Chiral Nematic Films Research Articles

Structural Color Controllable Humidity Response Chiral Nematic Cellulose Nanocrystalline Film.

Through self-assembly, environmentally friendly cellulose nanocrystals (CNCs) can form films with a photonic crystal structure whose pitch size can be adjusted in a variety of ways at the fabrication stage. Moreover, the films exhibit response performance to multiple stimuli, which offers extensive applications. Poly(ethylene glycol) (PEG) and CNCs combine to form a smaller chiral nematic domain that develops a solid film with a uniform spiral structure when slowly dried. By changing the composition of CNCs and PEG, flexible and flat photonic composite films with uniform structural colors from blue to red are prepared. Benefiting from the change in pitch size by insertion and detachment of water molecules into the chiral nematic structure, CNCs films and CNC-PEG composite films exhibit a reversible structural color change in response to different humidity. In addition, the chiral nematic films formed by the combination of glycerol and CNCs have a reversible stimulation response to hydrochloric acid gas. Similarly, adjusting the ratio of glycerol can control the pitch size of the films and, thus, the reflective color. In summary, the pitch size of the photonic crystal structure of the films can be precisely tuned by regulating the additive ratio, and the two prepared films have reversible responses to humidity and hydrochloric acid gas, respectively. The CNC-based films show promise in the application of colorimetric biosensors.

Cellulose nanocrystal and β-cyclodextrin chiral nematic composite films as selective sensor for methanol discrimination

Due to the serious threat of methanol to human health, the convenient, quick and specific detection of methanol is of great importance. Therefore, in this study, a biomass-derived chiral nematic composite film is fabricated by the co-evaporation of cellulose nanocrystals (CNC) and water-soluble polymer of β-cyclodextrin (PCD). The freestanding iridescent CNC-PCD film shows distinguishing structural colors of red and yellow green for methanol and ethanol respectively, realizing the identification of the two homologous alcohols by naked eyes. Besides, the quick, reversible and quantitative colorimetric sensing of the CNC-PCD film to methanol is verified. As its structural color and maximum reflection wavelength redshift to the greatest extent for methanol than other alcohols, the CNC-PCD film exhibits the specificity and selectivity for methanol detection in both single and mixed solvents. Such less-consumed and easy-to-handle CNC-PCD film may be useful as a colorimetric sensor to detect or discriminate methanol in some industrial products.

Electrically tunable collective motion of dissipative solitons in chiral nematic films

From the motion of fish and birds, to migrating herds of ungulates, collective motion has attracted people for centuries. Active soft matter exhibits a plethora of emergent dynamic behaviors that mimic those of biological systems. Here we introduce an active system composed of dynamic dissipative solitons, i.e. directrons, which mimics the collective motion of living systems. Although the directrons are inanimate, artificial particle-like solitonic field configurations, they locally align their motions like their biological counterparts. Driven by external electric fields, hundreds of directrons are generated in a chiral nematic film. They start with random motions but self-organize into flocks and synchronize their motions. The directron flocks exhibit rich dynamic behaviors and induce population density fluctuations far larger than those in thermal equilibrium systems. They exhibit “turbulent” swimming patterns manifested by transient vortices and jets. They even distinguish topological defects, heading towards defects of positive topological strength and avoiding negative ones.

Light-Induced Structures and Microparticle Transportation in a Free-Surface Frustrated Chiral Nematic Film

Local illumination with a light beam leads to thermo-orientational processes in a frustrated chiral nematic film with a free surface. Light-induced hydrodynamic flow and orientational structure create an adaptive platform for the collection, translation and rotation of suspended spherical microparticles. The demonstrated approach has potential applications in soft robotics, micro-object delivery systems, and other micro- and nanotechnologies.

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.

