid crystal structures[61]. The ions generated after electrolyte dissociation shield the negative charges on the surface of CNC particles, thereby reducing electrostatic repulsion between particles. This change increases the critical concentration required for forming a chiral nematic phase and decreases the pitch of the chiral liquid crystals[62]. For example, Edgar et al.[63] added NaCl into CNC suspensions obtained by sulfuric acid hydrolysis and found that the pitch of the CNC chiral liquid crystals decreased with increasing NaCl concentration, resulting in a blue shift in the maximum reflection wavelength of the film. However, further studies revealed that this regulation effect of external NaCl on the pitch only holds within a specific NaCl concentration range[58]; excessive addition of NaCl easily destabilizes the suspension, leading to nanoparticle aggregation and affecting the self-assembly behavior of CNCs[17,64]. Hirai et al.[64] confirmed this phenomenon: initially, the pitch of the CNC chiral liquid crystal film decreased with increasing NaCl concentration, reaching a minimum at approximately 0.75 mmol/L; subsequently, the pitch rapidly increased with further increase in NaCl concentration. When the NaCl concentration reached 2.0 mmol/L, the suspension entirely exhibited anisotropy without undergoing phase separation (Figure 12).
图12 a)不同NaCl添加量对CNCs相分离行为的影响[64];b)不同NaCl添加量CNCs的POM图(比例尺:1 mm)[64]

Fig.12 a)Effect of NaCl dosage on the phase separation behavior of CNCs[64];b)POM diagrams of CNCs with different concentrations of NaCl(scale: 1 mm)[64]

It can be seen that within an appropriate concentration range (0~0.75 mmol/L) of added electrolyte, the pitch of the CNCs chiral film decreases with increasing electrolyte concentration. However, excessively high electrolyte concentrations (greater than 2.0 mmol/L) will affect the self-assembly of CNCs and disrupt their chiral structure.

4.2.2 The Effect of Ultrasound

Ultrasonic treatment can release charged ions bound to the electric double layer on the CNCs surface, increasing the electrostatic repulsion between particles, thereby increasing the pitch.[23] Some researchers have indicated that during the dialysis process of CNCs prepared by sulfuric acid hydrolysis, many hydrogen ions are not completely removed through dialysis and instead combine with negative charges on the CNCs surface to form an electric double layer, weakening the electrostatic repulsion between particles; after ultrasonic treatment, hydrogen ions are released, enhancing the electrostatic repulsion and consequently increasing the pitch.[58]
Ultrasound treatment can regulate the chiral liquid crystal structure of CNCs by adjusting the intensity and duration of the applied ultrasound. Liu et al.[65] systematically investigated the effect of ultrasonication time on the pitch of CNCs chiral films. The results showed that increasing the ultrasonication time led to an increase in film pitch and a red shift in structural color (Fig. 13a~h). Liu S. et al.[66] studied the influence of ultrasonic power on the pitch of the films. Their study revealed that when the ultrasonic power increased from 100 W to 600 W, the film pitch increased from 4.6 μm to 10.4 μm (Fig. 13i~l). Lu et al.[67] further demonstrated that with increased ultrasonic energy, the size of CNCs decreased while the ionic conductivity of the suspension increased, resulting in an increased pitch of the chiral liquid crystal film and a red shift in its reflection band.
图13 a~h)不同超声时间CNCs手性薄膜的POM图[66];i~l)不同超声功率CNCs悬浮液的POM照片[67]

Fig.13 a~h)POM micrographs of CNCs chiral films with different treatment time of ultrasonic[66];i~l)POM photos of CNCs suspension with different treatment powers of ultrasonic[67]

As mentioned above, by adjusting the ultrasonication time or intensity, the chiral liquid crystal pitch of CNCs can be controlled, thereby enabling regulation of their chiral structure.

