Ionic liquid, ultrasound-assisted synthesis of lignin nanoparticles for barrier-enhanced all-cellulose nanocomposite films

The primary purpose of this work was to develop novel all-cellulose nanocomposite (ACNC) films by following a green approach that uses an ionic liquid as solvent and requires no additional reducing agents or stabilizers. In this way, biodegradable, UV-blocking ACNC films were obtained by partially dissolving cellulose to entrap lignin nanoparticles (LNP) within. Lignin particles in proportions of 3–7 wt% were modified by sonication in an ionic liquid (IL) to obtain lignin-IL dispersions. The influence of the LNP-IL medium on the chemical, physical, and morphological properties of the resulting nanocomposites was examined, and the properties compared with those of an all-cellulose composite (ACC) film and untreated paper. The TEM technique revealed the formation of unevenly spherical LNPs as small as 5.133 ± 0.003 nm, and XRD spectroscopy a transition from cellulose I to II and an increase in the proportion of non-crystalline cellulose as a result of partial dissolution and regeneration. In addition, SEM images confirmed the deposition of LNPs onto the surface of the regenerated cellulose matrix. The incorporation of LNPs considerably enhanced the UV-blocking, oxygen and water-barrier, biodegradation, antioxidant and antibacterial properties of the films. Moreover, LNPs in proportions of 3 and 5 wt% increased tensile strength, and a proportion of 7 wt% allowed the films to block 97% of light at 280 nm and increased antioxidant activity by 68% relative to the control sample. Interestingly, a 7 wt% LNP content in the films decreased antibacterial activity against Staphylococcus aureus and Escherichia coli by about 42.85% and 63.88%, respectively. The new, multifunctional biocomposite films are suitable for various uses in cellulose-based food packaging.