Sensitive chemoselectivity of cellulose nanocrystal films

Cellulose nanocrystals (CNCs), self-assembled into a chiral nematic structure film, create an advanced platform for the fabrication of remarkable sensing, photonic and chiral nematic materials. Despite extensive progress in the knowledge of functions of CNCs, their chemoselectivity has rarely been reported. Here, we report a brand-new perspective of CNCs in chemoselectivity, which shows sensitive selectivity even between isomers of monosaccharides and disaccharides by generating discernible crystal patterns. This sensitive selectivity of glucose homologs is attributed to the selective carbohydrate–carbohydrate interactions (CCIs) through generating hydrogen bonds between CNC units and the glucose homologs, endowing a remarkable color variation of the CNC films. Moreover, the CCIs are distinct for different immersion times and concentrations of glucose homologs, verified through Fourier Transform Infrared Spectroscopy (FTIR) spectra. Based on the sensitive CCIs, pristine CNC films are then used as templates to generate chiral mesoporous carbon films with tunable specific surface areas by assembly of CNC suspensions and the glucose homologs. We envision that the sensitive chemoselectivity of CNC films as well as precise structure modulation could provide insights into the recognition of carbohydrates and the preparation of mesoporous carbon in numerous practical applications. A pristine chiral nematic cellulose nanocrystal film with sensitive chemoselectivity to glucose homologues and even isomers of monosaccharides and disaccharide are prepared through self-assembly. For different kinds of glucose homologs, distinct crystal morphologies are formed due to the selective carbohydrate–carbohydrate interactions. Besides, the highly ordered left-helical layered structure of pristine cellulose nanocrystal film could be used as templates for the precise construction of delicate nanostructures.

Carboxylated cellulose nanocrystal films with tunable chiroptical properties

Carboxylated cellulose nanocrystals prepared by TEMPO-mediated oxidation exhibit a distinct ability to form nematic order, however, their ability to form chiral nematic films remains relatively unexplored. In this study, bleached cotton pulp hydrolyzed with hydrochloric acid and oxidized by TEMPO-mediated oxidation produce carboxylated cellulose nanocrystals with different aspect ratios 33.1, 32.8, 30.9, 29.0 and 28.9, and surface charge densities 0.16, 0.56, 1.00, 1.25, and 1.42 e·nm−2. By tuning the aspect ratio and surface charge density, the optimal carboxylated cellulose nanocrystals producing left-handed chiral nematic films by evaporation-induced self-assembly are obtained. The left-handed chiral nematic films enable selective reflection of left-handed circularly polarized light with the peak wavelength tunable from the visible to the near-infrared regime by modifying the characteristics of nanorods and suspensions. Such carboxylated cellulose nanocrystal films transform spontaneous luminescence to right-handed circularly polarized luminescence with the peak luminescence dissymmetry factor of −0.51.

One-Pot Synthesis of Magnetoplasmonic Au@FexOy Nanowires: Bioinspired Bouligand Chiral Stack.

One-dimensional hybrid nanostructures composed of a plasmonic gold nanowire core covered by a shell of magnetic oxide nanoparticles (Au@FexOy NWs) were synthesized by a one-pot solvothermal synthesis process. The effects of reaction temperature, time, reducing agent, and precursor as well as postsynthesis treatment were optimized to produce highly uniform NWs with a diameter of 226 ± 25 nm and a plasmonic core aspect ratio of 25 to 82. By exploiting the interaction of NWs with an external magnetic field, precise arrangements into highly periodic photonic structures were achieved, which can generate distinctive structural colors that are vividly iridescent and polarization-sensitive. Furthermore, a Bouligand-type chiral nematic film consisting of multistacked unidirectional layers of achiral NWs was fabricated using a modified layer-by-layer deposition method, which displays circular dichroism (CD) and chiral sensing capability. The addition of bovine serum albumin (BSA) as a model protein analyte induced a concentration-dependent wavelength shift of CD peaks. These intriguing properties of magnetoplasmonic anisotropic NWs and their self-assemblies could be consequently valuable for developing nature-inspired structural color imprints as well as solid-state chiral sensing devices.

