The self-assembly of chitin nanocrystals into hierarchically structured functional materials
To drive more sustainable technological innovations, highly abundant natural resources such as a biopolymer chitin need to be better exploited. In this work, chitin was used to produce rod-shaped nanoparticles, known as chitin nanocrystals (ChNCs) via acid hydrolysis. Firstly, hydrolysis in hydrochloric acid with varied acidity (3.0 and 5.0 M) and duration (90 to 540 min) was investigated to correlate hydrolysis conditions to colloidal and self- assembly properties of the resulting ChNCs. The post processing using tip sonication was investigated, showing that while it reduced the nanoparticle size, the self-assembly properties were not strongly affected, which contrasts with the previously reported findings for cellulose nanocrystals. Using ChNCs allowed to increase the surface charge by means of deacetylation while maintaining the nanoparticle dimensions unaffected, providing evidence that too high surface charge hinders the self-assembly. On the other hand, the process of deacetylation if applied on chitin before the acidic hydrolysis, allows to reduce the nanocrystal thickness without changing the surface charge strongly. As such, nanoparticles of higher aspect ratio can be produced. The importance of the chitin source was evaluated, revealing that ChNCs prepared from mushroom Agaricus bisporus were longer, higher aspect ratio, and less crystalline, when compared to shrimp derived ChNCs. Furthermore, fungal ChNCs exhibited self-assembly at lower nanoparticle concentrations and had smaller chiral nematic pitch in comparison to shrimp ChNC studied at comparable conditions. This broad investigation into the preparation conditions of ChNCs is the first of its kind; the field of cellulose nanocrystals is flourishing owing to comparable studies on cellulose nanocrystals. The effect of ionic strength and pH was revisited to demonstrate that these two parameters can be used to effectively tune the self-assembly of ChNCs, and in turn tune the helicoidal nanoarchitecture preserved in the solid state. While the shrimp ChNC system was found to be limited to pitch values ranging from 650 to 4,000 nm in solid state, fungal ChNC suspension could be evaporated to reach helicoidal pitch values small enough to manifest structural colouration. Optical properties of such ChNC films, reported for the first time, were studied in relation to previously published works and theory to understand their low reflectance, which could be increased by an in-situ post-treatment using a concentrated alkali. The resulting chitin conversion into chitosan retained the nanoarchitecture and increased the birefringence from 0.001-0.003 (chitin) to approximately 0.015 (chitosan). Overall, this work provides the basis for designing ChNC preparation conditions to obtain desired colloidal and liquid crystalline properties, outlines the differences between ChNCs and a much more studied system of cellulose nanocrystals, and shows that ChNCs can be successfully used to be used as a functional material.
chitin, nanocrystals, self-assembly, structural coloration, chiral nematic
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