Cellulose Nanocrystals (CNC) Liquid Crystalline State in Suspension: An Overview

Aref Abbasi Moud; Aliyeh Abbasi Moud

doi:10.3390/applbiosci1030016

Abbasi Moud, Aref;Abbasi Moud, Aliyeh

2022-11-03 00:00:00

Review Cellulose Nanocrystals (CNC) Liquid Crystalline State in Suspension: An Overview 1 , 2 Aref Abbasi Moud * and Aliyeh Abbasi Moud Department of Chemical and Biological Engineering, The University of British Columbia, Vancouver, BC V6T 1Z3, Canada Biomedical Engineering Department, AmirKabir University of Technology, PC36+P45 District 6, P.O. Box 15875/4413, Tehran 1591634311, Iran * Correspondence: aabbasim@ucalgary.ca Abstract: Films made from cellulose nanocrystals (CNCs) may have iridescent structural colours (pure or in combination with other materials). Numerous ﬁelds might beneﬁt from understanding how CNC self-assembly constructs these periodic structures. Herein, we looked at the colloidal characteristics of CNC particles as well as the development and behaviour of liquid crystals (LCs). We conducted a very brief literature analysis on the main issues related to the chiral structure creation of CNC LCs, including the origins of chirality, orientation, as well as its mechanical properties. Finally, by altering the pitch size, applications such as energy storage, humidity sensing, and photonic crystals were studied in a case-by-case manner. The manuscript, it is observed that the rational design of metamaterials built on CNCs allows for the reversible changing of colours through physical and chemical modiﬁcations by adding chemical or changing environmental factors. Examples of this alteration include the use of solvents, chemical penetration in applied ﬁelds (magnetic and electric), deﬂection, light, temperature change, acidity change, and molecular interaction detection. Reversible colours may be produced by altering the spacing between the particles, the ﬁller materials, or the structural elements of the system’s refractive indices. This article brieﬂy discusses the inner workings of CNCs, potential barriers to developing photonic structures, and several techniques and processes Citation: Abbasi Moud, A.; Abbasi for achieving changeable colours. Moud, A. Cellulose Nanocrystals (CNC) Liquid Crystalline State in Keywords: liquid crystals; cellulose nanocrystals; colloids; cellulose; chirality; chiral nematic structures Suspension: An Overview. Appl. Biosci. 2022, 1, 244–278. https:// doi.org/10.3390/applbiosci1030016 Academic Editor: Jangho Kim 1. Introduction The linear chain of glucose molecules known as cellulose is held together by an acetal Received: 1 August 2022 Accepted: 31 October 2022 oxygen covalent link between the C1 of one glucose ring and the C4 of the next. The cel- Published: 3 November 2022 lulose nanocrystals (CNC), created during the acid hydrolysis of cellulose, have lengths and widths of 100–300 nm and 3–10 nm, respectively. CNCs are bio-derived nanoparticles Publisher’s Note: MDPI stays neutral that can self-assemble in a colloidal solution into a left-handed chiral nematic (cholesteric) with regard to jurisdictional claims in phase. The chemical chirality of the glucose repeating unit enables it to twist; however, the published maps and institutional afﬁl- origin of chirality is still under debate [1]. Nature has created a wide range of materials iations. and organized them so their structural colours can be seen. Since they never fade and offer no toxicity, structured colours are more eco-friendly than pigment. The main cell walls of plants, algae, and oomycetes are structurally supported by the Copyright: © 2022 by the authors. linear polysaccharide known as cellulose, a naturally abundant polymer [2]. It is a chemical Licensee MDPI, Basel, Switzerland. molecule with a formula consisting of polysaccharides made up of hundreds or thousands This article is an open access article of linked D-glucose units [3–5]. Nanomaterials have considerable applications in various distributed under the terms and ﬁelds, including health [6], electronics, biomaterials, and energy storage [7] and genera- conditions of the Creative Commons tion. Nano-sized cellulose can be found in the form of nano-ﬁbrillated cellulose (NFC) (or Attribution (CC BY) license (https:// interchangeably cellulose nanoﬁbrils (CNF), CNCs, and bacterial cellulose (BC). These par- creativecommons.org/licenses/by/ ticles are favourable to a wide range of applications due to biocompatibility, degradability, 4.0/). Appl. Biosci. 2022, 1, 244–278. https://doi.org/10.3390/applbiosci1030016 https://www.mdpi.com/journal/applbiosci Appl. Biosci. 2022, 1 245 and outstanding mechanical characteristics [8]. Other possible uses for cellulose include aerogels for tissue scaffolds, coverings(packaging goods), Pickering emulsion agents, water- ﬁlled hydrogels, and reinforcing agents in ceramic, polymer, and metal matrixes, to name a few [8]. Even though some nanoparticles, such as nanotubes (CNT) [9,10], have excellent mechanical qualities, their toxicity [11], greater manufacturing costs, and the importance of environmental tolerance are limiting their adoption within the industry. Another factor to consider is digestibility when it comes to health-directed practices; with similar agenda, biodegradability is another important advantage of CNC particles in biological applications where tuneable gradual degradation of the particles is required. Acid hydrolysis transforms cellulose into CNCs, which are needle-shaped, rigid, negatively charged particles with a crystallinity of around 70% [8]; the majority of the mechanical delamination techniques used to manufacture CNF counters parts result in parts with decreased crystallinity and increased ﬂexibility. Due to the fulﬁlment of the requirement of having one scale in nanometric scale (1–100 nm) and moving with the thermal motion of media molecule, these particles, when suspended, are Brownian colloids. The main objective of CNC research is to fully exploit the outstanding physical and chemical properties of CNC particles in many applications. CNC particles’ distinct mechanical qualities have led to their widespread application as reinforcing agents in a wide range of polymers, including polyvinyl alcohol (PVA) [12], polypropylene (PP) [13,14], polyethylene glycol [15,16] and many others, as a load-bearing element; it is noteworthy that CNC particles can also be employed on their own to make ﬁlms and gels [17]. CNC particles can also function as liquid crystals or ﬁbres due to their geometry, and their rod shape allows them to generate a variety of geometries as nanostructures, resulting in a wide array of features and functionalities. However, issues such as ﬁne-tuning the interactions between CNC particles and polymer matrices, as well as achieving the agglomerate-free state of CNC particles in matrices, have remained challenging. When the CNC concentration rises, the suspended undergoes an LC-to-gel transi- tion [18,19]. Since the topology and mesostructure of the various CNC suspension states, including isotropic, biphasic, liquid crystalline, and liquid crystalline gel, differ funda- mentally [20,21], they immediately affect the rheological properties of CNC solutions. According to the literature study, the formation of liquid crystals affects rheology by caus- ing the viscosity maximum to appear in the plot of viscosity versus concentration curve [22]. This is because liquid crystalline domains may be orientated easily [18,23–25], the general ﬂow pattern of CNC solutions during shearing, as well as the effects of dosage, charge den- sity, sonication, and temperature levels, liquid crystal formation [18,23–25], and viscosity, have all been examined for CNC suspensions. Some material will be emphasized under the pertinent headlines of these reports to organize and further explain these reports in the sections that follow. After addressing the application in manuscript parts regarding the liquid crystalline phase creation of CNCs and presenting a section on its processing and rheology, the foundations and techniques of liquid crystal structure tuning are also provided in a separate portion. Researcher interest in chiral photonic crystals in biological systems has come from a range of disciplines; the wings of the Morpho butterﬂy [26], blue-skinned mandrill [27] and spotted parachute bird all feature structural colour [28,29] on their scales. When CNC is in its chiral condition, it may replicate nature’s periodic internal structure, which changes colour according to the viewing angle. Even though liquid crystal formation is connected to a state of suspension and gelation, details of the suspension, gelation, and colloidal behaviour have been omitted due to the current manuscript’s concision; these topics are covered in other references (refs) [8,29]. Table 1 provides an overview of varied yet focused on the main subject of chiral liquid crystalline phase formation and application formed on CNCs. Subjects include its chiral structure, left-handedness [1] and right-handedness [30], and methods of recognizing them using molecular dynamic simulations and microscopy [1], self-assembly, rheology [31–36], using rheology to recognize states, the interaction of liquid crystalline state under magnetic and electric ﬁeld [31] and its subsequent orientation, the Appl. Biosci. 2022, 1 246 effect of conﬁnement on liquid crystalline, and ﬁnally its application and blend with other chemical additives [12]. This table is an introductory representation of research focused on CNC with distinct agendas. Table 1. The table provides highlights on studies containing discussion and research on chiral LCs out of CNCs. Reference Highlight CNCs are bio-derived colloidal particles that can self-assemble into chiral photonic structures. Their mesophase [37] chirality’s genesis is unknown. CNC crystallite bundle might be the missing piece in the molecular-to-colloidal hierarchical transfer. Congo red displays induced optical activity when attached to regenerated cellulose in gel ﬁlms produced by [38] gradual precipitation from LiCl/N,N,dimethylacetamide solution. The colour generation is structural rather than the result of dye interaction with chiral centres on the cellulose chain. The bio-renewable resource CNCs spontaneously arrange into chiral nematic LCs. They reﬂect light, giving [39] them an iridescent appearance. Recent breakthroughs in photonic material development employing CNCs are described in this manuscript. In aqueous suspension, CNCs cause anisotropic order, which results in iridescence from the ﬂuid phase. The [28] effects of hydrolysis duration, wood pulp species, and sonication on LC phase separation are examined. Cellulose microcrystallites can be made from wood, cotton, or animal sources. When cellulose ﬁbres are acid [40] hydrolysed, they produce stable aqueous suspensions. These whiskers are arbitrarily suspended in the water and create an isotropic phase at very small doses. CNCs are elongated nanocolloids derived from nature that generate cholesteric phases in water and apolar [41] solvents. They are made up of bundles of crystalline microﬁbrils that are grouped together. The genesis of chiral interactions between CNCs is unknown. The major alcohol in cellulose I is in a trans-glycosidic (TG)-linkage orientation, resulting in the production of [42] twist-causing hydrogen bonds. Other crystalline forms of cellulose exhibit microﬁbril-scale twisting. This study indicates that it is partly attributable to the absence of secondary alcohol in the TG orientation in those forms. The helical self-assembly of cholesteric LCs is a strong, naturally formed assembly pattern. Attempts to emulate [43] these extraordinary materials frequently result in ﬁlms having a non-uniform mosaic-like quality. The researcher ’s results reconcile contradictory facts and pave the way for biomimetic artiﬁcial materials. Another contender for CNC, CNF can also form liquid crystalline phases; however, it is more challenging; even though at static conditions, CNF normally does not show birefringence in ﬂow, it does like CNC [44]. Cellulose nanoﬁbril (CNF) LC formation is more difﬁcult due to the greater length of CNFs and their intrinsic proclivity to produce entanglements [45]. A high aspect ratio CNF can also encourage the development of glassy regimes before the isotropic to nematic LCs transition occurs. A similar effect occurs with CNTs, which researchers shorten by treating with superacid to cause LC formation [46]. We will cover some of the principles of CNCs, LCs, and some of their applications in this review. The conclusions of this review research may provide a short guideline for the proper design of CNC LC-based products. Our earlier work provides a more thorough account of CNC chiral creation [29]. 2. CNC LCs 2.1. Origin of Chirality of CNCs Chiral mesophases are frequently produced by chiral LC molecules [37]. Asymmetry must be present in the entity attempting to generate chiral structures, and the system must not be racemic for the interaction of right- and left-handed entities to cancel out the chiral effects. A small amount of chiral dopant can produce otherwise mesophases due to the cooperative nature of LC ordering, which is typically sufﬁcient to establish one domain handedness [47]. As chiral twist develops, the system may interact with left- and right- handed circularly polarized systems differently, suggesting that these materials function as polarization ﬁlters [48]. Moreover, interesting optical interference can be noticed if the pitch Appl. Biosci. 