Chiral Photonic Liquid Crystal Films Derived from Cellulose Nanocrystals

Interface-tailored iridescent nanocellulose films with retentive antifouling and recyclable multi-environmental responsive properties

Cellulose nanocrystals (CNCs) with distinctive chiral nematic structures and attractive iridescent structural color after evaporation-induced-assembly have induced much interest. However, the hydrophilic nature and rigidity of CNCs materials greatly hindered their application in various environmental conditions. Herein, nature-derived xylose (Xyl) was introduced to regulate the chiral nematic structure of CNCs so as to improve their mechanical strength and flexibility. Facile solvent immersion strategy of materials in hydrophobic 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (C16H19F17O3Si, PFDTS) solution with various concentration was performed to endow hydrophobicity, as well as remaining their original cholesteric arrangements of the nanoparticles. Chemical and morphological characterizations proved the well distribution and intercalation of Xyl and PFDTS molecules into the chiral structures. The films after tailored interfacial modification exhibited satisfying hydrophobicity with highest water contact angle of ∼ 103°, and retentive anti-fouling capabilities were achieved for the films. Moreover, repeatable and highly sensitive humidity and acid response via iridescent change was fulfilled, well maintaining high mechanical strength (∼70 MPa) after recycling. Besides, excellent biocompatibility was confirmed for the modified materials via cell viability (>90 %) determination. Therefore, the proposed chiral CNCs-based hydrophobic films may greatly widen the application of cellulosic nanomaterials in areas of electrical devices, environmental protection and biomedical treatments, etc.

Cellulose-based circularly polarized luminescence nanofilm with superior optical intensity and controllable direction

Synthesizing the circularly polarized luminescent (CPL) assemblies with strong intensity and real-time handedness direction control requires an ingenious structure design, which is difficult in traditional optical materials. This work utilizes the self-assembled cellulose nanocrystals (CNCs) as the chiral template and packages the luminophores into the platform, to design a kind of CPL nanofilm. By optimizing the photonic bandgap structure, it possesses a considerable CPL and superior mechanical flexibility, which makes it a candidate for the next generation of wearable sensing devices. Additionally, it displays the reversible optical response in the humidity- or ammonia gas-stimulus. Next, this work designs two CPL amplification strategies, divided into selective reflection design and optical pathway length optimization. The intensified dissymmetry factor (glum) has reached -1.39. Since, by simply tuning the sequence of two films, the handedness direction is successfully switched, with the inverted glum of approximately 0.4442. This dual-direction CPL intensity has preceded most cellulose-based chiral luminescent materials. Importantly, this optical design avoids the incorporation of external stimuli and disruption of the inner structure. At last, this photonic film with direction- and intensity-controllable is used in the photo-induced polymerization reaction and optical anti-counterfeiting areas.

Self-Assembled Hydroxypropyl Celluloses With Structural Colors for Biomedical Applications

Hydroxypropyl cellulose (HPC), a cellulose derivative with biocompatibility, edibility, and exceptional solubility in many polar solvents, holds significant potential for biomedical applications. Within a specific concentration range, HPC undergoes self-assembly to form cholesteric liquid crystals, which display distinct structural colors. These colors result from the interaction between incident light and the periodic nano-architecture of HPC, providing long-lasting visual effects that can be dynamically adjusted by factors such as concentration, temperature, and functional additives. This review includes the mechanisms underlying the genesis of structural colors and the regulation of HPCs while summarizing advanced techniques for fabricating HPC-based materials with diverse configurations. Furthermore, through representative examples, we highlight the multifaceted applications of these materials in sensors, bionic skins, drug delivery, and anti-counterfeiting labels. We also propose strategies to address current research and application challenges with the goal of exploring the potential of structural color HPCs for scientific breakthroughs and societal well-being. We hope this review catalyzes HPC-based structural color materials' advancement and future biomedical applications.

Photonic Marvels: Exploring the Self-Assembly of Cellulose Nanocrystals for Sustainable Materials and Beyond

Cellulose nanocrystals (CNCs) are biodegradable, plant-derived colloidal particles that can self-assemble through evaporation-induced self-assembly (EISA) to form photonic films. The ability of CNCs to organize structurally colored films has garnered significant attention as a promising source of sustainable materials. CNCs serve as versatile photonic building blocks for creating biobased colored materials. This review provides a comprehensive overview of the latest advancements in chiral photonic CNC (CPCNC) materials. We delve into the chiral structures of these materials and factors affecting the EISA route, exploring their fundamental principles and bottom-up synthesis techniques. Additionally, various responsive CPCNCs are systematically introduced with a focus on their mechanisms, properties, and potential applications. The review concludes with a discussion of emerging applications, challenges, and future opportunities for CPCNCs. By leveraging the unique properties of CPCNCs within complex responsive polymer networks, we see significant potential for developing innovative physicochemical sensors, structural coatings, and optical devices.

Optically switchable chiral photonic crystal composite films for multimodal anti-counterfeiting labels and plant humidity sensing

The combination of chiral photonic crystals and fluorescent nanomaterials has garnered significant attention due to their exceptional light modulation abilities and versatile stimulus-responsive characteristics. In this study, we present a flexible chiral photonic crystal fluorescent film composed of rod-shaped cellulose nanocrystals (CNCs) and orthogonal upconversion nanoparticles (OUCNPs). This composite film maintains the photonic order of the CNCs chiral matrix while preserving the orthogonal excitation-emission properties of OUCNPs, with fluorescence shifting from blue to red as the excitation wavelength changes from 980 nm to 808 nm. By adjusting ultrasonic energy, the photonic bandgap (PBG) of the composite film can be tuned, and the bandgap effect is found to reduce OUCNPs fluorescence emission. Furthermore, by altering factors such as the observation background, viewing angle, polarizer, humidity, and excitation wavelength, the composite film can transition between distinct optical states, effectively integrating stimulus-responsive chiral structural color with fluorescence and exhibiting a sophisticated quintuple stimulus-response functionality. Due to the film's remarkable flexibility, its practical applications in multimodal anti-counterfeiting and humidity sensing are successfully demonstrated using a plant model. This work shows great potential for the development of innovative sensors with advanced environmental response capabilities.

