Covalent Modification of Nanocellulose Towards Advanced Functional Materials

Nanocelluloses including cellulose nanocrystals (CNC), cellulose nanofibrils (CNF) and bacterial cellulose (BNC) display properties that make them suitable for the development of new, advanced materials. They are expected to play key roles in the future bioeconomy, where sustainability and biomass valorization are important concepts. This thesis work mainly focused on proposing and testing effective chemical modification strategies to endow nanocelluloses with new properties and, developing novel hybrid materials. CNF and BNC as well as Carboxymethylated CNF (CM-CNF), TEMPO-oxidized CNF (TO-CNF) and TEMPO-oxidized CNC (TO-CNC) were studied with different microscopies including Atomic Force Microscopy, spectroscopic techniques such as X-ray photoelectron spectroscopy and Fourier Transform-Infrared Spectroscopy and other surface, thermo-mechanical and magnetic instrumentation. In order to prepare the hybrid materials, nanoparticles including gold nanoparticles (Au NP), carbon quantum dots (CQD) and magnetic nanoparticles (Fe3O4 NP) were synthesized. The binding of the respective NP with nanocellulose was accomplished by using Ethylcarbodiimide hydrochloride/N-hydroxysuccinimide (EDC/NHS) and Azide-alkyne Huisgen cycloaddition (CuAAC click coupling). In turn, the NP assemblies that were installed on the surface made the nanocelluloses truly functional. Significantly, the aforementioned reactions were performed in aqueous media under relatively mild reaction conditions. Several applications of the synthesized nanoparticle-nanocellulose hybrids were demonstrated. For instance, CQD-functionalized nanocellulose was utilized for preparing luminescent nanopaper and as cytocompatible probes for bio-imaging. Magnetically responsive hybrids were designed for protein separation and an impressive separation capacity was realized for isolation of lysozyme from egg white. As very important forms of nanocellulose networks, films and nanopapers were developed as alternatives to plastics made from non-renewable carbon sources. To this end, the change of the inherent hydrophilic character of the cellulosic materials was addressed by using different strategies. For example, photo-induced (thiol-ene and thiol-yne) click reactions were demonstrated to tailor the surface wettability of films and nanopapers. The reactions were shown to be scalable and effective and can be completed within ten minutes. The developed approaches facilitated the generation of properties that are otherwise not possible for unmodified nanocelluloses. For example, CNF films with patterned surface designs and super-slippery properties were developed, making them suitable for applications relevant to drug screening, cell culture, diagnostics, anti-fouling, etc. Overall, this thesis demonstrates a body of work that expands the possible utilization of nanocelluloses in advanced materials.