Transparent Depolarizing Organic and Inorganic Films for Optics and Sensors

Large-scale production of chiral nematic microspheres

The membrane emulsification technique enables the self-assembly of cellulose nanocrystals (CNCs) confined within a spherical geometry for large-scale production. The resulting solid microspheres show long-range ordering with chiral nematic structures, and this fascinating hierarchical architecture can even be transferred to mesoporous carbon or silica microparticles by a sacrificial template method.

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

Widespread concerns over the impact of human activity on the environment have resulted in a desire to replace artificial functional materials with naturally derived alternatives. As such, polysaccharides are drawing increasing attention due to offering a renewable, biodegradable, and biocompatible feedstock for functional nanomaterials. In particular, nanocrystals of cellulose and chitin have emerged as versatile and sustainable building blocks for diverse applications, ranging from mechanical reinforcement to structural coloration. Much of this interest arises from the tendency of these colloidally stable nanoparticles to self-organize in water into a lyotropic cholesteric liquid crystal, which can be readily manipulated in terms of its periodicity, structure, and geometry. Importantly, this helicoidal ordering can be retained into the solid-state, offering an accessible route to complex nanostructured films, coatings, and particles. In this review, the process of forming iridescent, structurally colored films from suspensions of cellulose nanocrystals (CNCs) is summarized and the mechanisms underlying the chemical and physical phenomena at each stage in the process explored. Analogy is then drawn with chitin nanocrystals (ChNCs), allowing for key differences to be critically assessed and strategies toward structural coloration to be presented. Importantly, the progress toward translating this technology from academia to industry is summarized, with unresolved scientific and technical questions put forward as challenges to the community.

Responsive Regulation of Energy Transfer in Lanthanide-Doped Nanomaterials Dispersed in Chiral Nematic Structure

The responsive control of energy transfer (ET) plays a key role in the broad applications of lanthanide-doped nanomaterials. Photonic crystals (PCs) are excellent materials for ET regulation. Among the numerous materials that can be used to fabricate PCs, chiral nematic liquid crystals are highly attractive due to their good photoelectric responsiveness and biocompatibility. Here, the mechanisms of ET and the photonic effect of chiral nematic structures on ET are introduced; the regulation methods of chiral nematic structures and the resulting changes in ET of lanthanide-doped nanomaterials are highlighted; and the challenges and promising opportunities for ET in chiral nematic structures are discussed.

Cellulose nanocrystals-based bio-composite optical materials for reversible colorimetric responsive films and coatings

Photonic materials with a tunable chiral nematic structure that can selectively reflect light dynamically are valuable for applications in smart responsive materials. Here, we prepared potential photonic composites with a chiral nematic structure by forming cellulose nanocrystals (CNCs) and waterborne polyurethane (WPU) composites with different compositions on different substrates by evaporation-induced self-assembly. With increasing WPU content, the reflected wavelength increased from 400 to 680 nm, which was mainly caused by the increase of the chiral nematic pitch. In addition, the mechanical properties were better for higher WPU content. WPU was sensitive to small amounts of moisture in ethanol owing to the swollen WPU after absorbing water will increase the helical pitch. The reversible red shift induced by moisture was approximately 100 nm. When wood was used as the substrate, the CNCs still self-assembled to form chiral nematic structures and the adhesion forces of the composites to the wood substrate were strong. By using MgCl₂ solution as an ink, invisible patterns can be written on the coating, which can be revealed temporarily by ethanol. In addition, the invisible pattern of photonic coating is rewritable. The easily prepared environmentally friendly photonic composite has great potential in sensors, anti-counterfeiting labels and smart decorative coatings.

Iridescent chiral nematic papers based on cellulose nanocrystals with multiple optical responses for patterned coatings

Chiral nematic papers (CNPs) with mesopores structure based on cellulose nanocrystals (CNCs) were fabricated successfully via a swelling and freeze-drying method. The order of the original chiral nematic cellulose nanocrystals film was preserved in CNPs, which was proved by scanning electron microscopy (SEM), polarized optical microscopy (POM) measurements and circular dichroism (CD) spectra. The CNPs exhibited excellent optical responsive properties to different solvents. Inspired by this feature, a colorable ink containing amounts of gel particles was prepared by pulverizing CNPs/water mixture into a suspension. Patterns written in suspension ink with various colors can be formed when soaked with different solvents. Moreover, CNPs displayed an irreversible color response to compression. Additionally, the hydrophilicity of CNPs was tuned by polyethyleneimine. Modified CNPs exhibited different colors under the identical solvent environment when compared to the original one. Aqueous PEI can be used as an ink to depict responsive photonic patterns on CNPs.