4.2.3 Influence of Magnetic Field Conditions

The external magnetic field can reduce the torsional force of the cellulose layers and decrease the twisting angle of the molecular layers, thereby altering the structure of CNCs chiral liquid crystals[68]. Therefore, the structure of CNCs chiral liquid crystals can also be regulated by applying an external magnetic field.
Kimura et al.[55] previously utilized a magnetic field to regulate the structure of chiral liquid crystals composed of CNCs. They found that the longer the exposure time to the magnetic field, the more aligned the CNC particles became along the direction of the magnetic field, resulting in an increased molecular twist period and thereby an increased pitch of the CNC chiral liquid crystals. Subsequently, Frka-Petesic et al.[69] controlled the orientation of the cholesteric phase structure using small commercial magnets (0.5-1.2 T), producing colorful CNC films with different optical properties (Figure 14a). However, when applying this method, the magnetic field strength must be greater than 0.5 T to be effective, limiting its applicability. To address this issue, Chen et al.[70] developed a method for adjusting the pitch using an ultra-small magnetic field by dispersing Fe3O4 nanoparticles within the CNC suspension, enhancing its sensitivity to magnetic fields. When the magnetic field strength was increased from 7 to 15 mT, the pitch of the chiral structure decreased from 302 nm to 206 nm (Figure 14b). Additionally, the diamagnetic anisotropy of CNCs can also be exploited to induce orientation of the helical axis during their self-assembly process through magnetic field application. Revol et al.[71] previously applied a strong magnetic field of 7 T to align the helical axis of CNCs parallel to the magnetic field direction, achieving a uniformly oriented single-domain assembly structure; upon removal of the magnetic field, the helical axis of the CNC chiral liquid crystal reverted back to its initially disordered state. Later, Li Ping[72] studied the self-assembly behavior of CNCs under a magnetic field of 0.54 T and found that the helical axis gradually shifted toward alignment with the magnetic field direction (Figure 14c). To better understand such behaviors of CNCs under magnetic field regulation, Frka-Petesic et al.[69] further compared the self-assembly of CNC suspensions under conditions without a magnetic field, with a vertical magnetic field, and with an inclined magnetic field. The results demonstrated that the magnetic field could alter the orientation of CNC particles and consequently affect the helical axis orientation of the chiral film. Under zero-field conditions, the film exhibited a layered periodic helical structure with randomly oriented multi-domain helical axes. In contrast, CNC chiral films prepared under magnetic fields showed magnetically regulated helical axis orientations, forming single-domain structures with highly uniform particle orientations and film pitches (Figure 14c).
图14 a)商用磁铁(0.5~1.2 T)对CNCs螺距的调控[69];b)含有Fe3O4纳米颗粒的CNCs悬浮液随着磁场强度增大的螺距情况[70];c)在磁场下制备的CNCs手性薄膜的横截面的SEM图[69,72]

Fig.14 a)Pitch adjustment with the magnetic field of commercial magnet(0.5~1.2 T)[69];b)Pitch of CNCs suspension containing Fe3O4 nanoparticles with increasing magnetic field intensity[70];c)Scanning electron microscopy(SEM)of cross sections of CNCs chiral films prepared under a magnetic field [69,72]

In summary, an external magnetic field can not only regulate the pitch of chiral CNCs films, but also induce the orientation of helical axes during their self-assembly process, resulting in chiral films with uniform orientation.

4.2.4 Effects of Other Conditions

In addition to the aforementioned influencing factors, the self-assembly structure of CNC chiral liquid crystals can also be regulated by changing the environmental temperature, humidity, pressure during the CNCs self-assembly process, or by applying macromolecular additives, electric fields and other methods.
Beck et al.[73] found that increasing the ambient temperature during the self-assembly process of CNCs suspensions can increase the pitch of the CNCs chiral films. This is because elevated temperatures alter the evaporation rate and thermodynamic behavior of the suspension, resulting in a red shift of the reflection wavelength of the CNCs chiral film. Adjusting the relative humidity of the environment can also regulate the pitch of the film; studies have shown that controlling the relative humidity between 16% and 98% enables reversible structural color changes from green to red (Figure 15a)[74]. Based on this, Yao et al.[75] developed a CNC/PEG chiral photonic crystal composite film responsive to environmental humidity: when humidity increases, the pitch of the composite film increases and its reflection wavelength red shifts; when humidity decreases, the pitch reduces and the reflection wavelength blue shifts (Figure 15b). Furthermore, the pitch of the chiral nematic liquid crystal formed by CNCs is also influenced by applied pressure. Kamita et al.[76] applied pressure to CNCs chiral films and observed that their chiral structure compresses under force, with higher pressures leading to smaller film pitches (Figure 15c).
图15 a)含有不同量甘油的固体CNCs手性薄膜的照片[74];b)相对湿度变化时CNC/PEG薄膜的照片[75];c)向CNCs手性薄膜施加压力前后薄膜螺距变化示意图[76];d)在电场增加时CNCs手性液晶的顺序取向和螺距变化示意图[80]

