Structural Color from Cellulose Nanocrystals or Chitin Nanocrystals: Self-Assembly, Optics, and Applications

Multi-scale investigation of the effect of photocurable polyethylene glycol diacrylate (PEGDA) on the self-assembly of cellulose nanocrystals (CNCs)

Cellulose nanocrystals (CNCs) are appealing building blocks to replace plastic for the fabrication of more sustainable photonic and barrier materials. Aqueous suspensions of CNCs are typically mixed with polymers to improve the mechanical flexibility of the dry nanocomposite, adding further functionalities. However, the role of the additives in the suspension state, and how they affect the processing and final properties of the material still requires further studies. In this work, the interactions between CNCs and polyethylene glycol diacrylate (PEGDA), a water-soluble, photocurable polymer, were investigated through a multi-scale approach involving small-angle X-ray and light scattering (SAXS and SALS). PEGDA was found to adsorb on the surface of CNCs, exhibiting two behaviour regimes depending on the mass ratio between PEGDA and CNCs (RPC). Below a threshold ratio (RPC=0.5), PEGDA weakened the chiral interactions between CNCs, reducing the anisotropic volume fraction and increasing the cholesteric pitch. Above this threshold (RPC>0.5), the adsorbed polymer enhanced the helical twisting power, resulting in higher amounts of anisotropic phase and shorter pitch values. This non-monotonic trend was attributed to the structure of the adsorbed PEGDA layer on the surface of the CNCs, first masking the chiral features of the twisted nanoparticles, then enhancing the transmission of chirality due to changes in particle morphology and effective particle volume fraction. A simultaneous decrease in the suspension dielectric constant amplified this effect, strengthening interparticle interactions. Finally, rheo-optical experiments provided further insights into the formation of the cholesteric structure, showing that larger amounts of polymer delayed the isotropic-cholesteric phase transition.

Tunable-color chiral liquid crystal film of cellulose nanocrystal with outstanding antibacterial and UV shielding capabilities for multifunction coating applications

Chiral nematic cellulose crystals (CNCs) have presented broad application prospects in information encryption, sensing and coating field due to their properties of tunable structural color, birefringence and sustainability and renewability. However, the controllable regulation of structural colors and the incorporation of functionalities still pose challenges for practical applications. Herein, we propose a facile method utilizing plant polyphenols to regulate the pitch of chiral nematic liquid crystal films, thereby enabling precise control over the structural color. The influence of varying amounts of tannic acid (TA) incorporation on the structure and optical properties of CNC iridescent films was investigated, and the underlying mechanisms were elucidated. Notably, TA enhances the UV absorption of the composite film, demonstrating nearly complete UVB absorption and partial UVA absorption. Furthermore, the films exhibit potent antibacterial activity against both Gram-positive and Gram-negative bacteria. The incorporation of polyphenol TA not only endows the composite film with outstanding antibacterial and UV-resistant properties but also allows for tuning the structural color of CNC films. The tunable structural color, along with the antibacterial and ultraviolet resistance properties exhibited by CNC-TA chiral cholesteric films, provides new prospects for anti-counterfeiting labels and multifunctional coating applications.

Chiral structural color from microdomes

Artificial chiral-structural-color materials can carry high-dimensional information based on multiple optical degrees of freedom, providing possibilities for advanced optical security and information storage. However, current artificial chiral-structural-color materials are hindered by their specific compositions, fine nanostructures, and single polarization modulation. Here, we found that microdomes made from common polymers have chiral structural colors with broadband tunability and multiple polarization-modulated chirality. The microdome patterns are easily fabricated by ordinary printing techniques and have inhomogeneous spatial distributions of full polarization states and customizable colors. Our chiral-structural-color microdomes (CSCMs) provide a promising roadmap for high-capacity information encryption and high-security anti-counterfeiting. We developed multidimensional tunable structural color displays and achieved encryption with high information capacity. To further highlight the application potential, we constructed contact lenses integrated with CSCMs for identity authentication with 232 distinctive cryptograms.

Chiral nematic cellulose nanocrystal films: Sucrose modulation for structural color and dynamic behavior

This study explores the effect of sucrose addition on the properties of chiral nematic cellulose nanocrystal (CNC) films for potential food industry applications, including biodegradable packaging and food coloring. The addition of sucrose altered the films' structural color, shifting from blue in pure CNC films to aqua blue, green, yellow-green, and red with increasing sucrose concentrations (up to 21 %). Surface analysis revealed a reduction in contact angle from 96° to 48° due to sucrose's hydrophilic nature and smoother film surfaces. XRD results indicated a decrease in crystallinity from 84.5 % to 15.6 %, linked to the disruption of CNC alignment by sucrose. Mechanical testing showed reduced tensile strength (138 MPa to 35 MPa) and Young's modulus (1.634 GPa to 70 MPa) with higher sucrose content. Notably, over the storage time, films with 21 % sucrose exhibited dynamic structural coloration caused by localized sucrose recrystallization, leading to pitch shifts and color transitions. These findings demonstrate the tunable optical and mechanical properties of CNC-sucrose films, positioning them as promising materials for sustainable food packaging and responsive coatings.

