Production of functionalized nanocelluloses from different sources using deep eutectic solvents and their applications

Nanocelluloses have gained increasing attention over the years due to their attractive intrinsic properties, such as high strength and stiffness, high biocompatibility, good film-forming ability, easy surface functionalization, tunable optical properties, etc. With these unique features, nanocelluloses have the potential to be applied in a wide range of applications. However, the economic and environmental problems associated to the conventional methods of producing nanocelluloses make it difficult to produce them on a large scale. Therefore, the scientific community has been studying new alternatives. One alternative that has recently emerged is the use of deep eutectic solvents (DESs) for the production of nanocelluloses. The biodegradable and biocompatible character of the DESs combined with their low toxicity, easy preparation, tunability and recyclability turn them promising alternatives for the nanocellulose isolation. In this sense, this article provides a comprehensive overview of the production of (ligno)cellulose nanofibrils ((L)CNFs) and (ligno)cellulose nanocrystals ((L)CNCs) from woody resources and non-woody/agricultural residues using DESs. Additionally, the applications of the produced DES- (L)CNFs and DES-(L)CNCs are also discussed. From this review, it was possible to conclude that by using different DES components, different types of surface chemical functionalization on the (L)CNFs are obtained, which confer to the final material distinct properties. Additionally, films produced from the DES-(L)CNFs showed very good mechanical properties. On the other hand, the DES-(L)CNCs can be produced with higher yields and showing better thermal stability compared to the conventional methods of CNC production. Despite the promising results, an in-depth economic analysis on the use of DES for nanocellulose production is still lacking. Notwithstanding, favorable DES recyclability and reuse results indicate that they are good candidates for the nanocellulose (and nanocellulose-based films) production in a large scale.