2022, 1 247 of the twist is on the order of the wavelength of the light; this interference with incident light is the reason behind the colourful appearance of photonic crystals. The right-hand twist of cellulose has already been observed across experiment and simulation studies [49–52], although the twist can be seen in transmission electron mi- croscopy (TEM) [49] and atomic force microscopy (AFM) [30]; it is still debatable the twist per length of ﬁbrils. In this regard, molecular dynamic simulation can be a valuable tool to determine the twist to a CNC. Generally, the twist depends on the diameter, i.e., it is controlled by the number of constituent chains forming the CNC [50]. In reference [53], the authors showed that the twisting angle varies between 9.9 and 1.3 /nm for CNC with cross-sectional that contains two by two and six by six chains. The results of molecular dynamic simulation point to a twist around 1.4 /nm [53]. One of the main subjects that have received attention from researchers is ascertain- ing individual cellulosic particles’ handedness and their assembly orientation. Several experiments and methodologies have been devised to distinguish this within cellulosic domains, such as using molecular dynamics simulation [54], dyes [55] and atomic force microscopy [30]; here we have shown one of those efforts. Figure 1a–c depicts an in- triguing experiment for determining the right-handedness of CNC’s chiral structure [56]. The “tactoid” rotated under the microscope allows researchers to detect the handedness of the cholesteric phase by observing the rotation-induced movement of the cholesteric bands; When the sample is rotated clockwise (right-handed); this observation is compatible with the left-handed rotation of the helix; this is the ﬁrst account of observing of a single isolated tactoid made of cellulose; it appears that rotation amount and rate of rotation is not strong enough to causes disruption in the structure of the isolated tactoid. Chirality seen in ref. [56] is comparable to CNF since there is chirality inversion (right-handed individual CNF leads to left-handed tactoid), which is similar to Deoxyribonucleic acid (DNA) [57] and virus [58] but different from amyloid ﬁbrils where left-handed particles generate right-handed cholesteric phases [59]. Moreover, temperature changes had no inﬂuence on the pitch of these tactoids within the temperature range of 10–43 degree Celsius in discordance with founding on DNA [60] and virus [61]. Therefore, changing temperature while knowing pitch is not inﬂuenced can produce an excellent system for rheological Appl. Biosci. 2022, 1, FOR PEER REVIEW examination of the system. Moreover, a liquid with suspended glittery 5 CNC sheets can exhibit colour as a nanoﬂuid even at elevated temperatures. Figure 1. (a) Simple determination of handedness of chiral structure of CNCs through manual Figure 1. (a) Simple determination of handedness of chiral structure of CNCs through manual rotation under the microscope through rotation of a single isolated tactoid. (b) Insensitivity of pitch size to temperature over the time range of 10–40 degrees Celsius. (c) Visual variation of rotation under the microscope through rotation of a single isolated tactoid. (b) Insensitivity of pitch pitch size as a function of temperature (no discernible changes are found). Adapted with per- size to temperature over the time range of 10–40 degrees Celsius. (c) Visual variation of pitch size mission from Ref [56]. Copyright 2018 American Chemical Society. as a function of temperature (no discernible changes are found). Adapted with permission from Cholesteric liquid crystals are frequently produced by adding chiral dopants to Ref. [56]. Copyright 2018 American Chemical Society. achiral nematic hosts [37]. Twisted nematic and super twisted nematic combinations are inducible with dopant, so chiral nematic formation is not the only beneficiary of dopants. Individual cellulose Iβ crystallites are elongated and have a propensity to twist along their axis [62–64], a property arising from molecular chirality of glucosic repeating units [53,65,66], but it is unclear if this twist is necessary or cause of chirality of CNC based chiral nematic structures [67]. According to Parton et al. [37], CNC bun- dles can also operate as chiral dopants, implying a new paradigm for colloidal self- assembly akin to chiral dopants in molecular LCs (See Figure 2a–c [37]). In some sense, these structures should serve as areas where tactoids can form. As a result, chi- ral structures should be produced when a small quantity of well-chosen nucleation agents is added to an achiral bulk phase. It has to be investigated if CNC twist, not just in CNC suspension but also in other suspensions, might be the reason chiral struc- tures like DNA filaments emerge [29,68]. Similarly, if nucleating agents help lower the thermodynamic barrier to the synthesis of these structures [46], the need to shorten CNF or CNC for the manufacture of chiral structures can be eliminated. Appl. Biosci. 2022, 1 248 Cholesteric liquid crystals are frequently produced by adding chiral dopants to achiral nematic hosts [37]. Twisted nematic and super twisted nematic combinations are inducible with dopant, so chiral nematic formation is not the only beneﬁciary of dopants. Individual cellulose I crystallites are elongated and have a propensity to twist along their axis [62–64], a property arising from molecular chirality of glucosic repeating units [53,65,66], but it is unclear if this twist is necessary or cause of chirality of CNC based chiral nematic structures [67]. According to Parton et al. [37], CNC bundles can also operate as chiral dopants, implying a new paradigm for colloidal self-assembly akin to chiral dopants in molecular LCs (See Figure 2a–c [37]). In some sense, these structures should serve as areas where tactoids can form. As a result, chiral structures should be produced when a small quantity of well-chosen nucleation agents is added to an achiral bulk phase. It has to be investigated if CNC twist, not just in CNC suspension but also in other suspensions, might be the reason chiral structures like DNA ﬁlaments emerge [29,68]. Similarly, if nucleating Appl. Biosci. 2022, 1, FOR PEER REVIEW 6 agents help lower the thermodynamic barrier to the synthesis of these structures [46], the need to shorten CNF or CNC for the manufacture of chiral structures can be eliminated. Figure 2. CNC bundle (slight agglomeration) as a chemical dopant (nucleus for chiral structure Figure 2. CNC bundle (slight agglomeration) as a chemical dopant (nucleus for chiral structure for- formation). (a) Sonication impact the formation of a chiral nematic structure that is structurally mation). (a) Sonication impact the formation of a chiral nematic structure that is structurally coloured coloured as a function of sonication (Sonication energy has changed from 0 to 3087 J/mL). (b) as a function of sonication (Sonication energy has changed from 0 to 3087 J/mL). (b) Transmission Transmission electron microscopy (TEM) image of CNC bundle, showing slight agglomeration. electron microscopy (TEM) image of CNC bundle, showing slight agglomeration. (c) Schematic c) Schematic of CNC composite bundle as a model (bundles also artificially change the aspect of CNC composite bundle as a model (bundles also artiﬁcially change the aspect ratio of CNCs). ratio of CNCs). Adapted from Ref [37]. Adapted from Ref. [37]. According to ref [56], sonication, a process for creating CNC from CNF, cutting According to ref. [56], sonication, a process for creating CNC from CNF, cutting of of fibrils happens not only at kinks but also at random positions inside kinks, leaving ﬁbrils happens not only at kinks but also at random positions inside kinks, leaving shorter shorter rods behind, causing the overall particle length distribution to change to lower rods behind, causing the overall particle length distribution to change to lower average average values. The liquid crystalline transition points based on aspect ratio and fibre values. The liquid crystalline transition points based on aspect ratio and ﬁbre polydispersity polydispersity anticipated by the theory of Onsager’s are undoubtedly impacted by anticipated by the theory of Onsager ’s are undoubtedly impacted by this evolution [29]. this evolution [29]. Based on modelling efforts, as illustrated in Figure 3 for the CNC length of 41.5 nm, the equilibrated structure is a left-handed helical ribbon with the CNC directions spinning in the planes [69]. It demonstrates the model’s capacity to recreate the right-handed CNC’s inclination to develop a left-handed cholesteric phase due to its inherent twist [56]. Fur- thermore, the simulation results show that increasing CNC length (e.g., through sonication) Appl. Biosci. 2022, 1, FOR PEER REVIEW 6 Figure 2. CNC bundle (slight agglomeration) as a chemical dopant (nucleus for chiral structure formation). (a) Sonication impact the formation of a chiral nematic structure that is structurally coloured as a function of sonication (Sonication energy has changed from 0 to 3087 J/mL). (b) Appl. Biosci. 2022, 1 249 Transmission electron microscopy (TEM) image of CNC bundle, showing slight agglomeration. c) Schematic of CNC composite bundle as a model (bundles also artificially change the aspect ratio of CNCs). Adapted from Ref [37]. can increase pitch size; this was seen in the results of ref. [70], where at the constant mass According to ref [56], sonication, a process for creating CNC from CNF, cutting fraction in solution, CNC pitch size increased from 7 micrometres to 18 micrometres when of fibrils happens not only at kinks but also at random positions inside kinks, leaving CNC length increased from 100 to 140 nm nanometer. Other comparable molecular dy- shorter rods behind, causing the overall particle length distribution to change to lower namic simulations were performed to investigate CNC’s inherent chirality and chirality average values. The liquid crystalline transition points based on aspect ratio and fibre that it induces in its assembled chiral form [50,53,65]. The model developed here can be polydispersity anticipated by the theory of Onsager’s are undoubtedly impacted by paramount to explore other parameters, such as the presence of defects or the presence of this evolution [29]. agglomerated bundles on pitch size and chiral formation. Figure 3. Molecular dynamic simulation of CNC chirality investigation. (a) Initial model structure with 180 CNCs of length 41.5 nm side-by-side with (110) surfaces facing each other and corre- sponding equilibrated cholesteric ribbons. Adapted with permission from Ref. [69]. Copyright 2020 American Chemical Society. (b) Pitch as a function of the CNC length. Adapted with permission from Ref. [69]. Copyright 2020 American Chemical Society. (c) Schematic of CNC composite bundle. (d) The modelled particle has a cholesteric ribbon for CNC with a length of 100 nm and undulation with a wavelength equal to . Adapted with permission from Ref. [69]. Copyright 2020 American Chemical Society. The CNC liquid crystalline allomorph assemblies were also reported to be distinct; CNC-I and CNC-II both have a right-handed twist, but CNC-II was longer and had a higher aspect ratio. Additionally, the CNC-II initial phase separates into an isotropic and lower nematic liquid crystalline phase before gradually reorganising into large-pitched chiral nematic assemblies [71]. Small angle neutron scattering (SANS) is very powerful in analysing the liquid crys- talline behaviour of CNCs, authors in ref. [72] analysed carboxymethylated CNCs, and samples were shown to be arranged laterally in 2-D stacks of similarly sulfuric acid hydrol- ysed CNCs are showed similarly arranged microstructure; these structures were formulated to be the initial nucleus for the formation of liquid crystals; these orderings were found using SANS and dynamic light scattering (DLS) to be also reversible under semi-dilute con- dition. This study [72] showed that the early stage of liquid crystalline formation in CNC is probable that can detect crystal aggregate at concentration way below those reported by birefringence of polarized optical microscopy (POM). Moreover, they constructed a graph to connect the trend in apparent hydrodynamic size and viscosity to the concentration of carboxymethylated CNCs and found that both viscosity and hydrodynamic size remained relatively constant till a concentration of 10 g/L, after which both parameters increased Appl. Biosci. 2022, 1 250 dramatically. This increase in hydrodynamic diameter was reversible and indicated the development of small liquid crystal domains of 2D aggregates. Other studies reported in the literature show weak or no structure at the concentration range of 7.8–13.7 g/L [73–75]; however, a direct comparison between these results and the results explored here is difﬁcult. Here viscosity was chosen as a method to track the development of aggregates; in ref. [76], similarly, rheology was found to be a more powerful tool in recognizing liquid crystal formation through the examination of ﬂow curves as opposed to POM. Moreover, it was reported that CNC attaches laterally. The lateral arrangement indi- cates that the surface properties of the sides of CNCs can be different. Cellulose has a crystal structure wherein polymer chains are oriented into ﬂat structures along a particle-long direction [77]. This signiﬁes that hydroxyl groups that are oriented parallel to the CNC plane can have two sides that are more polar than the two sides of the particle that are arranged in a perpendicular fashion [78,79]. Thus, the orientation of the polar side of two neighbouring particles in each direction leads to exposure of their hydrophilic side to the surrounding molecules of water and minimizes their free energy. Parallel stacking of CNC as the origin of chirality shown in Figure 2, therefore, can be seen as the true origin of chirality in CNC given results debuted in ref. [72]. The rheological, optical, and assembly behaviour of CNCs were shown to be sig- niﬁcantly affected by the isotropic to liquid crystalline transition. Table 2 is designed to show empirical values linked to the volume fraction of transition at the beginning of phase separation and point of completion that results in the production of a single phase entirely liquid crystalline phase. I-N (%) and LC (%) are the points at which the system transition from isotropic to nematic and subsequently fully liquid crystalline. It uses data from the literature. According to Onsager ’s theory, the point of transition and the point at which a completely liquid crystalline state forms scale with aspect ratio, as illustrated in equations 1 and 2. Table 2. Derived from the literature resources, a correlation between the volume fraction of the phase transition point and the volume fraction of the completely crystalline point was found with surfaces sulphate groups (0.89–1 wt%) and polydispersity (PDI) (standard deviation(std) around 30–50% of the average value). Reference Aspect Ratio (L/d) I-N (%) LC (%) [18] 5 2.5 6.8 [80] 67 2.1 4.9 [81] 28 1.2 5.1 [81] 19 0.5 4.3 [21] 31 0.5 2.6 [82] 14.3 2.0 5.1 [81] 22 3.0 5.8 [83] 18 3.5 8.9 [70] 28 3.4 7.8 [70] 30 3.1 7.4 [20] 16.5 3.1 5.1 [70] 24 4.7 9.1 [70] 24 3.5 8.2 3.29 C = (1) IN L/d 4.19 C = (2) LC L/d Appl. Biosci. 