Multi-axis alignment of Rod-like cellulose nanocrystals in drying droplets

Optically responsive dry cholesteric liquid crystal marbles

Dry liquid crystal marbles are structures that consist of cholesteric liquid crystal (CLC) droplets prepared by the mixture of chiral-doped thermotropic LCs encapsulated by cellulose nanocrystals (CNCs) that have been dried under ambient conditions. The characterizations revealed that CLC droplets were successfully encapsulated by self-standing CNC shells and responsive to the external gaseous stimulus. The dry LC marbles offer several advantages over previously reported LC-based gas sensors, such as fast response against minor external stimuli, and ease of handling, which make them particularly attractive for practical applications in sensing. We demonstrate the use of these marbles for detecting toluene vapor, a common industrial solvent and pollutant, which we also use to understand the response characteristics. The dry CLC marbles exhibit a significant response to toluene vapor with a detection limit below 500 ppm, attributed to the change of pitch size of the helical structure of CLC droplets induced by the toluene vapor. The CNC-capsulated CLC droplets were stable in emulsion for up to two weeks, and their dried form exhibited a sensitive response upon toluene exposure. The real-time experiments revealed that the LC marbles can be used multiple times without a significant loss of sensitivity, where 90 % of the maximum response was observed at 13.3 ± 4.7 s. These dry LC marbles can also be utilized in other areas, including drug delivery, optical devices, and biosensors.

Electric field-modulated evaporative thin film deposition of bio-particles for piezoelectric applications

Bio-based functional materials can be used to replace or limit the use of synthetic materials sourced from unsustainable sources. However, the potential of such materials remains largely unexplored. In this study, we demonstrate the use of weak AC electric fields to deposit ultra-thin piezoelectric films from cellulose nanocrystals (CNC). This is the first time electric fields are used to realize <50 nm thick uniform bio-based piezoelectric films wherein the bioparticles exhibit unidirectional arrangement. Interestingly, we found that the use of weak AC electric fields of suitable frequencies completely mitigates the coffee ring effect (CRE), which results in defect-free uniform ultra-thin films. Additionally, the electric fields appear to help in realizing unidirectional alignment of particles in the films, which enhances their piezoelectric properties. The method was also tested for chitin nanocrystals (ChNC), which have a similar aspect ratio but bear opposite polarity surface charges, and the influence of the field on coffee ring formation and particle orientation in CNC thin film deposition was validated. The phenomena can be attributed to the constant spatio-temporal curvature of the evaporating liquid film, the transient state between the three-phase contact (TPC) line, the electric field-dependent contact angle, and the permanent and field-induced dipole moments. These factors lead to particle polarization and alignment. The films have an optimum electrical frequency of deposition at which they are continuous and uniformly thin, have unidirectional alignment of particles, and function as a single dipole.

Evaporation-induced self-assembly of liquid crystal biopolymers

Evaporation-induced self-assembly (EISA) is a process that has gained significant attention in recent years due to its fundamental science and potential applications in materials science and nanotechnology. This technique involves controlled drying of a solution or dispersion of materials, forming structures with specific shapes and sizes. In particular, liquid crystal (LC) biopolymers have emerged as promising candidates for EISA due to their highly ordered structures and biocompatible properties after deposition. This review provides an overview of recent progress in the EISA of LC biopolymers, including DNA, nanocellulose, viruses, and other biopolymers. The underlying self-assembly mechanisms, the effects of different processing conditions, and the potential applications of the resulting structures are discussed.

Properties of APTES-Modified CNC Films

Sulfated cellulose nanocrystals' (CNCs') facile aqueous dispersibility enables producing films, fibers, and other materials using only water as a solvent but prevents using sulfated CNCs in applications that require water immersion. We report that modifying CNCs with 3-aminopropyl-triethoxysilane (APTES) via a simple, single-pot reaction scheme dramatically improves the hydrolytic stability of CNC films. The effects of APTES modification on CNCs' properties were studied using attenuated total reflectance Fourier transform infrared spectroscopy, atomic force and optical microscopy, thermogravimetric analysis, dynamic light scattering, and ultimate analysis. Substituting a mere 12.6% of the CNCs' available hydroxyl groups with APTES dramatically increased the hydrolytic stability of shear cast films while only having minor impacts on their mechanical properties. In addition, quartz crystal microbalance with dissipation monitoring (QCMD) and multiparametric surface plasmon resonance (MP-SPR) studies showed that the CNC-APTES films also had a greater irreversible binding with carbofuran, a pesticide and emerging contaminant. These results highlight that APTES modification is a promising method for increasing the utility of sulfated CNCs in sensors, adsorbents, and other applications requiring water immersion.

