By controlling the interaction of biological building blocks at the nanoscale, natural photonic nanostructures have been optimized to produce intense coloration. Inspired by such biological nanostructures, the possibility to design the visual appearance of a material by guiding the hierarchical self‐assembly of its constituent components, ideally using natural materials, is an attractive route for rationally designed, sustainable manufacturing. Within the large variety of biological building blocks, cellulose nanocrystals are one of the most promising biosourced materials, primarily for their abundance, biocompatibility, and ability to readily organize into photonic structures. Here, the mechanisms underlying the formation of iridescent, vividly colored materials from colloidal liquid crystal suspensions of cellulose nanocrystals are reviewed and recent advances in structural control over the hierarchical assembly process are reported as a toolbox for the design of sophisticated optical materials.

Complex hierarchical architectures are ubiquitous in nature. By designing and controlling the interaction between elementary building blocks, nature is able to optimize a large variety of materials with multiple functionalities. Such control is, however, extremely challenging in man-made materials, due to the difficulties in controlling their interaction at different length scales simultaneously. Here, hierarchical cholesteric architectures are obtained by the self-assembly of cellulose nanocrystals within shrinking, micron-sized aqueous droplets. This confined, spherical geometry drastically affects the colloidal self-assembly process, resulting in concentric ordering within the droplet, as confirmed by simulation. This provides a quantitative tool to study the interactions of cellulose nanocrystals beyond what has been achieved in a planar geometry. Our developed methodology allows us to fabricate truly hierarchical solid-state architectures from the nanometer to the macroscopic scale using a renewable and sustainable biopolymer.

The self-assembly of cellulose nanocrystals is a powerful method for the fabrication of biosourced photonic films with a chiral optical response. While various techniques have been exploited to tune the optical properties of such systems, the presence of external fields has yet to be reported to significantly modify their optical properties. In this work, by using small commercial magnets (≈ 0.5-1.2 T) the orientation of the cholesteric domains is enabled to tune in suspension as they assemble into films. A detailed analysis of these films shows an unprecedented control of their angular response. This simple and yet powerful technique unlocks new possibilities in designing the visual appearance of such iridescent films, ranging from metallic to pixelated or matt textures, paving the way for the development of truly sustainable photonic pigments in coatings, cosmetics, and security labeling.

We report a protocol that allowed us to fabricate nanoparticle aggregates from anionically coated 7 nm iron oxide nanocrystals and cationic-neutral block copolymers. The control of electrostatics resulted in the elaboration of spherical clusters or of highly persistent nanostructured rods, with lengths between 1 and 50 µm (see figure). The rods were shown to be superparamagnetic

Cellulose nanocrystal suspensions in apolar solvent spontaneously form iridescent liquid-crystalline phases but the control of their macroscopic order is usually poor. The use of electric fields can provide control on the cholesteric orientation and its periodicity, allowing macroscopic sample homogeneity and dynamical tuning of their iridescent hues, and is demonstrated here.

Cellulose nanocrystals (CNCs) are renewable plant-based colloidal particles capable of forming photonic films by solvent evaporation driven self-assembly. So far, the CNC self-assembly process has only been studied at a small scale, neglecting the limitations and challenges posed by continuous deposition processes that are required to exploit this sustainable material in an industrial context. Here, we address these limitations by using roll-to-roll (R2R) deposition to produce large area photonic films, which required optimisation of the formulation of the CNC suspension, the deposition and drying conditions. Furthermore, we show how metre-long structurally coloured films can be processed into effect pigments and glitters that are dispersible, even in water-based formulations. These promising effect pigments are an industrially relevant cellulose-based alternative to current products that are either micropolluting (e.g., non-biodegradable microplastic glitters) or based on carcinogenic, unsustainable or unethically-sourced compounds (e.g., titania, mica). MainMore sustainable approaches to produce effect pigments and functional nanomaterials are being intensively searched for to replace inorganic and plastic components [1][2][3][4][5][6] . In this context, the self-assembly of cellulose nanocrystals (CNCs) into structurally coloured films has attracted significant interest in the scientific community and beyond as a potential candidate to produce more sustainable photonic pigments 1,7,8 . However, while the understanding of the critical processes regulating the nanoscale self-assembly of CNCs has improved sufficiently to enable a wide range of optical applications [9][10][11][12] , and with several companies now supplying nanocellulose in large volumes 13,14 , the lack of scalable methodologies to produce large-area

By a simple two-step procedure, large photonic strain sensors using a biocompatible cellulose derivative are fabricated. Transient color shifts of the sensors are explained by a theoretical model that consideres the deformation of cholesteric domains, which is in agreement with the experimental results. The extremely simple fabrication method is suitable for both miniaturization and large-sale manufacture, taking advantage of inexpensive and sustainable materials

As with many other bio-sourced colloids, chitin nanocrystals (ChNCs) can form liquid crystalline phases with chiral nematic ordering. In this work, we demonstrate that it is possible to finely tune the liquid crystalline behavior of aqueous ChNC suspensions. Such control was made possible by carefully studying how the hydrolysis conditions and suspension treatments affect the colloidal and self-assembly properties of ChNCs. Specifically, we systematically investigate the effects of duration and acidity of chitin hydrolysis required to extract ChNCs, as well as the effects of the tip sonication energy input, degree of acetylation, pH and ionic strength. Finally, we show that by controlled water evaporation, it is possible to retain and control the helicoidal ordering in dry films, leading