2022, 1 251 C and C are volume fractions at transition points; L is the length of particles, and IN LC d is the diameter; based on the data points compiled here, Onsager ’s hypothesis has been violated slightly. The deviation from these equations can be due to the appropriateness of this equation’s applicability to aspect ratios higher than 15 [84]. The polydispersity of CNCs used in the tests may mask the transitional point, and the resolution limitations of precision optical microscopy may also contribute to the deviation. As a result, the commencement of the liquid crystalline transition is observed at greater concentrations. 2.2. Rheology of CNC LCs The rheology of CNC LC suspensions is extremely similar to those of rigid inclusions and LC polymers that resemble stiff rods [85,86]. For CNCs like carbon nanotubes (CNTs), Appl. Biosci. 2022, 1, FOR PEER REVIEW 9 the viscosity vs. concentration curves is not monotonic, but there is a point when viscosity initially reaches a maximum and then decreases as the concentration rises to a minimum value in a single nematic phase. Shear rheology and the apparent domain structure of liquid gion I display a shear thinning regime where viscosity declines with shear rate, fol- crystal states are shown in Figure 4a,b [33]. Region I display a shear thinning regime where lowed by a plateau and then another shear thinning phase connected to rod orienta- viscosity declines with shear rate, followed by a plateau and then another shear thinning tion, in contrast, to dilute samples. Additionally, it is known that the first normal dif- phase connected to rod orientation, in contrast, to dilute samples. Additionally, it is known ference turns negative in area III, where the cholesteric director oscillates back and that the ﬁrst normal difference turns negative in area III, where the cholesteric director forth in its flow direction. The negative represents a vertical extension and stream- oscillates back and forth in its ﬂow direction. The negative represents a vertical extension lined compression. The initial normal stress differential of the polymer solution and and streamlined compression. The initial normal stress differential of the polymer solution melts is positive when they are taken from the die [87]. This die swell warps extrudate and melts is positive when they are taken from the die [87]. This die swell warps extrudate forms, lowering the physical qualities of the finished product. Integrating CNC LCs forms, lowering the physical qualities of the ﬁnished product. Integrating CNC LCs with with polymer is a unique application for these materials to prevent die swell. More polymer is a unique application for these materials to prevent die swell. More research is research is needed to fully comprehend the causes of the negative first normal stress needed to fully comprehend the causes of the negative ﬁrst normal stress differential and differential and how to use it to improve CNC-based products [88,89]. how to use it to improve CNC-based products [88,89]. Figure 4. Correlation between mesostructure and rheology of liquid crystalline CNCs. (a) Schematic Figure 4. Correlation between mesostructure and rheology of liquid crystalline CNCs. (a) Schematic of domain orientation based on lyotropic liquid crystalline theory. (b) The shear rate dependence of of domain orientation based on lyotropic liquid crystalline theory. (b) The shear rate dependence of apparent viscosity for LC, dilute, semi-dilute, liquid crystalline and glassy rod’s suspension. apparent viscosity for LC, dilute, semi-dilute, liquid crystalline and glassy rod’s suspension. Adapted Adapted with permission from Ref [33]. Copyright 2019 Elsevier B.V. with permission from Ref. [33]. Copyright 2019 Elsevier B.V. Steady shear viscosity flow curves may generally be affected by changes in the ratio Steady shear viscosity flow curves may generally be affected by changes in the ratio of of isotropic to anisotropic (entering biphasic region) state in suspension or changes in liq- isotropic to anisotropic (entering biphasic region) state in suspension or changes in liquid uid crystalline domain size. Sonication, for instance, can directly impact the structure and crystalline domain size. Sonication, for instance, can directly impact the structure and rheology rheology of CNCs [24]. Agglomerate breakdown during sonication results in a reduction of CNCs [24]. Agglomerate breakdown during sonication results in a reduction in viscosity in viscosity by (i) generally increasing the aspect ratio of the CNC colloidal system, (ii) by (i) generally increasing the aspect ratio of the CNC colloidal system, (ii) expanding the expanding the localized dense LC zones, and (iii) decreasing the effective volume fraction localized dense LC zones, and (iii) decreasing the effective volume fraction of the CNCs in of the CNCs in contact with water. As the sonication or CNC aspect ratio rises, the critical viscosity—the level at which LC production begins—is pushed to a lower concentration in terms of viscosity. Although the gradient of the viscosity vs. concentration curves in reported reports on cellulose suspensions indicates a rise in the area of biphasic coexistence, a clearly de- fined maximum is non-existent thus far, even though it has been seen in other nano inclu- sions such as graphene oxide [85]. This is due to the polydispersity of CNCs that results in the formation of a broader biphasic region than the ones theoretically predicted; thus, the decline in viscosity ensues across the biphasic region; therefore, the maximum either disappears or is confounded [33]. As opposed to this set of observations, Li et al. [90] found that when particles interact, and the double layer around them is thin, a local max- imum in viscosity versus concentration develops. Therefore, maximum emergence may be more easily seen when the double layer is thin [90]. This needs further investigation in the literature. Appl. Biosci. 2022, 1 252 contact with water. As the sonication or CNC aspect ratio rises, the critical viscosity—the level at which LC production begins—is pushed to a lower concentration in terms of viscosity. Although the gradient of the viscosity vs. concentration curves in reported reports on cellulose suspensions indicates a rise in the area of biphasic coexistence, a clearly deﬁned maximum is non-existent thus far, even though it has been seen in other nano inclusions such as graphene oxide [85]. This is due to the polydispersity of CNCs that results in the formation of a broader biphasic region than the ones theoretically predicted; thus, the decline in viscosity ensues across the biphasic region; therefore, the maximum either disappears or is confounded [33]. As opposed to this set of observations, Li et al. [90] found that when particles interact, and the double layer around them is thin, a local maximum in viscosity versus concentration develops. Therefore, maximum emergence may be more easily seen when the double layer is thin [90]. This needs further investigation in the literature. 2.3. Phase Separation One of the distinctive properties of lyotropic rigid-rod polymers is the biphasic region in equilibrium where isotropic and liquid crystalline phases coexist. The capacity of polymeric chains to form LCs in a polydisperse environment is independent of their length; polymeric molecules having lengths less than the persistence length readily diffuse to an isotropic state, thus making their separation based on their size thermodynamically possible. As a result, fractionation depending on size, occurs in equilibrium with the isotropic state within the “Flory chimney” [46]. With a rise in concentration, the system undergoes an isotropic-to-nematic phase transition via a biphasic region, the so-called Flory chimney in the liquid-crystalline rigid polymers [91]. Rigid rod-like polymers already displayed this behaviour [92]. Similar to lyotropic (i.e., liquid crystalline phase forms as a result of a change in concentration) rigid polymers, CNC suspension above a certain concentration threshold develops a biphasic region that can separate into nematic and isotropic domains if given enough time [46]. The relaxation time to carry out phase separation depends on CNC’s aspect ratio and concentration. These relaxation times are dominated by kinetic arrest if concentration and aspect ratio is close to the glassy regime [43,93]. The separation time can be shortened if the biphasic suspension is allowed to be centrifuged [93]. Longer CNC must separate to the nematic phase at the bottom layer according to the topology of phase separation, whereas shorter CNC must ﬂoat with other impurities at the top layer. Such a procedure has lately been employed to improve or purify CNCs’ average aspect ratio. As cycles of puriﬁcation, whether by time passing or centrifugation, increase, the breadth of the chimney narrows, resulting in smaller length distribution of CNCs. This approach, as it has been used before [94], may result in the production of monodisperse LC, which might have profound scientiﬁc effects such as using these uniform distribution of CNCs for studying their migration on the interface for Pickering emulsion applications. Additionally, a shear fraction may be used to clean up a suspension of CNCs; as shear increases, longer CNCs ﬂow into the bulk while shorter CNCs remain at the moving border. [76,95]. The widening of the chimney also has been observed in polymeric liquid crystals before [96,97]. 2.4. Mechanical Properties Due to the rigidity of CNCs, which hinders the release of external stresses during deformation and speeds up the emergence of cracks, pure CNC ﬁlms are stiff and brittle [98]. By combining or blending CNC with other non-brittle components, such as water-soluble polymers, it is possible to create CNC-based photonic materials that can overcome this restriction [12]. To combat its brittle nature, CNC has previously been combined in the literature with surfactants [99], resins [100], and polymers [101]. The CNC’s cholesteric structure [102,103] enables it to interact with gold nanorods to induce chiroptic effects while supporting small particles without causing structural disruption. This accommodation Appl. Biosci. 2022, 1 253 is referred to as “templating” and will be brieﬂy explored in relation to its application in plasmonic [103] and energy storage devices [104]. A layered, spinning microstructure known as a “bouligand structure” commonly appears in naturally occurring materials. It looks like plywood. A key element of bio nanocomposite that has not been duplicated is the architecture of pore canals (aerogels or hydrogels), which can help stiffen the material and supply components for self-healing. This potential for real estate development is still open and serves as a solid foundation to produce pure CNC goods and CNC-based composites. Additionally, polymer molecules may be held inside the openings of the chiral geometries of CNCs when polymer and CNCs are mixed. This provides an excellent opportunity to research the properties of natural polymeric in conﬁned spaces, the cycling arrangement of CNCs and polymers in the CNC “bouligand” composites, and the adaptability of CNC-CNC spacing (pitch size). The polymers utilised may also respond to stimuli, making the “Bouligand” element sensitive to the environment; in other words, the level of conﬁnement might be altered. Additionally, the load bearing capacity and bending moment properties of carbon-ﬁber reinforced composites have been studied [105], as shown by biological samples [106]. The composite loading bearing capability is also increased by the Bouligand arrangement [107]. However, no measurements have been done for the properties of CNC gel and ﬁlm com- posites. To eliminate the ambiguity, more study is required in the area. AFM could be used to investigate the mechanical characteristics of CNCs’ chiral structures and the effect of force on pitch size. Additionally, CNC chiral structures can increase the products’ robustness. The most common microstructural arrangement in bioactive systems with higher mechanical strength and toughness, including such bone [106] and the mantis [108], is the twisting plywood, or Bouligand, architecture. These materials display consistent qualities in all directions because of the “Bouligand” design itself, a low volume fraction of soft, energy-dissipating polymer, and shear wave ﬁltering [109], fracturing twisting, deﬂection, and arrest [106]. Energy is removed at the nanoscale through delamination, matrix viscoelastic softening, and nanoparticle breaking. The process of removing heat often expands cooperatively over several length scales. The Bouligand structure lowers stress [108] by distributing the damage and ﬁltering off shear stress at the microscale. It has been discovered thus far that adding macromolecules or nanoparticles while the suspension is still wet leads to the development of nanomaterials with CNC seated in chiral architecture. According to the literature [101,103,110,111], the speciﬁcs that have so far been included are polyethylene glycol, PVA, glucose, and nanoparticles. This list comprises polymers that do not signiﬁcantly cause CNC agglomeration, preserving their ability to keep their chirality inside nanocomposite. Energy loss during crack propagation is expected to increase if the polymer has a strong inclination toward CNC. However, this propensity leads to clustering between CNC. The inﬁltration of CNC chiral nanostructures that have dried up is another remedy for this problem. Both monomer solutions and small molecules have attempted this [112,113]. Inﬁltration permits the creation of nanocomposite with an additional concentration of up to 50%. Despite the signiﬁcant possibility for agglomeration when additives are present in a moist setting, this enables the chiral nematic order to be maintained (suspension). The polymers [114,115], protein [116], and calcium chloride [117,118] were all subjected to this technique [119]. A vertically chiral-structured ﬁlm can also be maintained to reduce swelling under stress. The complexity and mechanical durability of aerogel production may be improved by chiral structure. CNCs are intriguing alternatives for the bottom-up synthesis of these structures because, when cast from liquids, they dynamically self-assemble into chiral nematic ﬁlms, simplifying the scaling-up process. Materials made by the self-assembly method are the best candidates for specialised application because altering a material’s nanoscale structure permits ﬁne-tuning of its physical features and related functionality. According to one study, chiral component orientation increased the speciﬁc strength and toughness of CNC aerogels by up to 137 and 60%, respectively [120]. According to the Appl. Biosci. 