Exploring the Circular Polarization Capacity from Chiral Cellulose Nanocrystal Films for a Photo-Controlled Chiral Helix of Supramolecular Polymers

Circularly polarized light (CPL) is key to asymmetric photochemistry as it could impart the chiral organization information into chemical products. Here, we demonstrate the circular polarization capacity of chiral cellulose nanocrystal (CNC) films to trigger photo-alignment of achiral supramolecular polymers into helical structures. Right-handed transmitted (T-) CPL was generated from self-assembled CNC films, which induced amorphous azobenzene (Azo) supramolecular polymers into chiral structures. The chiral induction effect of T-CPL is enhanced on Azo polymers with longer spacers. The absorptive dissymmetry factor (g_abs ) values of liquid-crystal supramolecular polymers can be amplified significantly (over 10 times) after T-CPL irradiation. Moreover, by integrating carbon dots into CNC films, CPL emission with a considerable luminescence dissymmetry factor (g_lum ) up to -0.66 was achieved, and it could be used for the photo-alignment of Azo polymers with high chiroptical properties. This work provides new insight for the photo modulation of supramolecular polymers by CPL-active materials.

Plant-Based Structures as an Opportunity to Engineer Optical Functions in Next-Generation Light Management

This review addresses the reconstruction of structural plant components (cellulose, lignin, and hemicelluloses) into materials displaying advanced optical properties. The strategies to isolate the main building blocks are discussed, and the effects of fibrillation, fibril alignment, densification, self-assembly, surface-patterning, and compositing are presented considering their role in engineering optical performance. Then, key elements that enable lignocellulosic to be translated into materials that present optical functionality, such as transparency, haze, reflectance, UV-blocking, luminescence, and structural colors, are described. Mapping the optical landscape that is accessible from lignocellulosics is shown as an essential step toward their utilization in smart devices. Advanced materials built from sustainable resources, including those obtained from industrial or agricultural side streams, demonstrate enormous promise in optoelectronics due to their potentially lower cost, while meeting or even exceeding current demands in performance. The requirements are summarized for the production and application of plant-based optically functional materials in different smart material applications and the review is concluded with a perspective about this active field of knowledge.

Marangoni Flow Manipulated Concentric Assembly of Cellulose Nanocrystals

Tunable assembly of cellulose nanocrystals (CNCs) is important for a variety of emerging applications in optics, sensing, and security. Most exploited assembly and optical property of CNCs are cholesteric assembly and corresponding circular dichroism. However, it still remains challenge to obtain homogenous and high-resolution cholesteric assembly. Distinct assembly and optical property of CNCs are highly demanded for advanced photonic materials with novel functions. Herein, a facile and programmable approach for assembling CNCs into a novel concentric alignment using capillary flow and Marangoni effect, which is in strike contrast to conventional cholesteric assembly, is demonstrated. The concentric assembly, as quantitatively evidenced by polarized synchrotron radiation Fourier transform infrared imaging, demonstrates Maltese cross optical pattern with good uniformity and high resolution. Furthermore, this Maltese cross can be readily regulated to "on/off" states by temperature. By combining with 3D inkjet technology, a functional binary system composed of "on"/"off" CNCs optical patterns with high spatial resolution, fast printing speed, good repeatability, and precisely controllable optical property is established for information encryption and decryption. This concentric assembly of CNCs and corresponding tunable optical property emerge as a promising candidate for information security, anticounterfeiting technology, and advanced optics.

Stimuli-responsive flexible membrane via co-assembling sodium alginate into assembly membranes of rod-like cellulose nanocrystals with an achiral array

Uniaxially assembling cellulose nanocrystals (CNCs) can induce strong solid-state emission based on optical inelastic scattering, whereas the CNC assembly membranes are not flexible enough for further applications. Thus, we introduced CNC into flexible sodium alginate (SA) and further controlled the assembly structure of CNC to increase the membrane toughness and maintain the emission properties. The results indicated that the stretchability increased from 0.027 % to 37 % when 33-37% when 33 % SA was introduced. The assembly achirality was controlled by tuning CNC concentration in suspension, and the co-assembly could further control the wavelength of the assembly-induced emission from 420 nm to 440 nm. Furthermore, the improved stretchability made assembly membrane an optical sensor, whose excitation wavelength blue-shifted about 30 nm under a 30 % strain. The emission of the co-assembly membrane could also respond to humidity, and this cellulose-based material should have great potential in biosensor and wearable devices.