Fig.15 a)Photos of solid CNCs chiral films containing different amounts of glycerol[74];b)photos of CNC/PEG films when relative humidity changes[75];c)Schematic diagram of pitch change of CNCs chiral film before and after pressure is applied to the film[76];d)Schematic diagram of sequence orientation and pitch change of the CNCs chiral liquid crystal when the electric field increases[80]

By adding other substances, such as polymer additives, into CNCs suspensions or grafting polymers onto CNCs, the surface charge of CNCs can be affected, leading to changes in their critical concentration for anisotropy and further influencing the structure of CNCs-based chiral liquid crystals[77]. To investigate the effect of externally added polymer additives on the pitch of CNCs chiral films, Guo Mengna[78] prepared polyvinyl alcohol (PVA) solutions with molecular weight of 67,000 at different gradient concentrations and added them into CNCs suspensions to fabricate CNC/PVA films via evaporation-induced self-assembly. The study found that the pitch of the films increased with the increasing content of PVA.
In addition, some researchers have controlled the helical axis orientation of CNCs chiral liquid crystals through electric fields and shear flow[79]. Frka-Petesic et al.[80] found that as the electric field voltage increased, the axial direction of the chiral liquid crystalline phase gradually twisted until it became perpendicular to the electric field direction, and the pitch also gradually increased; however, when the voltage was further increased to 2.2 kV/cm, the chiral helical structure disappeared (Figure 15d).

5 Applications of Cellulose Nanocrystal Chiral Nematic Liquid Crystals

CNCs can self-assemble into chiral nematic liquid crystal phases, thereby exhibiting unique optical properties such as circular polarization, structural color, and photoluminescence. Utilizing these properties, functional materials containing chiral structures can be prepared, showing great application potential in the fields of optical anti-counterfeiting, template materials, chiral catalysis, sensing, and biomedical sciences[81].

5.1 Anti-counterfeiting Materials

Based on the excellent mechanical properties and unique optical characteristics of CNCs, their films can be designed as security marks, labels, and optical components that sense external environmental changes through structural color variation. Moreover, CNCs chiral films exhibit non-fading structural colors and angle-dependent color variation, showing great potential in the field of anti-counterfeiting functional materials[82]. Many scholars both domestically and internationally have already developed CNCs anti-counterfeiting materials with humidity responsiveness, fluorescence responsiveness, and other properties.
Zhao et al.[83] designed and synthesized a CNC/PAA iridescent coating by utilizing the electrostatic interaction between CNCs and polyacrylic acid (PAA). This coating not only retains the chiral structure of CNCs and exhibits rapid humidity-responsive properties, but also shows angle-dependent color changes under crossed polarization conditions (Fig. 16a, b); based on these characteristics, a "RH-RA-color" ternary anti-counterfeiting code system was established. By adjusting the relative humidity and rotation angle, the appearance and disappearance of patterns on the film can be controlled, thereby achieving a dual anti-counterfeiting pattern effect.
图16 a)CNC/PAA薄膜在不同的相对湿度和旋转角度下表现出不同的颜色双重响应[83];b)CNC/PAA薄膜在不同的相对湿度表现出不同的颜色响应[83];c)CNC-Eu(DA)3-TPEC复合膜在不同紫外光照射下的图像(比例尺:100 μm)[84](上:不同紫外光下观察到蝴蝶图案的薄膜,中:用印章在薄膜上印上“笑脸”,下:在不同紫外光下观察到的纸上印上了“CNC”的字母);d)CNC/CF薄膜结构的双CPL发射产生机制的示意图[85];e)CNC/CF薄膜单光子带隙结构和双光子带隙结构构造的多色二维码图案的应用[85];f)通过溶胀-固化工艺制备CNC/PMTAC复合膜的示意图[86];g)示意图和光学图像显示了由两个CNC/PMTAC膜形成的夹层设计的加密和解码,两个CNC/PMTAC膜由透明单向拉伸膜分开,二维码只能在右手CPL下识别,底层润湿,顶层干燥[86]

