Abstract： Cellulose nanocrystals （CNCs） can self-assemble in suspension to form chiral cholesteric structure of liquid crystal with unique birefringence phenomena， and the structural parameters strongly depend upon the aspect ratio， surface structure， and physicochemical properties of CNC， along with suspension media. Many attempts have been carried out to keep this cholesteric structure in solid state via removing solvent， such as slower solvent-evaporation， rapid vacuum-filtration， and spin-casting under centrifugal force. The solid-state iridescence of the cholesteric CNC arrays has been used as structural color， and showed a great potential for the coding and securing of optical information. Moreover， to promote practical applications of such structural iridescence， the cholesteric CNC arrays have been embedded into many kinds of substrates via in-situ reaction of monomers or physical blending with polymers. However， this kind of structural iridescences may lead to misreading information. The uniaxial-orientation assembly of CNC has thus been proposed via regulating external force fields of CNC self-assembly， and successfully achieved nanoparticle assembly-induced solid-state monochrome emission based on enhanced inelastic collision theory of CNC dipoles and photons. This method can eliminate the chiral arrangement of CNC and the corresponding iridescence， and the structural monochromaticity can contribute to enhancing the accuracy of optical information. Overall， the CNC can be controllably assembled as the ordered arrays in solid state and presented structural color， and support optical anti-counterfeiting strategies different from the fluorescent technologies.
　　Keywords： cellulose nanocrystal; self-assembly; structural adjustment; structural color; optical applications
　　1    Introduction
　　Cellulose nanocrystal （CNC） is a rigid and rod-like renewable nanomaterial extracted from natural cellulose resources， and presents a diameter of 4～25 nm and a length of 100～1000 nm[1]. CNC is usually prepared from cellulose fibers by acid hydrolysis， i.e.， the amorphous cellulose region is removedby sulfuric acid to keep the rigid crystalline region with strongly hydrogen bonding[2]. As well-known， CNC has many advantages， such as low density （1.59 g/cm3）， high specific surface area （ca. 400 m2/g）， high modulus （up to 145 GPa）， easy-to-modification， biocompatibility， biodegradability， broad available sources， etc[3]. Especially， the most interesting feature is that the CNCs in suspension could self-assemble to form a cholesteric liquid crystal. As early as 1959， it was the first time that the lyotropic crystallization behavior and permanently birefringent property of CNC suspension were observed when heating on a steam bath[4]. Moreover， 3 wt% CNC suspension was found to possessa cholesteric （also called chiral nematic） phase as a liquid crystal unit[5]. Therefore， this chiral structure of cholesteric phase has been attempted to being immobilized in solid state[6] or embedded into the solid substrates[7-8]. Interestingly， the chiral cholesteric structure in sold-state materials showed the structural color of iridescences， and contributed to the application potential in the coding and security fields of optical information independent upon fluorescent technologies that could be easily photobleached and aggregation-quenched[9]. Hence， to avoid information misreading， the uniaxial-orientation assembly methods of CNC have been developed to achieve the monochromaticity of structural color and enhance the accuracy of optical information[10]. In this review， the mechanism and influencing factors of the CNC self-assembly in suspension is introduced， and how to immobilize and embed it in the solid-state materials is illustrated. At the same time， the methods of uniaxial-orientation assembly， i.e. the strategy of nanoparticle assembly-induced solid-state monochrome emission， are emphasized. Finally， the exploration and application of the structural iridescences and monochromaticity derived from the CNC assembly is summarized and prospected in the fields of optical information security and anti-counterfeiting. 　　2    CNC self-assembly in suspension
　　2.1    Mechanism of CNC self-assembly in suspension
　　The CNC in suspension could self-assemble to for liquid crystals with a cholesteric phase structure via the electrostatic repulsion force between these particles[5]. At a low CNC concentration， few isotropic phases of CNC would transform into an anisotropic phase. With an increase in CNC concentration or reduction in solvents， the anisotropic region expands in suspension. When the CNC concentration in suspension reaches a critical concentration， the anisotropic phase transforms into a cholesteric phase as liquid crystal. This evolution is because the rod-like structure of CNC easily leads to a cholesteric structure to minimize the electrostatic repulsion[3]. The phenomena of liquid crystal formation also occur in other rod-like particles， such as DNA fragments[11]， chitin crystallites[12]， tobacco mosaic viruses （TMV）[13]， and poly（tetrafluoroethylene） whiskers[14].
