Biochemical modification and functionalization of nanocellulose surface: Dissertation

Cellulose is an abundant biopolymer found in many different organisms ranging from microbes to plants and animals. The homopolymer, composed of repeating glucose units, forms mechanically strong nanosized fibrils and rods. In plants cellulose forms macroscopic fibers, which are incorporated in the cell walls. Recently, it has been shown that cellulose fibers can be disintegrated into the fibrils and rods by different chemical treatments. These materials are called nanocellulose. Nanocellulose is a promising material to replace fossil based materials because it is renewable, biodegradable and abundant. It holds great potential in many applications due to its superior mechanical properties and large surface area. For most applications modification of nanocellulose surface is needed due to its tendency to aggregate by hydrogen bonding to adjacent cellulose surfaces. In this thesis we took a biochemical approach on nanocellulose surface modification to achieve modified and functional materials. The advantages of this approach are that the reactions are done in mild aqueous ambient conditions and the amount of functionalities of biomolecules is broad. Four different approaches were chosen. First, genetically engineered cellulose binding proteins, were used to introduce amphiphilic nature to nanocellulose in order to create surface self-assembled nanocellulose films and to stabilize emulsions. This method was shown to be a good method for bringing new function to nanocellulose. (Publication I) Second, covalent coupling of enzymes directly onto modified nanocellulose surfaces provided a route for protein immobilization in bulk. Nanocellulose derivatives were shown to be well suited platforms for easy preparation of bioactive films. More over the film properties could be tuned depending on the properties of the derivative. (Publication II) Third, by modifying the nanocellulose surface with specific enzymes we could study the role of hemicellulose in nanocellulose fibril surface interactions. We showed that hemicellulose has an important role in nanofibrillated cellulose networks, yet its effects were different in aqueous and dry matrixes. (Publication III) Fourth, by modifying the specific function of cellulose binding protein via genetic engineering we showed how the binding properties can be altered and thus the functionalization properties can be tuned, and that the cellulose binding protein properties are substrate dependent. We also showed that nanocellulose as a model substrate in binding studies is a valuable tool for gaining new insight in protein binding behavior. (Publication IV) In conclusion, we showed that biochemical methods are feasible in nanocellulose modification and functionalization to study intrinsic properties of nanocellulose and cellulose binding proteins but also for creating new functional materials.