Contributed by Kai Zhang Wood Technology and Wood Chemistry, Georg-August-University of Göttingen, Büsgenweg 4
Nature provides a huge source of sustainable biopolymers, such as cellulose, hemicelluloses and lignin from plant. Their use in human history has a long tradition. However, they were and are still being widely used in their original, less or non-modified forms. On the molecular levels, they have similar polymeric chain structures and backbones as the synthetic polymers. In comparison, many framework molecules that are used in high-technology field in the last centuries are derived from petroleum, which has limited amount on earth. In contrary, the biopolymers, such as cellulose, are biosynthesized with an amount of millions of tonnes annually worldwide. In order to advance the application of biopolymers in high-technology fields, a promising strategy is to provide them special functions through chemical modifications.
Thanks to numerous hydroxyl groups at cellulose backbone, chemical reactions that can be carried out on hydroxyl groups, the feasibility to regioselectivelly control the derivatization of the hydroxyl groups within the building blocks of cellulose (the primary or secondary hydroxyl groups within anhydroglucose units that are the building blocks), and the contents of modified hydroxyl groups, a much bigger world of more diverse molecules is conceivable compared to the compounds derived from petroleum. In my group, we attempt to synthesize various novel polymeric compounds with particular functions from cellulose and then transform them into functional materials with various forms, including nanoparticles, flower-like particles, thin films and surface coatings. In particular, the materials should maintain the functions that have been pre-designed on the molecular level.
Among others, the pH-responsive nanoparticles with switchable sizes can be prepared using a modified nanoprecipitation method with the solution of cellulose derivatives containing terminal tertiary amine groups, more precisely, cellulose 11-(2-(dimethylamino)ethylthio)undecanoate and cellulose 11-(2-(diethylamino)ethylthio)undecanoate (Figure 1). As further steps, it is possible to introduce fluorescence compounds onto the terminal double bonds before or after the introduction of the pH-responsive tertiary amine groups, to endow the nanoparticles more functions. These stimuli-responsive nanoparticles are of great interest for future applications as drug-delivery systems or for biomedical imaging. In particular, the polysaccharides have further intrinsic advantages, such as excellent biocompatibilities, simple degradability and non-toxicity. By carefully choosing the groups that are introduced for the acquisition of functions, we could maintain these advantages of polysaccharides. However, the exact effect of the nanoparticles derived from cellulose derivatives or derivatives from other polysaccharides on human still needs more investigations due to the presence of dimethylaminoethyl or diethylaminoethyl groups. The investigations can be performed e.g. on cells and living organs. In comparison, their impact on environment should be much less considerable due to sensitive ester bonds as well as glycosidic bonds of polysaccharides, which can be simply degraded into mono sugars.
Although the chemical syntheses of cellulose provide a straightforward strategy for the preparation of novel compounds, their realizations still need improvements in the framework of green chemistry for less environmental impacts. Lots of challenges still remain, although some attempts have already been performed. For instance, green and renewable solvents including ionic liquids have been used as reaction medium. Mild reactions parameters and efficient reaction techniques are preferentially applied for the modifications. In the near future, solid-state reactions, catalyzed chemical reactions, one-pot synthesis strategy and further new synthetic strategies will gain their positions in this field. With all these optimized conditions for green and sustainable modifications on cellulose as well as many other polysaccharides, a broad library of novel compounds is not only achievable, but also are their syntheses on a bioeconomic and bioefficient basis.
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