Bioinspired manufacturing of oriented polysaccharides scaffolds for strong, optical haze and anti-UV/bacterial membranes

Here, biomimetic dual esterification strategy was proposed on natural polysaccharides cellulose nanocrystals (CNCs) and galactomannan (GM) in combination with tartaric acid (TA) and benzoic anhydride (BA) respectively. Evaporation-induced self-assembly (EISA) formed the oriented quasinematic structure of the nanocomposites membranes. The CNCs crystallites were modified by TA and intercalated by amorphous polysaccharides, building a complex supramolecular network. Thus, it presents excellent light scattering property with the optical haze of ~90%, which was rarely reported previously. TA and BA simultaneously contributed to satisfying UV adsorption capability for the membranes, showing almost whole-spectra UVA/UVB blocking. Super high mechanical strength (>150 MPa) and toughness (~8 kJ/m³) were revealed by the membranes with high addition amount of BA, together with the efficient antibacterial capability on both Gram-positive and negative bacteria. The diverse optical, mechanical and biological functions displayed by the polysaccharides membranes, propose new horizons on application for packaging, optoelectronic and biomonitoring sensors.

Fabrication of Moisture-Responsive Crystalline Smart Materials for Water Harvesting and Electricity Transduction

It is of profound significance with regard to the global energy crisis to develop new techniques and materials that can convert the chemical potential of water into other forms of energy, especially electricity. To address this challenge, we built a new type of energy transduction pathway (humidity gradients → mechanical work → electrical power) using moisture-responsive crystalline materials as the media for energy transduction. Single-crystal data revealed that a flexible zeolitic pyrimidine framework material, ZPF-2-Co, could undergo a reversible structural transformation (β to α phase) with a large unit cell change upon moisture stimulus. Dynamic water vapor sorption analysis showed a gate-opening effect with a steep uptake at as low as 10% relative humidity (RH). The scalable green synthesis approach and the fast water vapor adsorption-desorption kinetics made ZPF-2-Co an excellent sorbent to harvest water from arid air, as verified by real water-harvesting experiments. Furthermore, we created a gradient distribution strategy to fabricate polymer-hybridized mechanical actuators based on ZPF-2-Co that could perform reversible bending deformation upon a variation of the humidity gradient. This mechanical actuator showed remarkable durability and reusability. Finally, coupling the moisture-responsive actuator with a piezoelectric transducer further converted the mechanical work into electrical power. This work offers a new type of moisture-responsive smart material for energy transduction and provides an in-depth understanding of the responsive mechanism at the molecular level.

Cellulose Based Photonic Materials Displaying Direction Modulated Photoluminescence

Photonic materials featuring simultaneous iridescence and light emission are an attractive alternative for designing novel optical devices. The luminescence study of a new optical material that integrates light emission and iridescence through liquid crystal self-assembly of cellulose nanocrystal-template silica approach is herein presented. These materials containing Rhodamine 6G were obtained as freestanding composite films with a chiral nematic organization. The scanning electron microscopy confirms that the cellulose nanocrystal film structure comprises multi-domain Bragg reflectors and the optical properties of these films can be tuned through changes in the relative content of silica/cellulose nanocrystals. Moreover, the incorporation of the light-emitting compound allows a complementary control of the optical properties. Overall, such findings demonstrated that the photonic structure plays the role of direction-dependent inner-filter, causing selective suppression of the light emitted with angle-dependent detection.