Cholesteric Contact Lenses for Diabetics-Related Noninvasive Glucose Monitoring and Eye Healthcare

Blood sugar monitoring has crucial significance for diabetes mellitus diagnosis, and noninvasive continuous detection methods are the future development trend. Among various noninvasive detection methods, glucose detection in tears has the advantages of a high level of subject compliance, minimal pollution, and accuracy. However, sensors used for detecting glucose concentration in tears usually embed noble microelectrical components into contact lenses, making the process complicated and costly, and easily cause environmental pollution and resource wastage. Here, we propose a construction strategy for contact lenses based on the cellulose nanocrystal (CNC) cholesteric structure, preparing products that change color according to the concentration of glucose. In addition, the surface of the contact lenses can be loaded with drugs for adjuvant treatment of diabetic eye complications. Contact lenses offer advantages such as a fast response speed (<240 s), high sensitivity with distinct colors at specific glucose concentrations (green at 0 mM, yellow at 5 mM, and red at 10 mM), and a reversible response process. Furthermore, they exhibit good biocompatibility (90% cell viability by CCK-8 assay) and biodegradability (complete biodegradation in soil within 120 days). CNC cholesteric contact lenses realize noninvasive, wearable continuous glucose detection, providing a new strategy for health monitoring of diabetics.

Stable liquid crystal-infiltrated photonic crystal sensing film for facile detection of streptomycin

Liquid crystal (LC) biosensors have attracted interest due to their simplicity and ability to visualize results. However, their inherent instability including the fluidity of LCs and the complexity of operation, limits their potential as reliable and user-friendly detection tools. To enhance their practical applicability, a stable and facile LC-infiltrated photonic crystal (LCP) sensing film is developed through optimization of film substrate preparation and investigation of the response mechanism. The reflection peak of the LCP film, which is modulated by changes in LC orientation within the film, can be recorded using a fiber-optic spectrometer or observed visually. Molecular dynamics simulations, integrated with experimental data, were employed to improve LC induction efficiency and increase signal strength. This approach inherently improves the stability and sensitivity of LC biosensors, expanding their potential for use in compact devices. A triple-helix molecular conformational switch is introduced to establish a versatile and specific detection platform. When streptomycin was chosen as a model analyte, the LCP film exhibited a linear range from 5 nM to 10 μM, with a detection limit of 0.40 nM and a relative standard deviation of 2.19%, indicating high precision and reliability. Its practical application was further confirmed with food samples, highlighting its potential for at-home testing of antibiotic residues.

Mechanical and thermal responsive chiral photonic cellulose hydrogels for dynamic anti-counterfeiting and optical skin

Dynamic responsive structural colored materials have drawn increased consideration in a wide range of applications, such as colorimetric sensors and high-safety tags. However, the sophisticated interactions among the individual responsive parts restrict the advanced design of multimodal responsive photonic materials. Inspired by stimuli-responsive color change in chameleon skin, a simple and effective photo-crosslinking strategy is proposed to construct hydroxypropyl cellulose (HPC) based hydrogels with multiple responsive structured colors. By controlling UV exposure time, the structural color of HPC hydrogels can be effectively controlled in a full-color spectrum. At the same time, HPC hydrogels showcase temperature and mechanical dual-responsive structural colors. In particular, the microstructure of HPC hydrogels undergoes a transition from the chiral nematic phase to the nematic phase under the action of external stretching, leading to a significant reflection of circularly polarized light (CPL) to linearly polarized light (LPL). Given the diverse responsiveness exhibited by HPC hydrogels and their unique structural transition properties under external forces, we have explored their potential applications as dynamic anti-counterfeiting labels and optical skins. This work reveals the great possibility of using structural colored cellulose hydrogels in multi-sensing and optical displays, opening up a new path for the exploration of next-generation flexible photonic devices.

Improved Cell Adhesion on Self-Assembled Chiral Nematic Cellulose Nanocrystal Films

Chirality is ubiquitous in nature, and closely related to biological phenomena. Nature-originated nanomaterials such as cellulose nanocrystals (CNCs) are able to self-assemble into hierarchical chiral nematic CNC films and impart handedness to nano and micro scale. However, the effects of the chiral nematic surfaces on cell adhesion are still unknown. Herein, this work presents evidence that the left-handed self-assembled chiral nematic CNC films (L-CNC) significantly improve the adhesion of L929 fibroblasts compared to randomly arranged isotropic CNC films (I-CNC). The fluidic force microscopy-based single-cell force spectroscopy is introduced to assess the cell adhesion forces on the substrates of L-CNC and I-CNC, respectively. With this method, a maximum adhesion force of 133.2 nN is quantified for mature L929 fibroblasts after culturing for 24 h on L-CNC, whereas the L929 fibroblasts exert a maximum adhesion force of 78.4 nN on I-CNC under the same condition. Moreover, the instant SCFS reveals that the integrin pathways are involved in sensing the chirality of substrate surfaces. Overall, this work offers a starting point for the regulation of cell adhesion via the self-assembled nano and micro architecture of chiral nematic CNC films, with potential practical applications in tissue engineering and regenerative medicine.

