Light- and Humidity-Responsive Chiral Nematic Photonic Crystal Films Based on Cellulose Nanocrystals

A multi-scale layered helical structure composite using the co-dispersion of cellulose nanocrystals and the micro-nano Al sheets and its efficient near-infrared stealth performance

To design flexible functional materials with high efficiency, light weight, less metal consumption, stable structure for the thermal infrared stealth materials is a great challenge. We hypothesized that the use of crystal materials with different sizes to design composites with a chiral layered helical structure, the layered structures can repeatedly reflect infrared ray. Here, we reported the novel multi-scale layered helical chiral structure composite by self-assembly using the co-dispersion of cellulose nanocrystals (CNC) and micro-nano Al sheets. A new stable interlocking supermolecular structure is formed between the positively charged metal sheet and the negatively charged CNC photonic crystals. Metal sheets and CNC organic crystals were hybridized at the molecular level and form the Pickering-like CNC-Al co-dispersion system. The metal sheets in CNC chiral helical layered structure greatly improve its near-infrared reflection and stealth camouflage. Surprisingly, the CNC/Al composite on the heated glass substrate enabled the temperature drop 23 °C, and made its emissivity in the range of 7-14 μm significantly reduce. The synergetic effect of the Al sheets and the CNCs helical structure greatly improved the thermal infrared reflection and heat insulation properties. It is expected to provide a chiral layered material for the infrared stealth, and pattern camouflage fields.

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.

Cellulose nanocrystals in the development of biodegradable materials: A review on CNC resources, modification, and their hybridization

The escalating demand for sustainable materials has propelled cellulose into the spotlight as a promising alternative to petroleum-based products. As the most abundant organic polymer on Earth, cellulose is ubiquitous, found in plants, bacteria, and even a unique marine animal-the tunicate. Cellulose polymers naturally give rise to microscale semi-crystalline fibers and nanoscale crystalline regions known as cellulose nanocrystals (CNCs). Exhibiting rod-like structures with widths spanning 3 to 50 nm and lengths ranging from 50 nm to several microns, CNC characteristics vary based on the cellulose source. The degree of crystallinity, crucial for CNC properties, fluctuates between 49 and 95 % depending on the source and synthesis method. CNCs, with their exceptional properties such as high aspect ratio, relatively low density (≈1.6 g cm-3), high axial elastic modulus (≈150 GPa), significant tensile strength, and birefringence, emerge as ideal candidates for biodegradable fillers in nanocomposites and functional materials. The percolation threshold, a mathematical concept defining long-range connectivity between filler and polymer, governs the effectiveness of reinforcement in nanocomposites. This threshold is intricately influenced by the aspect ratio and molecular interaction strength, impacting CNC performance in polymeric and pure nanocomposite materials. This comprehensive review explores diverse aspects of CNCs, encompassing their derivation from various sources, methods of modification (both physical and chemical), and hybridization with heterogeneous fillers. Special attention is devoted to the hybridization of CNCs derived from tunicates (TCNC) with those from wood (WCNC), leveraging the distinct advantages of each. The overarching objective is to demonstrate how this hybridization strategy mitigates the limitations of WCNC in composite materials, offering improved interaction and enhanced percolation. This, in turn, is anticipated to elevate the reinforcing effects and pave the way for the development of nanocomposites with tunable viscoelastic, physicochemical, and mechanical properties.

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.

Chiral Materials: Progress, Applications, and Prospects

Chirality is a universal phenomenon in molecular and biological systems, denoting an asymmetric configurational property where an object cannot be superimposed onto its mirror image by any kind of translation or rotation, which is ubiquitous on the scale from neutrinos to spiral galaxies. Chirality plays a very important role in the life system. Many biological molecules in the life body show chirality, such as the "codebook" of the earth's biological diversity-DNA, nucleic acid, etc. Intriguingly, living organisms hierarchically consist of homochiral building blocks, for example, l-amino acids and d-sugars with unknown reason. When molecules with chirality interact with these chiral factors, only one conformation favors the positive development of life, that is, the chiral host environment can only selectively interact with chiral molecules of one of the conformations. The differences in chiral interactions are often manifested by chiral recognition, mutual matching, and interactions with chiral molecules, which means that the stereoselectivity of chiral molecules can produce changes in pharmacodynamics and pathology. Here, the latest investigations are summarized including the construction and applications of chiral materials based on natural small molecules as chiral source, natural biomacromolecules as chiral sources, and the material synthesized by design as a chiral source.