Synergistic enhancement of high-barrier polylactic acid packaging materials by various morphological carbonized cellulose nanocrystals

The environmental challenges linked to petroleum-based polymers have accelerated the search for alternative materials like polylactic acid (PLA). Diverse nanofillers, ranging from inorganic to organic and hybrid inorganic/organic varieties, are employed to bolster PLA performance. Yet, non-synergistic nanofillers often underperform due to inadequate dispersion and singular functionality within the PLA matrix. This work introduces carbonized cellulose nanocrystals (GCNC) with rod-like (R-GCNC) and spherical structures (S-GCNC), as synergistic reinforcements for PLA matrix. Unlike traditional nanofillers, the highly graphitized carbon layer on GCNC effectively mitigates CNC agglomeration while preserving cellulose morphology, fostering improved interfacial interactions and hydrogen bonding within PLA matrix. Moreover, GCNC acts as a nucleating agent, boosting the crystallization rate of PLA and enhancing its mechanical properties. Remarkable synergistic reinforcing effects of GCNC on PLA performances were observed. Particularly, the tensile strength of R-GCNC 5 % and S-GCNC 5 % composites surged by 93 % and 76 %, elongation at break increased by 29.4 % and 33.3 %, Young's modulus rose by 37 % and 18 %, and cold crystallization temperature decreased by 11.5 °C and 12.9 °C. Additionally, the GCNC/PLA composites exhibited exceptional thermal stability, UV resistance, and water vapor permeability reduced by 46 % for R-GCNC, and 35 % for S-GCNC, making them promising for industrial and sustainable packaging.

Mechanochromic and Conductive Gels Based on Cellulose Nanocrystals for Bioinspired Sensing

Soft mechanochromatic materials hold great promise for wearable sensing technologies. Bioinspired photonic crystal structures assembled from cellulose nanocrystals (CNCs) enable dynamic light modulation, providing a vibrant and easily controlled optical platform for real-time detection. This study constructed a CNC-based mechanochromic and conductive gel with a dual-signal response featuring interactive optical and electrical feedback. The mechanical response is achieved through the integration of encapsulating, interpenetration, and crystallization of a soft biocompatible poly(vinyl alcohol) (PVA) sandwich matrix, which propels the deformation of the encased CNC photonic crystal core, leading to dynamic changes in conductivity and color. The material exhibits the capability of detecting dynamic tensile/compressive strains (up to 130%/49% along with a visually discernible color shifting from red to blue) and recognizing finger bending and specific spoken words, which showcases its potential in pliable dual-signal sensing in human health monitoring and strain detection.

Spray-Assisted Fabrication of Cellulose Photonic Pigments on Superhydrophobic Surfaces

Photonic pigments, especially those based on naturally-derived building blocks like cellulose nanocrystals (CNCs), are emerging as a promising sustainable alternative to absorption-based colorants. However, the proposed manufacturing methods for CNC pigments, via either grinding films or emulsion-based production, usually require several processing steps. This limits their commercialization by increasing the costs, timescales, and environmental impacts of production. Toward addressing these challenges, it is reported that photonic pigments can be produced in a single process by drying microdroplets of aqueous CNC suspension on a superhydrophobic surface. Such liquid-repellent substrate ensures the microdroplets maintain a near-spherical shape, enabling the radial self-organization of the cholesteric phase. Upon drying under ambient conditions, the CNC mesophase becomes kinetically arrested, after which the strong capillary forces induced by water evaporation result in extensive buckling of the microparticle. This buckling, coupled with prior tuning of the CNC formulation, enables photonic pigments with adjustable color across the visible spectrum. Importantly, the elimination of an emulsifying oil phase to create microdroplets enables a much faster drying time (≈40 min) and improved color stability (e.g., polar solvents, elevated temperatures), while the reduction in reagents (e.g., oils, surfactants) and post-processing steps (e.g., solvent, heat) improves the sustainability of the fabrication process.

Synergistic color-changing and conductive photonic cellulose nanocrystal patches for sweat sensing with biodegradability and biocompatibility

Given the ongoing requirements for versatility, sustainability, and biocompatibility in wearable applications, cellulose nanocrystal (CNC) photonic materials emerge as excellent candidates for multi-responsive wearable devices due to their tunable structural color, strong electron-donating capacity, and renewable nature. Nonetheless, most CNC-derived materials struggle to incorporate color-changing and electrical sensing into one system since the self-assembly of CNCs is incompatible with conventional conductive mediums. Here we report the design of a conductive photonic patch through constructing a CNC/polyvinyl alcohol hydrogel modulated by phytic acid (PA). The introduction of PA significantly enhances the hydrogen bonding interaction, resulting in the composite film with impressive flexibility (1.4 MJ m-3) and progressive color changes from blue, green, yellow, to ultimately red upon sweat wetting. Interestingly, this system simultaneously demonstrates selective and sensitive electrical sensing functions, as well as satisfactory biocompatibility, biodegradability, and breathability. Importantly, a proof-of-concept demonstration of a skin-adhesive patch is presented, where the optical and electrical dual-signal sweat sensing allows for intuitive visual and multimode electric localization of sweat accumulation during physical exercises. This innovative interactive strategy for monitoring human metabolites could offer a fresh perspective into the design of wearable health-sensing devices, while greatly expanding the applications of CNC-based photonic materials in medicine-related fields.

Scalable and Precise Synthesis of Structurally Colored Bottlebrush Block Copolymers: Enabling Refined Color Calibration for Sustainable Photonic Pigments

Self-assembled bottlebrush block copolymers (BBCPs) offer a vibrant, eco-friendly alternative to traditional toxic pigments and dyes, providing vivid structural colors with significantly reduced environmental impact. Scaling up the synthesis of these polymers for practical applications has been challenging with conventional batch methods, which suffer from slow mass and heat transfer, inadequate mixing, and issues with reproducibility. Precise control over molecular weight and dispersity remains a significant challenge for achieving finely tuned color appearances. Here, we present an alternative strategy to overcome the challenges by integrating a rapid continuous flow technique with an in-line self-assembly procedure. This strategy enables the rapid, stable and large-scale synthesis of narrow-dispersed BBCPs, exceeding 2 kg/day, a significant improvement over conventional gram-scale methods. Furthermore, precise control over the degree of polymerization is achieved with an unprecedented interval accuracy of four repeat units. This level of precision enables refined color calibration in the resulting photonic pigments, effectively eliminating the need for labor-intensive and costly multiple batch syntheses.

