Reflectin and Cellulose-Based Bragg Reflectors with Chiral Photonic Response

Structural colors in nature are generated by elaborate hierarchical nanostructures, which have been exploited for the fabrication of functional bio-inspired photonic materials for sensing applications. One attractive model system for this is the structural coloration in cephalopods, which is achieved by the multilayer interference of periodically stratified reflectin platelets in iridophores that act as Bragg reflectors. Such multi-layered structures have the ability to respond to chemical stimuli via a complex response mechanism involving the change in the multilayer periodicity and absorbance through chromophores. While there are studies focused on using the reflectins in synthetic multilayered constructs, such optical materials are not scalable and the angular and polarisation response cannot be tuned easily. Here we designed a multilayered hybrid material system that is built by layer-by-layer (LbL) processing of reflectins and cellulose nanocrystals (CNCs). CNC films exhibit fascinating optical properties, mainly due to their chiral nematic structure and birefringence. The optical response of this hybrid system combines the chiral response from CNCs and is amplified by the addition of reflectins. LbL processing allows us to explore several parameters, such as dependence on the concentration, layer thickness, and the dynamics of cellulose suspension, in order to achieve optimal structural color-producing systems. The reflectance of these systems was analyzed by using an optical model based on the 4×4 Berreman matrix method (Mathematica^®). In addition, the effect on the retention of the chiral optical response of the CNC by the building up of the thin film was evaluated. Our findings define the optimal points and the limitations of the optical response from these multilayer systems. The multilayer construct with thin layered reflectin and CNCs at the gelation point suggests that the chiral response is affected by the ability to retain the helicoidal morphology during the LBL process and can be non-uniform. Our results show good agreement between the theoretical model and experiments. Overall, we will demonstrate that the unique optical properties of cellulosic materials can be combined with the dynamic coloration imparted by reflectin and this study will potentially broaden the application of these chiral photonic systems.