Highly Strong and Solvent-Resistant Cellulose Nanocrystal Photonic Films for Optical Coatings

Cellulose nanocrystals (CNCs) are powerful photonic building blocks for the fabrication of biosourced colored films. A combination of the advantages of self-assembled CNCs and multiple templating agents offers access to the development of novel physicochemical sensors, structural coatings, and optic devices. However, due to the inherent brittleness and water instability of CNC-derived materials, their further applications are widely questionable and restrictive. Here, a soft polymer of poly(vinyl alcohol) (PVA) was introduced into the rigid CNC system to balance molecular interactions, whereafter two hard/soft nanocomposites were fastened through a cross-linking reaction of glutaraldehyde (GA), resulting in a highly flexible, water-stable, and chiral nematic CNC composite film through an evaporation-induced self-assembly technique. For a 1.5 wt % GA-cross-linked 70 wt % CNC loading film, its treatment with harsh hydrophilic exposure (soaking in a strong acid, strong base, and seawater) and various organic solvents show exceptional solvent-resistant abilities. Furthermore, the film can even withstand a weight of 167 g cm^-2 without failure, which is a highly stiff and durable character. Importantly, the film remains a highly ordered chiral nematic organization, being able to act as a highly transparent substrate for selective reflection of left-handed circularly polarized light, preparing fully covered and patterned full-color coatings on various substrates. Our work paves the way for applications in low-cost, durable, and photonic cellulosic coatings.

Lateral Gradient Ambidextrous Optical Reflection in Self-Organized Left-Handed Chiral Nematic Cellulose Nanocrystals Films

Artificial photonic materials displaying ordered reflected color patterns are desirable in the field of photonic technologies, however, it is challenging to realize. Here we present that self-assembly of cellulose nanocrystals (CNC) in a tilted cuvette leads to the formation of rainbow color CNC films. We show that the self-organized CNC films enable simultaneous reflection of left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light with lateral gradient transmittance ratio (LCP/RCP: 8.7–0.9) and the maximum reflectance value up to ca. 72%. This unique ambidextrous optical reflection arises from left-handed chiral photonic architectures with lateral gradient photonic bandgaps and nematic-like defects at the film-substrate interface and between left-handed photonic bandgap layers acting as a half-wavelength retarder. We demonstrate that the tilted angle self-assembly method provides a feasible step toward color patterning of CNC-based photonic films capable of ambidextrous optical reflection.

Multiple Anti-Counterfeiting Composite Film Based on Cholesteric Liquid Crystal and QD Materials

A composite film with multiple anti-counterfeiting features was demonstrated by superposing quantum dots (QDs) polymer matrix (film A) and cholesteric liquid crystal film (film B) together. The first-line and second-line anti-counterfeiting characteristics were successfully implemented by employing thermochromic, angular photochromic, and circularly polarized discoloration of film B, respectively. By initiatively utilizing the different relative positions between the fluorescence emission peak (λ_em) of film A and the central selective reflection wavelength (λ_m) of film B at different temperatures, which resulted in changes in the fluorescence spectra or the different presence of latent patterns, the most important third-line anti-counterfeiting feature was successfully achieved.

Multidimensional Information Encryption and Storage: When the Input Is Light

The issue of information security is closely related to every aspect of daily life. For pursuing a higher level of security, much effort has been continuously invested in the development of information security technologies based on encryption and storage. Current approaches using single-dimension information can be easily cracked and imitated due to the lack of sufficient security. Multidimensional information encryption and storage are an effective way to increase the security level and can protect it from counterfeiting and illegal decryption. Since light has rich dimensions (wavelength, duration, phase, polarization, depth, and power) and synergy between different dimensions, light as the input is one of the promising candidates for improving the level of information security. In this review, based on six different dimensional features of the input light, we mainly summarize the implementation methods of multidimensional information encryption and storage including material preparation and response mechanisms. In addition, the challenges and future prospects of these information security systems are discussed.

Printing of Colorful Cellulose Nanocrystalline Patterns Visible in Linearly Polarized Light

This study addresses the inkjet printing approach for fabrication of cellulose nanocrystalline (CNC) patterns with tunable optical properties varied by the thickness of deposited layers. In particular, forming functional patterns visible only in linearly polarized light is of the primary interest. The possibility of controlling the bright iridescent color response associated with the birefringence in the chiral anisotropic structure of inkjet-printed layers of CNC with sulfo-groups (s-CNC) has been thoroughly investigated. In this connection, we have elaborated an appropriate synthesis sequence for deriving printable inks in the form of sedimentation-stable s-CNC colloids with various concentrations of solid phase and experimentally determined the optimal regimes of their inkjet printing. For this purpose, the rheological parameters and s-CNC particle concentration have also been optimized. The study is accomplished with a comprehensive optical characterization of the deposited s-CNC layers with variable thickness, drying conditions, and the polarization state. The experimental results demonstrate the feasibility of inkjet printing technology to perform the precise fabrication of optically active s-CNC patterns with variable optical properties. These results are particularly relevant for applications requiring special conditions of color demonstration in security printing for such as anticounterfeiting applications, polygraphy decoration printing, and color photo filters.

