Recyclable Universal Solvents for Chitin to Chitosan with Various Degrees of Acetylation and Construction of Robust Hydrogels

Chitin and chitosan are enticing natural polymers derived from seafood wastes, and their applications mostly depend on the degree of acetylation (DA). For their efficient utilization, a series of universal solvents for the direct dissolution from chitin to chitosan with various DA ranged from 5 to 94% were designed, and robust hydrogels were constructed from their solution-via a physical regeneration, for the first time. The NMR results demonstrated that K+ of KOH interacted easily with C=0 group to break the (NHO)-O-center dot center dot center dot=C intermolecular hydrogen bonds of chitin, whereas Li+ of LiOH could bound with NH2 group to promote the destruction of NH(center dot center dot center dot)06 hydrogen bonds of chitosan. Thus, a series of LiOH/KOH/urea aqueous solutions with weight ratios of LiOH to KOH from 0 to 2.5 were developed to directly dissolve these biomacromolecules with DAs ranging from 5 to 94%. Subsequently, a series of coagulants were also exploited for the regeneration of these chitin/chitosan solutions to construct the robust hydrogels with different DAs. These chitin/chitosan hydrogels exhibited homogeneous network structure consisting of nanofibers with a mean diameter of similar to 30 nm as well as excellent mechanical properties and high transparency. By adding a fluorescent agent with strong affinity with only the-NH2 group, the resulting fluorescent hydrogels with different DAs could be visually recognized because the intensity reflected the different contents of the active amino group. Furthermore, the wastewater after regeneration could be easily recycled via reduced pressure distillation to reuse without consuming any chemicals or producing byproducts, leading to a thoroughly green process. This work opens a new avenue for dissolving biomacromolecules from chitin to chitosan via universal solvents and to construct new materials via green transformation, which would be beneficial for global sustainable development.