　　2.2 CNC self-assembly in aqueous media and organic solvents
　　CNCs should first have stabilization and dispersibility in solvents to reach the critical concentration. Otherwise， agglomeration will happen to form precipitation， easily leading to the failure of self-assembly. As a result， the interaction between CNC and solvents should necessarily be improved to ensure enough dispersibility in solvents.
　　By sulfuric acid hydrolysis， extracted CNC spossess many sulfonic-acid groups on the surface， and concomitant negative charges increase the dispersion stability in water. The surface charges of CNCs result in electrostatic repulsion between particles， and hence favor the formation of cholesteric phase. At the same time， CNCs oxidized by TEMPO could also present higher dispersion stability due to carboxyl groups on their surface， and is propitious to form cholesteric phase.
　　In organic solvents， low-efficiency electrostatic repulsion and strong hydrogen bonds between CNC particles lead to coalescence and precipitation. By contrast， the chemically modified CNC， like by conjugating with poly（ethylene glycol）[16] and trimethoxysilane， could be uniformly dispersed in acetone， toluene or chloroform. CNCs coated with the lipophilic molecules on their surface could also self-assemble in organic solvents[17-18]. The Beycostat NA surfactant （phosphoric ester of polyoxyethylene（9） nonylphenyl ether）-coated CNCs could be dispersed stably in toluene and cyclohexane to form cholesteric liquid crystals[17]. CNC-X （X = Li， Na， K， etc.） could form cholesteric phase by evaporation-induced self-assembly （EISA） in organic solvents of DMSO， DMF， formamide， N-methylformamide， etc. by treating CNC with a strong alkali[19]. 　　2.3    Influencing factors of self-assembly structure
　　Fig.1 depicts cholesteric liquid crystal structure ascribed to chiral self-assembly of CNCs. It is well-know that numerous cellulose resources together with different preparation methods and conditions can produce many kinds of CNCs with various length， diameter and aspect ratio. Since the CNC is the basic structural unit of chiral self-assembly， the CNC dimension necessarily dominates the sub-structural parameters of cholesteric phase. Since the molecular-level electrostatic repulsion between CNCs and the microscopic dispersibility of CNC suspension are essential to chiral self-assembly in aqueous media and organic solvent mentioned above， the surface physicochemical properties of CNCs might regulate the assembly and sub-structure of cholesteric phase， which could be achieved via various physical and chemical modification methods towards CNCs. For example， nematic structure of CNC self-assembly is unusual due to its instability， and has been observed in the bacterial cellulose nanocrystal aqueous suspension with low concentration[20]， which is related to particle size and special structure of CNC as well as strong charge repulsion between CNC particles. Since such nematic structure requires harsh conditions， it is almost impossible to form the CNC self-assembled films with nematic structure by evaporation.
　　3 Solid-state cholesteric structure from self-assembled CNC suspension
　　3.1 Preparation methods of solid-state cholesteric structure
　　When the solvents were removed， the obtained CNC products have been verified to being able to retain the cholesteric structure[21]. Many methods have been attempted to preparing unique cholesteric structure materials. Simple and facile method of solvents removal is carried out by slow evaporation of solvents into the air[22]. However， the unstable environment conditions during evaporation might lead to uneven self-assembly structure， and even destroy the cholesteric structure in solid state. In this case， evaporation-induced self-assembly （EISA） could commonly be used to prepare solid-state cholesteric structure via evaporate solvents at a constant temperature and humidity chamber[23]. Under the given conditions， the solvents evaporation rate could be controlled， and so uniform self-assembly structure is easily obtained. On the other hand， the liquid crystal structure could be also regulated by adjusting temperature and humidity to dominate evaporation rate of solvents. Therefore， this method was widely used to fabricate solid-state cholesteric structure based on CNC self-assembly. To enhance the achieving efficiency of solid-state cholesteric structure， a rapid method， i.e. vacuum-assisted self-assembly （VASA）， was explored to remove the solvents via vacuum-filtration instead of longer-period solvent-evaporation[24]. This method could adjust the structure and optical properties of CNC self-assembly by controlling the vacuum degree in the preparation process of filtration. In addition， the spin-coating method has been also used to fleetly prepare the solid-state cholesteric structure of CNC in the film form， which takes only tens of seconds[25]. The structural arrangement of CNCs is affected by the centrifugal force and the CNC concentration. Moreover， this method could also be used to prepare layer-by-layer self-assembly of CNC-based composite films containing cholesteric structure[25]. The orientation of CNC arrangement at each layer could be also adjusted with high-concentration CNC suspension. 　　3.2    Structural adjustment of solid-state CNC assembly
　　As mentioned above， the self-assembled CNC film with solid-state cholesteric structure was obtained by removing solvents. This cholesteric structure of self-assembled CNC films shows a birefringence phenomenon and a fingerprint texture by the polarized optical microscope （POM）[26]. Because the spiral arrangement and corresponding orientation of CNCs are perpendicular to the substrate in the planar structure， the birefringence phenomenon is observed to give special iridescent of the CNC films， as shown in Fig.2. As regards to the fingerprint texture structure， the spiral arrangement-induced orientation of CNC， which is parallel to the substrate， is an important basis for determining CNC cholesteric structure[27].