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

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

Chiro-Optoelectronic Encodable Multilevel Thin Film Electronic Elements with Active Bio-Organic Electrolyte Layer

It is suggested that chiral photonic bio-enabled integrated thin-film electronic elements can pave the base for next-generation optoelectronic processing, including quantum coding for encryption as well as integrated multi-level logic circuits. Despite recent advances, thin-film electronics for encryption applications with large-scale reconfigurable and multi-valued logic systems are not reported to date. Herein, highly secure optoelectronic encryption logic elements are demonstrated by facilitating the humidity-sensitive helicoidal organization of chiral nematic phases of cellulose nanocrystals (CNCs) as an active electrolyte layer combined with printed organic semiconducting channels. The ionic-strength controlled tunable photonic band gap facilitates distinguishable and quantized 13-bit electric signals triggered by repetitive changes of humidity, voltage, and the polarization state of the incident light. As a proof-of-concept, the integrated circuits responding to circularly polarized light and humidity are demonstrated as unique physically unclonable functional devices with high-level logic rarely achieved. The convergence between functional nanomaterials and the multi-valued logic thin-film electronic elements can provide optoelectronic counterfeiting, imaging, and information processing with multilevel logic nodes.

Photo-Cross-Linked Polymeric Dispersants of Comb-Shaped Benzophenone-Containing Poly(ether amine)

Efficient dispersion of nanoparticles (NPs) is a crucial challenge in the preparation and application of composites that contain NPs, particularly in coatings, inks, and related materials. Physical adsorption and chemical modification are the two common methods used to disperse NPs. However, the former suffers from desorption, and the latter is more specific and has limited versatility. To address these issues, we developed a novel photo-cross-linked polymeric dispersant, comb-shaped benzophenone-containing poly(ether amine) (bPEA), using a one-pot nucleophilic/cyclic-opening addition reaction. The results demonstrated that the bPEA dispersant forms a dense and stable shell on the surface of pigment NPs through physical adsorption and subsequent chemical photo-cross-linking, which effectively overcome the drawbacks of the desorption occurred in physical adsorption and the specificity of the chemical modification. By means of the dispersing effect of bPEA, the obtained pigment dispersions show high solvent, thermal, and pH stability without flocculation during storage. Moreover, the NPs dispersants show good compatibility with screen printing, coating, and 3D printing, endowing the ornamental products with high uniformity, color fastness, and less color shading. These properties make bPEA dispersants ideal candidates in fabrication dispersions of other NPs.

Paper-Based Humidity Sensors as Promising Flexible Devices, State of the Art, Part 2: Humidity-Sensor Performances

This review article covers all types of paper-based humidity sensor, such as capacitive, resistive, impedance, fiber-optic, mass-sensitive, microwave, and RFID (radio-frequency identification) humidity sensors. The parameters of these sensors and the materials involved in their research and development, such as carbon nanotubes, graphene, semiconductors, and polymers, are comprehensively detailed, with a special focus on the advantages/disadvantages from an application perspective. Numerous technological/design approaches to the optimization of the performances of the sensors are considered, along with some non-conventional approaches. The review ends with a detailed analysis of the current problems encountered in the development of paper-based humidity sensors, supported by some solutions.

Multidimensional dynamic regulation of cellulose coloration for digital recognition and humidity response

Structural color is an eye-catching phenomenon in nature, which originates from the synergistic effect of cholesteric structure inside living organisms and light. However, biomimetic design and green construction of dynamically tunable structural color materials have been a great challenge in the field of photonic manufacturing. In this work, the new ability of L-lactic acid (LLA) to multi-dimensionally modulate the cholesteric structures constructed from cellulose nanocrystals (CNC) is revealed for the first time. By studying the molecular-scale hydrogen bonding mechanism, a novel strategy that electrostatic repulsion and hydrogen bonding forces jointly drive the uniform arrangement of cholesteric structures is proposed. Due to the flexible tunability and uniform alignment of the CNC cholesteric structure, different encoded messages were developed in the CNC/LLA (CL) pattern. Under different viewing conditions, the recognition information of different digits will continue to reversibly and rapidly switch until the cholesteric structure is destroyed. In addition, the LLA molecules facilitated the more sensitive response of the CL film to the humidity environment, making it exhibit reversible and tunable structural colors under different humidity. These excellent properties provide more possibilities for the application of CL materials in the fields of multi-dimensional display, anti-counterfeiting encryption, and environmental monitoring.

