Simulating the co-assembly of cellulose nanocrystals and gold nanorods-effect of size distribution and density

Co-assembly of nanoparticles with different shapes and sizes provides an exciting platform to produce multi-functional optical materials and devices. Cellulose nanocrystals (CNCs) self-assemble in a way to form left-handed cholesteric films with high reflectivity due to the Bragg reflection. Creating such helicoidal structures with plasmonic materials can lead to the development of meta-surfaces and security features. However, the experimental work on combining the self-assembly of Au-NRs with CNCs had limited success at a 3% maximum concentration of AuNRs was 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 particle systems of different sizes and densities. Our experimental and modeling work has a strong agreement that the charge and the density of Au-NRs have a significant influence 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.