　　The pitch （P） and angle are important physical quantity of cholesteric phase structure. Controlling the P and angle can cause the change in the iridescence of CNC films. The increase in P can come from the increase in the concentration of CNC in suspension， along with the decrease in particle size， surface charge density and shorter evaporation time， which causes a redshift in the dominant color of CNC films[6， 22]. Besides， reducing the vector angle can increase the P in the CNC cholesteric phase structure. Therefore， the P can also be directly controlled by adjusting the vector orientation of CNC arrangement.
　　It is possible to control the P accurately in the external field， such as magnetic field[28]， electric fields[29]， and shear forces[30]. For example， when CNCs self-assemble in the electric field， the P can be precisely controlled by adjusting the electric field[31]. The P increases in a stronger electric field， which causes redshift in the dominant color of CNC self-assembled films and even leads to the disappearance of the iridescence. In the Niobium （NdFeB） magnetic field （approximate to 0.5～1.2 T）， the P of CNC self-assembled films can also be controlled by changing the direction of the magnetic field， and CNC self-assembled films with different iridescence can be obtained[9].
　　4    Uniaxial-orientation assembly of CNC
　　4.1 Preparation methods of uniaxial-orientation CNC arrangement
　　The orientation of the CNC in the self-assembled structure can be controlled by shearing the CNC self-assembled suspension[32]， as shown in Fig.3（a）. At a high concentration， the CNC suspension behaves as a gel， which increases the viscosity of the medium. Soit is also easy to keep orientation alignment along the shear direction under high shearing rate. Hence， the iridescence intensity of self-assembled CNC film can be regulated by adjusting shear rate. Firstly， a completely holesteric structure with a strong iridescence color could be observed at zero shear rates. Moreover， some chiral-nematic structures could be destroyed at low shear rate， and increasing the shear rate might significantly reduce the chiral arrangement of CNCs to gradually present uniaxial orientation. This method resulting in a certain attenuation in the iridescence intensity of the assembled CNC film. However， the iridescence elimination effect was not very satisfactory and required a highly uniaxial orientation of CNCs. Full and perfect uniaxial orientation is very difficult under normal conditions. Fortunately， a new strategy to eliminate the iridescence of the assembled CNC film has been developed[10]. CNC can assemble at the vertical direction of the solvent level by EISA under gravity and capillary force to obtain structural monochrome of the vertical-assembled CNC films as shown in Fig.3（b）. When the CNC is uniformly assembled in the normal direction of the solvent surface， the included angle in contrast to the long axis of the assembled CNC films could be close to 0°. The direction deviation in the uniaxial-oriented film， which is determined by the half width （FWHM） of the included angle， is then very small. When the uniaxial-oriented CNC films were irradiated by light with the wavelength same as the Bragg diffraction wavelength of the array， a strong inelastic collision occurred between the CNC and photons to emit solid-state photoluminescence in the range of blue light. The excitation wavelength of solid-state illumination could be introduced into the ultraviolet region. Through the patterning process， the carrier information was naturally hidden under visible light， and information reading was realized under specific ultraviolet light. 　　4.2 Uniaxial-oriented arrays adjustment
　　The uniaxial-oriented arrays could be regulateed by pre-assembly treatment of ultrasound， assembly conditions of temperature and humidity， and surface chemical structure of CNC[10]， as shown in Fig.4.