2022, 1 254 author ’s research, chirality can result in chiral-nematically structured aerogels with precise meso- and microstructures. The amount of liquid absorbed by the aerogel can be observed by the colour it reﬂects [68] since the ﬁnal aerogels exhibit a signiﬁcant link between the mesopore% and selected light reﬂection (iridescence) caused by mechanical load. The scientists also pointed out that the mechanical performance of pore compression under load is markedly enhanced by chiral-nematic ordering. A recent review thoroughly summarises the bottom-up assembly process used to create CNC- or CNF-based aerogels, which are used as adsorbents [68]. However, tactoids can be employed as architecturally coloured pigments in melt processing rather than introducing additional CNCs. CNC can also be combined with water-soluble materials utilising solution processing methods or with water-insoluble poly- mers using melt processing. Liquid crystalline thermosets may also be produced using CNCs, a structure of initially non-mesogenic epoxy monomers, and a solution processing technique. These materials are cured at high temperatures to generate liquid crystalline phases [121]. Another strategy to embed CNC within polymers and simultaneously en- hance their mechanical properties [12] is to incorporate water-soluble polymers, such as PVA, into CNC ﬁlms. By doing this, polymer molecules improve stress transport through- out the network. Therefore, it is possible to incorporate chiral optical ﬁlms into coating applications that use PVA [122,123] or epoxy resin. Due to the lyotropic mesogen of rod-like negative CNCs, which is easily impacted by external inﬂuences such as hydrogen bonding interactions with PVA, some polymers may cause CNC to lose their LC property in the composites. However, the use of extra chemicals may help to solve this issue. For instance, in reference [124], sodium sulphate was employed as a coagulant during the wet spinning of PVA-CNC ﬁbres to produce nematic representations of CNC outcomes. After drying, CNC ﬁlms are likewise essentially irregular. CNC ﬁlms are not uniform due to two factors. The ﬁrst is that due to the ﬁlm’s randomly dispersed helix orientations and various pitch length values, the dried ﬁlm’s helix axes are oriented at an angle to the surface. After drying, the ﬁlm forms many tactoids with a cholesteric and well-organised structure that may be evaluated by scanning electron microscopy (SEM). The length and helix axis orientations of these tactoids change throughout time (drying speed affects pitch size). When these tactoids are gradually combined and deposited, they can form a stacked chiral nematic phase structure; as a result, the structure is not homogeneous. Additionally, after drying, the tactoid would be squeezed vertically to add extra helix axes. The second factor [125] is macroscopic non-homogeneity responsible for the coffee ring effects. As a result, the nanoparticles are merely forced toward the ring’s edge. The “coffee ring” effects that result from the creation of chiral tactoids with an unequal distribution give the ﬁlm its many hues. Ring-shaped deposits form because of capillary migration from the interior to the exterior, which transports suspended particles to the droplet’s edge and deposits them in a ring that forms at the border due to higher evaporation at the periphery than at the centre. Since the border is thicker than the centre, the corresponding location has different optical characteristics. The development makes the ﬁlm’s optical qualities inconsistent. Postprocessing is required to correct the structure for applications since the mechanical properties of coatings are determined by the uniform distribution of CNC tactoids over the ﬁlms. The merging of CNC tactoids is usually subpar during quick water evaporation, leaving just portions of the CNC ﬁlm in the LC structure as tactoids in the ﬁnal CNC ﬁlms, which commonly have uneven LC formation in addition to having poor mechanical properties. Additionally, water evaporates more quickly towards the petri dish’s edge than in the centre due to the “coffee ring” effect, transferring mass from the edge to the centre. As a result, reinforcement in solution casting is uneven [126]. The helix will not become uniform even if nearby tactoids with different helix orientations mix in the CNCs water suspension. A defect of this size would take a long time to ﬁx into a consistent helix shape [126]. However, continuing water evaporation quickly transforms the system into a glassy state and prohibits further helix or director rearrangement; therefore, this Appl. Biosci. 2022, 1 255 is unsustainable [127]. When tactoids fail to fuse completely, ﬂaws result, making the mechanically weak CNC pure ﬁlms further weaker. The tactoids rearrange and join to produce enormous tactoids during the phase separation, and higher concentration rises over time as water evaporates, comparable to the Ostwald ripening process of emulsions [126]. This, as we will brieﬂy discuss later, takes a lot of time; therefore, the process as it is now cannot be repeated to make goods at an industrial scale. Additionally, at the last step of the ﬁlm drying process [128], a vertical fracture parallel to the bottom edge of the capillaries was developed for CNC pure ﬁlms drying CNCs in a capillary. According to reports, those ﬁssures appeared as a result of a morphological mismatch between twisted structures and ﬂat rectangular walls. The origin was supposed to be a stiffness imbalance between adsorbed CNC to walls; these ﬁssures are proof of nano- crystals parallel alignment to the capillary bottom. Additionally, drying LC suspension results in additional types of defects because of the non-uniform deposition of materials at the edge of the droplets during water evaporation, which is caused by the “stick-slip” motion of the interface and Marangoni [126,129,130]. Rapid evaporation typically causes structural alterations to the ﬁlm and an increase in pitch size. It is essential to monitor and adjust these variables since they all impact how quickly things dry, including humidity, airspeed, casting area, heat, and other variables. Other techniques, including employing vacuum-aided ﬁlm production, are needed to produce homogeneous ﬁlms at reasonable time scales since the necessity for delayed drying restricts the scalability of ﬁlms. LC ﬁlms are naturally uneven when manufactured; however, the ﬁlm thickness may be controlled. The simplest approach applies mechanical force using vacuum-assisted self-assembly to produce a ﬁlm with homogeneous and controlled optoelectronic characteristics. Deheer et al. [131] employed this technique. Chen and associates [132] conducted additional research on this method in 2014. High ratings for the ﬁlms’ orientation and consistency of structure have been given. The spiral directions’ random distribution created brightly coloured ﬁlms. Mechanical crushing has been found to produce water-resistant ﬂakes [133] from CNCs with architecture colouring by utilising desulfation and the natural brittleness of CNC ﬁlms; as a result, CNC chiral tactoids may be employed as structurally coloured pigments [133]. In some applications, these pigment-free colourants should keep glittering inside a polar liquid; nevertheless, if they are not handled, the ﬂakes may re-dispersed in water or other polar mediums. The re-dispersion of CNC ﬁlms after immersion in water can be avoided by desulfation of the dry material using acidic desulfation [134], solvolytic desulfation [134], and alkaline desulfation [135,136]. One way to make ﬁlms more ﬂexible while creating an edible composite is by mixing them with safe chemicals like glucose. Kose et al. [113] and Mu and Gray [125] added glucose to CNC ﬁlms created in 2019 to increase ﬂexibility and change the pitch. The research by Kose et al. [113] is highly intriguing since it shows how the relationship between pitch and colour creation may be visually represented by stretching elastomeric composites and producing beautiful colours. These modern developments and cutting- edge utilisation methods can enhance the mechanical properties of CNC-inserted ﬁlms and composites. Chemical treatment is one of the alternatives. The durability and heat endurance of vacuum-ﬁltered CNC iridescent sheets can be considerably improved by a simple chemical process (alkali), as Nan et al. [137] show. Better mechanical properties could be attributed to increased load transfer among “chiral tactoids” because of the amalgamation of CNC nanoparticles with other compounds during processing and the subsequent fusion of ordered layers. The author proposed that the signiﬁcant increase in CNC-based ﬁlms’ mechanical characteristics was caused by the unstructured ﬁlm produced by alkali treatment. By altering the polymeric matrix around the liquid crystal or the nature of the charges on them, more liquid crystalline production is also feasible. In ref. [138], it was demon- strated how to produce carboxylated CNCs from cellulose by hydrolysis and esteriﬁcation with the help of oxalic acid. When synthetic CNCs were combined with PEG, composite Appl. Biosci. 2022, 1 256 ﬁlms that displayed spectacular colour snowﬂake patterns were formed that differed in characteristics from conventional CNCs. Through the addition of cations to the 1.27% CNC solution, the hydrogels of the treated CNCs also generated birefringence gels. Oxalic acid coupled to CNCs included 0.3–0.54 mmol/g, according to measurements. 2.5. Orientation The LC’s arrangement and direction are equally crucial and merit careful examination. Both small-angle x-ray and small-angle neutron scattering have been used to examine the structural properties of the ordered chiral nematic CNC phase. Findings from SANS demonstrate that in magnetically or shear-oriented chiral nematic suspension, CNC rods are more densely packed parallel to the chiral nematic axis than perpendicular to it [73,139]. Orts et al. also demonstrated that rod spacing decreases when CNC number density and electrolyte concentration increase [73,139]. Up until now, attempts to change CNC director direction have solely used exter- nal ﬁelds such as extension, shearing, and the use of magnetic or electric forces, all of which have had varying degrees of success. Fibres with diamagnetic dispersion also align in static magnetic ﬁelds. Carbon ﬁbres [140,141], carbon nanotubes [142], polyethene ﬁ- bres [143], cellulose ﬁbres, and other materials have been successfully used in the literature to demonstrate magnetic alignment. Studies using these similar rods’ liquid crystalline ﬁbres oriented magnetically may readily be expanded to CNCs. The chiral nematic axis aligns parallel to the static magnetic ﬁeld because cellulose ﬁbres are diamagnetic, and several studies on the chiral nematic phases produced by cellulose ﬁbre suspensions have been reported [144–146]. By inducing an induced magnetic ﬁeld in the opposite direction, which provides a repelling force, a magnetic ﬁeld repels diamagnetic materials. The application of a magnetic ﬁeld is the sole strategy that has been discovered so far to change the orientation of the helix. Although using microﬂuidics to create such structures has been discussed in the literature, this method has not been used to explain improving or changing orientation, particularly in relation to CNCs in their LC form. Due to the CNCs’ negative diamagnetic anisotropy, a rod aligned perpendicular to the magnetic ﬁeld, this approach is ideally suited to function and orient CNCs. As the result of their negative magnetization inhomogeneities, CNCs attempt to equal parallel to an external magnetic ﬁeld. At low magnetic ﬁelds (m H 0.5T), the cholesteric phases may be aligned empirically [147]. Since this rod of a cholesteric CNC suspension is all perpendicular to the helix axis, the helix will be evenly aligned in the ﬁeld direction when put in a suitably strong magnetic ﬁeld. Revol et al. [31] and Kimura et al. [146]; experimental data afﬁrmed this; The helical axis eventually organized parallel to the magnetic ﬁeld’s direction (CNC in the perpendicular direction), and the magnetic ﬁeld’s recurrent rotation caused the crystal to unwind as the suspension changed from LC to an orthotropic condition. The liquid crystalline phases were dissolved and relaxed upon the removal of the magnetic ﬁeld. In addition to displaying its magnetic qualities, the mere responsiveness of LC to magnetic ﬁelds may be further exploited by, for instance, enhancing its responsiveness by adorning it with iron oxide or other magnetic particles to make switching for electronic purposes quicker [148]. The current literature review states that magnetic ﬁelds have not been investigated for their effect on tactoids fusing. Furthermore, by observing how tactoid deposition spreads across the solution, tactoid deposition may be more homogenous. Look into strategies to increase or decrease tactoid nucleation as it may be organized and examined similarly to polymer crystallization. In research conducted by Qu and Zussman, the local crystal structure (LC) was disturbed using a magnetic ﬁeld, but the isotropic to nematic or nematic to chiral nematic transitions were not moved [149]. Tactoids’ orientation can be changed by depletion pressures in combination with magnetic ﬁelds. According to Kimura et al. [146], magnetic ﬁeld alignment occurs at 1 T in 5 h. How- ever, there are still unanswered questions regarding the effects of sodium chloride, depletion force, and concentration on the rate of orientation and the potential effects they may have Appl. Biosci. 