Upconversion/Downshifting Circularly Polarized Luminescence over 1200 nm in a Single Nanoparticle for Optical Anticounterfeiting and Information Encryption

Multimodal upconversion and downshifting circularly polarized luminescent materials hold significant potential for optical anticounterfeiting applications due to their exceptional chiroptical properties. However, constructing these materials within a single emitter remains challenging. In this study, a conceptual model of multimodal upconversion/downshifting circularly polarized luminescence (CPL) is realized within a single nanoparticle. A new type of nanoparticles with multilayer core-shell architecture is fabricated, capable of delivering upconversion/downshifting luminescence, when excited by a 980 nm laser. Utilizing a co-assembly strategy, multimodal upconversion/downshifting CPL emission, covering a broad emission range from ultraviolet (UV) to the second near-infrared (NIR-II) region, can be realized at the supramolecular level. These chiroptical properties closely follow the chirality of host matrix and are strongly dependent on the distribution mode of nanoparticles within the matrix films. The multimodal upconversion/downshifting CPL behavior enabled cutting-edge encryption applications including optical anticounterfeiting and information encryption. This work introduces a novel approach to designing multimodal upconversion/downshifting CPL materials and opens new avenues for the development of chiroptical functional materials.

Magnetic modulation on chiroptical activities of nematically assembled carbon dots

Effectively harnessing the assembly of achiral carbon dots into a chiral manner is a prominent step for applying carbon dots into the area of stereoselective optoelectronics and theranostics. Herein, magnetic-modulated and circularly polarized luminescence (CPL)-active photonic thin films were presented in this article via co-assembly and magnetic-mediation strategy of cellulose nanocrystals, carbon dots and magnetic nanoparticles. The photonic bandgap of the composite films is modulated via interfacial interactions between the building blocks, and more efficiently via external magnetic field which can further enhance the selective reflection of the films with a maximum CPL anisotropic factor as high as -0.92, indicating the optimized condition for achieving CPL signals is basically when the photonic bandgap (PBG) are close to the emission peaks of nanocomposite films, which may essentially facilitate the selective reflection effect and leads to the output of opposite CPL signals. Such strategy would inevitably boost the development of carbon dots based chiral devices and reagents into the realm of chirality-related biological issues and next generation chiral optoelectronics.

Active Photonic Glass for Hydrogen Generation

Chirality is vital in many living species since it is responsible for structural iridescent coloration and plays a key role in light harvesting during natural photosynthesis. Developing photoactive materials with such chiral structures is a challenging but promising strategy for energy applications. Here, we present a straightforward method to establish an active photonic glass obtained through the co-condensation of tetramethyl orthosilicate (TMOS) and titanium diisopropoxide bis(acetylacetonate) (TAA) dissolved in a liquid crystal formed from cellulose nanocrystalline (CNC). The inorganic glass maintains a long range of chiral nematic ordering, displaying iridescent colors characterized by a Bragg peak reflection. The reflected wavelengths are tuned all over the UV-visible range, demonstrating that the replica of the chiral nematic structure generates photonic properties. Incorporation of gold nanoparticles (Au NPs) into the films is further performed by impregnation/chemical reduction. We show that the charge carrier density and photocatalytic H2 generation were amplified when the photonic band gap edges matched the absorbance of the TiO2 and localized surface plasmon resonance (LSPR) of AuNPs. This photocatalytic glass with chiral nematic ordering and a tunable photonic bandgap paves the way for the development of metamaterials with new applications, such as asymmetric photocatalysis.

Bioinspired Mechanochromic Liquid Crystal Materials: From Fundamentals to Functionalities and Applications

Inspired by intriguing color changeable ability of natural animals, the design and fabrication of artificial mechanochromic materials capable of changing colors upon stretching or pressing have attracted intense scientific interest. Liquid crystal (LC) is a self-organized soft matter with anisotropic molecular alignment. Due to the sensitivity to various external stimulations, LC has been considered as an emerging and appealing responsive building block to construct intelligent materials and advanced devices. Recently, mechanochromic LC materials have becoming a hot topic in multifields from flexible artificial skins to visualized sensors and smart biomimetic devices. In this review, the recent progress of mechanochromic LCs is comprehensively summarized. Firstly, the mechanism and functionalities of mechanochromic LC is introduced, followed by preparation of various functional materials based on mechanochromic LCs. Then the applications of mechanochromic LCs are provided. Finally, the conclusion and outlooks of this field is given. This overview is hoped to provide inspiration in fabrication of advanced functional soft materials for scientists and engineers from multidisciplines including materials science, elastomers, chemistry, and physical science.

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.

Liquid crystal phase behavior of oxalated cellulose nanocrystal and optical films with controllable structural color induced by centripetal force

Cellulose nanocrystal (CNC) is a sustainable bio-nanomaterial. The distinctive left-handed polarization properties render cellulose nanocrystal a promising candidate for optical film. Due to eco-friendliness, reliability, mildness and simplicity, the oxalate hydrolysis method stands out among various preparation methods for CNC. This study delved into the liquid crystal phase behavior of oxalated cellulose nanocrystal derived from pulp, and discovered the influences of CNC concentration and pH on suspension stability and phase transition, and evaluated its optical properties. The results demonstrated that oxalated CNC presented two different liquid crystal phases, the nematic phase and the cholesteric phase. The stability mechanism of CNC suspension and the regulatory principle of the liquid crystal phase transition were revealed. A novel CNC film-forming technology, the multilayer spin-coating technique, was developed for cellulose nanocrystal optical films. Driven by centrifugal force, cellulose nanocrystals were induced to self-assembly and formed the optical film with circular dichroism and structural color. This simple and efficient film-forming technology promised rapid processing (1 h) and controllable film structure and optical properties compared to traditional technologies. This work provided a theoretical understanding and practical prospects for integrating oxalated cellulose nanocrystal into sustainable advanced optical film materials.