Flow and assembly of cellulose nanocrystals (CNC): A bottom-up perspective - A review

Cellulosic sources, such as lignocellulose-rich biomass, can be mechanically or acid degraded to produce inclusions called cellulose nanocrystals (CNCs). They have several uses in the sectors of biomedicine, photonics, and material engineering because of their biodegradability, renewability, sustainability, and mechanical qualities. The processing and design of CNC-based products are inextricably linked to the rheological behaviour of CNC suspension or in combination with other chemicals, such as surfactants or polymers; in this context, rheology offers a significant link between microstructure and macro scale flow behaviour that is intricately linked to material response in applications. The flow behaviour of CNC items must be properly specified in order to produce goods with value-added characteristics. In this review article, we provide new research on the shear rheology of CNC dispersion and CNC-based hydrogels in the linear and nonlinear regime, with storage modulus values reported to range from ~10^-3 to 10³ Pa. Applications in technology and material science are also covered simultaneously. We carefully examined the effects of charge density, aspect ratio, concentration, persistence length, alignment, liquid crystal formation, the cause of chirality in CNCs, interfacial behaviour and interfacial rheology, linear and nonlinear viscoelasticity of CNC suspension in bulk and at the interface using the currently available literature.

Light-Responsive Supramolecular Nanotubes-Based Chiral Plasmonic Assemblies

We describe the fabrication of dual-responsive (thermo/light) chiral plasmonic films. The idea is based on using photoswitchable achiral liquid crystal (LCs) forming chiral nanotubes for templating helical assemblies of Au NPs. Circular dichroism spectroscopy (CD) confirms chiroptical properties coming from the arrangement of organic and inorganic components, with up to 0.2 dissymmetry factor (g-factor). Upon exposure to UV light, organic molecules isomerize, resulting in controlled melting of organic nanotubes and/or inorganic nanohelices. The process can be reversed using visible light and further modified by varying the temperature, offering a control of chiroptical response of the composite material. These properties can play a key role in the future development of chiral plasmonics, metamaterials, and optoelectronic devices.

Research on deep eutectic solvents for the construction of humidity-responsive cellulose nanocrystal composite films

Photonic crystal materials based on cellulose nanocrystals (CNC), which are environmentally responsive and green, have attracted widespread attention. To overcome the brittleness of CNC films, many researchers have explored functional additives to improve their performance. In this study, a new green deep eutectic solvents (DESs) and an amino acid-based natural deep eutectic solvents (NADESs) were introduced into CNC suspensions for the first time, and hydroxyl-rich small molecules (glycerol, sorbitol) and polymers (polyvinyl alcohol, polyethylene glycol) were coassembled with the DESs and NADESs to form three-component composite films. The CNC/G/NADESs-Arg three-component film reversibly changed color from blue to crimson as the relative humidity rose from 35 % to 100 %; additionally, the elongation at break increased to 3.05 %, and the Young's modulus decreased to 4.52 GPa. The hydrogen bond network structure provided by trace amounts of the DESs or NADESs not only improved the mechanical properties of the composite films but also increased their water absorption capacities without destroying their optical activities. This allows for the development of more stable CNC films and creates potential for biological applications in the future.

Tailoring the humidity response of cellulose nanocrystal-based films by specific ion effects

HYPOTHESIS

The optical properties and humidity response of iridescent films made of cellulose nanocrystal (CNC) and polyethylene glycol (PEG) can be tailored by the incorporation of electrolytes chosen based on specific ion effects (SIE).

EXPERIMENTS

A series of inorganic salts comprising five different cations and five anions based on the Hofmeister series were mixed with CNC/PEG suspensions, followed by an air-dried process into iridescent solid films. These films were tested in changing relative humidity (RH) environments from 30% to 90% and their photonic properties and mass change monitored. The underlying structures and the mechanism of their formation were quantified in terms of interparticle distance derived from small angle X-ray scattering experiment and pitch size quantified by scanning electron microscope (SEM).

FINDINGS

The specific color and color range of CNC/PEG based films are controlled by a specific anion effect achieved by selection of the salt while the specific cation effect is negligible. The salting-in type anions with the same valency result in a red-shift color when films are in the dried state. The salting-in type leads to a greater color changing range during RH changes than the salting-out type. The resultant mass gain/loss trend is consistent with the color change. In contrast, cations do not show any relationships between salting-in effect and the measured properties as observed for anions. The observed SIE can be used to engineer CNC/polymer-based humidity and bio-diagnostic colorimetric indicator devices.

