Regular Linking of Cellulose Nanocrystals via Click Chemistry: Synthesis and Formation of Cellulose Nanoplatelet Gels

Over a number of years work in our laboratory has been developing new chemistry for the use of cellulose nanocrystals (CNCs) as scaffolds for the creation of nanomaterials with novel, stimuli responsive characteristics. Our work takes advantage of the rigid nature of CNCs, the unique nanopattern etched on their surface in the form of regularly spaced primary OH groups, and the fact that these materials have all reducing end groups located on one end. In this communication, a method for the grafting of amine-terminated monomers onto surface-modified CNCs followed by click chemistry is demonstrated. Initially the primary hydroxyl groups on the surface of the CNCs were selectively activated by converting them to carboxylic acids by the use of TEMPO-mediated hypohalite oxidation. Further reactions using the activated TEMPO-oxidized CNCs were carried out via carbodiimide-mediated formation of an amide linkage between precursors carrying an amine functionality and the carboxylic acid groups on the surface of the TEMPO-oxidized CNCs. Subsequently, two sets of CNCs were prepared, containing on their surface an azide derivative and an alkyne derivative, respectively. Finally, the click chemistry reaction, that is, the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition between the azide and the alkyne, surface-activated CNC was employed, bringing together the nanocrystalline materials in a unique regularly packed arrangement demonstrating a degree of molecular control for creating these structures at the nano level.