Chiral Nematic Films Research Articles

Chiral Nematic Graphene Films with a mesoporous Structure.

Ordering a crucial material like pure-graphene into the chiral nematic structure can potentially revolutionize the fields of photonics, chiral separation, and energy storage. However, the controlled fabrication of highly ordered graphene films with a long-range chiral nematic mesoporous structure is very challenging and as such, has not been achieved. Here, a chiral-template-directed chemical vapor deposition growth strategy is presented for the precise fabrication of mesoporous chiral nematic graphene films by using nanocrystalline cellulose as templates. It is evidenced that such graphene films possess the left-hand helical ordering, chiral nematic mesoporous structure with adjustable pore sizes (6.4-13.1nm), tailored pitch, excellent electrical conductivity (556 S cm-1), high specific surface area (1508 m2g-1) and mechanical flexibility. By coating Na anode with this film, uniform Na plating is achieved with no dendrite formation, resulting from its unique chiral nematic structure and tunable physicochemical properties. The films also achieved impressive thickness-dependent electromagnetic shielding, i.e., 35-63dB, demonstrating their potentially multi-disciplinary applications.

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

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

Self‐Assembled Biconvex Microlens Array Using Chiral Ferroelectric Nematic Liquid Crystals

AbstractRecently, it is shown (Popov et al, Sci. Rep, 2017, 7, 1603) that chiral nematic liquid crystal films adopt biconvex lens shapes underwater, which may explain the formation of insect eyes, but restrict their practical application. Here it is demonstrated that chiral ferroelectric nematic liquid crystals, where the ferroelectric polarization aligns parallel to the air interface, can spontaneously form biconvex lens arrays in air when suspended in submillimeter‐size grids. Using Digital Holographic Microscopy, it is shown that the lens has a paraboloid shape and the curvature radius at the center decreases with increasing chiral dopant concentration, i.e., with decreasing helical pitch. Simultaneous measurements of the imaging properties of the lenses show the focal length depends on the pitch, thus offering tunability. The physical mechanism of formation of the self‐assembled ferroelectric nematic microlenses is also discussed.

Effect of polymeric spherulite coatings on optical properties of chiral nematic cellulose nanocrystal films

AbstractCellulose nanocrystals (CNCs) have received an abundance of attention because of their distinctive chiral nematic structure and exceptional optical characteristics. However, the use of free‐standing films is constrained by the inherent stiffness and brittleness. In this work, CNC films with robust mechanical properties are prepared by simply coating them with polymeric spherulites. Maleic anhydride‐grafted polypropylene (PP‐g‐MA) is used to coat CNC films. The optical properties of coated films were also analyzed using polarized optical microscope. It is observed that the circular extinction patterns for the coated CNC film were created by the radial symmetry of banded spherulites, which were responsible for the perception of the optical transmission bands of the CNC film. Interfacial anchoring plays a crucial role in enhancing the overall performance and properties of the composite system. The future development of numerous portable functional components needing improved optical, mechanical, and thermal qualities of CNC films is made possible by this protective coating approach.

All‐Cellulose‐Based Flexible Photonic Films

AbstractCellulose nanocrystals (CNCs) are environmentally friendly building blocks used in the preparation of chiral nematic iridescent films through evaporation‐induced self‐assembly. However, the brittleness of CNC films limits their versatility, especially in applications that require mechanical flexibility. In this study, it is innovatively engineered flexible colored CNC films entirely composed of cellulose by introducing cross‐linked chemically modified cellulose nanofibers (CNFs) as a supporting network. By reducing the surface charge content of CNFs and anchoring them in a network, the disruption to the self‐assembly process of CNCs caused by CNFs is minimized. This effectively preserves the structural colors of CNCs films. Furthermore, the CNFs network enhances the tensile strength and elongation of these all‐cellulose composite films, enabling them to be folded with excellent flexibility while still maintaining a high Young's modulus. Vibrant red, green, and blue‐colored all‐cellulose photonic films can also be achieved by simply adjusting the sonication energy input on the suspension of CNCs. The all‐cellulose‐based flexible films have the potential to be applied as an eco‐friendly and sustainable photonic material when flexibility is essential, for instance, in applications like flexible sensors, anti‐counterfeit labels, or foldable display components.

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 24h 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.

