Chiral Self-Assembly of Cellulose Nanocrystals—The Role of Composite Morphology

Naturally-sourced cellulose nanocrystals (CNCs) are elongated nanoparticles that spontaneously form a left-handed cholesteric liquid crystal phase in aqueous suspension. This self-assembly process has been shown to be a promising way to create structurally colored films from sustainable materials (<i>Adv. Mater.</i> (2018) 30, 1704477), but the underlying mechanism by which chiral ordering emerges has remained unclear. Although the morphology of individual CNCs is believed to play an important role, most particles are not significantly twisted, and suspensions exhibit considerable polydispersity in both particle size and shape.<br/><br/>Here we will present an in-depth morphological analysis of sub-populations of individual CNCs, as imaged by transmission electron microscopy (TEM) and atomic force microscopy (AFM). We gradually tuned the size and shape of the CNCs using ultrasonication and correlated the morphology of individual particles with their ensemble behavior in suspension and in photonic films. This analysis revealed that CNC “bundles” (i.e. clusters of laterally-bound elementary crystallites) are essential for the formation of a cholesteric phase. These bundles act as chiral dopants, analogous to those used for molecular liquid crystals, and therefore explain how chirality is transferred across length-scales from the morphology of individual CNCs to the photonic nanostructure (<i>Nat. Commun.</i> (2022) 13, 2657).