Regioselective functionalization of cellulose nanomaterial for advanced application

Cellulose nanomaterials (CNMs) with their remarkable properties and abundant natural sources have emerged as a versatile platform for material science. However, their widespread adoption to develop novel applications often hinges on precise control over their surface chemistry. Regioselective functionalization, i.e., the ability to modify specific hydroxy groups on the cellulose backbone or aldehyde reducing end group (REG), offers unparalleled control on their surface chemistry. This review highlights the exciting developments in regioselective functionalization of CNMs and their impacts on structure-property relationships. Key factors that influence regioselectivity are examined and exciting applications of regioselectively functionalized CNMs are reviewed. This review also highlights the need for efficient, large-scale regioselective functionalization techniques and identifies key areas for future research.

Infrared Stealth Coating with Tunable Structural Color Based on ZnO Spheres

It is important to develop low infrared (IR) emissive coating with tunable structure color to improve the infrared-visible stealth performance of military equipment. In this work, uniform ZnO spheres are used as building units to construct photonic structures with both bright adjustable structure color and low IR emissivity due to the relatively high refractive index and low IR emissivity of ZnO. The vivid tunable structural colors are provided by the photonic bandgap of ZnO photonic crystals (PCs) or the quasi-bandgap of amorphous photonic crystals (APCs), respectively. Both ZnO PCs and APCs exhibited low IR emissivity in 3-5 µm. The IR emissivity of 255 nm ZnO PC is 0.483 and the IR emissivity of 255 nm ZnO APC is 0.492 at 25 °C. With the increase of temperature, the IR emissivity of further decreased to 0.295 and 0.312 at 300 °C. These structures can be applied to a variety of surfaces, and all these structures have good thermal and light stability as well. This work may open a simple and effective way to fabricate materials with good infrared-visible stealth performance, expanding the application of ZnO PCs and APCs coatings in the camouflage area.

Butterfly wing-inspired superhydrophobic photonic cellulose nanocrystal films for vapor sensors and asymmetric actuators

Cellulose nanocrystals (CNCs)-based stimuli responsive photonic materials demonstrate great application potential in mechanical and chemical sensors. However, due to the hydrophilic property of cellulose molecular, a significant challenge is to build a water-resistant photonic CNCs material. Here, inspired by butterfly wings with vivid structural color and superhydrophobic property, we have designed a CNCs based superhydrophobic iridescent film with hierarchical structures. The iridescent colored layer is ascribed to the chiral nematic alignment of CNCs, the superhydrophobic layer is ascribed to the micro-nano structures of polymer microspheres. Specially, superhydrophobic iridescent CNCs film could be used as an efficient colorimetric humidity sensor due to the existence of 'stomates' on superhydrophobic layer, which allowed the humid gas to enter into and out from the humidity responsive chiral nematic layers. Meanwhile, superhydrophobic iridescent films show out-standing self-cleaning and anti-fouling performance. Moreover, when the one side of the CNCs film was covered with superhydrophobic layer, the Janus film displays asymmetric expansion and bending behaviors as well as responsive structural colors in hydrous ethanol. This CNCs based hierarchical photonic materials have promising applications including photonic sensors suitable for extreme environment and smart photonic actuators.

Polarization and reflectance are linked to climate, size and mechanistic constraints in a group of scarab beetles

Beetles exhibit an extraordinary diversity of brilliant and colourful appearances and optical effects invisible to humans. Their underlying mechanisms have received some attention, but we know little about the ecological variables driving their evolution. Here we investigated environmental correlates of reflectivity and circular polarization in a group of optically diverse beetles (Scarabaeidae-Rutelinae). We quantified the optical properties of 261 specimens representing 46 species using spectrophotometry and calibrated photographs. Then, we examined associations between these properties and environmental variables such as temperature, humidity and vegetation cover, controlling for body size and phylogenetic relatedness. Our results showed larger beetles have higher visible reflectivity in drier environments. Unexpectedly, near-infrared (NIR) reflectivity was not correlated with ecological variables. However, we found a correlation between humidity and polarization (chiral nanostructures). We identified trade-offs between optical properties: beetles without polarization-associated nanostructures had higher NIR reflectivity. By contrast, visible reflectivity was negatively correlated with the accumulation of pigments such as melanin. Our study highlights the value of a macroecological approach for testing alternative hypotheses to explain the diversity of optical effects in beetles and to understand the link between structure and function.

Modulation of the Chirality and Dynamics of Self-Assembled Nanocellulose-Chiral C-Dot Film for Chiral Sensing Applications

The detection and sensing of chirality using chiral biomaterials are growing areas of research in advanced bioelectronics. As a result, chiral-controlled biomaterials are crucial for advancing current technologies in chiral sensing applications within biosystems. A chiral carbon dot (C-dot) modulated self-assembled emissive cellulose nanocrystal (CNC) film is developed where the chirality of the CNC film can be tempered between left-handed and right-handed chirality after being doped with chiral L/D-C-dots in CNCs (C-dot-CNC film), transferring the chirality from C-dots to CNCs. The interaction between C-dots, CNCs, and carrier dynamics is investigated using a variety of steady-state and time-resolved PL spectroscopy techniques. The chiral C-dot enhanced the protonic conductivity across the CNC via the formation of hydrogen bonds with its surface functional groups and water molecules. Further, the chiral CNC-C-dots photoelectrodes demonstrate an excellent ability to distinguish between left-handed and right-handed small molecules. These findings on the underlying mechanism of spin selectivity between chiral CNC-C-dot and chiral ligand hold promise for the development of efficient chiral-sensing electronic devices.

Recent advances in nanocellulose pretreatment routes, developments, applications and future prospects: A state-of-the-art review

In a quest to find eco-friendly materials from renewable resources, researchers have focused on cellulose materials, which is the primary reinforcing component of plant cell walls. Nanocellulose is at the forefront of research due to its wide range of sources, biocompatibility, large surface area and tunable surface chemistry. It has gained considerable attention in various industries as a nano-reinforcement for polymer matrices due to its hierarchical structure (medical and healthcare, oil and gas, packaging, paper, board, composites, printed and flexible electronics, 3D printing, aerogels). In this paper, we have reviewed the recent advances in nanocellulose production, physical properties, structural characterization, surface modification strategies, pretreatment methods, applications, limitations and future directions. This review emphasizes the quantification of nanocellulose extraction and applications of the most prevalent areas of nanocellulose research. In view of its increasing and broader applications, the demand for nanocellulose is expected to increase in the future.

