TAILORING MULTIPLE PHYSICOCHEMICAL PROPERTIES OF SUSTAINABLE LIGNIN NANOPARTICLES

Lignin nanoparticles (LNP) are multifunctional plant-derived nanostructures useful as UV-blockers, antioxidants, and antimicrobials [1]. Herein, we assessed the effects of prior oxidation and different production methods on some key properties of LNP, such as color, morphology, colloidal stability, antioxidant capacity, and UV-absorption [2].
Pure lignin (>98%) isolated from elephant grass was mildly oxidized with 1% (m/v) NaOH and 10% (m/v) H2O2. Neat and oxidized lignins were then converted by (i) a bottom-up (antisolvent) route, yielding spherical/self-assembled LNP, or (ii) a top-down (ultrasonication) route, yielding irregular/globular LNP.
Neat lignin (Mw ~8000 g/mol), which contains UV/VIS chromophores, presented typical dark brown color. In turn, mild oxidation imparted a lighter coloration to oxidized lignin (Mw ~5000 g/mol) due to macromolecule fragmentation. Accordingly, oxidized lignin nanospheres (⌀ ~20 nm) obtained by (i) were smaller than neat lignin nanospheres (⌀ ~50 nm), but the oxidation did not affect the average size of the larger ultrasonicated LNP (~200 nm). Although all neat LNP had a higher content of anionic surface groups (up to 1.9 mmol/g) as compared to neat bulk lignin (~0.2 mmol/g), prior oxidation had paramount importance to produce LNP with surface charge densities as high as 5 mmol/g and outstanding colloidal stability (ζ-potentials below – 30 mV over pH 3–4 to 12). Nevertheless, oxidation reduced LNP antioxidant activity from ~80% to ~20% and slightly decreased its UV-shielding properties.
Therefore, oxidizing the starting lignin and performing different production methodologies are straightforward strategies to tailor the properties of LNP designed for diverse value-added target applications, including nanocomposites and coatings.
1. Österberg, M. et al. Spherical Lignin Particles: A Review on Their Sustainability and Applications. Green Chem. 2020, 22 (9), 2712–2733.
2. Camargos, C. H. M.; Rezende, C. A. Antisolvent versus Ultrasonication: Bottom-up and Top-down Approaches to Produce Lignin Nanoparticles (LNPs) with Tailored Properties. Int. J. Biol. Macromol. 2021, 193, 647–660.