Grating Assembly Dissected in Periodic Bands of Poly (Butylene Adipate) Modulated with Poly (Ethylene Oxide)

Polarized optical microscopy (POM), scanning electron microscopy (SEM), and synchrotron microbeam wide-angle X-ray diffraction (WAXD) were used to investigate the mechanisms of periodic assemblies leading to ring-banded crystal aggregates with light-grating capacity for iridescence in poly (1,4-butylene adipate) (PBA) modulated with poly (ethylene oxide) (PEO). A critical finding is that the PBA crystal assembly on the top surface and in the interior constitutes a grating architecture, with a cross-bar pitch equaling the inter-band spacing. The inner lamellae are arranged perpendicularly to the substrate under the ridge region, where they scroll, bend, and twist 90° to branch out newly spawned lamellae to form the parallel lamellae under the valley region. The cross-hatch grating with a fixed inter-spacing in the PBA aggregated crystals is proved in this work to perfectly act as light-interference entities capable of performing iridescence functions, which can be compared to those widely seen in many of nature’s organic bio-species or inorganic minerals such as opals. This is a novel breakthrough finding for PBA or similar polymers, such as photonic crystals, especially when the crystalline morphology could be custom-made and modulated with a second constituent.

Anisotropic chitosan/tunicate cellulose nanocrystals hydrogel with tunable interference color and acid-responsiveness

A robust chitosan/tunicate cellulose nanocrystals (TCNCs) anisotropic hydrogel with bright interference colors was fabricated via combining the prestretching orientation method and chemically-physically dual cross-linking. The oriented regenerated chitosan nanofibrous network enabled the TCNCs alignment by covalent interaction and hydrogen bonding. The stretching alignment endows the chitosan/TCNCs hydrogel with enhanced tensile strength, from 0.63 MPa (draw ratio 1.0) to 2.06 MPa (draw ratio 3.5). Moreover, the orientation of chitosan nanofibers led to birefringence appearance, which could be regulated with the TCNCs introduction or draw ratios. The hydrogel swelled completely in 2 min in pH = 3 solution and the interference color disappeared. The oriented chitosan/TCNCs hydrogels showed distinct color change under acid stimulation, which could be quantitatively measured or directly observed under crossed polarizers. This work demonstrated a strategy for fabricating the interference color regulatable hydrogels with acid-response property for sensors and environmental monitoring.

Self-Assembled Nanodot Actuator with Changeable Fluorescence by π-π Stacking Force Based on a Four-Armed Foldable Phthalocyanine Molecule and Its Supersensitive Molecular Recognition

Designing intelligent molecules and smart nanomaterials as molecular machines is becoming increasingly important in the nanoscience fields. Herein, we report a nanodot actuator with changeable fluorescence by π-π stacking force based on a four-armed foldable phthalocyanine molecule. The assembled nanodot possessed a three-dimensional molecular space structure and multiple supramolecular interactions. The arms of the nanodot could fold and open intelligently in response to environmental molecular stimuli such as natural plant mimosa, which could lead to multiple variable fluorescence emissions. The nanodot was highly sensitive to the biomolecule thyroxine at the molecular level. The accurate molecular recognition and the changeable fluorescence conversion of the nanodot were attributed to multiple supramolecular interactions, including photoinduced electron transfer (PET), intramolecular fluorescence resonance energy transfer (FRET), and π-π stacking of the nanodots, resulting in an intelligent "nanodot machine with folding arms". The self-assembled nanodot actuators with changeable fluorescence have potential applications in advanced intelligent material fields.

Scalable and Sensitive Humidity-Responsive Polymer Photonic Crystal Films for Anticounterfeiting Application

In this study, we fabricate a new kind of ultrasensitive humidity-responsive photonic crystal (HPC) films based on emulsion polymerization and the open mill and bending-induced ordering technique (OM-BIOT) method, which is simple and scalable. The HPC film senses relative humidity (RH) from 9 to 98% for the polymer matrix swells up in high RH and shrinks in low RH, leading to a large reflectance shift (81 nm) and distinct color change. Based on the double-peak reflective spectra of the HPC film, we confirm the gradient swelling hypothesis and find that the thickness is another important factor for controlling the sensitivity and response rate of the HPC film. Except for static humidity, the HPC film can also respond to the dynamic humid flow of blowing and polar solvents, which broadens its application potential. This kind of HPC film shows a vivid structural color, and the humidity-responsive behavior is quick, distinct, energy-free, and reversible, having a great prospect for anticounterfeiting application.

