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

Abstract

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.

Inkjet Printed Photonic Cellulose Nanocrystal Patterns

“…This demonstrates the trade-off between increasing resolution and decreasing visibility that arises from printed photonic structures, where for thin films the intensity scales quadratically with the number of cholesteric pitch repeats. [7,32] It is noteworthy that even this low DPI is comparable to that of dot matrix printers, or commonly used in large-scale signage, such as billboards. Additionally, it is apparent that the smaller drops are more severely impacted by printing through an oil layer with a greater degree of misalignment due to deflection by the air-liquid interface.…”

“…As such, by tuning both the initial composition and the nature of the post-treatment, information can be encoded into the printed CNC arrays. This behavior can be further combined with the polarization-selective reflection (Figure 4c; and Video S2, Supporting Information) and angle-dependent hue (Figure 4d; and Figure S8, Supporting Information) that is inherent to well-aligned CNC microfilms, [7] or the real-time colorimetric response to humidity that arises from their thin dimensions (Figure 4e; and Video S3, Supporting Information). [29] Together, this suite of functional properties promotes photonic CNC arrays as being highly relevant for smart labeling or anticounterfeit applications.…”

“…For example, microfilms tuned via electrolytes will reversibly swell in ethanol (resulting in a transient redshift), whereas for microfilms tuned by additives the hydrophilic polymer can be dissolved and removed by such a post-treatment, resulting in an irreversible blueshift once redried (Figure S7, Supporting Information). and d) angular-dependent color (i.e., iridescence, following Fergason's law [7] ). The photographs in (c) and (d) were recorded under specular illumination.…”

“…[1][2][3][4] Naturally-derived cellulose nanocrystals (CNCs) are elongated, birefringent nanoparticles that, due to their high aspect ratio, self-organize into a cholesteric (i.e., chiral nematic) liquid crystal when colloidally dispersed in water. [5][6][7] Upon drying, the relative orientations of the CNCs within this mesophase can be preserved, leading to a helicoidal nanoarchitecture in the solid state. Crucially for their application as a photonic material, the periodicity of this helicoid (referred to as the pitch, p) is typically on the order of the wavelengths of visible light, giving rise to vibrant, structural color.…”

“…Alternatively, the replacement of water with a nonvolatile hydrophilic additive can lead to a redshifted film, [11,12] attributed to the reduced compression experienced by a kinetically arrested cholesteric domain upon complete loss of water. [7] While the cholesteric self-assembly of CNCs into photonic films and their resultant optical properties have been extensively studied at laboratory scale, the development of photonic CNC coatings toward real-world applications necessitates moving beyond small-scale or batch-based casting methods. For example, while the dimensions of a dish-cast film are typically defined by the geometry of the container, meter-scale films can be continuously cast using Roll-to-Roll deposition.…”