2022, 1 257 on accelerating or slowing the rate of orientation. To make particles more sensitive to mag- netic ﬁelds and faster at aligning, diamagnetic particles, such as Fe O [150], can also be 2 3 sprayed onto the particles. In 2016, magnetically aligned CNC-Fe O were reported to copy 3 4 plywood structures by combining individual layers of unidirectional CNCs. To achieve orientation, Poly (L-lactic acid) (PLLA)/CNC-Fe O suspensions in chloroform were left to 3 4 become dry in a magnetic ﬁeld with a strength of 60 mT at different orientation parallel and perpendicular to the surface [148]. However, one research [151] asserts that viscosity affects chiral direction. This study examined this in a 0.7 T magnetic ﬁeld while using a ﬂuid that changed from water to n-methyl formamide (NMF). In comparison, the suspensions with a higher viscosity, exhibited almost little temporal change after the initial ﬁeld hit, therefore, highlighting the role of surrounding viscosity on ordering. These ﬁndings represent a major step in extending the application areas of large-scale cellulose-based composites with anisotropic characteristics by simplifying the manufacture of globally ordered CNCs in a range of polymers in the presence of a mag- netic ﬁeld. Viscosity’s impact on CNC order Compared to CNC-water, CNC-NMF has a greater viscosity. Rotating in a magnetic gets more challenging as a result. Therefore, CNC tactoids may be used as microrheological probes, with the orientation rate corre- lated with the ﬂuid’s viscosity. This probing, similar to the ﬂuorescence recovery after the photobleaching probe [152], can reveal details about the pore structure and level of surrounding conﬁnement. The rate of rod orientation in different suspensions can also be related to tactoid changes. Magnetic ﬂux was also used to produce well-aligned nematic liquid crystalline and colloidal crystalline samples of the essentially monodisperse tobacco mosaic virus (TMV) [153]. Studies show that the pitch of CNC-based ﬁlms decreases as the magnetic ﬁeld strength increases [154]. The ﬁlm’s pitch might be changed using this technique, which would also increase the ﬁlm’s homogeneity. To produce highly distorted low concentrations of CNC distributed in cyclohexane, Habibi et al. [155] employed an AC ﬁeld. The results showed that rod-shaped particles turned [155] degrees parallel to the direction of the electric ﬁeld rather than the magnetic ﬁeld direction. In the case of tunicate CNCs, there was no alignment in the samples exposed to AC electric ﬁeld lower than 2000 V/cm for all frequencies applied. A better orientation was achieved at starting ﬁeld strength of 4 6 2500 V/cm. Perfect orientation was observed for frequencies ranging from 10 and 10 Hz. According to De France et al. [145], cooperative ordering favours ﬁeld direction. To orient samples containing 1.65 and 4.13 wt percent CNC, a target recently attained for tunicates-derived CNC [156], magnetic ﬁelds of 0.56–1.2 T were insufﬁcient. In exposure to the magnetic ﬁeld, the helix was untangled, which led to an increase in pitch size (redshift) [154,157]. The method works well with suspensions that have low viscosities and low cholesteric concentration sizes. Films with consistently aligned helices may also be produced by starting with a low CNC concentration and slowly emptying the solvent under a constant magnetic ﬁeld. Due to CNC’s positive dielectric anisotropy, the helix may be unwound by an electric ﬁeld. In ﬁlms made by drying CNC suspension in the presence of electric ﬁelds with frequencies ranging from moderate to high, Habibi et al. [155] achieved homogenous CNC alignment. De France et al. [145] examine the impact of CNC concentration (1.65–8.25 wt%) and weak magnetic ﬁelds (0–1.2 T) on the kinetics and level of CNC ordering. For CNC suspensions above C* (4.8 wt%) in a 1.2 T magnetic ﬁeld, partial alignment occurred in less than 2 min, proceeded by slower cooperative ordering to reach almost perfect alignment in less than 200 min. Magnetic alignment was not seen in suspensions below C*. Frka-Petesic et al. [156] describe magnetic birefringence measurements of dilute aqueous solutions of CNCs generated from tunicate up to 17.5 T (high ﬁelds). The birefringence reached its maximum at 17.5 T, suggesting that almost all nanocrystals originated with their long axis parallel to the ﬁeld. To appropriately regulate crystal alignment, the authors suggested utilizing a magnetic ﬁeld, mechanical deformation, or neither an electric ﬁeld nor a magnetic ﬁeld. After Appl. Biosci. 2022, 1 258 extensive annealing at ambient temperature in a magnetic ﬁeld, shear-induced morphology disappeared, and a distinctive nematic “schlieren texture” appeared [158]. A string of irregular director orientation points characterizes the “schlieren texture” of nematic LCs. From the end, these dots correspond to disclination lines. Other liquid crystalline materials have these “schlieren textures,” but they are created by physics and superstructure rather than chemistry [158–160]. After some time, the alignment caused by a high magnetic ﬁeld (0.25 T) could be observed; domains with discrete LC orientations that were initially torn apart by disclinations were subsequently reoriented and joined into larger domains. Due to the low magnetic repulsion of the bare platelets, perfect alignment required many hours. The same magnetic ﬁeld strength was used to adorn the platelets with iron oxide (Fe O ), which preserved liquid crystallinity in an aqueous environment and ﬁnished 2 3 alignments caused by ﬁling swiftly. Crystallinity may be preserved if the suspended liquid is coupled with a polymer matrix. When polarizers were crossed, nanocomposites made of poly (acrylic acid) (PAA) and graphene oxide showed birefringent schlieren texture. The scientists discovered that platelets were precisely aligned using hand drawing and SEM. With the gap among nematic planes along the cholesteric axis being less than the spacing among rods in a nematic plane [139], microﬁbril suspension exhibits anisotropic chiral nematic ordering in a 2.4 T magnetic ﬁeld. Kvien and Oksman [161] used PVA with CNC made from micro-crystalline CNC. The authors claimed that in that circumstance, CNC alignment was accomplished using a powerful 7 T magnetic ﬁeld. They developed translucent, birefringent ﬁlms with a ﬁnal concentration of 2 weight percent CNC (4.21 GPa) and discovered that it was considerably lower than the storage modulus determined in the transverse direction (6.19 GPa). This result veriﬁed that CNCs are parallel to the applied ﬁeld. The shear ﬂow will also give uniaxial CNC alignment, another manipulating tool for directing the direction of the unwinding helix. The ﬁlms with uniform nanorod alignment produced by shearing and the X-ray examination of the LC CNC alignment during shear ﬂow displayed complicated behaviour with signiﬁcantly different outcomes depending on the shear rate [162,163]. A microﬂuidic chip’s CNC orientation has also resulted in the complete orientation of each CNCs [164]; however, there is no information on orienting LCs in the same manner. Under intense hydrodynamic shear, the cholesteric order (chiral nematic) breaks down and transforms into nematic [165,166]. According to studies by Lagerwall and associates [127,167], a small circular shear applied to a drying solution in a circular dish resulted in a locally increased vertical cholesteric orientation. However, when it got closer to the edge in the later example, the helicoidal structure started to sag. The reaction of anisotropic ﬂuids to shear is challenging and intricate, especially for polymeric and colloidal ﬂuids where viscoelastic processes may be prominent [168]. Contrary to popular belief, the transition from nematic (shear-induced) to chiral nematic states (and vice versa) is thought to involve only a simple twisting or untwisting of the chiral nematic axis. It was found that shear generated chiral structural unwinding in an elastomer with CNC embedded in it and that this process induced the CNC nanoparticles to align all along the ﬂow axis. However, as soon as the ﬂow was cut off, the chiral nematic form fully recovered [169]. The effect of shear on the unwinding of tactoids of CNC liquid crystals can also be examined through models such as Folgar-Tucker set models [170] that are developed based on the Jeffery equation [171]; account of the application of these models on CNC orientation prediction can be found in ref. [12]. 2.6. Pitch Length The pitch of LC is the very important distinguishing factor that controls colour in photonic crystals made from this self-assembly process. A helix is characterized by its 2p periodicity deﬁned mathematically as q = as well as its handedness, i.e., a clockwise twist means right-handed while a negative denotes left-handed or counter-clockwise. Chirality is extendable to particles (individuals) for CNC and CNF; chirality is right- Appl. Biosci. 2022, 1 259 handed [30]. The wavelength of reﬂected light is controlled by the pitch of LC [172], and it is conﬁrmed to be the case for CNC using experiments done in ref [173,174]. The wavelength is describable by the equation proposed by Bragg l = nP sin(q) [175,176], where l is the reﬂected wavelength, P is the helical pitch, q is the angle ( ) of the incident light, sin(q) = 1 , where incident light is perpendicular to crystal plane, n also signiﬁes a value that is the average refractive index of the material bombarded by the light [177]. Figure 5a depicts the chiral structure of CNC under POM, showing the ﬁngerprint texture of the chiral nematic structure [178]. Figure 5b displays the maximal reﬂection wavelength at 360 nm, and the CNC iridescent ﬁlm was violet. Figure 5c display of FE-SEM of the structure at a higher resolution displaying the periodicity of the developed structure and the spectra of the CNC ﬁlm recorded by a reﬂectance spectrometer and transformed into a Commission on Illumination (CIE) chromaticity value (x = 0.175, y = 0.006) as shown in Appl. Biosci. 2022, 1, FOR PEER REVIEW 17 Figure 5d [178]. The graphic depicts visible colours as a function of chromaticity coordinates, which are x (red) and y (green) components. Maximum reﬂection was 360 nm at 43 percent relative humidity; increasing humidity to 75, 86, and 99 percent induced a signiﬁcant shift reflected wavelength, P is the helical pitch, 𝜃 is the angle (°) of the incident light, in reﬂection maxima to 451, 513, and 525 nm, respectively [178]. Therefore, this assembly sin(𝜃 ) =1 , where incident light is perpendicular to crystal plane, n also signifies a value based on UV-vis spectra analysis can act as a humidity sensor. that is the average refractive index of the material bombarded by the light [177]. Figure 5. (a) polarized optical microscopy (POM) images of the CNC suspension that contained an Figure 5. (a) polarized optical microscopy (POM) images of the CNC suspension that contained an increasing CNC concentration during the evaporation process. (b) UV−vis absorbance spectra of the increasing CNC concentration during the evaporation process. (b) UVvis absorbance spectra of the CNC iridescent film show a maximum reflection point of 360 nm that determines how the colour of CNC the devel iridescent opment is perce ﬁlm show ived a by the naked eye. ( maximum reﬂection c) Fpoint ield em of ission 360 nm scanni that ng electron m determines icroscopy how the colour (FE-SEM) images of the CNC iridescent film. (d) Commission on Illumination (CIE) chromaticity of the development is perceived by the naked eye. (c) Field emission scanning electron microscopy diagram of the CNC film. Adapted with permission from Ref [178]. Copyright 2020 American (FE-SEM) images of the CNC iridescent ﬁlm. (d) Commission on Illumination (CIE) chromaticity Chemical Society. diagram of the CNC ﬁlm. Adapted with permission from Ref. [178]. Copyright 2020 American Chemical Society. Figure 5a depicts the chiral structure of CNC under POM, showing the fingerprint texture of the chiral nematic structure [178]. Figure 5b displays the maximal reflection Interestingly, when CNC was treated with ionic liquid, it developed a distinctive wavelength at 360 nm, and the CNC iridescent film was violet. Figure 5c display of FE- assembly behaviour and showed no signs of phase separation since tactoid and isotropic SEM of the structure at a higher resolution displaying the periodicity of the developed solutions structure a coexisted nd the spe at ctr concentrations a of the CNC filbetween m recorded by 1.0 and a re 4flwt%. ectanceIn sp contrast ectromete to r aCNCs, nd the concentration transformed in of to tr a C eated omm CNCs issionincr on I eased llumina the tion pitch (CIE of ) chrom chiral a nematic ticity value tactoids (x = 0. [17 179 5,]. y Polymers = 0.006) as shown in Figure 5d [178]. The graphic depicts visible colours as a function of like polyethylene glycol (PEG) can be added to alter pitch size. For example, the half-pitch chromaticity coordinates, which are x (red) and y (green) components. Maximum reflec- dimension increased with PEG addition from 103 nm to 143 nm with an increase in PEG tion was 360 nm at 43 percent relative humidity; increasing humidity to 75, 86, and 99 percent induced a significant shift in reflection maxima to 451, 513, and 525 nm, respec- tively [178]. Therefore, this assembly based on UV-vis spectra analysis can act as a humid- ity sensor. Interestingly, when CNC was treated with ionic liquid, it developed a distinctive as- sembly behaviour and showed no signs of phase separation since tactoid and isotropic solutions coexisted at concentrations between 1.0 and 4 wt%. In contrast to CNCs, the concentration of treated CNCs increased the pitch of chiral nematic tactoids [179]. Poly- mers like polyethylene glycol (PEG) can be added to alter pitch size. For example, the half- pitch dimension increased with PEG addition from 103 nm to 143 nm with an increase in Appl. Biosci. 