Nature-Inspired Emerging Chiral Liquid Crystal Nanostructures: From Molecular Self-Assembly to DNA Mesophase and Nanocolloids

Liquid crystals (LCs) are omnipresent in living matter, whose chirality is an elegant and distinct feature in certain plant tissues, the cuticles of crabs, beetles, arthropods, and beyond. Taking inspiration from nature, researchers have recently devoted extensive efforts toward developing chiral liquid crystalline materials with self-organized nanostructures and exploring their potential applications in diverse fields ranging from dynamic photonics to energy and safety issues. In this review, an account on the state of the art of emerging chiral liquid crystalline nanostructured materials and their technological applications is provided. First, an overview on the significance of chiral liquid crystalline architectures in various living systems is given. Then, the recent significant progress in different chiral liquid crystalline systems including thermotropic LCs (cholesteric LCs, cubic blue phases, achiral bent-core LCs, etc.) and lyotropic LCs (DNA LCs, nanocellulose LCs, and graphene oxide LCs) is showcased. The review concludes with a perspective on the future scope, opportunities, and challenges in these truly advanced functional soft materials and their promising applications.

Retrieving the Coassembly Pathway of Composite Cellulose Nanocrystal Photonic Films from their Angular Optical Response

Aqueous suspensions of cellulose nanocrystals (CNCs) are known to self-assemble into a chiral nematic liquid crystalline phase, leading to solid-state nanostructured colored films upon solvent evaporation, even in the presence of templating agents. The angular optical response of these structures, and therefore their visual appearance, are completely determined by the spatial arrangement of the CNCs when the drying suspension undergoes a transition from a flowing and liquid crystalline state to a kinetically arrested state. Here, it is demonstrated how the angular response of the final film allows for retrieval of key physical properties and the chemical composition of the suspension at the onset of the kinetic arrest, thus capturing a snapshot of the past. To illustrate this methodology, a dynamically evolving sol-gel coassembly process is investigated by adding various amounts of organosilica precursor, namely, 1,2-bis(trimethoxysilyl)ethane. The influence of organosilica condensation on the kinetic arrest can be tracked and thus explains the angular response of the resulting films. The a posteriori and in situ approach is general; it can be applied to a variety of additives in CNC-based films and it allows access to key rheological information of the suspension without using any dedicated rheological technique.

Structural Color Materials for Optical Anticounterfeiting

The counterfeiting of goods is growing worldwide, affecting practically any marketable item ranging from consumer goods to human health. Anticounterfeiting is essential for authentication, currency, and security. Anticounterfeiting tags based on structural color materials have enjoyed worldwide and long-term commercial success due to their inexpensive production and exceptional ease of percept. However, conventional anticounterfeiting tags of holographic gratings can be readily copied or imitated. Much progress has been made recently to overcome this limitation by employing sufficient complexity and stimuli-responsive ability into the structural color materials. Moreover, traditional processing methods of structural color tags are mainly based on photolithography and nanoimprinting, while new processing methods such as the inkless printing and additive manufacturing have been developed, enabling massive scale up fabrication of novel structural color security engineering. This review presents recent breakthroughs in structural color materials, and their applications in optical encryption and anticounterfeiting are discussed in detail. Special attention is given to the unique structures for optical anticounterfeiting techniques and their optical aspects for encryption. Finally, emerging research directions and current challenges in optical encryption technologies using structural color materials is presented.

Multi-responsive nanocomposite membranes of cellulose nanocrystals and poly(N-isopropyl acrylamide) with tunable chiral nematic structures

By imitating the unique structure of nature creatures, photonic membranes with periodic chiral helical structure can be assembled by cellulose nanocrystals (CNCs). It is still an issue to fabricate CNC photonic structures tunable in the entire visible spectrum with multiple stimuli-response capacities. Herein, a multi-responsive nanocomposite photonic membrane is fabricated by co-assembly of poly(N-isopropyl acrylamide) (PNIPAM) grafted CNCs with waterborne polyurethane (WPU) latex on the basis of the chiral nematic structure of CNCs, the thermo-responsibility of PNIPAM, and the flexibility of WPU. The flexible photonic membranes with uniform structural colors from blue to red are obtained by tuning the PNIPAM content. The membrane exhibits reversible responses to solvents, and iridescence changes in response to relative humidity with excellent repeatability. Interestingly, the membrane can be transparent or opaque depending on the ambient temperature. The photonic membranes are appealing in applications as humidity sensor, camouflage materials, and even smart windows.