Biomimetic Robust Starch Composite Films with Super-Hydrophobicity and Vivid Structural Colors

The starch composite films (SCFs) will be one of the best alternative packaging materials to petroleum based plastic films, which mitigates white pollution and energy consumption. However, weak mechanical stability, water resistance, and dyeability has hindered the application of SCFs. Herein, a bioinspired robust SCFs with super-hydrophobicity and excellent structural colors were prepared by fiber-reinforcement and assembling SiO₂/Polydimethylsiloxane (PDMS) amorphous arrays on the surface of SCFs. The properties of the designed SCFs were investigated by various methods including scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), a tensile test, contact angle (CA) test, and an optical test. The results showed that the obtained SCFs possessed a higher tensile strength (55.17 MPa) attributed to the formed abundant hydrogen bonds between the molecular chains of the starch, cellulose fiber, and polyvinyl alcohol. Benefiting from the nanostructure with rough surface which were modified by materials with low surface free energy, the contact angle and sliding angle of the film reached up to 154° and 2°, respectively. The colors which were produced by the constructive interference of the coherent scattered light could cover all of the visible regions by tuning the diameters of the SiO₂ nanoparticles. The strategy in the present study not only reinforces the mechanical strength and water resistance of SCFs but also provides an environmentally friendly way to color the them, which shows unprecedented application potential in packaging materials of the starch composite films.

Ultrasonication pretreatment assisted rapid co-assembly of cellulose nanocrystal and metal ion for multifunctional application

Co-assembly of metal ion and cellulose nanocrystals (CNC) is a promising strategy to fabricate novel iridescent CNC materials with advanced applications. By combining ultrasonication pretreatment and vacuum-assisted self-assembly (VASA) technique, a facile and rapid strategy is proposed to prepare the Mn²⁺-doped carboxylated CNC (C-CNC) iridescent films with multifunctional application. The ultrasonication pretreatment temporarily disassembles the aggregates of C-CNC nanorods caused by the electrostatic interaction between negative charged C-CNC and Mn²⁺. The subsequent VASA process accelerates the self-assembly of chiral liquid crystals prior to the re-agglomeration of C-CNC by the bridge effect of Mn²⁺. Furthermore, the as-prepared Mn²⁺/CNC film exhibits a rapid and visible color change in ammonia atmosphere along with the formation of MnO₂. The reversible change can be realized by the stimulation of reducing agent. The derived MnO₂/C-CNC composite film displays efficient removal of methylene blue dye in aqueous solution by both of adsorption and degradation procedure.

Multi-stimuli-responsive photonics films based on chiral nematic cellulose nanocrystals

Multiple-stimuli-responsive bio-based materials have received considerable attention for intelligent packaging and anti-counterfeiting applications. Herein, we present a unique biobased photonics film with multi-stimuli responsive behavior based on cellulose nanocrystals (CNCs), sorbitol (S) and anthocyanin (Anth). The resulting photonics film exhibits multi-stimuli responsive behavior to humidity, solvent and pH stimuli. Notably, the photonics film showed dramatic invertible color from blue to fuchsia and high sensitivity at a relative humidity from 50% to 100%. Moreover, the photonics film exhibited fast response and good reversibility under different ethanol concentrations. Significant color changes of the photonics film were also observed in response to pH change in the range of 2 to 12. Particularly, the humidity, solvent and pH responsiveness of the photonics film did not interfere with each other.

Silica-templated photonic crystal sensors for specific detection of Cu2+

A novel Cu-IOPC sensor was developed by combining the SiO2-based inverse opals with imidazolyl. Such mechanism is based on the specific binding of imidazole with copper ions, then the copper ions can be detected with high accuracy and efficiency.