Homeotropic Concentric Helix Orientations in Chiral Nematic Cellulose Nanocrystal Films by Local Magnetic Fields

AbstractMany fascinating device applications of chiral nematic materials are determined by their helical axis orientations. Cellulose nanocrystals (CNCs) self‐assemble producing left‐handed chiral nematic films with predominant planar textures. While varied techniques have been exploited to tune the helical axis orientations, it remains challenging to produce concentrically aligned chiral nematic state. Herein, the authors report novel findings that left‐handed chiral nematic CNC films formed with homeotropic concentric helix orientations in the planar textures can be realized by local radial magnetic field‐directed self‐assembly of CNCs. These chiral nematic CNC films display left‐handed circular polarization patterns and transform spontaneous luminescence to right‐handed circularly polarized luminescence on both film plane and lateral surfaces owing to the underlying photonic bandgaps. These plane‐lateral chiroptical properties can be tuned simultaneously from the visible to the near‐infrared spectral regime. Their work presents a step towards the development of self‐organized chiroptical materials encoded with complex polarization features for photonic applications.

On‐Demand Circularly Polarized Room‐Temperature Phosphorescence in Chiral Nematic Nanoporous Silica Films

AbstractCircularly polarized room‐temperature phosphorescence (CPRTP) is of paramount importance for applications in information encoding, bioimaging, and optoelectronic devices, however, it remains challenging to organize highly efficient CPRTP materials with controlled handedness and long afterglow. Herein, as an outstanding novel finding, it is diclosed that ultralong CPRTP with on‐demand chiroptical properties is simple to realize using the earth‐abundant cellulose and silica. It is shown that left‐handed chiral nematic nanoporous silica films featuring defect photoluminescence enable ultralong CPRTP with an unprecedented control of the handedness. This work presents that the chiral nematic nanoporous silica films display right‐handed CPRTP and left‐handed CPRTP with the phosphorescence dissymmetry factors up to −0.130 and 0.093 as well as the afterglow lifetimes of 1.094 and 0.949 s, respectively. The potential of the transparent chiral nematic nanoporous silica films, capable of ultralong CPRTP and selective reflection of left‐handed circularly polarized light, for the development of anticounterfeiting optical labels is showcased. This work presents a versatile strategy and a step toward the development of ultralong CPRTP materials with on‐demand chiroptical properties from readily available defect phosphors for photonic applications.

Glass-Forming Chiral Liquid Crystalline Dimers Based on an Oligo(phenylenevinylene) Unit Exhibiting Circularly Polarized Photoluminescence

Oligo(p-phenylenevinylene)-based chiral nematic liquid crystals were synthesized. Mixtures of dimeric and monomeric materials exhibited a glassy chiral nematic phase at room temperature, and the reflection band was tunable from near UV to near IR areas by the temperature and mixing ratio of the chiral nematic materials. The reflection band could be tuned to cover the emission band of the oligo(p-phenylenevinylene) chromophores completely, resulting in the circularly polarized fluorescence with the dissymmetric factor exceeding 1.4. The reflection band could be immobilized by cooling film samples of the mixtures in ice water. The dissymmetric factor of the glassy chiral nematic films exceeded 1.3 at room temperature. From the film samples of the S-form of the chiral nematic liquid crystals emitted left-handed circularly polarized light while right-handed circularly polarized light was enhanced at the edge of the reflection band due to the Purcell effect.

Annealing and melting of active two-dimensional soliton lattices in chiral nematic films.

In this work, thousands of electrically driven dissipative solitons, called directrons, are generated in a chiral nematic liquid crystal. The directrons start with random motions but soon synchronize their motions and self-organize into a two-dimensional hexagonal lattice. The directron lattice moves collectively and forms a hexatic phase. By increasing the applied voltage, the lattice exhibits a first-order hexatic-to-liquid phase transition.

Tunable Circularly Polarized Luminescence of Excited‐State‐Proton‐Transfer‐Based Chiral Guanidine

Circularly polarized luminescence (CPL) of chiral materials responsive to external stimuli has drawn widespread attention due to their important applications in smart photonic materials. Here, a CPL‐active chiral emitter derived from guanidine‐substituted 1,8‐naphthalimide (R/S‐1) is reported, which could emit tunable circularly polarized light through regulating the situation of protonation/deprotonation in aqueous, as well as the solvent polarity. Tunable fluorescence emission from the protonated compound can be observed, which is contributed to excited‐state proton transfer of guanidine moiety, as well as the intramolecular charge transfer of naphthalimide group. Subsequently, color‐tunable CPL is obtained, enabling a single‐molecule‐white‐color circularly polarized light emission. Additionally, by introducing R/S‐1 into chiral nematic liquid crystal film, CPL emission is adjusted by exposing to acid/alkaline environment, which can be directly distinguished by naked eyes due to the extremely large dissymmetry factor (|glum| = 1.1). This work opens new venues for photonic applications of CPL active materials.