Fig.16 a)CNC/PAA films show different color dual responses under different relative humidity and rotation angles[83];b)CNC/PAA films show different color responses at different relative humidity[83];c)Image of CNC-EU(DA)3-TPEC composite film under different UV irradiation(scale: 100 μm)[84](Top: butterfly pattern film observed under different UV light,middle: stamp on the film with "smiling face",bottom: paper observed under different UV light with the letter "CNC");d)Schematic diagram of double CPL emission generation mechanism of CNC/CF film structure[85];e)Application of multicolor two-dimensional code pattern of CNC/CF film constructed with single-photon bandgap structure and two-photon bandgap structure[85];f)schematic diagram of CNC/PMTAC composite film prepared by swell-curing process[86];g)schematic and optical images showing the encryption and decoding of a sandwich design formed by two CNC/PMTAC films separated by a transparent unidirectional stretch film. The QR code can only be recognized under the right hand CPL,with the bottom layer wet and the top layer dry[86]

Constructing CNCs hybrid materials with multiple optical states as anti-counterfeiting labels and information encryption materials can significantly enhance their anti-counterfeiting capabilities to prevent information theft or counterfeiting. Xing et al.[84] selected a Eu3+ complex exhibiting red fluorescence emission and a green fluorescence-emitting TPE derivative, tetrakis[4-(4'-carboxyphenyl)phenyl]ethylene (TPEC), as fluorescent guests to fabricate dual-emissive CPL materials with photonic switching properties composed of CNCs and emissive guests via the EISA process. The material can be customized into films of various shapes, and the resulting films exhibit bright red and cyan fluorescence colors under excitation at 254 nm and 365 nm, respectively (Figure 16c top); when both 254 nm and 365 nm UV lamps are turned on simultaneously, the pattern displays a light pink fluorescence (Figure 16c top). Additionally, when used as ink, a smiley printed via stamping becomes invisible under a 365 nm UV lamp but is clearly observable only under 254 nm UV irradiation (Figure 16c middle); "CNC" written as fluorescent ink remains colorless under natural light but exhibits bright colors under UV irradiation, making it easily identifiable and showing great potential for application in the field of anti-counterfeiting (Figure 16c bottom).
In order to obtain intelligent photonic materials capable of real-time switching of circularly polarized signals, Duan et al. [85] initially incorporated fluorescent carbon quantum dots (F-CQDs) into the chiral structure of CNCs to obtain multicolored circularly polarized fluorescence (CF) films. They further constructed a novel dual photonic bandgap structure composed of CNC and CF (CNC/CF) films, which not only enables flexible switching of the intensity, wavelength, and direction of circularly polarized fluorescence (CF), but also excites dual circularly polarized reflections for advanced multi-level encryption (Figure 16d). For example, the encrypted information within the composite film can be recognized under right-handed circular polarizers or fluorescent illumination, whereas the composite QR code pattern cannot be identified under sunlight or left-handed circular polarizers (Figure 16e).
Chen et al.[86] reported a rewritable photonic chiral paper composed of CNCs and polycations. The designed "sandwich" structure (Fig. 16f): the top layer can only be decoded under right-handed polarized light, while the bottom layer can be decoded by wetting with water. Due to the depolarization effect of the sandwich structure, an invisible QR code pattern inkjet-printed on the bottom film can only be recognized under right-handed circularly polarized light when the bottom film is wet and the top film is dry, thereby achieving multidimensional encryption (Fig. 16g).