Formation and Application of Polymer Spherulite-like Patterns of Halloysite Nanotubes by Evaporation-Induced Self-Assembly

Halloysite nanotubes (HNTs) are one-dimensional clay nanomaterials featuring distinct tubular structures and unique surface charges. HNTs can readily form ordered assembly structures under specific conditions, which shows significant potential applications in optical and biological fields. In this study, sodium hexametaphosphate (SHMP) was employed as a stabilizer to prepare polymer spherulite-like patterns via the evaporation-induced self-assembly (EISA) technique. The incorporation of SHMP enhanced the repulsion force among the nanotubes and the surface potential, which facilitated the orderly deposition of HNTs. The influence of HNT concentration, SHMP concentration, drying temperature, and substrate on the polymer spherulites-like pattern has been investigated in detail. The optimal conditions were 10 wt % HNT dispersion, 0.6 wt % SHMP concentration, 30 °C as drying temperature, and glass substrates. In addition, by changing the droplet volume and shape of the three-phase contact line, patterns of different sizes and shapes can be achieved. Bovine serum albumin or metal salt compounds were incorporated into the dispersion of SHMP-modified HNTs, which altered the charge and the self-assembled patterns with different area ratios. Thus, this technology can be utilized for the analysis and comparison of protein and metal ion concentration accurately. This study creates the correlation between the structural parameters and the preparation process involved in creating polymer spherulite-like patterns of modified HNTs and offers fresh insights into potential applications for the self-assembly of HNT droplets in the realms of anticounterfeiting and solution concentration analysis.

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

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

Gas barrier coating based on cellulose nanocrystals and its preservation effects on mango

Mango is the "king of tropical fruits" because of its attractive appearance, delicious taste, rich aroma, and high nutritional value. However, mango keeps fast metabolizing after harvest, leading to water loss, starch conversion into sugar, texture softening, and decay. Here, a gas barrier coating based on cellulose nanocrystals (CNCs) is proposed to control the post-harvest metabolism of mango. The results of gas barrier permeability show that CNCs enhance the barrier ability of the chitosan (CS) membrane on mango by 202 % and 63 % for oxygen and water vapor, respectively. The gas-barrier coating reduces the climb in pH and the decrease in firmness by 84.9 % and 45.8 %, respectively, decelerating the conversion process from starch to sugar. Besides, introducing clove essential oil (CEO), the CEO mainly adsorbs and crystalizes on the hydrophobic facets of CNCs, presenting high compatibility, increases the antibacterial rate to nearly 100 %. As a consequence, the preservation period of the mango coated by the CNC-based membrane is at least 7-day longer than the control group. Such a gas-barrier coating based on eco-friendly composites must have excellent potential in the preservation of mango, and even for other tropical fruits.

Flow induced attrition of cellulose nanocrystals

To study the potential impacts of shear stress on cellulose nanocrystals (CNCs), a microcapillary rheometer was employed to repeatedly shear approximately 10 mL of 6 wt% aqueous CNC suspension at 25 °C and rates ranging from 1,000 s-1 to 501,000 s-1. A 9 wt% CNC suspension was also tested at 316,000 s-1 for comparison of concentration effects on the behavior of the suspensions. After monitoring viscosity for 25 steady shear measurements, the suspensions processed at 1,000 s-1 decreased in viscosity by approximately 20 %. Higher shear rates produced smaller changes in viscosity, while increasing the concentration produced higher general viscosities. Atomic force microscopy (AFM) and X-ray diffraction (XRD) probed physical changes between the neat and sheared CNC samples. AFM images showed up to a 24 % reduction in length after shearing, but an insignificant reduction in cross-section. XRD showed a slight increase in the ratio of amorphous to crystalline fractions of the CNCs. Additionally, conductometric titration showed insignificant differences between neat and sheared samples. These findings suggest that viscosity changes in CNC suspensions during steady shear flow arise from physical fracturing of the CNCs perpendicular to their length, and not significantly from chemical degradation or reduction in residual amorphous content.