Curvature-directed anchoring and defect structure of colloidal smectic liquid crystals in confinement

Rod-like objects at high packing fractions can exhibit liquid crystalline ordering. By controlling how the rods align near a boundary, i.e. the anchoring, the defects of a liquid crystal can be selected and tuned. For smectic phases, the rods break rotational and translational symmetry by forming lamellae. Smectic defects thereby include both discontinuities in the rod orientational order (disclinations), as well as in the positional order (dislocations). In this work, we use experiments and simulations to uncover the geometrical conditions necessary for a boundary to set the anchoring of a confined, particle-resolved, smectic liquid crystal. We confine a colloidal smectic within elliptical wells of varying size and shape for a smooth variation of the boundary curvature. We find that the anchoring depends upon the local boundary curvature, with an anchoring transition observed at a critical radius of curvature approximately twice the rod length. Surprisingly, the critical radius of curvature for an anchoring transition holds across a wide range of rod lengths and packing fractions. The anchoring controls the defect structure. By analyzing topological charges and networks composed of maximum density (rod centers) and minimum density (rod ends), we quantify disclinations and dislocations formed with varying confinement geometry. Circular confinements, characterized by planar anchoring, promote disclinations, whereas elliptical confinements, featuring antipodal regions of homeotropic anchoring, promote long-range smectic order and dislocations. Our findings demonstrate how geometrical constraints can control the anchoring and defect structures of liquid crystals-a principle that is applicable from molecular to colloidal length scales.

Evaluating the strategies to improve strength and water-resistance of chitin nanofibril assembled structures: Molecule-bridging, heat-treatment and deacidifying

Chitin nanofibril (ChiNF) is a promising building block used to fabricate chitin fibers, films or gels via self-assembly from its aqueous suspension. Although mechanical strengthening of its assembled structures has made great advances, the unsatisfactory water-resistance is still a crucial obstacle to practical application and even rarely referred to. Herein, ChiNF was prepared via deacetylation-ultrasonication treatment and the strategies of molecule-bridging, heat-treatment and deacidifying that aiming to improve the strength and water-resistance of its assembled films were evaluated. Molecule-bridging, including tannic acid (TA) or/and chitosan (CS), improved the mechanical properties to some extent, but had no obvious positive effects on water-resistance; heat-treatment was a useful route to enhance both strength and water-resistance; interestingly, deacidifying was more efficient than heat-treatment with respect to improving strength and water-resistance, implying the presence of acid was the major reason for deteriorating assembled structures. Combining molecule-bridging, deacidifying and heat-treatment produced a strong ChiNF-TA/CS cast film with excellent water-resistance. Different from the commonly-used approach of vacuum filtration, these strategies are very suitable for large-scale production of the ChiNF-based self-supported films or coatings via solution casting. Furthermore, the reverse dialysis deacidification simultaneously produced highly concentrated suspensions suitable for dry-spinning, and thus strong chitin macrofibers were successfully fabricated.

Multi-scaled regulation for cholesteric organization of cellulose nanocrystals based on internal and external factors

Cellulose nanocrystals (CNCs), as one of the most promising bio-sourced materials, have been drawing increasing attention as they offer an attractive route for the rational design and sustainable manufacturing of photonic materials owing to their cholesteric self-assembly from the suspension to solid state. Such an organization process can be readily regulated depending on either internal factors or external forces. In this review, recent advances in the control over the self-assembly process and photonic organizations of CNCs are summarized. Furthermore, an in-depth understanding of diverse factors affecting the nano-scaled periodicity and micro-scaled alignment of CNC cholesteric organization is obtained from perspectives of bulk building blocks, solution environment, extra additives, and external forces. Additionally, the roles of the multi-sized photonic architecture associated with photonic-photonic coupling and the macrogeometry related to the complex confined self-assembly are highlighted for sophisticated CNC optical materials. Finally, insights into the future challenges in the field of CNC photonic materials, regarding the precise mechanism of CNC self-assembly and translation of CNC photonic technology from academia to industry, are proposed.

Using Rheology and Image Processing to Study the Effects of Cellulose Nanocrystal Sedimentation

The effect of sedimentation on lyotropic liquid crystalline dispersions is both an interesting subject in colloidal science and is of practical importance for understanding changes that can occur during dispersion storage. This research explored how the seemingly subtle changes in average length resulting from a single sedimentation step affected the rheological properties and self-assembly of aqueous dispersions of sulfated cellulose nanocrystals. Sedimentation of a primarily isotropic aqueous cellulose nanocrystal dispersion for 1 month at ambient conditions resulted in an isotropic top phase and a biphasic bottom phase, which were separated for further study. Both atomic force microscopy measurements and intrinsic viscosities determined via Fedor's equation showed preferential fractionation of longer cellulose nanocrystals (CNCs) to the bottom phase. The effects of this fractionation on dispersion rheology and phase behavior were studied by preparing a range of concentrations from each phase. As expected, the concentration for the isotropic-biphasic phase transition was significantly lower for the bottom phase than for the top phase or parent dispersion. Rheological changes were generally subtle, but significant differences were seen in the storage modulus at concentrations approaching rheological gelation. Most notably, quantitative image processing showed that even this simple, single-step fractionation process had a significant impact on the relative proportion of tactoids and chiral helices with planar and homeotropic anchoring in self-assembled cellulose nanocrystal films. These results highlight the impact that changes in polydispersity due to sedimentation can have on the self-assembly of nanomaterial mesogens.