5.2 Template Material

The research on using CNCs as a chiral source for preparing templated functional materials can be traced back to 1993, when Revol et al.[9] found that the chiral structure was preserved in solid films obtained from CNCs through the EISA process, which laid a solid theoretical foundation for using CNCs as templates for preparing chiral materials. In recent years, with the rapid development of CNC-based chiral research, significant progress has been made in the co-assembly of CNCs as chiral templates with polymers or inorganic materials to prepare advanced materials, which show broad application prospects in fields such as sensors, optoelectronic devices, and smart displays.
Shopsowitz et al.[87] synthesized mesoporous silica films using CNCs as templates. After removal of the CNC templates, the chiral structures and high specific surface areas of the CNCs were accurately replicated in the inorganic solids, demonstrating for the first time that template methods can produce free-standing mesoporous silica films with long-range ordered structures. The chiral helical structure of the mesoporous film imparts birefringent properties, with a reflection peak wavelength that can be tuned across the entire visible spectrum and near-infrared range (Figure 17a). Based on the above research, Shopsowitz[88] et al. further utilized the chiral mesoporous silica film obtained through the aforementioned method as a template to prepare mesoporous anatase titanium dioxide materials, wherein the chiral structures were again replicated and preserved within the titanium dioxide material (Figure 17b). This chiral mesoporous titanium dioxide film material can be applied in photocatalysis and dye-sensitized solar cells. Sun et al.[89] prepared photonic crystal composite films (GCP) of different colors by combining cellulose nanocrystals (CNCs) with two different surface charge densities and particle sizes, phenol-glutaraldehyde resin, and graphene oxide (GO) at varying ratios. Flexible photonic film actuators (GP) were obtained after alkaline treatment to remove the CNCs. Due to uneven distribution of GO throughout the film, the upper and lower surfaces of GP exhibited differing swelling behaviors. This characteristic enables GP to simultaneously display structural color changes and solvent-responsive actuation, allowing rapid water-induced color change and bending when exposed to water (Figure 17c). These features indicate promising applications for detecting the presence of water in various solvent mixtures, as well as in optical actuators and solvent sensors.
图17 a)手性介孔氧化硅薄膜不同颜色的照片[87];b)CNCs基手性介孔锐钛矿相二氧化钛材料的制备路线图[88];c)GO/CNC复合膜的示意图和物理图像及其表现出快速的水响应变色和弯曲图像[89];d)制备 SAL-CNC复合膜的示意图[90];e)SAL-CNC薄膜在水中浸泡不同时间后的照片[90];f)SAL-CNC薄膜在水中浸泡30 s和干燥之后薄膜的最大反射波长的变化[90]

Fig.17 a)Photos of chiral mesoporous silica films in different colors[87]; b)Preparation roadmap of CNCs-based chiral mesoporous anatase phase titanium dioxide[88]; c)schematic and physical images of the GO/CNC composite film and its rapid water-responsive discoloration and bending images[89]; d)Schematic diagram of preparing SAL-CNC composite film[90];e)Photos of SAL-CNC film after soaking in water for different time[90]; f)Maximum reflected wavelength of SAL-CNC film after soaking in water for 30 s and drying[90]

Leng et al.[90] proposed a strategy for preparing flexible photonic latex films using CNCs chiral liquid crystals as templates. This strategy fabricated composite films of organosilane-modified acrylate emulsion and CNCs through the EISA method (Fig. 17d), where the structural color of the composite film originates from the chiral structure of the CNCs, and its color can be adjusted by varying the emulsion content. After selective removal of the CNCs via alkali treatment, two-dimensional porous latex films retaining the chiral structure were obtained. These films exhibit reversible humidity-responsive structural color, which can be tuned through water absorption and desorption (Fig. 17e, f), showing significant potential for application in humidity indicators.

