Simulating the Co-Assembly of Cellulose Nanocrystals and Gold Nanorods - Effect of Size Distribution and Density

The self-assembly of anisotropic particles holds great potential in fabricating multifunctional optical materials through cholesteric structures. Specifically, cholesteric structures formed by these particles can exhibit unique optical properties, making them ideal for various applications for sensing and fabrication of optical security features. In particular, integrating plasmonic materials into cholesteric phases that can interact with the visible spectrum can be used in building metasurfaces that can alter the interaction of the light completely. While there have been efforts to validate this hypothesis, the experimental work on combining the self-assembly of gold nanorods (Au-NRs) with cellulose nanocrystals (CNCs) had limited success as the plasmonic particle loading was minimal and didn’t allow coupling between the plasmonic states of the Au-NPs possible without disrupting the cholesteric order. To understand the physical and theoretical limitations of this co-assembly system, we used molecular dynamic coarse-grained methods with Gay-Berne potential with anisotropic particle systems of different aspect ratios and densities to complement our experimental work.<br/>Our experimental and modeling work demonstrated good agreement with regard to identifying the surface charge and the density of Au-NRs as the main driving force on the co-assembly. Negatively charged AuNRs can distribute uniformly and align in the same direction with CNCs and the particle systems with higher aspect ratio distributions would have successfully co-assembled, as AuNRs would prefer to fill the vacancy between the cholesteric structures formed by CNC particles. Our further studies explain the limit of maximum concentration from density, as lighter particles (CNCs) would reach assembly conditions compared to the AuNRs. Our work sheds new light on the understanding of the self-assembly of multi-component systems with polydispersity and charge variations and takes our understanding one step further in developing optical materials.