Controlling self-assembly of cellulose nanocrystals for multiphase colloids and chiral photonics

The growing concern over the environmental costs of synthetic materials has motivated a shift toward renewable, biodegradable, and biocompatible alternatives. Cellulose nanocrystals (CNCs) serve as such building blocks, being notable for their ability to self-organize in water into a lyotropic cholesteric liquid crystal whose helicoidal order can be preserved in the solid state. This doctoral thesis establishes a novel framework for controlling the cholesteric self-assembly behaviour of CNCs via aqueous two-phase systems (ATPSs) and harnesses the cholesteric CNC structure as substrates for engineering plasmonic chiral metamaterials. The thesis first demonstrates the controllable cholesteric self-assembly of CNCs within ATPSs formed by the liquid-liquid phase separation (LLPS) of uncharged poly(ethylene glycol) (PEG) and dextran polymer solutions. The polymer acting as depletants modulate the interaction between CNCs, while the LLPS-induced two distinct polymer compartments provide the spatial confinement for CNCs assembly. By controlling the thermodynamics of the CNC-PEG-dextran aqueous mixtures, we obtain a series of multiphase LC colloids that display hierarchical organization from the micro- to the macroscale. Specifically, under equilibrium conditions, the multicomponent aqueous mixtures separate into two, three or four coexisting phases, displaying alternating stacks of cholesteric and isotropic phases. The structural heterogeneity in the fluid state can be retained upon drying, producing stratified cholesteric CNC films with a compartmentalized polymer distribution and an anisotropic microporous morphology. In contrast, when phase separation in the multicomponent aqueous system is arrested before reaching equilibrium, gradients in osmotic and chemical potential drive the uneven partitioning of CNCs of the two polymer compartments. At a critical nanoparticle concentration, CNCs confined within one polymer phase self-assemble into a cholesteric structure, while those in the other remain isotropic, resulting in a bicontinuous structured LC emulsion. Our findings provide a promising platform for designing novel heterogeneous complex colloidal systems whose functionalities can mimic the intricate structures observed in biology. Cholesteric CNC structures retained in the dried films exhibit birefringence and polarizationselective optical responses, providing a bio-based platform to manipulate light. In the final part of the thesis, cholesteric CNC films are used as substrates to support plasmonic metasurfaces composed of linearly assembled gold nanoparticles (AuNPs), enabling the engineering of chiral lightmatter interactions. The centimetre-scale hybrid plasmonic CNC composites are produced by casting CNC dispersions onto pre-assembled achiral AuNP arrays. During the drying process, the CNCs co-assemble with the AuNPs at the liquid-solid interface, maintaining the array’s linear arrangement and keeping it isolated from the overlying cholesteric CNC layers. This unique configuration displays both the linear dichroism (LD) of the plasmonic arrays and the linear birefringence (LB) of CNC substrate. Despite the achiral geometry of the AuNP arrays, the obtained composites exhibit strong and spectrally tuneable plasmonic circular dichroism, with a maximum value of 1217 mdeg and a dissymmetry factor of −0.19 at the surface lattice resonance wavelength. The enhanced chiroptical activity originates from synergistic LB-LD coupling, offering a new biobased design principle for scalable and controllable chiral photonic materials. Overall, this thesis advances the understanding and control of cholesteric self-assembly of CNCs, enabling the design of multiphase LC colloids and hierarchical photonic architectures that combine compositional heterogeneity, interfacial organization and structural gradients across multiple length scales.