5.3 Other Functional Materials

The chiral nematic structure formed by CNC self-assembly endows materials with responsiveness to external environments (such as pressure, temperature, humidity, pH, etc.)[91], enabling them to sense environmental changes and provide detectable visual or color signals. This chiral structure is tunable, allowing adjustment of the material's morphology and an increase in specific surface area. Therefore, utilizing CNCs to fabricate functional materials with environmental responsiveness and adjustable morphology can be applied in fields such as mechanically stretchable materials, indicators, electromagnetic interference (EMI) shielding materials, and pollutant degradation. However, due to their highly crystalline structure and ordered molecular arrangement, CNCs inherently exhibit poor mechanical properties, limiting their practical applications. The mechanical strength of CNC-based films can be enhanced through blending with other reagents.
Kose et al.[92] developed a stretchable CNC/elastomer composite material with a chiral structure (Fig. 18a), which exhibits reversible stretch-induced chromatic stimulus-responsive properties due to the incorporation of CNCs. When stretched, noticeable color changes can be observed under crossed polarizers; upon removal of the tensile force, the material rapidly recovers its original shape and color (Fig. 18b). This material enables visualization of mechanical stress and can be applied in the development of multifunctional sensors for automatic crack detection in bridges, buildings, and other structures.
图18 a)CNC/弹性体复合材料制备示意图[92];b)复合膜在交偏光镜下的拉伸照片[92];c)具有珍珠质样层状结构的CNC/RGO复合膜的示意图[93];d)穿过CNC/RGO复合材料的电磁波传递的示意图[93];e)通过EISA制备CNCs/OS/TA膜示意图[94];f)CNCs/OS/TA薄膜在强酸性溶液(pH = 0.5)、碱性溶液(pH = 14)和常用有机溶剂中浸泡不同时间的照片[94];g)CNCs/OS/TA薄膜在甲醇中浸渍24 h之前和之后的紫外透射光谱[94];h)CNC@AuNP的制备和催化应用的示意图[95]

Fig.18 a)Schematic diagram of the preparation of CNC/ elastomer composites[92]; b)Stretching photos of composite film under orthogonal polarizer[92]; c)Schematic diagram of CNC/RGO composite film with pearl-like layer structure[93]; d)Schematic diagram of electromagnetic wave transmission through CNC/RGO composites[93]; e)Schematic diagram of CNCs/OS/TA membrane prepared by evaporation-induced self-assembly[94]; f)Photos of CNCs/OS/TA film soaked in strong acidic solution(pH = 0.5),alkaline solution(pH = 14)and common organic solvent for different time[94]; g)Ultraviolet transmission spectra of CNCs/OS/TA films before and after 24 hours impregnation in methanol[94]; h)schematic diagram of the preparation and catalytic application of CNC@AuNP[95]

Jin et al.[93] intercalated CNCs between reduced graphene oxide (RGO) sheets as reinforcements and dispersants to form highly ordered nacre-like structured CNC/RGO films through a self-assembly process (Fig. 18c). The highly ordered layered structure improved the mechanical strength and electromagnetic interference (EMI) shielding effectiveness of the film (Fig. 18d). Furthermore, the morphology and aspect ratio of the CNCs influenced the thickness, mechanical properties, electrical conductivity, and EMI shielding effectiveness of the film. Higher aspect ratios and smaller diameters resulted in lower film porosity, denser structures, and more uniform distributions, thereby achieving higher electrical conductivity and shielding effectiveness, demonstrating great application potential as EMI shielding materials in aerospace and flexible electronics fields.
Feng et al.[94] prepared a CNCs composite film with high strength and excellent water resistance through co-assembly of CNCs with oxidized starch and tannic acid. The three-dimensional cross-linked network formed by hydrogen bonding interactions among CNCs, oxidized starch, and tannic acid endows the composite film with broad solvent resistance and outstanding mechanical properties, showing excellent stability in water, common organic solvents, strong acid (pH=0.5), and strong base (pH=14) environments (Fig. 18f). Additionally, due to the high swelling capacity of cellulose in methanol and its easy permeability into the amorphous regions of starch, the film exhibited a red shift in color before and after immersion in methanol (Fig. 18g), making it an effective test paper for distinguishing methanol from water and ethanol, and enabling its use as a colorimetric sensor for specific detection of methanol in methanol-ethanol mixtures.
Wang et al.[95] developed a novel method to control the size of Au nanoparticles by utilizing the chiral structure of CNCs. This approach regulates the chiral liquid crystal structure formed by CNC self-assembly by adjusting the concentration of CNCs in solution, thereby controlling the size of Au nanoparticles at the nanoscale (Figure 18h). The synthesized CNC@Au exhibited excellent catalytic performance in the reduction reaction of the 4-nitrophenol model substrate, achieving high catalytic efficiency (conversion rate of 98.0%) within 30 minutes. This study demonstrated that the chiral nematic structure of CNCs has great potential in regulating the size of functional nanomaterials.

