Nanocellulose for bio-inspired nanocomposites - surface modification with recombinant proteins

The increased concern about the environment and legislation pressure, such as plastic bag ban, has generated the fast-grown demand for renewable and sustainable materials to replace the petroleum-based products. Cellulose is the most abundant natural polymer on earth, which can be found in many different organisms ranging from microbes to plants and animals. Nanocellulose, including cellulose nanofibrils and cellulose nanocrystals, can be extracted from plant cell wall by mechanical treatment or acid hydrolysis. Due to its extraordinary mechanical properties, low density and good biocompatibility, nanocellulose attracts increasing interests for materials scientist. In this work, we used genetically engineered recombinant proteins containing multiple functional domains to functionalize nanocellulose and make nanocomposites together with other building components. First, we needed to understand how the proteins interact with nanocellulose. The cellulose binding domains were coupled with gold nanoparticles through EDC-NHS chemistry and their binding to nanocellulose was visualized with electron microscopy. Further, the in situ interaction of recombinant protein and nanocellulose surface was investigated with QCM-D. It showed that the recombinant proteins containing resilin like polypeptide and cellulose binding module could bind to cellulose surface and form a pH responsive layer. From the rheology study, we also found that the recombinant protein could cross-link the cellulose nanofibrils. This finding led to our next study, improve the mechanical properties of nanocellulose film by cross linking the fibrils with cellulose binding proteins. The stiffness of the cellulose film was clearly increased due to the cross-linking of fibrils. In order to make conductive cellulose film, carbon nanotubes were added to the cellulose matrix. A bifunctional protein containing a hydrophobic patch and a glycosylated domain was used to disperse carbon nanotube into the cellulose matrix. In addition, the protein-protein interaction was also investigated in this work. The recombinant protein contains resilin like polypeptides could form salt induced coacervates. The effect of ionic strength, pH, protein concentration and different termimal domains were studied with dynamic light scattering and electron microscopies. In conclusion, we demonstrated that genetically engineered proteins provide a new toolbox for the functionalization of nanocellulose and modify the interface between different building components in nanocomposites