Recent Advances in the Preparation, Antibacterial Mechanisms, and Applications of Chitosan

Chitosan, a cationic polysaccharide derived from the deacetylation of chitin, is widely distributed in nature. Its antibacterial activity, biocompatibility, biodegradability, and non-toxicity have given it extensive uses in medicine, food, and cosmetics. However, the significant impact of variations in the physicochemical properties of chitosan extracted from different sources on its application efficacy, as well as the considerable differences in its antimicrobial mechanisms under varying conditions, limit the full realization of its biological functions. Therefore, this paper provides a comprehensive review of the structural characteristics of chitosan, its preparation methods from different sources, its antimicrobial mechanisms, and the factors influencing its antimicrobial efficacy. Furthermore, we highlight the latest applications of chitosan and its derivatives across various fields. We found that the use of microbial extraction shows promise as a new method for producing high-quality chitosan. By analyzing the different physicochemical properties of chitosan from various sources and the application of chitosan-based materials (such as nanoparticles, films, sponges, and hydrogels) prepared using different methods in biomedicine, food, agriculture, and cosmetics, we expect these findings to provide theoretical support for the broader utilization of chitosan.

Chiral Nematic Cellulose Nanocrystal Films for Enhanced Charge Separation and Quantum-Confined Stark Effect

Efficient charge separation is essential in various optoelectronic systems, yet it continues to pose substantial challenges. Building upon the recent evidence that chiral biomolecules can function as electron spin filters, this study aims to extend the application of chirality-driven charge separation from the molecular level to the mesoscale and supramolecular scale. Utilizing cellulose nanocrystals (CNCs) derived from cellulose, the most abundant biomaterial on Earth, this research leverages their self-assembly into chiral nematic structures and their dielectric properties. A device is introduced featuring a chiral nematic hybrid film composed of CNCs and quantum dots (QDs), decorated with iron oxide nanoparticles. Using the quantum-confined Stark effect (QCSE) to probe charge separation, we reveal significant sensitivity to the circular polarization of light and the chiral nematic structure of the film. This approach achieves effective, long-lasting charge separation, both locally and across length scales exceeding 1 μm, enabling potential applications such as self-assembled devices that combine photovoltaic cells with electric capacitance as well as optical electric-field hybrid biosensors.

Cellulose nanomaterial metrology: microscopy measurements

Cellulose nanomaterials are increasingly used for a wide variety of applications. Adequate characterization of these materials is required for quality control during production, to distinguish between materials synthesized by different methods, by different suppliers or from difference cellulose biomass sources, to facilitate development of applications and for regulatory purposes. Here we review recent microscopy measurements for the three main types of cellulose nanomaterials: cellulose nanocrystals, individual cellulose nanofibrils and cellulose nanofibrils. Atomic force microscopy and both scanning and transmission electron microscopy are covered with a focus on recent studies that have metrological rigor, rather than qualitative investigations. In some cases results are compared to those obtained by other methods that are more likely to see widespread use for routine quality control measurements. Detailed studies that use microscopy to provide insight on fundamental material properties (e.g., chiral properties) are also included. Particle size and morphology are important properties but are challenging to measure for cellulose nanomaterials due to the rod or fibril shaped particles, their propensity to agglomerate and aggregate, their low contrast for electron microscopy and, for cellulose nanofibrils, the complex branched and interconnected structures. Overall, the results show that there are now a number of studies in which attention to metrological detail has resulted in measurements that allow one to compare and distinguish between different materials, although there are still examples for which it is not possible to draw conclusions on size differences. The use of detailed microscopy protocols that yield accurate and reliable results will be beneficial in material production and addressing regulatory requirements and will allow the validation of other methods that are more amenable to routine measurements.

Multiscale study of the chiral self-assembly of cellulose nanocrystals during the frontal ultrafiltration process

The structural organization of cellulose nanocrystal (CNC) suspensions at the membrane surface during frontal ultrafiltration has been characterized, for the first time, at the nano- and microscale by in situ small-angle X-ray and light scattering (SAXS and SALS, respectively). During filtration, the particles assembled at the membrane surface and formed the so-called concentration polarization layer (CPL), which contains CNCs arranged in a chiral nematic (cholesteric) helicoidal structure, with the long axis of the CNCs oriented parallel to the membrane surface, and the helical axis of the cholesteric structure oriented perpendicular to the membrane surface. The self-organization of CNCs in the form of oriented cholesteric structures was further characterized by a pitch gradient in the CPL. The structure of the CPL was also investigated upon release of the transmembrane pressure. SAXS data revealed a relaxation process associated with a diffusion of the CNCs from the membrane surface towards the bulk, while SALS measurements revealed a re-organization of the cholesteric phase that was preserved all along the deposit. The preservation of the observed structure after 14 days of continuous filtration followed by air-drying was confirmed using scanning electron microscopy and wide-angle X-ray diffraction, demonstrating the feasibility of the process scale-up.

Enzyme-Triggered Assembly of Glycan Nanomaterials

A comprehensive molecular understanding of carbohydrate aggregation is key to optimize carbohydrate utilization and to engineer bioinspired analogues with tailored shapes and properties. However, the lack of well-defined synthetic standards has substantially hampered advances in this field. Herein, we employ a phosphorylation-assisted strategy to synthesize previously inaccessible long oligomers of cellulose, chitin, and xylan. These oligomers were subjected to enzyme-triggered assembly (ETA) for the on-demand formation of well-defined carbohydrate nanomaterials, including elongated platelets, helical bundles, and hexagonal particles. Cryo-electron microscopy and electron diffraction analysis provided molecular insights into the aggregation behavior of these oligosaccharides, establishing a direct connection between the resulting morphologies and the oligosaccharide primary sequence. Our findings demonstrate that ETA is a powerful approach to elucidate the intrinsic aggregation behavior of carbohydrates in nature. Moreover, the ability to access a diverse array of morphologies, expanded with a non-natural sequence, underscores the potential of ETA, coupled with sequence design, as a robust tool for accessing programmable glycan architectures.

