A Tunable Hydrophilic-Hydrophobic, Stimulus Responsive, and Robust Iridescent Structural Color Bionic Film with Chiral Photonic Crystal Nanointerface

Electrostatic fence induced assembly of low-concentration colloidal nanospheres to form liquid photonic crystals

Liquid photonic crystals (LPCs) have great application potential in sensors, anti-counterfeiting materials and detection due to their sensitive dynamic responsiveness. Currently, the preparation of LPCs mainly relies on the supersaturation of colloidal nanospheres. However, the supersaturation method usually fails to obtain LPCs based on low-concentration colloidal nanospheres. In turn, the use of high-concentration colloidal nanospheres results in poor mobility of LPCs, which makes them inappropriate for subsequent utilization in responsive systems. In this study, self-assembly of LPCs with vibrant structural colors is achieved through the electrostatic fence effect by introducing an anionic carboxylate-containing polymer as an inducer into the system of low-concentration negatively charged colloidal nanospheres (≥1.25 wt%). It is shown that the anionic carboxylate groups on the inducer molecules and the appropriate molecular chain length are the decisive factors for the inducing effect. The obtained LPCs exhibit a typical non-close-packed structure with a face-centered cubic arrangement of nanospheres. The nearest inter-nanosphere surface distance is 12.10 nm, and the farthest one is as long as 40.30 nm. The LPCs possess good dynamic recovery performance and sensitive optical response characteristics, which are conducive to application in responsive optical sensors directly or after filling.

The Correlations between Microstructures and Color Properties of Nanocrystalline Cellulose: A Concise Review

Cellulose nanocrystals (CNCs) are a green resource which can produce photonic crystal films with structural colors in evaporation-induced self-assembly; CNC photonic crystal films present unique structural colors that cannot be matched by other colored materials. Recently, the mechanisms of CNC photonic crystal films with a unique liquid crystal structure were investigated to obtain homogenous, stable, and even flexible films at a large scale. To clarify the mechanism of colorful CNC photonic crystal films, we briefly summarize the recent advances from the correlations among the preparation methods, microstructures, and color properties. We first discuss the preparation process of CNCs, aiming to realize the green application of resources. Then, the behavior of CNCs in the formation of liquid crystal phases is studied, considering the influence of the CNCs' size and shape, surface properties, and the types and concentrations of solvents. Finally, the film formation process of CNCs and the control of structural colors during the film formation are summarized, as well as the mechanisms of CNC photonic crystal films with full color. In summary, considering the above factors, obtaining reliable commercial CNC photonic crystal films requires a comprehensive consideration of the subsequent preparation processes starting from the preparation of CNCs.

Polysaccharide Nanocrystals-Based Chiral Nematic Structures: From Self-Assembly Mechanisms, Regulation, to Applications

Chiral architectures, one of the key structural features of natural systems ranging from the nanoscale to macroscale, are an infinite source of inspiration for functional materials. Researchers have been, and still are, strongly pursuing the goal of constructing such structures with renewable and sustainable building blocks via simple and efficient strategies. With the merits of high sustainability, renewability, and the ability to self-assemble into chiral nematic structures in aqueous suspensions that can be preserved in the solid state, polysaccharide nanocrystals (PNs) including cellulose nanocrystals (CNCs) and chitin nanocrystals (ChNCs) offer opportunities to reach the target. We herein provide a comprehensive review that focuses on the development of CNCs and ChNCs for the use in advanced functional materials. First, the introduction of CNCs and ChNCs, and cellulose- and chitin-formed chiral nematic organizations in the natural world, are given. Then, the self-assembly process of such PNs and the factors influencing this process are comprehensively discussed. After that, we showcased the emerging applications of the self-assembled chiral nematic structures of CNCs and ChNCs. Finally, this review concludes with perspectives on the challenges and opportunities in this field.