A cellulose nanocrystal-based dual response of photonic colors and fluorescence for sensitive benzene gas detection

Benzene, as a common volatile organic compound, represents serious risk to human health and environment even at low level concentration. There is an urgent concern on visualized, sensitive and real time detection of benzene gases. Herein, by doping Fe3+ and graphene quantum dots (GQDs), a cellulose nanocrystal (CNC) chiral nematic film was designed with dual response of photonic colors and fluorescence to benzene gas. The chiral nematic CNC/Fe/GQDs film could respond to benzene gas changes by reversible motion. Moreover, chiral nematic film also displays reversible responsive to humidity changes. The resulting CNC/Fe/GQDs chiral nematic film showed excellent response performance at benzene gas concentrations of 0–250 mg/m3. The maximal reflection wavelength film red shifted from 576 to 625 nm. Furthermore, structural color of CNC/Fe/GQDs chiral nematic film change at 44 %, 54 %, 76 %, 87 %, and 99 % relative humidity. Interestingly, due to the stability of GQDs to water molecules, CNC/Fe/GQDs chiral nematic film exhibit fluorescence response to benzene gas even in high humidity (RH = 99 %) environment. Besides, we further developed a smartphone-based response network system for quantitively determinization and signal transformation. This work provides a promising routine to realize a new benzene gas response regime and promotes the development of real-time benzene gas detection.

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

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

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.

Chiral nematic cellulose nanocrystal composites: An organized review

AbstractCellulose nanocrystals (CNCs) are commercially available materials derived from cellulose, the most abundant biopolymer on our planet. Due largely to their high strength, high surface area‐to‐volume ratio, tailorable surface chemistry, and the abundance of biomass feedstocks with which to produce them, CNCs have attracted significant interest in applications spanning the paints and coatings, composites, packaging, and biomedical sectors. However, and perhaps most interestingly, CNCs will self‐assemble (or, as I've teased in the title, organize) to form highly ordered chiral nematic liquid crystal phases when concentrated in suspension. Upon complete solvent evaporation, this chiral nematic order is ‘locked’, yielding films with structural colour—colour arising not due to chemical pigments, but rather due to the physical structure of a material itself. In the pursuit of novel multi‐functional materials, research interest has shifted recently towards the incorporation of functional additives to form composite chiral nematic films. Along with introducing the basics of liquid crystals and self‐assembly, this review discusses the main approaches used in order to form CNC‐based composite films: co‐assembly, templating, and post‐processing, and highlights exceptional examples in each case. Finally, I give my uniquely Canadian perspective on the current status, future prospects, and major challenges associated with the development of CNC‐based chiral nematic composite materials.

Effects of Ferric Ions on Cellulose Nanocrystalline-Based Chiral Nematic Film and Its Applications.

Chiral nematic materials have been attracting attention in fields of advanced functional applications due to their unique iridescent colors and tunable helical structure. A precisely decreased pitch is of importance for construction and applications of chiral nematic materials; however, it remains a huge challenge. Herein, cellulose nanocrystal (CNC) is selected as a constructed matrix for chiral nematic films, and ferric chloride (FeCl3) is used as a modification agent. We investigate the effects of the ferric ion loads on the helical structure and optical characteristics of iridescent film. Subsequently, the influence of ferric ions on the assembly process of CNC liquid crystal and the regulation of the structure color of self-assembled monolayers are discussed. Therefore, the CNC/FeCl3 chiral nematic films showed a blueshifted structural color from orange to blue, which highlights a simple route to achieve the regulation of decreased pitch. Further, we have applied this CNC/FeCl3 chiral nematic film for benzene gas detection. The sensing performance shows that the CNC/FeCl3 chiral nematic film reacts to benzene gas, which can be merged into the nematic layer of the CNC and trigger the iron ions chelated on the CNC, consequently arousing the redshift of the reflected wavelength and the effective colorimetric transition. This CNC/FeCl3 chiral nematic film is anticipated to boost a new gas sensing mechanism for faster and more effective in-situ qualitative investigations.

Controlling the optical properties of chiral nematic mesoporous organosilica films with bioadditives.

Chiral nematic mesoporous organosilica (CNMO) films have unique iridescent properties that make them attractive candidates for decorations, sensing and photonics. However, it has proven difficult to control the colour and porosity of CNMO films. Here, we have explored the addition of a range of biodegradable and eco-friendly additives to tune the helical pitch and, hence, the colour of the CNMO materials. It was found that the controlled integration of additives allows for the colour of the materials to be tuned across the visible spectrum, but cannot be used to tune the porosity of the films. This work opens up new prospects for preparation of CNMO materials with adjustable optical properties.

