Self-assembly of polysaccharide nanocrystals: from aggregation in suspensions to optical materials

Bottom-up synthesis strategies to construct nano-architectonic material exhibiting specific properties by controlling the spatial distribution of the material units are challenging. Native polysaccharide nanocrystals, primarily cellulose and chitin nanocrystals (CNCs and ChNCs), possess excellent intrinsic biodegradability, biocompatibility, tailorable surface chemistry, and unprecedented optical and mechanical properties. These nanocrystals, in particular CNCs, have attracted considerable attention within the last years for constructing optical materials via bottom-up self-assembly. Here, the physicochemical mechanisms underlying the self-assembly of CNC nanocrystals and the structure-property relations of CNC nanocrystal assembly structures are summarized, including the transition from the isotropic phase at low concentrations to the cholesteric phase at high concentrations, and finally to dry films in a fixed state. The properties of aggregated and self-assembled CNCs are described in detail. Based on the dimensions of self-assembled structures as divided in zero-, one, two and three-dimensional constructions, recent advances of polysaccharide nanocrystals-based optical materials are discussed. Finally, the challenges of the methods for the environmentally benign preparation of polysaccharide nanocrystals are identified and the opportunities for realizing novel functional materials based on polysaccharide nanocrystal assembly are described.