　　The CNC was pre-assembled by ultrasonic to improve the dispersion of CNCs in the aqueous system and the uniformity of CNC size. The results showed that the order of the vertical-assembled CNC structure increased with the increase of ultrasonic time. When ultrasonically applied for 25 min， the FWHM of the angle between the long axis of CNC in the vertical-assembled CNC film and the vertical direction was the narrowest， which meant the order of CNC self-assembly was the highest. When the ultrasonic time was too short， it was not conducive to the dispersion of CNC. If the ultrasonic time was too long， CNC would be destroyed and its size uniformity would be reduced， which were not conducive to the uniaxial-orientation assembly of CNC along the vertical derecation of the solvent.
　　During the uniaxial-orientation assembly of CNC， the assembly temperature affected the solvent evaporation rate during CNC self-assembly， which affected the assembly rate of CNC. The experimental results showed that the FWHM of the angular distribution of the as-prepared CNC self-assembled film was the narrowest when the assembly temperature was 35℃. At this time， the order of the vertical-assembled CNC film was also the best. When the preparation temperature was too high， the evaporation rate of the solvent was fast， which led to the evaporation of the solvent before CNC could be self-assembled.
　　5    Self-assembled CNC arrays in composites
　　5.1    Direct mixing with polymer
　　CNC can also be co-assembled to obtain composites when mixed with other substances or polymer matrix in aqueous dispersion. By introducing other substances or matrix during co-assembly， CNC films can gain better properties. For example， CNC/reduced graphene oxide （RGO） composite films can be simply prepared by introducing RGO into CNC suspension in the VASA method[33]. This kind of film has iridescent colors and good electrical conductivity under certain conditions， which expands the application of CNC self-assembled film in the field of electricity. Small molecules like D-glucose[34] have also been chosen to adjust the helical organization of self-assembled CNC films， which changes the optical performance of the composites. Besides the optical properties， D-glucose also offers the self-assembled CNC films a high strain-at-break with a stretch response， which can be applied in mechanical and chemical responsive selective reflection. Another way to improve the mechanical properties of self-assembled CNC films properties is to introduce flexible polymer substrates， such as soft poly（ethylene glycol） （PEG）[7]， poly（vinyl alcohol）[35]， and poly（2-hydroxyethyl methacrylate）[36]. Neat CNC assembled films are usually crisp， and the polymer chains are flexible and can readily surround the surface of CNC to improve the flexibility of the composite film， thus increasing its tensile strength and elongation at break. For example， with the different content of PEG， CNC/PEG composite films show different structural color， and these colors are sensitive with humidity， which suggests that the self-assembled CNC films composited with hydrophilic polymer may have application potential in humidity sensor. 　　5.2    In-situ polymerization of monomers
　　CNC can also self-assemble to form cholesteric structure in the special polymer precursor solution， such as acrylic acid （AAc）[37]， urea formaldehyde （UF）[38]， and phenol-formaldehyde （PF） solution[39]. The CNC self-assemble structure can be maintained during the in-situ polymerization of polymer precursors， and thus self-assembled CNC gels can be prepared， as shown in Fig.5（a）. The temperature and pH value of the precursor can strongly affect the crosslinking density of the self-assembled CNC gel， and ambient conditions like the humidity can also influence the interactions of hydrogen bonding， which may lead to a pitching change of CNC. For example， the chiral nematic structure can be found in the CNC/AAc composite film. After photoinitiation polymerizing， the structure color of the composite films can red-shift significantly as the pH value increases， as shown in FIg.5（b）. UF and PF have also been used to prepare mesoporous photonic resins， which means that the CNC need to be removed after CNC-composite polymer is prepared. The CNC-UF and CNC-UF composites have a network structure， and the optical characteristics can be maintained after thermal curing. These composite films are sensitive to solvent， and the ratio of water and ethanol can influence the swelling property of the composite films， thus affecting the P and structure color.