Multi-Modal Melt-Processing of Birefringent Cellulosic Materials for Eco-Friendly Anti-Counterfeiting

Ubiquitous anti-counterfeiting materials with a rapidly rising annual consumption (over 1010 m2) can pose a serious environmental burden. Biobased cellulosic materials with birefringence offer attractive sustainable alternatives, but their scalable solvent-free processing remain challenging. Here, a dynamic chemical modification strategy is proposed for multi-modal melt-processing of birefringent cellulosic materials for eco-friendly anti-counterfeiting. Relying on the thermal-activated dynamic covalent-locking of the spatial topological structure of preferred oriented cellulose, the strategy balances the contradiction between the strong confinement of long-range ordered structures and the molecular motility required for entropically-driven reconstruction. Equipped with customizable processing forms including mold-pressing, spinning, direct-ink-writing, and blade-coating, the materials exhibit a wide color gamut, self-healing efficiency (94.5%), recyclability, and biodegradability. Moreover, the diversified flexible elements facilitate scalable fabrication and compatibility with universal processing techniques, thereby enabling versatile and programmable anti-counterfeiting. The strategy is expected to provide references for multi-modal melt-processing of cellulose and promote sustainable innovation in the anti-counterfeiting industry.

Stimuli-Responsive Chiral Cellulose Nanocrystals Based Self-Assemblies for Security Measures to Prevent Counterfeiting: A Review

The proliferation of misleading information and counterfeit products in conjunction with technical progress presents substantial worldwide issues. To address the issue of counterfeiting, many tactics, such as the use of luminous anticounterfeiting systems, have been investigated. Nevertheless, traditional fluorescent compounds have a restricted effectiveness. Cellulose nanocrystals (CNCs), known for their renewable nature and outstanding qualities, present an excellent opportunity to develop intelligent, optically active materials formed due to their self-assembly behavior and stimuli response. CNCs and their derivatives-based self-assemblies allow for the creation of adaptable luminous materials that may be used to prevent counterfeiting. These materials integrate the photophysical characteristics of optically active components due to their stimuli-responsive behavior, enabling their use in fibers, labels, films, hydrogels, and inks. Despite substantial attention, existing materials frequently fall short of practical criteria due to limited knowledge and poor performance comparisons. This review aims to provide information on the latest developments in anticounterfeit materials based on stimuli-responsive CNCs and derivatives. It also includes the scope of artificial intelligence (AI) in the near future. It will emphasize the potential uses of these materials and encourage future investigation in this rapidly growing area of study.

Structural Color Out of the Blue: A Quantitative Framework for the Self-Assembly Kinetics of Cholesteric Cellulosic Mesophases

Cholesteric mesophases based on cellulose ethers, such as ethyl cellulose and hydroxypropyl cellulose, have been studied widely for their remarkable ability to display macroscopic structural color. However, the typical time scales involved in the multiscale self-assembly of cholesteric liquid crystals, from individual nanoscale helical arrangements to discrete microscopic domains, and their dependence on the gel's viscoelastic properties remain underexplored. Here, we establish a quantitative relationship between the kinetics of structural color formation after shear deformation and cholesteric order development at the nano- and microscales. Utilizing rheology in tandem with static and time-resolved reflectivity measurements, we underscore the strong influence of polymer diffusivity and chain elasticity on self-assembly kinetics in cholesteric cellulose ether gels. We show that our phenomenological model can be employed to assess the structure-property relationships of multiple polysaccharide systems, elucidating key design guidelines for the development and processing of structurally colored cholesteric mesophases.

Fundamental Aspects of Stretchable Mechanochromic Materials: Fabrication and Characterization

Mechanochromic materials provide optical changes in response to mechanical stress and are of interest in a wide range of potential applications such as strain sensing, structural health monitoring, and encryption. Advanced manufacturing such as 3D printing enables the fabrication of complex patterns and geometries. In this work, classes of stretchable mechanochromic materials that provide visual color changes when tension is applied, namely, dyes, polymer dispersed liquid crystals, liquid crystal elastomers, cellulose nanocrystals, photonic nanostructures, hydrogels, and hybrid systems (combinations of other classes) are reviewed. For each class, synthesis and processing, as well as the mechanism of color change are discussed. To enable materials selection across the classes, the mechanochromic sensitivity of the different classes of materials are compared. Photonic systems demonstrate high mechanochromic sensitivity (Δnm/% strain), large dynamic color range, and rapid reversibility. Further, the mechanochromic behavior can be predicted using a simple mechanical model. Photonic systems with a wide range of mechanical properties (elastic modulus) have been achieved. The addition of dyes to photonic systems has broadened the dynamic range, i.e., the strain over which there is an optical change. For applications in which irreversible color change is desired, dye-based systems or liquid crystal elastomer systems can be formulated. While many promising applications have been demonstrated, manufacturing uniform color on a large scale remains a challenge. Standardized characterization methods are needed to translate materials to practical applications. The sustainability of mechanochromic materials is also an important consideration.

Cellulose nanocrystals-based optical organohydrogel fiber with customizable iridescent colors for strain and humidity response

Stimuli-responsive optical hydrogels are widely used in various fields including environmental sensing, optical encryption, and intelligent display manufacturing. However, these hydrogels are susceptible to water losses when exposed to air, leading to structural damage, significantly shortened service lives, and compromised durability. This study presents mechanically robust, environmentally stable, and multi-stimuli responsive optical organohydrogel fibers with customizable iridescent colors. These fibers are fabricated by incorporating tunicate cellulose nanocrystals, alginate, and acrylamide in a glycerol-water binary system. The synthesized fibers exhibit high strength (1.38 MPa), moisture retention capabilities, and elastic properties. Furthermore, a sensor based on these fibers demonstrates high- and low-temperature resistance along with stimuli-responsive characteristics, effectively detecting changes in environmental humidity and strains. Moreover, the fiber sensor demonstrates continuous, repeatable, and quantitatively predictable moisture discoloration responses across a humidity range of 11 % and 98 %. During strain sensing, the optical-organohydrogel-based sensor demonstrates a large working strain (50 %) and excellent cycling stability, underscoring its potential for effectively monitoring a wide range of intricate human motions. Overall, the synthesized fibers and their simple fabrication method can elicit new avenues for numerous related applications including the large-scale implementation of advanced wearable technology.