2022, 1 260 content from 10 to 20 wt% [180]. Similar to chiral structures, the helical pitch increases as polymer concentration rises, and polymer addition causes the wavelength of the reﬂected wave to increase from 332 nm to 625 nm [181]. The rate at which ﬁlms dry out under the CNC anisotropic environment can change pitch length because phase separation takes time, and the evolution of chiral structure depends on time. In ref. [182], CNC suspensions that were dried slowly had the smallest pitch size, which was 330 nm, whereas samples dried quickly had a larger pitch size, which was 440 nm. This study signiﬁed the impact of time given to form tactoids from the original isotropic suspension into a phase-separated suspension (top isotropic and bottom being liquid crystalline) and the impact of drying rate on developed pitch sizes. The average reported pitch of CNC ﬁlms was measured as roughly 1.88 micrometres with SEM, although it was substantially smaller than POM pictures (approximately 349 nm) in ref. [183]. The discrepancy found between dry and wet states is important as drying, if done through regular evaporation, can lead to an increase in concentration over time and therefore pitch size changes; the differences between POM measurement and SEM can become smaller if samples become super critically dried thus freezing the developed microstructure [68]. This variation in drying rate can also produce discrepancies with pitch size measurements from POM and SEM images. According to the drying mechanism, CNCs become biphasic and start to form tactoids as the concentration of the suspension increases over time. These tactoids expand until they reach a threshold volume, mixing with nearby tactoids to form larger anisotropic domains [29]. Salt can impact on the electrostatic attraction between CNCs in chiral systems. It has been demonstrated that salt eventually turns CNC ﬁlms black and white. The development might be explained by two variables. First, if the pitch size is at a region that does not reﬂect the visible electromagnetic spectrum, the ﬁlm may appear transparent. Another factor is the breakdown of chiral structures caused by salt, which can result in microstructures that are either completely collapsed or partially damaged [183]. Furthermore, circular dichroism was unaffected by the presence of salt since CNC ﬁlms with even high concentrations of salt-preserved positive circular dichroism [183]. In presence of salt tested, i.e., NaCl, KCl, MgCl , and CaCl , CNC ﬁlm maintained positive circular dichroism over the wavelength 2 2 examined; therefore, the handedness of the cholesteric helix is invariant to the presence of electrolyte and ionic strength. The results showing how salt affects pitch size are consistent with those showing how pitch size changes in response to negative charges on CNCs, where pitch size increases as the zeta potential of charged CNCs increases with an increase in ionic strength [184]. Though phase transition and pitch sizes are unaffected by the type of additional coagulant cations [20], bigger ions have been proposed to enhance pitch sizes [185], whereas adding polyvalent ions results in lower pitches [186]. Finally, it is possible to alter the surface charge of carboxylated CNC or, for example, heat sulfated-CNC; pitch drop is noticeable after sulfuric acid removal [134]. If additional nanorods are used in co-assembly, their presence may affect the pitch size in CNCs. For example, gold nanorod assembly with CNC using SANS experiments were described [187]; ﬁndings highlight that a low concentration of gold nanorods scarcely affects CNC liquid crystal phase development. A tighter packing of the CNCs in the liquid crystalline phase may be caused by possible columbic interaction due to the adsorption of gold nanorods, according to SANS results that showed an increase in correlation distance compared to pure CNC systems. In another report [188], to change the magnetic response of the CNC, Fe O nanoparticles were dispersed within the clean CNC. A 7 mT magnetic ﬁeld 3 4 was then applied to ﬁne-tune the self-assembly and microstructure of the CNCs. The pitch decreased as the magnetic ﬁlm thickness increased from 302 to 206 nm, with the application ﬁeld’s strength increasing from 7 to 15 mT. In contrast to the researcher ’s ﬁnding that pitch increased with magnetic ﬁeld intensity, this discovery showed that the helicoidal structure was being compressed rather than unwound. Appl. Biosci. 2022, 1 261 Surprisingly, in ref [189], pitch length reduced as virus length increased, according to the author ’s tuning of fd-viruses with varied lengths. The same argument may be made for CNCs. The aspect ratio of the CNC signiﬁcantly impacts the rate of phase separation in biphasic suspension. Long CNCs exhibit quick phase separation with gravity and achieve their ﬁnal equilibrium in a day, whereas shorter CNCs did not, even after three months [43]. As a result, it is conceivable to hypothesize that rate of phase separation can also be used as an extra tool for characterization by indirectly correlating it to the aspect ratio. This unique suspension can be used in coatings thanks to the rigorous supervision of the CNCs author in ref. [43], who also generated mosaic-free ﬁlms that were consistent in colour. 2.7. Conﬁnement The effects of reﬁnement, advanced issues centred on liquid crystal forms and their speciﬁc applications, as well as certain under-researched areas in the literature will all be addressed in addition to the themes that have already been brieﬂy discussed here. For the study of soft matter, it is crucial to do basic research on the geometric conﬁne- ment of nanoparticles, molecules, and liquid crystals. The symmetry of their equilibrium states may be broken by conﬁnement in 1D, 2D, and 3D dimensions, stabilising topological faults and producing distinctive structural patterns (such as disinclination). As a result of the restriction of liquid crystals in 2 dimensions to round droplets [190] and small capillaries, technological advances in optics and sensing are anticipated [191,192]. This study may also open up new research into liquid crystalline formations similar to those in living organisms like the cornea [193,194] and insect exoskeletons [195,196]. Conﬁnement Monitoring the evolution of the pitch as a function of the concentration over a much larger range is made possible by observing the diminishing microdroplet. As a result, in micro- spheres, contraction is more in pitch length, which is expected as opposed to cases of 2-D contraction in a ﬁlm [197,198]. The pitch at low concentration matches pretty well with the bulk concentration and follows a power law of approximately p µ c above a certain connection attributed to the kinetic arrest scaling law correlating pitch to concentration 1/3 changes as p µ c . Additionally, containment in spherical particles with a diameter of less than 90 micrometres led to the development of an isotropic core and cholesteric shell [199]; when the diameter was decreased below 20 micrometres, concentric layers were formed parallel to the interface, changing the morphology to bipolar planar [200] Latex, gold, magnetic nanoparticles, and other substances were segregated into an isotropic core. Like these ﬁndings, scientists in reference reported splitting ﬂuorescent latex nanoparticles into an isotropic core [201] during conﬁnement in capillaries. This method of partitioning, which adopts the formation of liquid crystals in one phase, can be beneﬁcial for the natural separation of colloids from one another. 3. Applications 3.1. Responsive Materials The cholesteric helix can be tuned to provide stimuli response materials that may match the complexity of biological systems, which is a big opportunity (may operate on moisture, heat, phase transition, etc.). Low-cost biodegradable optical sensors are made possible by implementing sustainable enhancements like CNC because the underlying nanostructure regulates how the reflected colour structurally responds to an external stimulus. It has been demonstrated that responsive materials can measure pressure in [202], detect humidity in [203,204], solvents in [202], ultraviolet (UV) light in [205], and temperature in [205]. For instance, CNCs-based ﬁlms have an advantage over other inclusions with compa- rable properties because of their inherent sensitivity to water. Their low water resistance, which results in the loss of their prized iridescent colour even after being gently inﬂated by water, restricts their use in humid environments. However, because of their sensitivity to humidity, they make excellent humidity sensors. Recently, the co-assembly of CNC with oxidised starch and tannic acid was researched as a replacement for humidity variations to improve CNC’s solvent sensitivity. A full day of submersion in water did not affect the Appl. Biosci. 2022, 1 262 composite’s structural integrity, vibrant colour, or mechanical characteristics; cross-links were made possible using tannic acid [206]. Starch or tannic acid additions also kept the self-assembly mostly unaltered. As a result of the information provided here, the chirality sensitivity of CNCs is adjustable; that is, the degree of assembly with other substances, such as starch or tannic acid, can affect how sensitive the chirality is to humidity. He et al. [207] produced CNC composite sheets that responded to moisture and mechanical stress using glycerol as a plasticiser. The structural colours of the ﬁlms could have their chiral structures altered [207]. There was a reversible colour change when the ﬁlm was exposed to relative humidity values between 16 and 98 percentBy altering its iridescent hue, the material can also monitor compression pressure quantitatively. In addition to responding to humidity and formaldehyde gas, ﬁlms created using CNC technology can also change their structural colour and have gas detection capabilities [178]. At RHs of 43, 75, 86, and 99%, the maximum reﬂection wavelengths of CNC ﬁlms were measured to be 360, 451, 513, and 525 nm, respectively. Due to their abundance of hydroxyl groups and porosity, CNC can also react to gas [208]. Formaldehyde gas concentration was varied to achieve maximum reﬂection wavelengths of 378, 404, and 470 nm, respectively, in order to evaluate the response of chiral structures to the gas. CNCs can be utilised as solvent sensors since their chiral-optical properties can be altered in relation to solvents. Based on this, the researchers developed a unique technique for producing tuneable-colour mesoporous CNC ﬁlms. Giese and colleagues [202] created mesoporous photonic cellulose (MPC) ﬁlm by treating a composite of CNCs and a urea- formaldehyde (UF) resin with an alkaline solution. This allowed for quick and reversible structural colour changes in the visible light spectrum. They discovered that the peak reﬂection wavelength of the composite material was 430 nm in 100 percent ethanol but 840 nm in pure water. The colour will “redshift” when there is more water present. The synthetic cellulose ﬁlms displayed excellent ﬂexibility due to their reduced crystallinity (in comparison to the initial CNC ﬁlms’ crystallinity) and the mesoporous structure produced by super-critical drying. Furthermore, the fast and reversible colour change this MPC ﬁlm undergoes upon swelling makes it ideal for pressure sensing. These new active mesoporous cellulose materials could be useful for biosensing, functional membranes, tissue engineering, and chiral separation. Instances of materials where PEG-induced “depletion afﬁnity” has been acknowledged are DNA and lyotropic LC [209,210]. CNCs with asymmetric nematic structures can alter their optical properties by altering the PEG’s molecular weight [180]. As pure CNC ﬁlms are delicate, several water-soluble polymers, such as poly(vinyl alcohol) [12], PEG, and polyurethane [181], have been added to increase the elasticity and good mechanical of the ﬁlm. With these water-soluble polymers, the pitch size can also be changed. Due to depletion forces, PEG has two effects. First, it enhances the CNC ﬁlm’s mechanical characteristics. Second, it alters the pitch size due to the effects of depletion. However, the inclusion of polymers can also alter mechanical characteristics. By altering the polymer fraction, it is possible to change the depletion effect’s iridescent colours from red to blue. The concurrent improvement of mechanical properties enables a wide range of applications, including pressure sensors [113], humidity sensors, and anti-counterfeiting sheets [48]. For example, the reflectance spectra of pure CNC films show a peak at 242 nm, which rises to 361 nm when the PEG weight fraction reaches 30 wt%. Additionally, home furniture and other products can be created by altering the reflection spectrum [180,211] since the reflectance band grows when the PEG level exceeds 25 weight percent. The half-pitch diameter increases from 103 nm to 143 nm when the PEG weight percentage increases from 10% to 20%. Similar conduct was displayed in reference [180]. When the molecular weight of the non-adsorbing polymer rises, the pitch size can also change. The creation of thin ﬁlms on a Petri plate because of the evaporation of a tiny volume of an iridescent solution is the most basic demonstration of LC in conﬁnement. Brilliant structural colours can still be created even when the thickness of such ﬁlms is only a few orders of magnitude greater than the pitch length [212,213]. A planar orientation of the Appl. Biosci. 