Amplified Photoluminescence of CsPbX3 Perovskites Confined in Silica Film with a Chiral Nematic Structure

AbstractMetal halide perovskites (MHPs, CsPbX3: X = Cl, Br, I) have advanced the field of optoelectronic devices due to their remarkable light‐emitting capabilities, stemming from the large overlap between their emission and absorption spectra, offering the possibility to reabsorb their own emitted photons. Herein, a straightforward method is reported to confine CsPbBr3 into mesoporous silica films with a chiral nematic structure, allowing the amplification of the photoluminescence (PL). The simple room temperature ligand‐free synthesis allows facile growth of CsPbBr3 in silica photonic films, in which the Bragg peak position can be tuned from the UV to the visible range. The perovskite/silica films demonstrate a remarkable improvement in PL intensity and lifetime compared to the as‐synthesized non‐confined perovskite nanocrystals (NCs) due to the overlap of the Bragg peak position of the chiral nematic photonic films and CsPbBr3 absorption band. Such a PL enhancement stems from the slow photon effect induced at blue and red Bragg peak edges that facilitates the photon recycling of the emitted photons. This innovative approach offers a new way to fabricate highly emissive and long‐lived photoluminescent films at ambient conditions, potentially advancing perovskite utilization in light‐emitting devices.

Mechanical responsive visible structural colors based on chiral-nematic cellulose nanocrystals photonic hydrogels

It has been extensively reported that cellulose nanocrystals (CNCs) are capable of exhibiting structural colors due to their unique chiral nematic self-assembly. However, further investigation is required to effectively utilize these remarkable properties for addressing real-world challenges, such as development of advanced materials like mechanical sensors and smart wearable devices. One particular challenge has been maintaining the refractive properties of structural colors resulting from the CNCs structure within the visible spectrum. In this study, we introduce a one-step hydrogel fabrication method involving chiral nematic CNC films embedded within poly(acrylic acid). These hydrogels exhibit naked-eye-visible structural colors and exhibit responsiveness to mechanical stresses. The resulting photonic hydrogels exhibit a blue shift in response to mechanical stretching. The hydrogels demonstrated self-recovery capabilities, as observed through cyclic loading–unloading stress tests. The color of the materials was characterized using UV–Vis spectroscopy to determine the wavelength of the reflected light. Additionally, we investigated the mechanism of CNCs orientation change using 2D-X-ray diffraction. Demonstrated by our simulation study, the morphology of the CNC bundles significantly influenced the characteristics of the chiral nematic structure as they orient due to the force application. Notably, the color change of the photonic hydrogels was reversible within the optimal range of applied force and can be repeated numerous times. This work presents a new strategy for fabricating optically functional photonic hydrogels with stimuli-responsive properties, enabling their potential applications in biomedical engineering and wearable responsive sensor technologies.

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.

Excellent optical, flexible, and mechanical properties of chiral cellulose nanocrystal films for structural coloration and fluorescent anti-counterfeiting applications

Cellulose nanocrystal (CNC)-based chiral nematic films have recently received attention due to their unique photophysical properties. However, their fragility and single structural color mode (birefringence) restricts their further development. Herein, a flexible optoelectronic, chiral, CNC films was reported composed of carbon quantum dots (CQDs) and CNCs produced by evaporation-induced self-assembly (EISA). Due to weak interactions between CQDs and CNCs, which limit molecular sliding, the optimal tensile strength and curvature reached 65.5 MPa and 2 × 102, respectively. In addition, these products retained not only the structural color of cellulose chiral films in the birefringent mode, but also introduced fluorescence in the CQD ultraviolet excitation mode. Related work also addressed the issue of cellulosic chiral film fragility and provided a new method for their production.

Mechanochromic and Conductive Chiral Nematic Nanostructured Film for Bioinspired Ionic Skins.

Chameleon skin is naturally adaptive and can sense environmental changes and transform sensing into bioelectrical and optical signals by manipulating ion transduction and photonic nanostructures. The increasing interest in mimicking biological skins has considerably promoted the development of advanced photonic materials with an increasing ionic conductivity. Herein, we report the judicious design and fabrication of a bioinspired mechanochromic chiral nematic nanostructured film with good ionic conductivity by infiltrating fluorine-rich ionic liquids (FILs) into a swollen self-assembled cellulose nanocrystal (CNC) film with helical nanoarchitectures. Notably, the introduction of 2-hydroxyethyl acrylate considerably enhances the compatibility of hydrophobic FILs and hydrophilic CNCs. The resulting FIL-CNC nanostructured films exhibited excellent mechanochromism, good ionic conductivity, and outstanding optical/electrical dual-signal sensing performance when used as a bioinspired ionic skin for real-time monitoring of human motions. Owing to the integration of FILs, the underwater stability of the chiral liquid crystal nanostructures of CNCs was significantly enhanced. Notably, underwater contact/contactless sensing modes and encrypted information transmission have been achieved with the FIL-CNC nanostructured film. This study can offer great insights for the advancement of biomimetic multifunctional artificial skins and emerging interactive devices, which can find important applications in wearable iontronics, human-machine interactions, and intelligent robots.