Supramolecular DNA Photonic Hydrogels for On-Demand Control of Coloration with High Spatial and Temporal Resolution

Hydrogels embedded with periodic arrays of nanoparticles display a striking photonic crystal coloration that may be useful for applications such as camouflage, anticounterfeiting, and chemical sensing. Dynamically generating color patterns requires control of nanoparticle organization within a polymer network on-demand, which is challenging. We solve this problem by creating a DNA hydrogel system that shows a 50 000-fold decrease in modulus upon heating by ∼10 °C. Magnetic nanoparticles entrapped within these DNA gels generate a structural color only when the gel is heated and a magnetic field is applied. A spatially controlled photonic crystal coloration was achieved by photopatterning with a near-infrared illumination. Color was "erased" by illuminating or heating the gel in the absence of an external magnetic field. The on-demand assembly technology demonstrated here may be beneficial for the development of a new generation of smart materials with potential applications in erasable lithography, encryption, and sensing.

Chiral Nematic Coatings Based on Cellulose Nanocrystals as a Multiplexing Platform for Humidity Sensing and Dual Anticounterfeiting

The need for a precise regulation of the properties of chiral nematic structures in response to external stimuli is addressed. Self-assembled iridescent coatings are produced under the effect of electrostatic interactions between cellulose nanocrystals and poly(acrylic acid), endowing a high anisotropic dissymmetry (>0.3) and sensitivity to environmental humidity (13.1 nm/1% at 68-75% relative humidity, RH). The phenomena associated with shifts in selective light reflection (green to orange) and polarization, facilitate tunable transmitted colors (blue to orange) at given rotation angles (RA). Such properties are conveniently integrated into a "RH-RA-color" ternary code that is introduced as an anticounterfeiting technology, taking advantage of multicolor patterns that conveniently track with changes in RH and RA. The proposed charge-driven assembly opens new opportunities for chiral nematic materials that enable precise optical sensing and information encryption.

Construction of Functional Materials in Various Material Forms from Cellulosic Cholesteric Liquid Crystals

Wide use of bio-based polymers could play a key role in facilitating a more sustainable society because such polymers are renewable and ecofriendly. Cellulose is a representative bio-based polymer and has been used in various materials. To further expand the application of cellulose, it is crucial to develop functional materials utilizing cellulosic physicochemical properties that are acknowledged but insufficiently applied. Cellulose derivatives and cellulose nanocrystals exhibit a cholesteric liquid crystal (ChLC) property based on rigidity and chirality, and this property is promising for constructing next-generation functional materials. The form of such materials is an important factor because material form is closely related with function. To date, researchers have reported cellulosic ChLC materials with a wide range of material forms-such as films, gels, mesoporous materials, and emulsions-for diverse functions. We first briefly review the fundamental aspects of cellulosic ChLCs. Then we comprehensively review research on cellulosic ChLC functional materials in terms of their material forms. Thus, this review provides insights into the creation of novel cellulosic ChLC functional materials based on material form designed toward the expanded application of cellulosics.

Artificial Chameleon Skin with Super-Sensitive Thermal and Mechanochromic Response

Both the nonclose-packed structure and the large refractive index contrast of guanine nanocrystals and cytosols in iridophores play a vital role in the dynamic camouflage of chameleons, including the bright skin color and color tuning sensitivity to external stimulus. Here, the nonclose-packed photonic crystals consisting of ZnS nanospheres and polymers, which have similar refractive indices with guanine nanocrystals and cytosols, respectively, are constructed by a two-step filling strategy. ZnS@SiO₂ nanospheres are self-assembled to build intermediate close-packed photonic crystals followed by filling polymers in their interstices. The nonclose-packed photonic crystal is successfully achieved when the silica portion is etched by HF solution and refilled by polymers. Excitingly, the stimulus response of the designed photonic crystal is as sensitive as the skin of chameleons due to the similar contrast of refractive indices and nonclose-packed structure. The reflection peak of the structure can blue-shift more than 200 nm as the temperature increases from 30 to 55 °C or under 20% compressional strain. This work not only builds the nonclose-packed photonic crystals by introducing a two-step filling strategy but also proves that high refractive contrast in photonic crystals is an effective strategy to achieve ultrasensitivity, which is highly desirable for various applications.

Green and sustainable cellulose-derived humidity sensors: A review

Cellulose, as the most abundant natural polysaccharide, is an excellent material for developing green humidity sensors, especially due to its humidity responsiveness as a result of its rich hydrophilic groups. In combination with other components including carbon materials and polymers, cellulose and its derivatives can be used to design high-performance humidity sensors that meet various application requirements. This review summarizes the recent advances in the field of various cellulose-derived humidity sensors, with particular attention paid to different sensing mechanisms including resistance, capacitance, colorimetry and gravity, and so on. Furthermore, the roles of cellulose and its derivatives are highlighted. This work may promote the development of cellulose-derived humidity sensors, as well as other cellulose-based intelligent materials.