Tunable Upconverted Circularly Polarized Luminescence in Cellulose Nanocrystal Based Chiral Photonic Films

Integrating chromophores into chiral photonic crystals to fabricate materials that exhibit circularly polarized luminescence (CPL) is promising as this method allows efficient manipulation of the spontaneous emission within photonic bandgaps (PBGs). However, tuning the wavelength of CPL and the dissymmetry factor ( g_lum) in a convenient and accurate manner remains a significant challenge. Here, right-handed, tunable upconverted CPL (UC-CPL) emission was achieved by integrating multiple emissive, upconverting nanoparticles into cellulose nanocrystal based chiral photonic films that had tunable PBGs. Glycerol was used to tune the PBGs of the chiral photonic films, which yielded tunable UC-CPL emission at 450 and 620 nm with a tailored g_lum. Moreover, humidity responsive UC-CPL at blue wavelength was obtained from glycerol-composite photonic film, with a g_lum that ranged from -0.156 to -0.033. It was possible because the PBG and chirality of photonic composite was responded to the relative humidity. This work gives valuable insight into tunable and stimuli-responsive CPL photonic systems.

Single-Stimulus-Induced Modulation of Multiple Optical Properties

Stimuli-responsive smart optical materials hold great promise for applications in active optics, display, sensing, energy conversion, military camouflage, and artificial intelligence. However, their applications are greatly restricted by the difficulty of tuning different optical properties within the same material, especially by a single stimulus. Here, magnetic modulations of multiple optical properties are demonstrated in a crystalline colloidal array (CCA) of magnetic nanorods. Small-angle X-ray scattering studies reveal that these nanorods form an unusual monoclinic crystal in concentrated suspensions. The CCA exhibits optical anisotropy in the form of a photonic bandgap and birefringence, thus enabling magnetic tuning of the structural color and transmittance at a rate of 50 Hz. As a proof-of-concept, it is further demonstrated that the fabrication of a multifunctional device for display, anticounterfeiting, and smart-window applications based on this multiple magneto-optical effect. The study not only provides a new model system for understanding colloidal assembly, but also opens up opportunities for new applications of smart optical materials for various purposes.

Chemoselectivity of Pristine Cellulose Nanocrystal Films Driven by Carbohydrate-Carbohydrate Interactions

Biological photonic nanostructures comprising a hierarchically self-assembled cellulose nanocrystal (CNC) have been exploited for the development of sensing, optoelectronics, and energy materials. Although multiple techniques are used for controlling the optical response and chiral nematic structure of CNC-derived materials, the presence of external studies that pristine CNC has chemoselectivity is not yet reported to implement this destination. Here, we report that the CNC film without modification shows a high optical sensitivity for glucose through color variation from blue to red. Moreover, various glucose homologs or analogs that only differ in terms of the orientation of a hydroxyl group are selectively distinguished through the naked eye. The excellent chemoselectivity of CNC is attributed to carbohydrate-carbohydrate selective hydrogen-bonding interactions. Close binding with glucose induces the rearrangement of a CNC chain and strengthens the repulsive interaction, thus increasing the helical pitch of the chiral nematic structure of the CNC film and changing its macroscopic color. This CNC chemoselectivity presents an unprecedented control of chiral nematic mesoporous carbon through monosaccharide species. The results provide a simple but highly efficient method to tune the optical and structural properties of CNC nanomaterials and to apply them for practical biosensors, chiral separation, and energy applications.