5.4 Biomedical

CNCs' excellent biocompatibility and degradability make them ideal candidates for constructing biomedical fields such as drug carriers and medical detection[96]. Ganguly et al.[97] constructed a magnetically controlled three-dimensional wound repair scaffold by utilizing the responsiveness of CNCs under magnetic field stimulation. This scaffold, prepared from alginate-silk fibroin and CNCs, exhibited high anisotropy and could control the alignment direction of the scaffold through induced ordering of CNCs under an external magnetic field, thereby inducing three-dimensional ordered orientation growth of cells (Fig. 19a). Physiological responses of cells cultured on the scaffold confirmed enhanced cell activity, and the wound healing capability of the scaffold was also evaluated in a rat model, further proving its bioactivity (Fig. 19b). This research provides a theoretical basis for using CNCs in tissue engineering materials. Liu et al.[98] constructed a ratiometric upconversion fluorescent nanosensor for pH monitoring and imaging by using CNCs as bridges to chemically bond upconversion nanoparticles donors with fluorescein isothiocyanate and rhodamine receptors. The rod-like structure of CNCs improved the stability of the probe, solving issues such as dye aggregation, long distance between donor and acceptor, and poor water dispersibility of the probe. The probe showed good reversibility, selectivity, and biocompatibility, and demonstrated excellent pH monitoring and imaging capabilities in live cells and in vivo, making it applicable for early diagnosis and treatment of certain diseases (Fig. 19c).
图19 a)在低强度磁场(MF)存在下制备藻酸盐、丝素蛋白和纤维素纳米晶体的各向异性支架的示意图[97];b)CNCs定向排列图案化促进快速细胞浸润、蛋白质释放和拓扑引导的伤口愈合[97];c)基于CNCs的上转换荧光纳米探针的合成工艺[98]

Fig.19 a)Schematic diagram of preparing anisotropic scaffolds of alginate,fibroin and cellulose nanocrystals in the presence of low intensity magnetic field(MF)[97];b)Patterning of CNCs directed arrangement promotes rapid cell invasion,protein release,and topologically guided wound healing[97];c)Synthesis process of upconversion fluorescent nanoprobes based on CNCs[98]

6 Conclusion and Prospect

The unique optical properties of CNCs chiral liquid crystals have shown great application value in fields such as anti-counterfeiting, sensing, and template materials. The raw materials of CNCs are widely available, inexpensive, easily accessible, and green renewable. Against the background of advocating green chemistry and sustainable technologies, CNCs have become an ideal choice for constructing various functional materials. In the research on building CNCs-based chiral functional materials, significant progress has been made in preserving and regulating the chiral structures within functional materials, and many research achievements have also been obtained in areas such as anti-counterfeiting and template materials. However, there still remain a series of issues to be resolved in the development of CNCs chiral liquid crystals.
(1) There are still many limitations in the traditional preparation methods of chiral liquid crystals from CNCs, such as expensive and non-recyclable reagents, complicated preparation processes, and environmental pollution. Therefore, the development should gradually shift towards green, efficient, sustainable, and scalable directions. For example, using recyclable organic acid hydrolysis to prepare CNC-based chiral liquid crystals exhibits excellent industrial prospects.
(2) The regulation of CNCs' chiral liquid crystal structure at the microscale can directly affect the performance of their chiral materials at the macroscale; developing simpler, more convenient, and greener regulatory methods will effectively enhance the application of CNCs-based chiral materials across various fields.
(3) CNCs-based chiral functional materials have shown great application potential in anti-counterfeiting, sensing, and optical applications; however, their development towards high-end and precise directions remains a research hotspot and challenge. Meanwhile, the practical application and commercialization of CNCs-based chiral functional materials also represent one of the major challenges in the future.
In conclusion, although there have been numerous reports on the preparation and application of chiral liquid crystals based on CNCs, significant challenges remain in the future. It is still necessary for more researchers to continue exploring new advanced functional materials and expanding their broader application prospects.
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