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.

The Correlations between Microstructures and Color Properties of Nanocrystalline Cellulose: A Concise Review

Cellulose nanocrystals (CNCs) are a green resource which can produce photonic crystal films with structural colors in evaporation-induced self-assembly; CNC photonic crystal films present unique structural colors that cannot be matched by other colored materials. Recently, the mechanisms of CNC photonic crystal films with a unique liquid crystal structure were investigated to obtain homogenous, stable, and even flexible films at a large scale. To clarify the mechanism of colorful CNC photonic crystal films, we briefly summarize the recent advances from the correlations among the preparation methods, microstructures, and color properties. We first discuss the preparation process of CNCs, aiming to realize the green application of resources. Then, the behavior of CNCs in the formation of liquid crystal phases is studied, considering the influence of the CNCs' size and shape, surface properties, and the types and concentrations of solvents. Finally, the film formation process of CNCs and the control of structural colors during the film formation are summarized, as well as the mechanisms of CNC photonic crystal films with full color. In summary, considering the above factors, obtaining reliable commercial CNC photonic crystal films requires a comprehensive consideration of the subsequent preparation processes starting from the preparation of CNCs.

Polysaccharide Nanocrystals-Based Chiral Nematic Structures: From Self-Assembly Mechanisms, Regulation, to Applications

Chiral architectures, one of the key structural features of natural systems ranging from the nanoscale to macroscale, are an infinite source of inspiration for functional materials. Researchers have been, and still are, strongly pursuing the goal of constructing such structures with renewable and sustainable building blocks via simple and efficient strategies. With the merits of high sustainability, renewability, and the ability to self-assemble into chiral nematic structures in aqueous suspensions that can be preserved in the solid state, polysaccharide nanocrystals (PNs) including cellulose nanocrystals (CNCs) and chitin nanocrystals (ChNCs) offer opportunities to reach the target. We herein provide a comprehensive review that focuses on the development of CNCs and ChNCs for the use in advanced functional materials. First, the introduction of CNCs and ChNCs, and cellulose- and chitin-formed chiral nematic organizations in the natural world, are given. Then, the self-assembly process of such PNs and the factors influencing this process are comprehensively discussed. After that, we showcased the emerging applications of the self-assembled chiral nematic structures of CNCs and ChNCs. Finally, this review concludes with perspectives on the challenges and opportunities in this field.

Uncovering the Recent Progress of CNC-Derived Chirality Nanomaterials: Structure and Functions

Cellulose nanocrystal (CNC), as a renewable resource, with excellent mechanical performance, low thermal expansion coefficient, and unique optical performance, is becoming a novel candidate for the development of smart material. Herein, the recent progress of CNC-based chirality nanomaterials is uncovered, mainly covering structure regulations and function design. Undergoing a simple evaporation process, the cellulose nanorods can spontaneously assemble into chiral nematic films, accompanied by a vivid structural color. Various film structure-controlling strategies, including assembly means, physical modulation, additive engineering, surface modification, geometric structure regulation, and external field optimization, are summarized in this work. The intrinsic correlation between structure and performance is emphasized. Next, the applications of CNC-based nanomaterials is systematically reviewed. Layer-by-layer stacking structure and unique optical activity endow the nanomaterials with wide applications in the mineralization, bone regeneration, and synthesis of mesoporous materials. Besides, the vivid structural color broadens the functions in anti-counterfeiting engineering, synthesis of the shape-memory and self-healing materials. Finally, the challenges for the CNC-based nanomaterials are proposed.

Tunable Construction of Chiral Nematic Cellulose Nanocrystals/ZnO Films for Ultra-Sensitive, Recyclable Sensing of Humidity and Ethanol

The investigation of functional materials derived from sustainable and eco-friendly bioresources has generated significant attention. Herein, nanocomposite films based on chiral nematic cellulose crystals (CNCs) were developed by incorporating xylose and biocompatible ZnO nanoparticles (NPs) via evaporation-induced self-assembly (EISA). The nanocomposite films exhibited iridescent color changes that corresponded to the birefringence phenomenon under polarized light, which was attributed to the formation of cholesteric structures. ZnO nanoparticles were proved to successfully adjust the helical pitches of the chiral arrangements of the CNCs, resulting in tunable optical light with shifted wavelength bands. Furthermore, the nanocomposite films showed fast humidity and ethanol stimuli response properties, exhibiting the potential of stimuli sensors of the CNC-based sustainable materials.

Potential Application Prospects of Biomolecule-Modified Two-Dimensional Chiral Nanomaterials in Biomedicine

Chirality, one of the most fundamental properties of natural molecules, plays a significant role in biochemical reactions. Nanomaterials with chiral characteristics have superior properties, such as catalytic properties, optoelectronic properties, and photothermal properties, which have significant potential for specific applications in nanomedicine. Biomolecular modifications such as nucleic acids, peptides, proteins, and polysaccharides are sources of chirality for nanomaterials with great potential for application in addition to intrinsic chirality, artificial macromolecules, and metals. Two-dimensional (2D) nanomaterials, as opposed to other dimensions, due to proper surface area, extensive modification sites, drug loading potential, and simplicity of preparation, are prepared and utilized in diagnostic applications, drug delivery research, and tumor therapy. Current advanced studies on 2D chiral nanomaterials for biomedicine are focused on novel chiral development, structural control, and materials sustainability applications. However, despite the advances in biomedical research, chiral 2D nanomaterials still confront challenges such as the difficulty of synthesis, quality control, batch preparation, chiral stability, and chiral recognition and selectivity. This review aims to provide a comprehensive overview of the origins, synthesis, applications, and challenges of 2D chiral nanomaterials with biomolecules as cargo and chiral modifications and highlight their potential roles in biomedicine.