Modulation of Tannic Acid on the Cholesteric Structure of Cellulose Nanocrystals

The chiral nematic phase structure, formed by the self-assembly of cellulose nanocrystals (CNCs) in an aqueous suspension and maintained in a solid film, shows great potential for optical applications. To achieve complex structures in optical devices, it is crucial to subject CNCs to specific shearing processes, such as spinning and printing. Understanding the structural and property changes of the CNC liquid crystal phase in these processes is of utmost importance. In this study, we investigated the effect of adding tannic acid (TA) on the rheological properties and cholesteric phase structures of CNCs/TA mixed suspensions. By calculating the surface site interaction points, we observed that TA can adsorb onto the surface of CNC rods in suspensions through hydrogen bonding. Through characterization techniques, such as polarized optical microscopy, rheology, and synchrotron SAXS, we examined the effects of TA addition on the microstructure and rheological properties of the CNC liquid crystal phase and clarified the change relating to the system composition. Under the same CNC concentration, the volume fraction of the anisotropic phase, the pitch, and the rod spacing of the cholesteric phase were not significantly affected by the addition of TA. However, the system viscosity was significantly reduced with the appropriate amount of TA (2 wt %), in a wide range of CNC concentrations (up to 15 wt % CNCs). The flow indexes (n) in Region I and Region III of steady-state shear curves of CNCs/TA systems (11-15 wt % CNCs) were compared. Moreover, we introduced the well-established theoretical models for liquid crystal polymers to tentatively interpret Region I of the CNCs/TA cholesteric phase and realized that increased numbers of smaller cholesteric-phase domains in the CNCs/TA system and interfacial modification by TA may contribute to the fluidity change. The feature of the domain texture of CNCs/TA systems is verified by polarized optical microscopy observations.

Controlling the Nematic Liquid Crystallinity of Cellulose Nanocrystals with an Alcohol Ethoxy Sulfonate Surfactant

Cellulose nanocrystals (CNCs) are biobased colloidal nanorods that have unlocked new opportunities in the area of sustainable functional nanomaterials including structural films and coatings, biomedical devices, energy, sensing, and composite materials. While selective light reflection and sensing develop from the typical chiral nematic (cholesteric, Nem*) liquid crystallinity exhibited by CNCs, a wealth of technologies would benefit from a nematic liquid crystal (LC) with CNC uniaxial alignment. Therefore, this study answers the central question of whether surfactant complexation suppresses CNC chirality in favor of nematic lyotropic and thermotropic liquid crystallinity. Therein, we use a common surfactant having both nonionic and anionic blocks, namely, oligo(ethylene glycol) alkyl-3-sulfopropyl diether potassium salt (an alcohol ethoxy sulfonate (AES)). AES forms complexes with CNCs in toluene (a representative for nonpolar organic solvent) via hydrogen bonding with an AES' oligo(ethylene glycol) block. A sufficiently high AES weight fraction endows the dispersibility of CNC in toluene. Lyotropic liquid crystallinity with Schlieren textures containing two- and four-point brush defects is observed in polarized optical microscopy (POM), along with the suppression of the cholesteric fingerprint textures. The results suggest a nematic (Nem) phase in toluene. Moreover, thermotropic liquid crystallinity is observed by incorporating an excess of AES, in the absence of an additional solvent and upon mild heating. The Schlieren textures suggest a nematic system that undergoes uniaxial alignment under mild shear. Importantly, replacing AES with a corresponding nonionic surfactant does not lead to liquid crystalline properties, suggesting electrostatic structural control of the charged end group of AES. Overall, we introduce a new avenue to suppress CNC chirality to achieve nematic structures, which resolves the long-sought uniaxial alignment of CNCs in filaments, composite materials, and optical devices.

Nature-Inspired Helicoidal Nanocellulose-Based Multi-Compartment Assemblies with Tunable Chiroptical Properties

Cellulose-based nanocomposites are highly appealing for the development of next-generation sustainable functional materials. Although many advances have been made in this direction, the true potential of fibrillar nanocomposites has yet to be realized because available fabrication approaches are inadequate for achieving precise structural control at the sub-micrometer scale. Here a spray-assisted alignment methodology of cellulose nanofibrils is combined with the layer-by-layer assembly into an additive manufacturing process in which the alignment direction of each cellulose layer is rationally selected to achieve thin films with a helicoidal arrangement of the nanofibrils. The helicoidal structure of the films is verified by measuring the circular dichroism (CD) of the samples. The sign and position of the structural CD peak show that the handedness and the pitch of the chiral structures can be easily tuned by deliberately selecting simple parameters, such as the number of consecutive cellulose layers sprayed in the same direction, and the angle of rotation between successive stacks of layers. To the authors' knowledge, this approach is unique as it offers the possibility to prepare complex nanocomposite architectures with various nanoscale-controlled sub-structures from different anisometric objects, which is enabling novel designs of composite films with damage-resistant and/or optical filtering functionalities.