Recent Advances on Cellulose Nanocrystals and Their Derivatives

Nanocellulose, typically cellulose nanocrystals (CNCs), has excellent properties and is widely used. In particular, CNC has a small dimension, high chemical reactivity, and high sustainability, which makes it an excellent candidate as a starting material to be converted into nanocellulose derivatives. Chemical modification is essential for obtaining the desired products; the modifications create different functional attachment levels and generate novel microstructures. Recent advances on nanocellulose derivatives have not yet been reviewed and evaluated for the last five years. Nanocellulose derivative materials are being used in a wide variety of high-quality functional applications. To meet these requirements, it is essential for researchers to fully understand CNCs and derivative materials, precisely their characteristics, synthesis methods, and chemical modification approaches. This paper discusses CNC and its derivatives concerning the structural characteristics, performance, and synthesis methods, comparing the pros and cons of these chemical modification approaches reported in recent years. This review also discusses the critical physicochemical properties of CNC derivative products, including solubility, wetting performance, and associated impacts on properties. Lastly, this paper also comments on the bottlenecks of nanocellulose derivatives in various applications and briefly discusses their future research direction.

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.

Bioinspired Multi-Stimuli Responsive Actuators with Synergistic Color- and Morphing-Change Abilities

The combination of complex perception, defense, and camouflage mechanisms is a pivotal instinctive ability that equips organisms with survival advantages. The simulations of such fascinating multi-stimuli responsiveness, including thigmotropism, bioluminescence, color-changing ability, and so on, are of great significance for scientists to develop novel biomimetic smart materials. However, most biomimetic color-changing or luminescence materials can only realize a single stimulus-response, hence the design and fabrication of multi-stimuli responsive materials with synergistic color-changing are still on the way. Here, a bioinspired multi-stimuli responsive actuator with color- and morphing-change abilities is developed by taking advantage of the assembled cellulose nanocrystals-based cholesteric liquid crystal structure and its water/temperature response behaviors. The actuator exhibits superfast, reversible bi-directional humidity and near-infrared (NIR) light actuating ability (humidity: 9 s; NIR light: 16 s), accompanying with synergistic iridescent appearance which provides a visual cue for the movement of actuators. This work paves the way for biomimetic multi-stimuli responsive materials and will have a wide range of applications such as optical anti-counterfeiting devices, information storage materials, and smart soft robots.

Additive printing of recyclable anti-counterfeiting patterns with sol-gel cellulose nanocrystal inks

The assembly of cellulose nanocrystals (CNCs) that produce attractive structural color shows great potential in anti-counterfeiting application, but their processability and recyclability remain unsatisfactory due to the strong hydrogen bonds between CNCs. For the first time, optical anti-counterfeiting patterns are obtained by additive printing of surface-functionalized CNC inks (CNC-DC5700-NPES). The surface-functionalized CNC inks are prepared by sequential modification of CNCs with organosilane (DC5700) and polyoxyethylene ether (NPES), which show good flowability under shearing force and transform into a gel-like phase rapidly after printing, making possible ink-jet printing without additives. The printed patterns are transparent under natural light but show vivid interference color, showing anti-counterfeiting features between crossed polarizers. The texture and optical properties of the printed patterns can be facilely controlled by tuning the printing parameters, such as nozzle diameter, writing angle, and filling width. Moreover, the CNC-DC5700-NPES patterns with a core-shell structure could be collected in various solvents and reprinted after removing solvents. This work provided a new pathway for the preparation of optical anti-counterfeiting patterns from biomass resources.

Bottom-Up Assembled Photonic Crystals for Structure-Enabled Label-Free Sensing

Photonic crystals (PhCs) display photonic stop bands (PSBs) and at the edges of these PSBs transport light with reduced velocity, enabling the PhCs to confine and manipulate incident light with enhanced light-matter interaction. Intense research has been devoted to leveraging the optical properties of PhCs for the development of optical sensors for bioassays, diagnosis, and environmental monitoring. These applications have furthermore benefited from the inherently large surface area of PhCs, giving rise to high analyte adsorption and the wide range of options for structural variations of the PhCs leading to enhanced light-matter interaction. Here, we focus on bottom-up assembled PhCs and review the significant advances that have been made in their use as label-free sensors. We describe their potential for point-of-care devices and in the review include their structural design, constituent materials, fabrication strategy, and sensing working principles. We thereby classify them according to five sensing principles: sensing of refractive index variations, sensing by lattice spacing variations, enhanced fluorescence spectroscopy, surface-enhanced Raman spectroscopy, and configuration transitions.