Accelerating Cellulose Nanocrystal Assembly into Chiral Nanostructures

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

Bioinspired Photonic Materials from Cellulose: Fabrication, Optical Analysis, and Applications

Polysaccharides are a class of biopolymers that are widely exploited in living organisms for a diversity of applications, ranging from structural reinforcement to energy storage. Among the numerous types of polysaccharides found in the natural world, cellulose is the most abundant and widespread, as it is found in virtually all plants. Cellulose is typically organized into nanoscale crystalline fibrils within the cell wall to give structural integrity to plant tissue. However, in several species, such fibrils are organized into helicoidal nanostructures with a periodicity comparable to visible light (i.e., in the range 250-450 nm), resulting in structural coloration. As such, when taking bioinspiration as a design principle, it is clear that helicoidal cellulose architectures are a promising approach to developing sustainable photonic materials. Different forms of cellulose-derived materials have been shown to produce structural color by exploiting self-assembly processes. For example, crystalline nanoparticles of cellulose can be extracted from natural sources, such as cotton or wood, by strong acid hydrolysis. Such "cellulose nanocrystals" (CNCs) have been shown to form colloidal suspensions in water that can spontaneously self-organize into a cholesteric liquid crystal phase, mimicking the natural helicoidal architecture. Upon drying, this nanoscale ordering can be retained into the solid state, enabling the specific reflection of visible light. Using this approach, colors from across the entire visible spectrum can be produced, alongside striking visual effects such as iridescence or a metallic shine. Similarly, polymeric cellulose derivatives can also organize into a cholesteric liquid crystal. In particular, edible hydroxypropyl cellulose (HPC) is known to produce colorful mesophases at high concentrations in water (ca. 60-70 wt %). This solution state behavior allows for interesting visual effects such as mechanochromism (enabling its use in low-cost colorimetric pressure or strain sensors), while trapping the structure into the solid state enables the production of structurally colored films, particles and 3D printed objects. In this article, we summarize the state-of-the-art for CNC and HPC-based photonic materials, encompassing the underlying self-assembly processes, strategies to design their photonic response, and current approaches to translate this burgeoning green technology toward commercial application in a wide range of sectors, from packaging to cosmetics and food. This overview is supported by a summary of the analytical techniques required to characterize these photonic materials and approaches to model their optical response. Finally, we present several unresolved scientific questions and outstanding technical challenges that the wider community should seek to address to develop these sustainable photonic materials.

The Colloidal Properties of Nanocellulose

Nanocelluloses are anisotropic nanoparticles of semicrystalline assemblies of glucan polymers. They have great potential as renewable building blocks in the materials platform of a more sustainable society. As a result, the research on nanocellulose has grown exponentially over the last decades. To fully utilize the properties of nanocelluloses, a fundamental understanding of their colloidal behavior is necessary. As elongated particles with dimensions in a critical nanosize range, their colloidal properties are complex, with several behaviors not covered by classical theories. In this comprehensive Review, we describe the most prominent colloidal behaviors of nanocellulose by combining experimental data and theoretical descriptions. We discuss the preparation and characterization of nanocellulose dispersions, how they form networks at low concentrations, how classical theories cannot describe their behavior, and how they interact with other colloids. We then show examples of how scientists can use this fundamental knowledge to control the assembly of nanocellulose into new materials with exceptional properties. We hope aspiring and established researchers will use this Review as a guide.

Evaporation-Driven Liquid-Liquid Crystalline Phase Separation in Droplets of Anisotropic Colloids

Drying a colloidal droplet involves complex physics that is often accompanied by evaporation-induced concentration gradients inside of the droplet, offering a platform for fundamental and technological opportunities, including self-assembly, thin film deposition, microfabrication, and DNA stretching. Here, we investigate the drying, liquid crystalline structures, and deposit patterns of colloidal liquid crystalline droplets undergoing liquid-liquid crystalline phase separation (LLCPS) during evaporation. We show that evaporation-induced progressive up-concentration inside the drying droplets makes it possible to cross, at different speeds, various thermodynamic stability states in solutions of amyloid fibril rigid filamentous colloids, thus allowing access to both metastable states, where phase separation occurs via nucleation and growth, as well as to unstable states, where phase separation occurs via the more elusive spinodal decomposition, leading to the formation of liquid crystalline microdroplets (or tactoids) of different shapes. We present the tactoids "phase diagram" as a function of the position within the droplet and elucidate their hydrodynamics. Furthermore, we demonstrate that the presence of the amyloid fibrils not only does not enhance the pinning behavior during droplet evaporation but also slightly suppresses it, thus minimizing the coffee-ring effect. We observed that microsize domains with cholesteric structure emerge in the drying droplet close to the droplet's initial edge, yet such domains are not connected to form a uniform cholesteric dried film. Finally, we demonstrate that a fully cholesteric dried layer can be generated from the drying droplets by regulating the kinetics of the evaporation process.