Polyethylene glycol regulates the pitch and liquid crystal behavior of cellulose nanocrystal-based photonic crystals

Inspired by iridescent color in natural creations, cellulose nanocrystal (CNC) photonic crystals artificially created by nanotechnology have great application prospects due to their potential to control light propagation in the linear and nonlinear regimes. One of the most important development directions of photonic crystals is the diversification of colors, usually by adjusting the pitch. However, few researchers notice the effect of polymer molecular weight and content on pitch regulation and the interaction between polymer and CNC liquid crystals. Polyethylene glycol (PEG) were used as polymers to regulate the pitch of CNC photonic crystals and investigate the changes in microstructure, crystal structure, thermal properties, and liquid crystal texture of the composites by changing the PEG content and molecular weight. Different photonic crystal construction systems show that when the molecular weight of PEG is 0.4 k, it can be filled between CNCs to regulate the pitch of photonic crystals, while when the molecular weight of PEG is 20 k, it cannot always be filled between CNCs in evaporation-induced self-assembly (EISA) process due to the depletion interaction, which cannot effectively regulate the pitch. This study reveals the relationship between PEG and CNC liquid crystals, which supports the development of photonic crystals and the pitch regulation.

Recent Advances in Flexible CNC-Based Chiral Nematic Film Materials

Cellulose nanocrystal (CNC) is a renewable resource derived from lignocellulosic materials, known for its optical permeability, biocompatibility, and unique self-assembly properties. Recent years have seen great progresses in cellulose nanocrystal-based chiral photonic materials. However, due to its inherent brittleness, cellulose nanocrystal shows limitations in the fields of flexible materials, optical sensors and food freshness testing. In order to solve the above limitations, attempts have been made to improve the flexibility of cellulose nanocrystal materials without destroying their structural color. Despite these progresses, a systematic review on them is lacking. This review aims to fill this gap by providing an overview of the main strategies and the latest research findings on the flexibilization of cellulose nanocrystal-based chiral nematic film materials (FCNM). Specifically, typical substances and methods used for their preparation are summarized. Moreover, different kinds of cellulose nanocrystal-based composites are compared in terms of flexibility. Finally, potential applications and future challenges of flexible cellulose nanocrystal-based chiral nematic materials are discussed, inspiring further research in this field.

Photonic features of blue phase liquid crystals under curved confinement

Blue phase (BP) liquid crystals represent a fascinating state of soft matter that showcases unique optical and electro-optical properties. Existing between chiral nematic and isotropic phases, BPs are characterized by a three-dimensional cubic lattice structure resulting in selective Bragg reflections of light and consequent vivid structural colors. However, the practical realization of these material systems is hampered by their narrow thermal stability and multi-domain crystalline nature. This feature article provides an overview of the efforts devoted to stabilizing these phases and creating monodomain structures. In particular, it delves into the complex relationship between geometrical confinement, induced curvature, and the structural stability and photonic features of BPs. Understanding the interaction of curved confinement and structural stability of BPs proves crucially important for the integration of these materials into flexible and miniaturized devices. By shedding light on these critical aspects, this feature review aims to highlight the significance of understanding the coupling effects of physical and mechanical forces on the structural stability of these systems, which can pave the way for the development of efficient and practical devices based on BP liquid crystals.

Constructing a cellulose based chiral liquid crystal film with high flexibility, water resistance, and optical property

Cellulose nanocrystal (CNC) derived from cellulose can form a liquid crystal film with bright structural color by evaporative-induced self-assembly (EISA). As a new class of photonic liquid crystals material, it has attracted much attention because of its intrinsic unique structural characteristics and excellent optical properties. However, the brittleness and water sensitivity of CNC film have hindered its practical application. Herein, multiple cross-linked networks CNC/(polyethylene glycol diacrylate:polyethylene oxide) (PEGDA:PEO) composite film was prepared through EISA and UV irradiation strategies. The as-prepared film exhibits high-flexibility with a fracture strain of up to 36.40 % and strong water resistance, with water absorption at an equilibrium of only 17.41 %. Moreover, the film retains its structural color in aqueous solution for a long time due to its water stability. The outstanding flexibility and water resistance of CNC composite film are attributed to multiple crosslinked networks (i.e. PEGDA, PEO, and PEDGA-PEO networks), which endow the film with excellent stress dispersion and transferability when stretched and limit film swelling in water without affecting chiral nematic structures of CNC. Overall, this work provides a promising strategy to prepare CNC-based film with high-flexibility, water resistance, and optical property for applications like decoration, sensor, and anti-counterfeiting.

