Matrix-Engineered Cellulose Nanocrystals for Robust and Water-Stable Chiral Photonic Devices

Conspectus Cellulose nanocrystals (CNCs), derived from biomass via chemical extraction, exhibit a remarkable capacity for spontaneous chiral self-assembly, leading to hierarchically structured materials that integrate chemical, physical, optical, and chiral properties. Capitalizing on the innate renewability and biodegradability of CNCs, their organization into chiral photonic architectures has emerged as a promising strategy for developing sustainable optical materials, offering eco-friendly alternatives to conventional pigments and coatings. Co-assembly with diverse functional precursors further facilitates the construction of chiral nematic nanomaterials, broadening their potential applications, including circularly polarized light emission and optical sensing. Recent advances underscore the expanding role of CNC-based materials in photonics, enabled by tunable structural coloration, tailored circularly polarized luminescence, and stimuli-responsive optical switching. Consequently, helical CNC frameworks constitute a compelling platform for developing advanced optical materials within sustainability-driven design paradigms. This Account highlights how matrix engineering transforms inherently fragile and water-labile CNC assemblies into robust, environmentally stable functional materials, marking a pivotal paradigm shift in sustainable photonics. Despite these merits, the practical deployment of CNC-based structural color films and chiral photonic devices is hindered by their inherent water sensitivity and mechanical brittleness. These limitations stem from the high hydrophilicity of CNCs, which causes swelling and structural disintegration in humid environments, and their dense hydrogen-bonded network, which restricts chain mobility and leads to brittle fracture. This Account delineates our recent progress in addressing these limitations through matrix engineering of CNC derivatives. We have developed strategies based on compatible chemical cross-linking and innovative intermolecular interactions to convert water-sensitive chiral photonic CNC assemblies into solvent-resistant or controllably swellable systems, while preserving their essential chiral nanostructure. Through methods such as photo-cross-linking polymerization and hydration-induced molecular chain polarization, these materials achieve enhanced mechanical robustness, with high strength, strain, and toughness. The exceptional stability and flexibility unlock a broad prospect, which we demonstrate in optical coatings, mechanochromic devices, chiral luminescence anticounterfeiting, and flexible sweat sensors. Furthermore, our work on high-performance CNC-based chiral photonic hydrogels extends their applicability to biowearable devices and human–machine interfaces. By spatially confining CNC self-assembly or coassembly within defined geometries (e.g., spheres or cylinders) in conjunction with cross-linking, we have engineered chiral photonic microspheres and filaments with tailored topological features. This Account summarizes our contributions along with relevant advances from other researchers, focusing on: (1) strategies to improve the environmental and mechanical stability of CNC photonic materials, and (2) functionalization pathways that facilitate their practical deployment. We also offer perspectives on the development of robust CNC-derived photonic biointerfaces and optical fibers, aligning with the growing demand for intelligent, multifunctional, and sustainable technologies.