Fast Self-Assembly of Scalable Photonic Cellulose Nanocrystals and Hybrid Films via Electrophoresis

Replacing existing nonrenewable engineering materials with bio-based and nano-enabled functional materials has attracted significant research attention. These functional materials would play a significant role in facilitating the transition to a sustainable society, owing to their nontoxicity and excellent performance. “Thanks to their extraordinary optical, self-assembly, and electrochemical properties, cellulose nanocrystals (CNCs) are expected to play a significant role in fabricating optoelectronic, photonic, and functional materials,” explains Dr Wadood Hamad, Lead Scientist and Research Manager at FPInnovation. Typically, CNCs are produced via acid hydrolysis of biomass. Beyond a critical concentration, CNCs form iridescent films endowed with photonic properties. “The self-assembly of CNCs into chiral nematic structures gives rise to creating structural colour leading to useful photonic properties,” explains Dr. Hamad. “Structural coloration is a phenomenon observed extensively in nature in some plants and animals, like exoskeletons, e.g., birds (kingfisher), butterfly wings, beetle wing cases, etc.” he continued. “This phenomenon has drawn considerable research attention over the past two decades for the fabrication of nontoxic, bio-inspired sensors, pigments, coatings and templating functional materials and structures.”

Currently, all reported chiral CNC films have been synthesized via evaporation-induced self-assembly (EISA), also referred to as solution casting. However, this technique has several drawbacks that limit its fabrication capabilities. For example, EISA is highly sensitive to electric and magnetic fields and external stimuli like temperature, which may induce difficulty in controlling particulate contamination. Although the chiral nematic CNC films could be fabricated via other methods like capillary confinement, they are time intensive. Hence the need to develop a rapid and robust technique for producing commercially and industrially scalable CNC chiral nematic structures becomes vital in facilitating its technological viability. Dr Hamad and his team at FPInnovations, Canada’s leading non-for-profit R&D organization for valorizing high-value products from forestry materials, developed an industrially viable and scalable approach to create photonic CNC films or CNC-based composite films within 1-5 minutes using electrophoresis. Their invention, CNCs-based polymer films using electrochemical techniques, was awarded US Patent No. 10,081,880 and Canadian Patent No. 2,911,750.

Thereafter, Dr. Wadood Hamad and his team from FPInnovations, Drs Siham Atifi and Mehr-Negar Mirvakili, collaborated with Professor Silvia Vignolini and her students, Drs. Cyan Williams and Mélanie Bay, from the University of Cambridge to study the mechanics of CNC formation using this novel method for rapid industrial production of photonic CNC films with superior properties. The technique for electrophoretically induced self-assembly of CNCs, and the underlying mechanisms of CNC self-assembly and the influence of process parameters was investigated. The work is currently published in the journal, Advanced Materials.

The research team reported the rapid fabrication (within minutes) of photonic films exhibiting long-range chirality on flexible, rigid, or conductive substrates (see Figure a). Moreover, CNC films with different sizes could be fabricated by controlling the electrophoretic deposition parameters and CNC surface properties. The reflected wave intensity of the CNC films could be tuned by varying the number and duration of the electrodeposition cycles of the pulsed electrophoretic deposition. On the other hand, the film coloration could be controlled by the characteristics of the CNC suspensions before the electrodeposition process by either shortening the pitch or reducing the interparticle electrostatic repulsion between the nanoparticles.

Furthermore, the compatibility and the potential to co-assemble CNCs with other nanoparticle types to produce hybrid photonic-plasmonic films via one-pot electrodeposition without reducing agents was successfully demonstrated. For instance, the resulting CNCs were compatible with in situ reduction of gold precursors in the absence of toxic reducing agents. As a result, hybrid photonic films with controllable plasmonic responses were successfully prepared by altering the nature and amount of gold precursors.

In summary, a fast, scalable electrodeposition technique for producing long-range chiral nematic CNC films on different substrates was reported, successfully overcoming the inherent challenges of conventional methods. Regardless of the charge functionality of the CNC surface, the prepared dry films retained their chiral nematic structure. In a statement to Advances in Engineering, Dr. Wadood Hamad the corresponding author and lead investigator, explained their study “provides valuable insights that would positively impact the potential to industrially produce renewable and environmentally friendly photonic films based on CNCs for developing functional hierarchical materials and structures suitable for photonic and optoelectronic applications, as well as for a more sustainable colour production and management in developing a new generation of coating materials.”