Accelerating Cellulose Nanocrystal Assembly into Chiral Nanostructures

Cellulose nanocrystal (CNC) suspensions self-assembled into chiral nematic liquid crystals. This property has enabled the development of versatile optical materials with fascinating properties. Nevertheless, the scale-up production and commercial success of chiral nematic CNC superstructures face significant challenges. Fabrication of chiral nematic CNC nanostructures suffers from a ubiquitous pernicious trade-off between uniform chiral nematic structure and rapid self-assembly. Specifically, the chiral nematic assembly of CNCs is a time-consuming, spontaneous process that involves the organization of particles into ordered nanostructures as the solvent evaporates. This review is driven by the interest in accelerating chiral nematic CNC assembly and promoting a long-range oriented chiral nematic CNC superstructure. To start this review, the chirality origins of CNC and CNC aggregates are analyzed. This is followed by a summary of the recent advances in stimuli-accelerated chiral nematic CNC self-assembly procedures, including evaporation-induced self-assembly, continuous coating, vacuum-assisted self-assembly, and shear-induced CNC assembly under confinement. In particular, stimuli-induced unwinding, alignment, and relaxation of chiral nematic structures were highlighted, offering a significant link between the accelerated assembly approaches and uniform chiral nematic nanostructures. Ultimately, future opportunities and challenges for rapid chiral nematic CNC assembly are discussed for more innovative and exciting applications.

Nacre-inspired layered composite gels with broad tunable mechanical strength and structural color for stress visualization

The brick-and-mortar architecture of nacre shells brings radiant structural colors, high toughness, and strength, inspiring numerous designs for structural and optical materials. However, constructing structural color is not always easy, especially among soft materials where aligning components against random and dynamically active environments is generally difficult. Here, we propose a composite organohydrogel capable of visualizing multiple levels of stress, featuring broad tunable mechanical properties, dynamic mechanochromism, deep low working temperatures, and anti-drying attributes. In the composite gels, the intercalation between α-zirconium phosphate (α-ZrP) nanoplates and poly-(diacetone acrylamide-co-acrylamide) is induced by shear-orientation-assisted self-assembly followed by solvent replacement. The highly tailorable (from ∼780 nm to ∼445 nm) range of colors was achieved by regulating the concentration of α-ZrP and glycerol inside the matrix. With the help from glycerol, the composite gels exhibited long-term stability (7 d) in the arid condition and remarkable low-temperature tolerance (-80 °C). The extraordinary mechanical property (compressive strength up to 119 MPa) of composite gels is achieved by the assembled α-ZrP plates with a small aspect ratio, high negative charge repulsion, and abundant hydrogen bonding sites. As a result, the mechanochromic sensor based on the composite gel enjoys a wide range of stress detection (0-1862 KPa). This study provides a new strategy for constructing high strength structural-colored gels, opening up opportunities for sensitive yet strong mechanochromic sensors in extreme environments.

Fabrication of Advanced Cellulosic Triboelectric Materials via Dielectric Modulation

The rapid rise of triboelectric nanogenerators (TENGs), which are emerging energy conversion devices in advanced electronics and wearable sensing systems, has elevated the interest in high-performance and multifunctional triboelectric materials. Among them, cellulosic materials, affording high efficiency, biodegradability, and customizability, are becoming a new front-runner. The inherently low dielectric constant limits the increase in the surface charge density. However, owing to its unique structure and excellent processability, cellulose shows great potential for dielectric modulation, providing a strong impetus for its advanced applications in the era of Internet of Things and artificial intelligence. This review aims to provide comprehensive insights into the fabrication of dielectric-enhanced cellulosic triboelectric materials via dielectric modulation. The exceptional advantages and research progress in cellulosic materials are highlighted. The effects of the dielectric constant, polarization, and percolation threshold on the charge density are systematically investigated, providing a theoretical basis for cellulose dielectric modulation. Typical dielectric characterization methods are introduced, and their technical characteristics are analyzed. Furthermore, the performance enhancements of cellulosic triboelectric materials endowed by dielectric modulation, including more efficient energy harvesting, high-performance wearable electronics, and impedance matching via material strategies, are introduced. Finally, the challenges and future opportunities for cellulose dielectric modulation are summarized.

Multimodal-Responsive Circularly Polarized Luminescence Security Materials

Nations, industries, and aspects of everyday life have undergone forgery and counterfeiting ever since the emergence of commercialization. Securing documents and products with anticounterfeit additives shows promise for authentication, allowing one to combat ever-increasing global counterfeiting. One most-used effective encryption strategy is to combine with optical-security markers on the required protection objects; however, state-of-the-art labels still suffer from imitation due to their poor complexity and easy forecasting, as a result of deterministic production. Developing advanced anticounterfeiting tags with unusual optical characters and further incorporating complex security features are desired to achieve multimodal, unbreakable authentication capacity; unfortunately, this has not yet been achieved. Here, we prepare a series of stable circularly polarized luminescence (CPL) materials, composed of toxicity-free, high-quality-emitting inorganic quantum dots (QDs) and liquid crystals, using a designed helical-coassembly strategy. This CPL system achieves a figure of merit (FM, assessing the performance of both luminescence dissymmetry and quantum yield) value of 0.39, fulfilling practical demands for anticounterfeiting applications. Based on these CPL structures, we produce a type of multimodal-responsive security materials (MRSMs) that exhibits six different stimuli-responsive modes, including light activation, polarization, temperature, voltage, pressure, and view angle. Thus, we show a proof-of-principle blockchain-like integrated anticounterfeiting system, allowing multimodal-responsive, interactive/changeable information encryption-decryption. We further encapsulate the obtained security materials into a fiber to expand our materials to work on flexible fabrics, that is, building an intelligent textile with a color-adaptable function along with environmental change.