2022, 1 263 cholesteric nanostructure is easily produced in such thin-ﬁlm conﬁnement [214], even under quick-drying conditions when disclinations in the cholesteric order are supposed to be kinetically conﬁned. The discontinuous pitch shift brought on by these disclinations becomes critical in very thin ﬁlms (about 1–2 nm) and can produce an appealing colour mosaic [174]. According to some reports, conﬁnement conceals translational order in smectic crystals [215] and turns the bulk isotropic-to-nematic transition into a continuous ordering from an isotropic to a nematic phase. In addition, due to uneven ﬁlm production on various surfaces, suspension can offer a variety of iridescent colours that can be adjusted. The effect of the substrate on the formation of the CNC cholesteric phase was investigated in ref. [216]. When CNC dispersion was dropped on the substrates, the inﬂuence of the substrate on the fading out of CNC suspension and its LC formation behaviour was investigated. On glass and stainless steel (SS), CNCs demonstrated initial contact angles of 37.83 and 57.32 , respectively. On the hydrophilic glass surfaces, the cholesteric phase self-assembled from the droplet’s bottom centre and diffused to the edges [217]. The iridescent coating ﬁlms created on polystyrene (PS), SS, glass, Cu-Zn alloy, and Cu-Ni alloy exhibit distinctive “coffee rings” as a result of the edge-centre ordered drying procedure [125]. The study’s conclusions can be used to spray coat a variety of substrates, but more investigation is required to see whether the surfaces in question inherently display shimmering colour. When utilised as a stimulus-response material, developed iridescence may also be used to determine the surface’s composition. Additionally, coated components and a chemical resistance polymer can produce a vibrant, long-lasting coating, making this a suitable colouring technique for urban development. Thermotropic LCs can act as a thermal switch and affect the average refractive index of periodic structures by being associated with the chiral structure of CNCs. This has already been done in silica ﬁlms [218]. Thermotropic LCs can also undergo orientation due to temperature ﬂuctuations. The ﬁlm became colourless as the temperature increased because the thermotropic agent, originally nematic, changed into an isotropic state around 40 C. On the other hand, ﬁlms were iridescent at room temperature. As there was no indication of a reﬂection signal in the UV-vis spectra, these visual cues were coupled with the optical properties of LC-loaded ﬁlms. In reference [219], the texture of the suspension of the CNC- grafted poly(N, N-dimethylamino ethyl methacrylate was also altered by temperature. This colour-changing technique has remained largely unexplored. Pitch was measured using SEM on CNC ﬁlm cross-sections and optical microscopy on CNC photonic ﬁlms; the outcomes revealed a low-temperature dependency. The cholesteric stripe’s bright and dark margins were not equal, and their difference varied greatly with temperature and nematic phase. The change from one biaxial cholesteric to (calamitic cholesteric, discotic cholesteric and biaxial cholesteric), the crossovers between biaxial cholesteric and calamitic cholesteric as well as discotic cholesteric and biaxial cholesteric were seen using optical microscopy [220]. Changing the optical properties of photonic substances is a crucial objective in the creation of reﬂecting displays, screens, and sensors. The optical properties of photonic materials can be modiﬁed by adjusting their periodicity or refractive index contrast. Visitors that allow stimuli-induced changes in refractive index and hence ﬂuid modulation of the optical properties of the composite may be present in a chiral nematic mesoporous host. As a result, temperature changes in CNC chiral structures can be programmed. 3.2. Energy Storage Applications Due to its exceptional biocompatibility, adaptable surface chemistry, renewable and carbon-neutral nature, unsurpassed optical and mechanical capabilities, and great biocom- patibility, nanocellulose is becoming increasingly popular [8]. This study’s objective is to present a current assessment of recent nanomaterial advancements and their prospective uses in soft robotics, energy storage, and medical research; therefore, the discussion on energy storage applications ﬁts very well. Appl. Biosci. 2022, 1 264 As cutting-edge technology (such as portable electronic gadgets, electric cars, and big intermittent battery systems) is integrated into our everyday lives, the need for so- phisticated energy storage systems with high energy density, high power density, and extended lifespan has continued to rise [221]. Due to their lengthy lifespan, superior performance, and dependable stability, lithium ion batteries (LIBs) have been the most extensively used candidate systems in commercial electronic products [222,223]. Due to their extended cycle life, high speciﬁc power, and energy density, rechargeable lithium-ion batteries (LIBs) are also viable options for sustainable energy storage devices [224,225]. Since the introduction of commercial LIBs in 1991, carbonaceous materials have received a great deal of attention as candidate anode materials due to their respectable theoretical capacity (372 mAhg ) [226], good electrical conductivity, and exceptional mechanical- chemical stability. Examples include graphite [227,228], CNT [229], and its associated composites [230,231]. At the same time, environmental concerns have sparked a lot of interest in adopting ecologically benign materials for Li/Na ion batteries sourced from sustainable resources such as [232,233] and [234]. The idea of employing carbon produced from fungi as an anode material for LIBs was put out by Tang et al. [235]. Several re- sources have been used as carbon sources and have shown outstanding electrochemical performances for Li/Na ion batteries. These resources include banana peels [236], packing peanuts [237], wheat [238], and numerous others [239,240]. The capacity of a battery is measured in milliamp hours (mAH). For example, if a battery has 250 mAH capacity and delivers 2 mA average current to a load, it should last 125 h. However, the investigations have mostly concentrated on modifying carbon precursors or enhancing material composition. These material-oriented methodologies have not considered additional potential characteristics, such as electrode shape, dispersion, and alignment, that might impact the overall electrochemical kinetics and cycle stability of battery electrodes. For rechargeable energy storage devices, research into the implications of carbon precursors’ structural orientation (i.e., alignment)has thus far attracted little attention. Even if the fundamental elements that make up the electrode are the same, it is crucial to note that the architecture (or alignment) of materials can greatly alter the electrochemical kinetics and stability in energy storage applications [241,242]. For instance, Liu et al. looked at how three inorganic ﬁllers with various alignments affected the ionic conductivity of composite polymer electrolytes [243]. Aligned inorganic nanowires (NW) have ten times higher ionic conductivity than randomly dispersed NWs. In this case, inorganic nanowires of different orientations were mainly utilized as ﬁllers to improve the mobility of Li ions inside the polymer medium and to compensate for the drawbacks of polymer electrolytes. There is no evidence that the structural orientation of inorganic (or organic) components has a direct impact on the overall electrochemical performances of LIBs. To tackle the intriguing issues stated above, cellulose nanocrystals (CNCs) ﬁlms with different structural orientations were utilized as carbon precursors for this experiment. In a different study, the sol-gel technique was used to integrate Germanium (IV) oxide (GeO ) onto chiral nematic CNCs. The procedure maintained the original arrangement of the chiral nematic CNC aerogels. It led to hybrid aerogels with a large proportion of randomly dispersed GeO nanoparticles and concentrated regions up to 705 m /g. Car- bonizing the composite material produced a crystalline structure material with really no pressure collapse and good form restoration following release. By fusing the carbonaceous skeleton’s electrochemical double-layer capacitance with the pseudocapacitive contribution of the GeO nanoparticles, materials with a maximum capacitance (Cp) of 113 F/g and high capacitance retention were created [244]. Similar to this, in work by Lizundia et al. [245], conductive polymers were applied to modiﬁed chiral nematic CNC sheets on polymeris- ing pyrrole in place. The TEMPO-oxidation, acetylation, desulfation, and cationization processes did not affect the chiral structures. These innovative materials offer appealing possibilities for eco-friendly sensors and energy storage devices since they are simple to make. The chiral structure was unaffected by TEMPO oxidation, acetylation, desulfation, or cationization. Appl. Biosci. 2022, 1 265 Synthetic chiral material created by nano-micro-sized matrices has aided chemical synthesis, chiral sensing, chiral catalysis, and metamaterial-based improved optical devices. Both hard template methods and soft template approaches are often used to create chiral substances. This method could be used to produce new chiral nanostructured materials. (1) Chiral material synthesis typically uses mesoporous hosts as hard templates to transfer their nanostructure to other materials. (2) Soft template method: material is created by partially removing the host template. The template uses techniques including hyper molec- ular aggregation, rapid self-assembly, and molecular evaporation. For nano-mesoporous materials, soft template creation is more ﬂexible than hard template preparation. Chiral compounds based on cellulose have grown in prominence as chiral research shifts from the molecule to the nanoscale. Template synthesis from nano colloidal LCs in limited shape can be used as a model for the organization of nanoparticles (chiral structure guides their assembly in 3-D space). In reference [201], 3-D conﬁnement of cholesteric LC under two-dimensional containment was investigated. The phase-separated cholesteric shell was built with an isotropic inner thread parallel to the capillary’s long axis and a helicoidal axis parallel to the inner surface of the walls. The generated core-shell LCs might be used as optical waveguides since the geometry of the LCs changed as the amount of conﬁnement increased. The structure was examined over time using POM, and it was discovered that an isotropic core had been precisely constructed after six hours. It took its core 168 h to begin to relax. The time- consuming nature of isotropic and cholesteric CNC rearranging was thus highlighted [201] The average pitch recorded in the core-shell structure of optical waveguides in ref. [201] after switching substrates to a glass capillary was about 9 micrometres, but the structure created in Teﬂon tubes was 5.8 micrometres under the same conditions. As a result, the pitch object served in the glass capillary was much larger than the one created in Teﬂon. The mismatch results from surface energy variations between the two surfaces. In ref. [246], mesoporous black titanium dioxide (TiO ) with the chiral nematic 2-X structure of core-shell nanorod was created utilising the templating method once more. Carbonized TiO /CNC helical materials were created by chiral depositing TiO nanopar- 2 2 ticles onto gelatine functionalized CNCs and calcinating to recover TiO copies after 2-X the carbon is removed. The black TiO was composed of nanorods of chiral nematic crystalline-amorphous TiO and had a mesoporous semiconducting structure. Highly porous nanocarbon networks support the chiral black TiO nanocrystals that make up 2-X the anode electrodes of lithium-ion batteries. In addition to the current efforts, these black TiO - materials and composites might be useful in ﬁelds including energy storage and 2 X catalysis. Using silica precursors and evaporation-induced self-assembly of CNC, refer- ence [247] claims that nanocomposites containing chiral nematic structures can be created. Following the pyrolysis and etching of the silica, chiral nematic mesoporous carbon ﬁlms 1 1 are produced. Using a speciﬁc capacitance of 170 Fg at 230 mAg , mesoporous carbon sheets displayed nearly ideal capacitor behaviour in a symmetric capacitance with H SO 2 4 as the electrolyte. It has also been described in the literature [248,249] to produce energy storage devices employing cellulose ﬁlaments and other novel nanomaterials. To facilitate different interactions with light, templating can also be used. Due to the scripting technique [36,250], the helical shape can be applied to a variety of media. Compared to a dried CNC film, the inorganic replica’s optical sensitivity may differ. While an optically isotropic material displays continuously shifting refractive indices due to rotation of the birefringent CNC rods, an amorphous inorganic film with a template exhibits recurrent dis- continuous differences between the refractive indices of air and the substance. By introducing an index-matching fluid into the gaps and allowing it to evaporate, the reflection can be activated and deactivated [36,251]. If a thermotropic nematic was used to replace the gaps, the material’s absence from the gaps would cause its refractive index to change whenever an electric field is applied; a thermotropic nematic loses index matching, which causes the film to seem coloured. A birefringent matrix material lacked an index matching fluid, which prevents such ON/OFF switching of pho-tonic crystal characteristics even when the original Appl. Biosci. 