Chiral Nematic Cellulose Nanocrystal Films Cooperated with Amino Acids for Tunable Optical Properties.

The exploration of functional materials relies greatly on the understanding of material structures and nanotechnologies. In the present work, chiral nematic cellulose nanocrystal (CNC) films were prepared by incorporation with four types of amino acids (AAs, glycine, histidine, phenylalanine, and serine) via evaporation-induced self-assembly. The films present ideal iridescence and birefringence that can be tuned by the amount of AAs added. The intercalation of AAs enlarged the pitch values, contributing to the red-shift trend of the reflective wavelength. Among the AAs, serine presented the most compatible intercalation into cellulose crystals. Interestingly, histidine and phenylalanine composite films showed high shielding capabilities of UV light in diverse wavelength regions, exhibiting multi-optical functions. The sustainable preparation of chiral nematic CNC films may provide new strategies for materials production from biocompatible lignocellulose.

Monolithic Chiral Nematic Organization of Cellulose Nanocrystals under Capillary Confinement.

We demonstrate bioenabled crack-free chiral nematic films prepared via a unidirectional flow of cellulose nanocrystals (CNCs) in the capillary confinement. To facilitate the uniform long-range nanocrystal organization during drying, we utilized tunicate-inspired hydrogen-bonding-rich 3,4,5-trihydroxyphenethylamine hydrochloride (TOPA) for physical cross-linking of nanocrystals with enhanced hydrogen bonding and polyethylene glycol (PEG) as a relaxer of internal stresses in the vicinity of the capillary surface. The CNC/TOPA/PEG film is organized as a left-handed chiral structure parallel to flat walls, and the inner volume of the films displayed transitional herringbone organization across the interfacial region. The resulting thin films also exhibit high mechanical performance compared to brittle films with multiple cracks commonly observed for capillary-formed pure CNC films. The chiral nematic ordering of modified TOPA-PEG-CNC material propagates through the entire thickness of robust monolithic films and across centimeter-sized surface areas, facilitating consistent, vivid iridescence, and enhanced circular polarization. The best performance that prevents the cracks was achieved for a CNC/TOPA/PEG film with a minimal, 3% amount of TOPA. Overall, we suggest that intercalation of small highly adhesive molecules to cellulose nanocrystal-polymer matrices can facilitate uniform flow of liquid crystal phase and drying inside the capillary, resulting in improvement of the ultimate tensile strength and toughness (77% and 100% increase, respectively) with controlled uniform optical reflection and enhanced circular polarization unachievable during regular drying conditions.

Highly strong luminescent chiral nematic cellulose nanocrystal/PEI composites for anticounterfeiting

Cellulose nanocrystal (CNC)-based materials have gained increasing attention because of their great potential applications in optical devices, anticounterfeiting and structural coatings. However, due to the inherent brittleness of CNC, fabricating highly strong and solvent-resistant CNC derived materials is still an important and challenging task. Here, a straightforward, matrix swelling and in-situ insertion method is developed to fabricate chiral nematic composite films by intercalating poly(ethylene imine) (PEI) into the as-prepared swollen matrix of CNC films. The resulting composite CNC films exhibit a great increase in ultimate strain and toughness and especially have an extremely high folding endurance. Notably, the intercalation of PEI into CNC films also leads to a fluorescence enhancement effect, which endows the films circularly polarized luminescence (CPL) properties. The CPL handedness and dissymmetry factors (glum) of the composite films can be controlled. Furthermore, the CNC composite films display optical responsiveness (structure color, fluorescence intensity) to various stimuli, including ethanol and formaldehyde environments. We expect the current in-situ intercalation technique to be promising in manufacturing foldable encryption materials.