Microstructural Periodic Arrays in Poly(Butylene Adipate) Featured with Photonic Crystal Aggregates

Poly(butylene adipate) (PBA) self-aggregation into unique periodicity correlating to its interfacial photonic properties is probed in detail. Investigations on the unique periodic morphology and top-surface and interior architectures in specifically crystallized PBA are focused on its novel photonic patterns with periodic gratings. Detailed analysis of the interior lamellae from ringless to periodically ordered aggregates (crystallized at 33-35 °C vs. T_c = 30 °C) serves as ideal comparisons. Each interior arc-shape shell is composed of tangential and radial lamellae mutually intersecting at 90^o angle. The interior layer thickness in SEM-revealed arc-shape shish-kebab shell is exactly equal to the optical inter-band spacing (≈6 µm). A 3D assembly mechanism of periodically banded PBA crystals is proposed, where the orderly arrays on top surfaces as well as the interior microstructures of strut-rib alternate-layered assembly resemble nature's photonic crystals and collectively account for the interfacial photonic properties in the ring-banded PBA crystal that is novel and has potential applications in future.

High Structural Stability of Photonic Crystals on Textile Substrates, Prepared via a Surface-Supported Curing Strategy

Over the past years, photonic crystals (PCs) with a periodically ordered nanostructure have attracted great attention due to their potential as advanced optical materials for structural coloration of textiles. However, the weak structural stability of PCs on flexible textile substrates makes them vulnerable to strong external forces, hampering their large-scale application. In this work, a waterborne polyurethane (wPU) is chosen for enhancing the structural stability of PCs. The composite PCs (PCs/wPU) show both brilliant structural colors and significantly improved structural stability. The structural color produced by the encapsulated PCs is found to depend on the properties of encapsulating agents. The wPU with high surface tension solidifies mainly on the PC surface in the form of a transparent film, protecting the overall structure of PCs. Meanwhile, a small amount of wPU, infiltrating into the interior of PCs, provides strong adhesion and ensures stability among nanospheres. In turn, polydimethylsiloxane (PDMS) with low surface tension is easy to infiltrate into the interior of PCs, forming fully encapsulated PCs. This reduces the brightness of structural color produced by the final PCs/PDMS composite over the original PCs, due to the replacement of air by PDMS, and thus the decrease in the refractive index contrast of PCs. The supported curing strategy using the encapsulating agent with high surface tension is shown to not only improve the structural stability of PCs but also exert almost no influence on the optical properties of PCs, facilitating the practice application of structural coloration in the textile industry.

High-Performance Colorimetric Humidity Sensors Based on Konjac Glucomannan

High-humidity conditions (85-100% relative humidity (RH)) have very diverse effects on many aspects of people's daily lives. Despite remarkable progress in the development of structural coloration-based humidity sensors, how to significantly improve the sensitivity and visual humidity resolution of these humidity sensors under a high-humidity environment remains a great challenge. In this study, high-performance colorimetric humidity sensors based on environment-friendly konjac glucomannan (KGM) via thin-film interference are developed using a simple, affordable, and scalable preparation method. An effective strategy is demonstrated for substantially improving the sensor sensitivity and visual humidity resolution under a high-humidity environment via synergistic integration of multiorder interference peaks, sensor array technology, and superior water-absorbing polymer. The KGM full-range humidity sensors exhibit fast and dynamic response toward the humidity change without power consumption, and they also show high sensitivity and selectivity, little hysteresis, and excellent stability against high-humidity conditions. The KGM humidity sensors display extraordinary red shift of the reflection peak (e.g., 385 nm) and the visual humidity resolution as high as 1.5% RH in the visible range from 85 to 100% RH, which represent the largest spectra shift and highest visual humidity resolution, respectively, for structural coloration-based humidity sensors in high-humidity conditions.

Bioinspired structural color nanocomposites with healable capability

This minireview summarizes the recent development of healable structural color nanocomposites from the perspective of the construction strategies.