　　6    Optical applications and prospects
　　6.1    Applications of CNC cholesteric structure
　　Since the cholesteric structure of CNC films can be controlled， the iridescence could be easily changed， promoting its potential applications as optical materials. The visible iridescence changes of self-assembled CNC films are observed by adjusting the cholesteric structure. The change of environmental factors can lead to the change in the P of self-assembled CNC films. Therefore， CNC self-assembled films can be used as a sensor according to the iridescence change. For example， some changes can be easily detected， such as humidity[21]， pH value[37]， pressure[38-40]， solvents[37]， and other stimulations[33]. It was discovered that the iridescence of self-assembled CNC films had a reversible transition from dry blue-green to wet red-orange iridescence at humidity[21]. It also proved that the change of iridescence was caused by the increase of pitch with the rise in humidity. Thinner self-assembled CNC films as humidity indicator could change color in less than 2s in a high relative humidity. 　　The iridescent self-assembled CNC films can also be applied in coatings and safety labeling[9]. The controllability of self-assembled CNC films iridescence can be applied to information encryption. However， cholesteric structures are prone to vary sharply， leading to erroneous transmission of information. The complex color can also lead to misunderstandings. Furthermore， the application of self-assembled CNC films has been achieved as a safety feature in information security[41]. The P of self-assembled CNC films and Zeta potential were adjusted by controlling the addition of the optical brightener agent （TINOPAL）. The Zeta potential was used as a hidden high-level encryption potential.
　　6.2    Applications of uniaxial-orientated CNC arrays
　　The chiral cholesteric structure of self-assembled CNC films can be eliminated by uniaxial-orientation assembly in evaporation process. Therefore， there is no pitch and iridescent disappearing. Fortunately， theoretical calculations show that this uniaxial-orientation CNC arrays can enhance Stoke scattering[10]. The uniaxial-orientation CNC films （CNC extracted by sulfuric acid hydrolysis， without conjugated group structure） present 377 nm diffraction peaks， and blue light has been observed under ultraviolet lamp. Besides， the orderliness of the uniaxial-orientation CNC arrays affects the intensity of Stoke scattering. Uniaxial-orientation CNC films with different diffraction wavelengths can be obtained by controlling the included-angle of between CNC aligned-orientation and the vertical direction of the solvent. However， when the order is completely destroyed， Stoke scattering disappears and the diffraction peaks disappear.
　　The uniaxial-orientation CNC films， unlike self-assembled CNC cholesteric films， have no complex iridescence， and only appears blue light under ultraviolet lamp[10]. Due to the fact that the structural monochromaticity reduces the probability of information misunderstanding， the information security could be significantly improved. The uniaxial-oriented structure of CNC can enter the UV zone. This enables the encoded CNC film carrying information to be naturally concealed under visible light to achieve encryption of the encoded information. As shown in Fig.6， the patterned uniaxial-orientation CNC arrays are embedded in the famous painting to realize the hiding of information under natural light. And the encoded information can be read under the specific wavelength of ultraviolet light. Furthermore， due to the high compatibility between CNC and paper-based materials， it will be conducive to preparation of the high-quality paper. The uniaxial-orientation assembly of CNC shows an application potential in the anti-counterfeiting of paper currency and other paper materials. 　　6.3    Future Prospect
　　Combining patterning technologies， the structural colors of self-assembled CNC have been applied in information coding and anti-counterfeiting. The uniaxial assembled CNC arrays can further induce solid-state emission， hide the information in background and reducethe misreading of information when decrypted. Developing program-control patterning technologies will allow various background design to hide the true information in false background patterns. Since the assembly-induced emission is free of aggregation-induced quench， the emission can theoretically obtain a higher quantum efficiency further via facile emission-enhancement technologies， like plasmon resonance. The co-assembly technology of CNC-based composites will also allow new encryption strategies to introduce CNC assemblies into responsible matrices， making the information luminous under designed stimuli. Those potential technologies based on various CNC assemblies can develop new methods of optical information security with encryption of multi levels.
　　Acknowledgments
　　Jin Huang and Lin Gan are grateful to the National Natural Science Foundation of China （51603171）， Fundamental Research Funds for the Central Universities （XDJK2016C032）， and the Talent Project of Southwest University （SWU115034）.
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