Luminescence regulation of lanthanide-doped nanorods in chiral photonic cellulose nanocrystal films

A new design for chiral photonic cellulose nanocrystal films was developed by co-assembling lanthanide-doped nanorods (NRs) into chiral cellulose nanocrystals, in which the photonic band gap (PBG) could be tuned in the visible range by changing the mass fraction of flexible agents, such as polyvinyl alcohol (PVA) and ethylene glycol (EG). Due to the PBG effect, the luminescence modulation in such nanocrystal films had been realized. The down-conversion luminescence from NaGd₃₀Y₆₀F₄:5%Tb³⁺, 5%Eu³⁺ NRs and up-conversion luminescence from NaGd₄₀Y₄₀F₄:18%Yb³⁺, 2%Er³⁺ NRs could be enhanced by 28 % and 18 % respectively, on account of the band edge effect. The luminescence would be inhibited when the luminescence overlapped with the stop band of the PBG. These results implied that the biocompatible photonic cellulose nanocrystal films are ideally suited for applications in optical coding, optical resonators and biocompatible lasers.

Cellulose Nanocrystal Films with NIR-II Circularly Polarized Light for Cancer Detection Applications

Near-infrared circularly polarized light is attractive for wide-ranging applications. However, high-performance near-infrared circularly polarized light is challenging to realize. Here, we show that left-handed chiral photonic cellulose nanocrystal (CNC) films produced from ultrasonicated suspensions enable right-handed circularly polarized luminescence with a dissymmetry factor of -0.330 in the second near-infrared window (NIR-II). We present a theoretical analysis of the adverse effect of structural defects and luminescence intensity heterogeneity on the right-handed circularly polarized luminescence g_lum inside the bandgap and the occurrence of left-handed circularly polarized luminescence at the band edges. We demonstrate the potential of the chiral photonic CNC films with NIR-II circularly polarized light for cancer cell discrimination. The present work identifies key scientific questions in CNC-based circularly polarized luminescence materials research.

Recent Advances in Ultrathin Chiral Metasurfaces by Twisted Stacking

Artificial chiral nanostructures have been subjected to extensive research for their unique chiroptical activities. Planarized chiral films of ultrathin thicknesses are in particular demand for easy on-chip integration and improved energy efficiency as polarization-sensitive metadevices. Recently, controlled twisted stacking of two or more layers of nanomaterials, such as 2D van der Waals materials, ultrathin films, or traditional metasurfaces, at an angle has emerged as a general strategy to introduce optical chirality into achiral solid-state systems. This method endows new degrees of freedom, e.g., the interlayer twist angle, to flexibly engineer and tune the chiroptical responses without having to change the material or the design, thus greatly facilitating the development of multifunctional metamaterials. In this review, recent exciting progress in planar chiral metasurfaces are summarized and discussed from the viewpoints of building blocks, fabrication methods, as well as circular dichroism and modulation thereof in twisted stacked nanostructures. The review further highlights the ever-growing portfolio of applications of these chiral metasurfaces, including polarization conversion, information encryption, chiral sensing, and as an engineering platform for hybrid metadevices. Finally, forward-looking prospects are provided.

Switchable Circularly Polarized Signals with High Asymmetric Factor Triggered by Dual Photonic Bandgap Structure

Currently, the smart photonic materials that can switch circularly polarized signals in real-time have attracted extensive attention due to numerous potential applications in information storage and photonics displays. However, the dynamically reversible switching of circularly polarized signals requires precise structural reconfiguration, which is rarely achieved in traditional biomaterials. Herein, a dual photonic bandgap (PBG) structure is constructed based on the optical propagation principle of cellulose-based photonic crystals, enabling the flexible switching of the intensity, wavelength, and direction of circularly polarized luminescence (CPL). By adjusting the fluorescence intensity and the matching degree of chiral structure, the asymmetric factor value of dual PBG structure is up to -1.47, far exceeding other cellulose-based materials. Importantly, it is demonstrated that dual CPL emission can be efficiently induced by two different PBGs, opening a new approach for on-demand switching of single and dual CPL emission. In addition, the dual PBG structure exhibits dual circularly polarized reflected signals under the circular polarizer, which perfectly embodies the applicability of multiple encryptions in QR codes. This work provides new insights into the real-time manipulation of circularly polarized signals by chiral photonic materials.

Chiral Liquid Crystalline Properties of Cellulose Nanocrystals: Fundamentals and Applications

By using an independent self-assembly process that is occasionally controlled by evaporation, cellulose nanocrystals (CNCs) may create films (pure or in conjunction with other materials) that have iridescent structural colors. The self-forming chiral nematic structures and environmental safety of a new class of photonic liquid crystals (LCs), referred to as CNCs and CNC-embedded materials, make them simple to make and treat. The structure of the matrix interacts with light to give structural coloring, as opposed to other dye pigments, which interact with light by adsorption and reflection. Understanding how CNC self-assembly constructs structures is vital in several fields, including physics, science, and engineering. To constructure this review, the colloidal characteristics of CNC particles and their behavior during the formation of liquid crystals and gelling were studied. Then, some of the recognized applications for these naturally occurring nanoparticles were summarized. Different factors were considered, including the CNC aspect ratio, surface chemistry, concentration, the amount of time needed to produce an anisotropic phase, and the addition of additional substances to the suspension medium. The effects of alignment and the drying process conditions on structural changes are also covered. The focus of this study however is on the optical properties of the films as well as the impact of the aforementioned factors on the final transparency, iridescent colors, and versus the overall response of these bioinspired photonic materials. Control of the examined factors was found to be necessary to produce reliable materials for optoelectronics, intelligent inks and papers, transparent flexible support for electronics, and decorative coatings and films.