2022, 1 266 dried CNC film is porous. Inorganic and air layer thicknesses, as well as their combined thickness (which establishes the local optical period), are likely to have a greater impact on the colours produced than visible light wavelengths. One potential use for transparent CNC-templated inorganic materials is cholesteric-based mirrorless lasing [36]. According to the sizes, structures, and surface chemical performances of lignocellulosic biomass, new materials and devices for energy storage should be researched. These materi- als and devices should use various biomass resources and their combinations as building blocks. According to reference [8], nanocellulose can be created from a wide range of sources and processes. For example, TEMPO-oxidized CNCs can be used with transparent conductors to create ﬂexible and optically transparent electrodes [252]. Both forms of CNC contain negatively charged rod particles, and sulfuric acid hydrolysed CNC can be utilised as a template to make chiral nematic mesoporous electrode materials. Research into new materials and production techniques may lead to new opportunities in a range of applica- tions. Even though supercapacitors and lithium-ion batteries (LIBs) have extensively used nanocellulose and its derivative materials [253], only a small number of research on the use of nanocellulose in sodium-ion and Li-S batteries has been published [254,255]. Addition- ally, several cutting-edge energy storage technologies, including Mg (Al, Mn)-ion batteries, have not received enough attention. Due to its physical robustness, adaptable structure, and surface/interface chemistry, nanocellulose and its derivatives are an environmentally friendly material alternative for rapidly developing energy storage devices. 3.3. Optical and Optoelectronic Applications The colour of ﬁlms (produced with liquid crystalline CNCs) is produced by reﬂected light, which is determined by the wavelength and angle of incoming light. The material looks colourful to the human eye when the incoming light’s wavelength is in the visible range. The usage of the perfect ﬁlms is constrained by their fragility and uneven structural makeup; therefore, an additional supplementary chemical is needed to address mechanical property weakness. This shortcoming is manageable considering the optical characteristics of the CNC chiral nematic LC that cannot be matched by other rod particles; inherent properties such as handedness, sensitivity to humidity, ability to be surface modiﬁed and mixed with other chemicals and wealth of information currently available in the literature. The CNC is also excellent for wettability control and adhesion because of its built-in periodic spirally structured LC structure. As mentioned in the energy storage usage, templating may enhance optical properties. In addition to other uses, novel materials have promise for chiral separation [256], enantioselective adsorption [257], catalysis [258], sensing, optoelectronics, and lithium-ion batteries [259]. Polymers and CNC have been used to boost the mechanical qualities of naturally inspired structures, as some accounts of this research were outlined earlier. For improved mechanical or optical qualities, the insertion of CNC ﬁlms into larger laminar structures has been investigated [260]. Thin sandwiched structures with bright structural colour, improved mechanical characteristics, and shape memory capabilities were developed by incorporating CNC ﬁlms into the polymer. Similar to the jewel beetle chrysin resplen- dent [203], CNC ﬁlms may be built up on either side of a birefringent membrane to enable the simultaneous reﬂection of left- and right-handed circularly polarised light. A simi- lar birefringent layer was produced for the sample optical mechanism by impregnating millimetre-scale planar gaps in CNC ﬁlms with a nematic LC. This layer may be triggered by heat or electric ﬁelds to modify the reﬂected spectrum or its polarization state [261]. Additionally, CNCs sheets only reﬂect light with a left circular polarisation (LCP). The odd manner chiral cellulose nanorods self-assemble is what causes this property. It is possible to control the switch from right to left-handedness by adjusting the temperature or using an electric ﬁeld [262]. This method might modify the optical characteristics of different chiral LC systems, which could be used as coatings, polarizers, and ﬁlters in optical devices. Using references and highlights, Table 3 presents some of the most important Appl. Biosci. 2022, 1 267 ﬁndings in the literature on the modiﬁcation of optical characteristics of CNC pure and composite ﬁlms. Table 3. Modulation of optical properties of CNC pure and composite ﬁlms using references and highlights. Reference Highlight [263] Widening of left circularly polarization reﬂected band with addition of micelles [264] Distortion of helix during drying phase; impact of vertical compression [184] Molecular dynamic simulations proving that increase surface charge increases pitch size The pitch of the LC in the box varied due to the isomerization of photosensitive molecules when exposed [265] to alternating ultraviolet and blue light, resulting in a shift in the reﬂection wavelength. [133] Using grinded CNC iridescent ﬁlm as pigments [266] Coffee ring effect leading to non-uniform optical characteristics. [127] Employing circular shear ﬂow is applied in the drying process to improve CNC ﬁlms uniformity. 3.4. Advanced Applications Liquid crystal production can be considered a purifying technique since CNC can self-partition out of a mixture of colloids. In ref. [267] the co-assembly of CNCs and solid latex nanoparticles was investigated by the author; the nanoparticles were also ﬂuorescently labelled, and co-assembly was investigated in both solution and solid ﬁlms. The creation of two types of structures—CNC-rich layers with cholesteric structures and randomly distributed latex-rich layers—was demonstrated by the CNC-latex combination. These layers were arranged on the ﬁlm’s plane, and a sizeable number of latex nanoparticles were dispersed in the cholesteric area of the CNC matrix. With the use of circular dichroism spectroscopy, the chiroptical properties of ﬁlms were investigated. The CD spectra of a latex-free ﬁlm revealed a positive single from the deliberate transmission of right-handed circularly polarized light by CNCs. With the addition of latex particles, the volume portion of the 0.31 positive CD signal remained constant, but the peak signiﬁcantly grew, indicating a wider range of pitch values. The loss of cholesteric order and the 57% drop in signal were both correlated with the CNC percentage. The position maximum also remained constant, indicating that the addition of spherical latex particles did not affect the cholesteric domains. Following ﬁndings from earlier studies on wood-sourced CNCs, measurements of the distance among fringe lines in ﬁngerprint texture revealed that half pitch of domains increased from 14 3 micrometres in latex-free CNC suspension to 2410 micrometres in the presence of latex particles [268]. Subsequent analysis of the ﬂuorescence signal revealed that ﬂuorescence intensity was 30% less cholesteric phase than in the isotropic phase, meaning there was a lower concentration of spherical particulates. In ﬁlms, the pitch of the organization is as follows phase decreased from 48 micrometre to 220 80 nm for samples that contain latex particles while it changed to 220 60 nm in samples with no latex particles, following the knowledge that assembly within cholesteric particles occurs primarily in suspension and not in the dried-out form. This is because ﬁlms formed after drying will have lower pitch values. It was determined that excluded volume effects resulting from depletion pressures between latex spherical nanoparticles [269,270] were responsible for the creation of latex-rich “islands” in CNC-latex ﬁlms. Due to depletion effects, the addition of tiny colloids to a solution of larger colloids can cause ﬂocculation of the larger colloids; as a result, bigger latex nanoparticles may exhibit attraction in the presence of CNCs [271,272]. The author went on to infer that particle geometry regulates the assembly of a binary mixture of rods and spheres in the absence of interaction [273]. Phase separation between sphere- and rod-rich phases occurs in a liquid state, but at large sphere volume fractions, only one isotropic phase emerges [273]. If an application calls for such arrangements, Appl. Biosci. 2022, 1 268 having spheres within rod-rich phases is crucial in situations when it is intended to prevent rod particles from forming liquid crystalline phases, such as viruses. 4. Conclusions While CNC-based LC is still a young ﬁeld of study, it is already extending into intriguing new ﬁelds. Currently, CNC-based nanomaterials can be placed artfully into unique self-assembled structures. Here, we have given a thorough overview of the relevant topic, emphasized its conception and discovery while skipping over a few subgenres of developing applications. All CNC-based materials, whether modiﬁed or unaltered, should create colloidal LC phases over a certain threshold concentration due to the exceptionally substantial shape anisotropy. It was discovered that the main interaction stabilizing the equilibrium LC phase was repulsive contact between negatively charged CNC rods. Other research projects that were conducted afterwards led to the rediscovery of the nematic phase as well as other mesophases, including the lamellar and helically chiral LC phases. These ﬁndings sug- gested high-performance applications, including wet spinning of CNC ﬁbres, in addition to reporting unique phase behaviour. Due to their ability to control the internal structure of the photonic crystal, cellulose-based LCs are amazing novel LC materials with extraordi- nary optical characteristics. A special kind of LC material has the potential for large-scale commercial use due to its straightforward manufacturing procedure, straightforward raw material availability, and low energy consumption. This review brieﬂy discussed how to distinguish left- and right-handed CNCs using molecular dynamic simulations, microscopy, self-assembly, and rheology. Potential applications for producing non-fading colours and energy devices included the use of energy storage and photonic crystals. A product made from CNC and other polymers may be structurally coloured, used as a template, mag- netically changeable, and able to change colour in response to humidity. Chirality may make its goods more durable by employing CNC helicoidal organization. The goods may also be made responsive by calcination or coating them with conductive materials, which makes it possible to employ them for applications like intriguing energy storage systems for batteries and a variety of other purposes. Due to the nanostructure’s poor compatibility with the host medium, scalable fab- rication of such materials using mesostructured liquids like LCs is still difﬁcult. The photo-switchable chirality of plasmonic DNA-origami in cellulose nanoﬁber-based LCs has already been proven. With the help of UV light, the DNA-origami plasmonic nanostructure characteristics of the composite material may be deleted and recovered. Finally, by modify- ing any parameter that may affect the point at which an isotropic material transitions to a nematic one, it is feasible to modify the chirality of CNCs to suit a range of applications. Due to the wide range of applications, they could serve, iridescent inks created by CNCs are likely to become more well-known shortly. Author Contributions: Conceptualization, A.A.M. (Aref Abbasi Moud); methodology, A.A.M. (Aref Ab- basi Moud); software, A.A.M. (Aref Abbasi Moud); validation, A.A.M. (Aref Abbasi Moud) and A.A.M. (Aliyeh Abbasi Moud); formal analysis, A.A.M. (Aref Abbasi Moud); investigation, A.A.M. (Aref Abbasi Moud); resources, A.A.M. (Aref Abbasi Moud); data curation, A.A.M. (Aref Abbasi Moud); writing— original draft preparation, A.A.M. (Aref Abbasi Moud); writing—review and editing, A.A.M. (Aliyeh Abbasi Moud); visualization, A.A.M. (Aliyeh Abbasi Moud); supervision, A.A.M. (Aref Abbasi Moud); project administration, A.A.M. (Aref Abbasi Moud); funding acquisition, A.A.M. (Aref Abbasi Moud). All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. Appl. Biosci. 2022, 1 269 References 1. Parton, T.G.; van de Kerkhof, G.T.; Narkevicius, A.; Haataja, J.S.; Parker, R.M.; Frka-Petesic, B.; Vignolini, S. Chiral self-assembly of cellulose nanocrystals is driven by crystallite bundles. arXiv 2021, arXiv:2107.04772. [CrossRef] 2. George, J.; Sabapathi, S. Cellulose nanocrystals: Synthesis, functional properties, and applications. Nanotechnol. Sci. Appl. 2015, 8, 45. [CrossRef] [PubMed] 3. 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Cellulose Nanocrystals (CNC) Liquid Crystalline State in Suspension: An Overview

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DOI: 10.3390/applbiosci1030016
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Abstract

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Applied Biosciences – Multidisciplinary Digital Publishing Institute

Published: Nov 3, 2022

Keywords: liquid crystals; cellulose nanocrystals; colloids; cellulose; chirality; chiral nematic structures

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Cellulose Nanocrystals (CNC) Liquid Crystalline State in Suspension: An Overview

Cellulose Nanocrystals (CNC) Liquid Crystalline State in Suspension: An Overview

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Cellulose Nanocrystals (CNC) Liquid Crystalline State in Suspension: An Overview

Cellulose Nanocrystals (CNC) Liquid Crystalline State in Suspension: An Overview

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