Flexible cellulose nanocrystal-based bionanocomposite film as a smart photonic material responsive to humidity

A flexible bionanocomposite film with a visible structural color was developed by co-assembling cellulose nanocrystals with glycerol, glucose, and poly(vinyl alcohol), and its application as a photonic humidity-responsive material was investigated. This chiral nematic film exhibited an indigo, cyan, olive green, orange, pink, and transparent color when exposed to relative humidity of 33%, 75%, 85%, 89%, 96%, and 100%, respectively. The color change was caused by reversible change in chiral nematic pitch due to hydration and dehydration. The nanocomposite displayed a stable structural color after alternatively conditioned at 33% and 100% relative humidity for ten cycles. Notably, material cost for a piece of such a smart film with an area of 28 cm2 is less than $0.1 USD. This flexible, uniform bionanocomposite film has great potential to be used as a smart photonic material responsive to humidity.

Thermal annealing of iridescent cellulose nanocrystal films

The properties of chiral nematic and iridescent cellulose nanocrystal films with different monovalent cations (CNC-X) obtained through evaporation-induced self-assembly (EISA) can be modified by a variety of external stimuli. Here, we study the transformations of their optical and structural properties when the films are thermally annealed at 200 °C and 240 °C for up to 2 days. The chiral nematic structure of the most thermally stable films is not destroyed even after extensive heating due to the thermochemical stability of the cellulose backbone and the presence of surface alkali counterions, which suppress catalysis of early stage degradation. Despite the resilience of the cholesteric structure and the overall integrity of heated CNC-X films, thermal annealing is often accompanied by reduction of iridescence, birefringence, and transparency, as well as formation of degradation products. The versatility, sustainability, and stability of CNC-X films highlight their potential as temperature indicators and photonic devices.

Effect of thermal treatments on chiral nematic cellulose nanocrystal films

The ability to manipulate the optical appearance of materials is essential in virtually all products and areas of technology. Structurally coloured chiral nematic cellulose nanocrystal (CNC) films proved to be an excellent platform to design optical appearance, as their response can be moulded by organising them in hierarchical architectures. Here, we study how thermal treatments influence the optical appearance of structurally coloured CNC films. We demonstrate that the CNCs helicoidal architecture and the chiral optical response can be maintained up to 250 °C after base treatment and cross-linking with glutaraldehyde, while, alternatively, an exposure to vacuum allows for the helicoidal arrangement to be further preserved up to 900 °C, thus producing aromatic chiral carbon. The ability to retain the helicoidal arrangement, and thus the visual appearance, in CNC films up to 250 °C is highly desirable for high temperature colour-based industrial applications and for passive colorimetric heat sensors. Similarly, the production of chiral carbon provides a new type of conductive carbon for electrochemical applications.

Chiral Photonic Liquid Crystal Films Derived from Cellulose Nanocrystals.

As a nanoscale renewable resource derived from lignocellulosic materials, cellulose nanocrystals (CNCs) have the features of high purity, high crystallinity, high aspect ratio, high Young's modulus, and large specific surface area. The most interesting trait is that they can form the entire films with bright structural colors through the evaporation-induced self-assembly (EISA) process under certain conditions. Structural color originates from micro-nano structure of CNCs matrixes via the interaction of nanoparticles with light, rather than the absorption and reflection of light from the pigment. CNCs are the new generation of photonic liquid crystal materials of choice due to their simple and convenient preparation processes, environmentally friendly fabrication approaches, and intrinsic chiral nematic structure. Therefore, understanding the forming mechanism of CNCs in nanoarchitectonics is crucial to multiple fields of physics, chemistry, materials science, and engineering application. Herein, a timely summary of the chiral photonic liquid crystal films derived from CNCs is systematically presented. The relationship of CNC, structural color, chiral nematic structure, film performance, and applications of chiral photonic liquid crystal films is discussed. The review article also summarizes the most recent achievements in the field of CNCs-based photonic functional materials along with the faced challenges.