A Surface Diffusion Barrier Strategy toward Water-Resistant Photonic Materials for Accurate Detection of Ethanol

Photonic materials that enable visual detection of chemicals have shown great potential for wide applications in chemical, environmental, biotechnological, and food industries, but until now, using hydrophilic photonic materials for tracing water-soluble chemicals remains a major challenge due to the strong water interference. Here, we demonstrate a two-step co-assembly and subsequent surface coating strategy to develop an ethanol-sensitive and anti-water interference photonic crystal film. By using citric acid as a co-assembly phase, high ethanol sensing is realized because of the strong intermolecular affinity. By controlling the thickness of the outer polyvinyl butyral layer, selective ethanol penetration but a water barrier is enabled. Notably, the composite photonic films are free-standing, highly flexible, and controllably structurally colored. We further present using the composite film to quantitatively trace ethanol/water mixtures and potentially track drunk driving as a colorimetric sensor. The heuristic two-step modification strategy proposed in this work not only overcomes the limitation of water interference for hydrophilic colorimetric sensors but also provides references to develop more new photonic materials with water resistance that need to be applied in water/humid environments.

Biomimetic Chiral Photonic Materials with Tunable Metallic Colorations Prepared from Chiral Melanin-like Nanorods for UV Shielding, Humidity Sensing, and Cosmetics

Many biological species combine the helical organization of cellulose or chitin microfibrils with broadband light absorption of black melanin to produce brilliant structural colors with metallic and glossy effects and other diverse functions. In this work, based on core-shell CNC@PDA chiral nanorods consisting of cellulose nanocrystals (CNCs) as the core and melanin-like polydopamine (PDA) as the shell that can form well-defined chiral liquid crystal phases, we report chiral photonic materials that closely mimic the unique coloration mechanisms and functionalities mastered by several biological species. The photonic films formed by such single CNC@PDA nanorods have brilliant iridescent structural colors originating from selective reflection of circularly polarized lights by the helical organization of CNC@PDAs across the films. Furthermore, the colors of such films have background-independent brightness, high visibility, and metallic effects that arise from the light absorption of the PDA component. Especially, the color ranges and metallic effects of the films can be conveniently tuned by varying the thickness of the PDA shell. In addition, the UV absorption and hygroscopic properties of PDA endow these CNC@PDA films with efficient broadband UV shielding and sensitive humidity-induced dynamic color changes. Due to the mussel-like superior adhesion of PDA, CNC@PDA-based photonic coatings can be formed conformably onto diverse kinds of substrates. A shiny eye shadow with viewing angle-dependent colorful patterns was used to demonstrate the potential applications. With combinations of multiple unique properties in one photonic material fabricated from a single building block, these CNC@PDA-based films are expected to have potential applications in cosmetics, UV protection, anticounterfeiting, chiral reflectors, etc.

Advanced Functional Liquid Crystals

Liquid crystals have been intensively studied as functional materials. Recently, integration of various disciplines has led to new directions in the design of functional liquid-crystalline materials in the fields of energy, water, photonics, actuation, sensing, and biotechnology. Here, recent advances in functional liquid crystals based on polymers, supramolecular complexes, gels, colloids, and inorganic-based hybrids are reviewed, from design strategies to functionalization of these materials and interfaces. New insights into liquid crystals provided by significant progress in advanced measurements and computational simulations, which enhance new design and functionalization of liquid-crystalline materials, are also discussed.

Template-assisted self-assembly of achiral plasmonic nanoparticles into chiral structures

The acquisition of strong chiroptical activity has revolutionized the field of plasmonics, granting access to novel light-matter interactions and revitalizing research on both the synthesis and application of nanostructures. Among the different mechanisms for the origin of chiroptical properties in colloidal plasmonic systems, the self-assembly of achiral nanoparticles into optically active materials offers a versatile route to control the structure-optical activity relationships of nanostructures, while simplifying the engineering of their chiral geometries. Such unconventional materials include helical structures with a precisely defined morphology, as well as large scale, deformable substrates that can leverage the potential of periodic patterns. Some promising templates with helical structural motifs like liquid crystal phases or confined block co-polymers still need efficient strategies to direct preferential handedness, whereas other templates such as silica nanohelices can be grown in an enantiomeric form. Both types of chiral structures are reviewed herein as platforms for chiral sensing: patterned substrates can readily incorporate analytes, while helical assemblies can form around structures of interest, like amyloid protein aggregates. Looking ahead, current knowledge and precedents point toward the incorporation of semiconductor emitters into plasmonic systems with chiral effects, which can lead to plasmonic-excitonic effects and the generation of circularly polarized photoluminescence.

Chiral Cellulose Nanocrystal Humidity-Responsive Iridescent Films with Glucan for Tuned Iridescence and Reinforced Mechanics

The fabrication of biomimetic photonic materials with environmental stimuli-responsive functions from entirely biobased materials is becoming increasingly challenging with the growing demand for biodegradable materials. Herein, the effect of glucan with different molecular weights on the mechanical performance and tunable structural color of iridescent CNC composite films was investigated. The existence of glucan did not influence the self-assembly performance of CNCs, but rather led to an improvement in the mechanical performance, enabling cholesteric CNC composite films with an adjustable structural color. Simultaneously, the iridescent films showed a conspicuous redshift and enlarged initial pitch without obstruction of the chiral structure. In response to environmental humidity, the structural colors of the iridescent composite films can be changed by regulating their chiral nematic structure. In particular, the films demonstrate a reversible structural color change between blue and red at RH between 50 and 98%. The resulting biobased iridescent composite films have potential applications in decorative coating, optical and humidity sensing, and anticounterfeiting.

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.