All bio-based chiral nematic cellulose nanocrystals films under supramolecular tuning by chitosan/deacetylated chitin nanofibers for reversible multi-response and sensor application

Application of green and renewable all bio-based resources on functional nanomaterials has been of much interest, but often limited by their complex supramolecular construction. Herein, two kinds of most abundant biopolymers on earth, cellulose and chitin were selected as raw materials to prepare all bio-based iridescent films. Cellulose nanocrystals (CNCs) were cooperated with chitosan (CS) or deacetylated chitin nanofibers (DChN) at various content ratios to fabricate chiral nematic films via evaporation-induced self-assembly (EISA). The iridescent films showed increased crystallinity (80%∼ 90%) than pristine CNCs as well as lowered crystallite sizes, due to hydrogen bonding linkages and hydrophobic interactions between CNCs and amphiphilic CS/DChN with diverse degrees of acetylation. Obviously enlarged helical pitches were achieved by intercalation of CS/DChN, which reflected optical light with red-shifted wavelength bands. CS and DChN also improved the hydrophobicity of the nanocomposite films with various degrees, but both maintaining good biocompatibility proved by cell viability assay. The films showed wide-range/fast humidity and pH stimuli response capabilities via visible colorimetric changes. Moreover, the materials were equipped with satisfying reversible response, which was confirmed by repeated water ink writing-drying and acidic/alkaline vapor evaporation tests for 10 cycles. A food freshness sensor device was assembled on quick response (QR) code for sensitive and convenient recognition by smart phone, exhibiting high potential on multi-functional sensors of the all bio-based sustainable materials.

Coassembly of Cellulose Nanocrystals and Neutral Polymers in Iridescent Chiral Nematic Films.

Photonic materials based on composite films of cellulose nanocrystals (CNCs) and polymers are promising as they can be renewable and show tunable optical and mechanical properties. However, the influence of polymers on CNC self-assembly is not always well understood, and conflicting results are present in the literature. In this study, we incorporate three neutral, water-soluble polymers-poly(ethylene glycol) (PEG), poly(vinyl pyrrolidone) (PVP), and poly(acrylic acid) (PAA)-with different molecular weights into CNC suspensions at various concentrations prior to obtaining iridescent composite thin films by solvent evaporation. Through spectroscopic, potentiometric, and rheological analyses, we find that PVP physically adsorbs to the surface of CNCs resulting in a bathochromic shift in film color with both increasing concentration and polymer molecular weight. In contrast, PEG induces depletion interactions that result in a decrease in the size of chiral nematic CNC domains, with a negligible change in film color. Finally, PAA hydrogen bonds to the hydroxyl groups of CNCs, resulting in a bathochromic color shift along with interesting rheological and liquid-state properties. This work demonstrates a deeper understanding of CNC-polymer interactions during coassembly and formation of iridescent chiral nematic films, allowing for greater control over optical properties of future CNC-based materials.

Design of experiments to investigate multi-additive cellulose nanocrystal films.

Cellulose nanocrystal (CNC) suspensions can self-assemble into chiral nematic films upon the slow evaporation of water. These films are brittle, as indicated by their fracturing instead of plastically deforming once they are fully elastically deformed. This aspect can be mediated to some extent by plasticizing additives, such as glucose and glycerol, however, few reports consider more than one additive at a time or address the influence of additive content on the homogeneity of the self-assembled structure. In this work, design of experiments (DoE) was used to empirically model complex film compositions, attempting to relate additive concentrations in dilute suspension to film properties, and to understand whether outcome specific predictions are possible using this approach. We demonstrate that DoE can be used to predict film properties in multi-additive systems, without consideration given to the different phenomena that occur along the drying process or to the nature of the additives. Additionally, a homogeneity metric is introduced in relation to chiral nematic organization in CNC films, with most of the additive-containing compositions in this work found to reduce the homogeneity of the self-assembly relative to pure CNC films.