Solid-state 23Na NMR spectroscopy studies of ordered and disordered cellulose nanocrystal films

Cellulose nanocrystal films with either disordered or chiral nematic structures of varying helical pitch were investigated using ²³Na solid-state nuclear magnetic resonance (NMR) spectroscopy. Spin lattice relaxation of ¹H correlated with ²³Na analyzed by indirect observation using polarization transfer from ¹H nuclei to ²³Na nuclei showed that the Na⁺ cations are well hydrated in the cellulose nanocrystal films. Linewidth analysis in solid-state ²³Na NMR showed that the Na⁺ cations move in confined spaces, and that the Na⁺ cations in the film having disordered structure are more dynamic than in the films having ordered structure. From lineshape analysis of the ²³Na 2D nutation NMR spectra, we can distinguish the Na⁺ environments within the ordered and disordered films, and find trends in anisotropic interaction parameters between ordered samples with different pitches. These are the first detailed ²³Na NMR spectroscopic studies of CNC-Na⁺ films, and they show that this technique may be a powerful probe for characterizing the extent of order in nanocellulose samples.

Unwinding a spiral of cellulose nanocrystals for stimuli-responsive stretchable optics

Cellulose nanocrystals (CNCs) derived from biomass spontaneously organize into a helical arrangement, termed a chiral nematic structure. This structure mimics the organization of chitin found in the exoskeletons of arthropods, where it contributes to their remarkable mechanical strength. Here, we demonstrate a photonic sensory mechanism based on the reversible unwinding of chiral nematic CNCs embedded in an elastomer, leading the materials to display stimuli-responsive stretchable optics. Vivid interference colors appear as the film is stretched and disappear when the elastomer returns to its original shape. This reversible optical effect is caused by a mechanically-induced transition of the CNCs between a chiral nematic and pseudo-nematic arrangement.

Bioinspired Smart Moisture Actuators Based on Nanoscale Cellulose Materials and Porous, Hydrophilic EVOH Nanofibrous Membranes

Biomimetic actuators with rapid response speed, high sensitivity, and selectivity to external stimulus have found potential applications in smart switches, artificial muscles, and soft robots. The nanoscale structures of actuators enhance the exposed area to stimulus as well as enable versatile control of the actuation behaviors. Freestanding, flexible, and porous water-driven actuators with poly(vinyl alcohol- co-ethylene) (EVOH) nanofibers as the substrate and super hydrophilic nanoscale cellulose materials (cellulose nanofibers, cellulose nanocrystals, bacterial cellulose) as the active substance via uniform mixing or surface depositing were fabricated. The effects of the EVOH nanofiber substrate, the structures and concentrations of nanoscale cellulose materials, as well as the different environmental stimuli like humidity and temperature on the performance of actuators were studied. The water-driven actuation mechanism was proposed from the macroscopic and molecular aspects and the analysis of Gibbs free energy and mechanical energy. The actuator could bend to an angle of 180° and recovered less than 1 s for more than 100 circles without compromising properties when the environmental moisture changed. Furthermore, the multidimensional deformation behaviors of the water-stimulated actuators could also be well tuned by varying the orientations of the nanoscale materials. Additionally, the applications of the prepared actuator were demonstrated.

High energy oxidation and organosolv solubilization for high yield isolation of cellulose nanocrystals (CNC) from Eucalyptus hardwood

Cellulose nanocrystals (CNC) have been widely used as responsive materials, chiral templates, and tough nano-composites due to its unparalleled properties. Acid and enzyme hydrolyses are extensively employed to prepare CNC. These traditional approaches exhibit inherent limitations of corrosion hazards, time-consuming process, and/or low yield. Herein, irradiation oxidation and organosolv solubilization are conducted to cause rapid degradation with simultaneous crystallization of cellulose to achieve approx. 87% yield of CNC. The morphology, spectroscopic, and stability properties of the as-prepared CNC are characterized through UV-vis spectroscopy, zetal potential, XRD, TEM, DLS, GPC, FT-IR and TGA techniques. The resultant CNC suspension presents unique property with high stability after 9 months storage at 4 °C. Moreover, CNC liquid crystal phase is successfully generated by addition of anions or cations solution to the CNC aqueous dispersion without stirring. The innovative approach in this work opens an avenue to obtain CNC directly from lignocellulosic biomass through irradiation oxidation and organosolv solubilization without acid hydrolysis and washing procedure.

The Self-Assembly of Cellulose Nanocrystals: Hierarchical Design of Visual Appearance

By controlling the interaction of biological building blocks at the nanoscale, natural photonic nanostructures have been optimized to produce intense coloration. Inspired by such biological nanostructures, the possibility to design the visual appearance of a material by guiding the hierarchical self-assembly of its constituent components, ideally using natural materials, is an attractive route for rationally designed, sustainable manufacturing. Within the large variety of biological building blocks, cellulose nanocrystals are one of the most promising biosourced materials, primarily for their abundance, biocompatibility, and ability to readily organize into photonic structures. Here, the mechanisms underlying the formation of iridescent, vividly colored materials from colloidal liquid crystal suspensions of cellulose nanocrystals are reviewed and recent advances in structural control over the hierarchical assembly process are reported as a toolbox for the design of sophisticated optical materials.