Interfacial Engineering of Soft Matter Substrates by Solid-State Polymer Adsorption

Polymer coating to substrates alters surface chemistry and imparts bulk material functionalities with a minute thickness, even in nanoscale. Specific surface modification of a substate usually requires an active substrate that, e.g., undergoes a chemical reaction with the modifying species. Here, we present a generic method for surface modification, namely, solid-state adsorption, occurring purely by entropic strive. Formed by heating above the melting point or glass transition and subsequent rinsing of the excess polymer, the emerging ultrathin (<10 nm) layers are known in fundamental polymer physics but have never been utilized as building blocks for materials and they have never been explored on soft matter substrates. We show with model surfaces as well as bulk substrates, how solid-state adsorption of common polymers, such as polystyrene and poly(lactic acid), can be applied on soft, cellulose-based substrates. Our study showcases the versatility of solid-state adsorption across various polymer/substrate systems. Specifically, we achieve proof-of-concept hydrophobization on flexible cellulosic substrates, maintaining irreversible and miniscule adsorption yet with nearly 100% coverage without compromising the bulk material properties. The method can be considered generic for all polymers whose Tg and Tm are below those of the to-be-coated adsorbed layer, and whose integrity can withstand the solvent leaching conditions. Its full potential has broad implications for diverse materials systems where surface coatings play an important role, such as packaging, foldable electronics, or membrane technology.

Beyond Color Boundaries: Pioneering Developments in Cholesteric Liquid Crystal Photonic Actuators

Creatures in nature make extensive use of structural color adaptive camouflage to survive. Cholesteric liquid crystals, with nanostructures similar to those of natural organisms, can be combined with actuators to produce bright structural colors in response to a wide range of stimuli. Structural colors modulated by nano-helical structures can continuously and selectively reflect specific wavelengths of light, breaking the limit of colors recognizable by the human eye. In this review, the current state of research on cholesteric liquid crystal photonic actuators and their technological applications is presented. First, the basic concepts of cholesteric liquid crystals and their nanostructural modulation are outlined. Then, the cholesteric liquid crystal photonic actuators responding to different stimuli (mechanical, thermal, electrical, light, humidity, magnetic, pneumatic) are presented. This review describes the practical applications of cholesteric liquid crystal photonic actuators and summarizes the prospects for the development of these advanced structures as well as the challenges and their promising applications.

Structural, Optical, and Mechanical Insights into Cellulose Nanocrystal Chiral Nematic Film Engineering by Two Assembly Techniques

Iridescent cellulose nanocrystal (CNC) films with chiral nematic nanostructures exhibit great potential in optical devices, sensors, painting, and anticounterfeiting applications. CNCs can assemble into a chiral nematic liquid crystal structure by evaporation-assisted self-assembly (EISA) and vacuum-assisted self-assembly (VASA) techniques. However, there is a lack of comprehensive examinations of their structure-property correlations, which are essential for fabricating materials with unique properties. In this work, we gained insights into the optical, mechanical, and structural differences of CNC films engineered using the two techniques. In contrast to the random self-assembly at the liquid-air interface in EISA, the continuous external pressure in the VASA process forces CNCs to assemble at the filter-liquid interface. This results in fewer defects in the interfaces between tactoids and highly ordered cholesteric phases. Owing to the distinct CNC assembly behaviors, the films prepared by these two methods show great differences in the nanostructure, microstructure, and macroscopic morphology. Consequently, the highly ordered cholesteric structure gives VASA-CNC films a more uniform structural color and enhanced mechanical performance. These fundamental understandings of the relationship of structure-property nanoengineering through various assembly techniques are essential for designing and constructing high-performance chiral iridescent CNC materials.

Angle-resolved optical spectroscopy of photonic cellulose nanocrystal films reveals the influence of additives on the mechanism of kinetic arrest

Cellulose nanocrystals (CNCs) are rod-like nanoparticles whose chiral self-assembly into photonic films has been promoted as a sustainable source of colouration. Upon drying, an aqueous CNC suspension passes through two regimes: first, a liquid phase, where the CNCs self-organise into a cholesteric liquid crystal, followed by a kinetically-arrested phase, where the helicoidal structure compresses upon loss of solvent, resulting in a solid film with vibrant structural colour. The transition between these two regimes plays an important role in the visual appearance of photonic CNC films, but details on when and how kinetic arrest occurs have remained elusive. In this work, we combine angle-resolved optical spectroscopy of photonic films (approx. 100 vol% CNC) with a model for compressed helicoidal structures to retrieve the suspension conditions during kinetic arrest (approx. 10 vol% CNC). This analysis indicates a shift in the mechanism of kinetic arrest from a glass transition at lower ionic strength to gelation at higher ionic strength, explaining the trends in domain size and film colour. In contrast, neutral additives (glucose, poly(ethylene glycol)) appear to primarily reduce the compression upon drying without affecting cholesteric behaviour, as supported by a general analytical model. These findings deepen our understanding of CNC co-assembly with various commonly-used additives, enabling better control over the production of multifunctional structurally coloured materials.

Inkjet Printed Photonic Cellulose Nanocrystal Patterns

Naturally-sourced cellulose nanocrystals (CNCs) are elongated, birefringent nanoparticles that can undergo cholesteric self-assembly in water to produce vibrant, structurally colored films. As such, they are an ideal candidate for use as sustainable and cost-effective inks in the printing of scalable photonic coatings and bespoke patterns. However, the small volume and large surface area of a sessile CNC drop typically leads to rapid evaporation, resulting in microfilms with a coffee-stain-like morphology and very weak coloration. Here, it is demonstrated that inkjet printing of CNC drops directly through an immiscible oil layer can immediately inhibit water loss, resulting in reduced internal mass flows and greater time for cholesteric self-assembly. The color of each microfilm is determined by the initial composition of the drop, which can be tuned on-demand by exploiting the overprinting and coalescence of multiple smaller drops of different inks. This enables the production of multicolored patterns with complex optical behaviors, such as angle-dependent color and polarization-selective reflection. Finally, the array can be made responsive to stimuli (e.g., UV light, polar solvent) by the inclusion of a degradable additive. This suite of functional properties promotes inkjet-printed photonic CNC arrays for smart colorimetric labeling or optical anticounterfeiting applications.