One-step hydrolysis for preparation of carboxylated cellulose nanocrystals with high yield at room temperature and their iridescent phenomenon

Here, a facile method to extract the carboxylated cellulose nanocrystals (CNCs) with high yield from microcrystalline cellulose (MCC) at room temperature (RT) is put forward that uses a solvent system of zinc chloride (ZnCl₂) and citric acid (CA) aqueous solution. The rod-like carboxylated CNCs were achieved with a maximum yield of 72.3% and a maximum carboxyl group content of 0.75 mmol/g. Meanwhile, the crystal transformation from cellulose I to II was detected with variation of solvent composition. In addition, to the best of our knowledge, this is the first time that the iridescent phenomenon was observed in extremely thin films which were prepared from carboxylated CNC suspension with a concentration of only 0.08–0.15 wt% via evaporation-induced self-assembly. The possible formation mechanism is considered to be related to the synergistic effects of film confinement and drying stresses. Furthermore, the interaction between Petri dish substrate and the film played a role. As the first example of isolation of carboxylated CNCs with high yield at RT using ZnCl₂/CA aqueous solution, this work provides a facile and practical strategy for the preparation of functional cellulose nanomaterials.