Uncovering the Circular Polarization Potential of Chiral Photonic Cellulose Films for Photonic Applications

Circularly polarized light (CPL) is central to photonic technologies. A key challenge lies in developing a general route for generation of CPL with tailored chiroptical activity using low-cost raw materials suitable for scale-up. This study presents that cellulose films with photonic bandgaps (PBG) and left-handed helical sense have an intrinsic ability for circular polarization leading to PBG-based CPL with extraordinary |g | values, well-defiend handedness, and tailorable wavelength by the PBG change. Using such cellulose films, incident light ranging from near-UV to near-IR can be transformed to passive L-CPL and R-CPL with viewing-side-dependent handedness and |g | values up to 0.87, and spontaneous emission transformed to R-CPL emission with |g | values up to 0.68. Unprecedented evidence is presented with theoretical underpinning that the PBG effect can stimulate the R-CPL emission. The potential of cellulose-based CPL films for polarization-based encryption is illustrated. The evaporation-induced self-assembly coupled with nanoscale mesogens of cellulose nanocrystals opens new venues for technological advances and enables a versatile strategy for rational design and scalable manufacturing of organic and inorganic CPL films for photonic applications.

Anisotropic flexible transparent films from remaining wood microstructures for screen protection and AgNW conductive substrate

Flexible transparent conductive films or substrates prepared from plastics or cellulose are widely used in optoelectronic devices. However, all of these films or substrates are fabricated by complex and expensive methods, which consume much energy and time. In this work, we report for the first time a remarkably facile and effective approach for fabricating flexible transparent films directly from wood. The resulting films exhibit an array of exceptional optical and mechanical properties. The well-aligned cell structures in natural wood are maintained during delignification, leading to anisotropic films with high transparency (≈90% transmittance). These anisotropic films with well-aligned cell structures show mechanical tensile strengths higher than those of the original wood, and can be used as screen protection films for cellphones. Furthermore, ultrathin, highly transparent, and outstandingly conductive films have been prepared from such films and silver nanowires (AgNWs) using the Meyer technique. A conductive film with an optimal area density (341 mg m^-2) of AgNWs showed outstanding synergistic properties, with a transmittance of 80% and a sheet resistance of 11 Ω sq^-1, equal to the conductivity of ITO. Of importance here is that the low-cost anisotropic transparent wood film shows promising potential for electronics applications in solar cells, flexible displays, and other products.

Tactoid Annealing Improves Order in Self-Assembled Cellulose Nanocrystal Films with Chiral Nematic Structures

The self-assembly process in cellulose nanocrystal (CNC) film formation was studied as a function of evaporation time. It is known that the total evaporation time of CNC dispersions affects the structure of the film obtained, but the extension of different phases of the evaporation has not been explored. By extending the evaporation time of CNC suspensions after the onset of liquid crystallinity, the homogeneity of the resulting films could be improved as observed by polarized optical microscopy and scanning electron microscopy. Here, we show that an intermediate stage of self-assembly, between phase separation and gel vitrification, called tactoid annealing, helps explain the discrepancies in order for chiral nematic CNC films dried at varying evaporation times. This intermediate stage of self-assembly may be useful for designing highly ordered and homogenous CNC-based materials.

Controlling the Photonic Properties of Cholesteric Cellulose Nanocrystal Films with Magnets

The self-assembly of cellulose nanocrystals is a powerful method for the fabrication of biosourced photonic films with a chiral optical response. While various techniques have been exploited to tune the optical properties of such systems, the presence of external fields has yet to be reported to significantly modify their optical properties. In this work, by using small commercial magnets (≈ 0.5-1.2 T) the orientation of the cholesteric domains is enabled to tune in suspension as they assemble into films. A detailed analysis of these films shows an unprecedented control of their angular response. This simple and yet powerful technique unlocks new possibilities in designing the visual appearance of such iridescent films, ranging from metallic to pixelated or matt textures, paving the way for the development of truly sustainable photonic pigments in coatings, cosmetics, and security labeling.