Lignin nanoparticles (LNPs) are sustainable nanomaterials obtained from agro-industrial residues, with increasing interest for different applications. Their biocompatibility and functional properties have made LNPs one of the most explored materials in food packaging, drug delivery, and photonic applications. The challenge with LNPs is the lack of uniformity in their production, as broad particle size distributions can limit their effectiveness in advanced applications that require size precision and homogeneity. This study presents the extraction of lignin and the synthesis of size-controlled LNPs from eucalyptus bark, using an ethanol-organosolv process followed by antisolvent precipitation. By valuing a low-cost waste material, this work also contributes to circular economy and sustainability goals, positioning LNPs as viable bio-based nanomaterials for high-value applications.

Lignin was extracted under non-isothermal conditions at 230 °C using 65% ethanol, resulting in an extraction yield of 49% and purity of 96.07%. The dried lignin was dissolved in ethanol to prepare a precursor solution, which was then added dropwise into deionized water at controlled dilution rates to form nanoparticles, without the use of surfactants or chemical additives. Size control was achieved by adjusting the water-to-solvent ratio during precipitation. Higher water content increased supersaturation, promoting rapid nucleation and the formation of smaller nanoparticles, while lower water content reduced nucleation rates, allowing more particle growth and larger sizes. Dynamic Light Scattering revealed tunable nanoparticle sizes ranging from 148.0 ± 3.2 nm to 274.9 ± 8.1 nm, with polydispersity index values below 0.2, indicating narrow size distributions. Zeta potential measurements confirmed the colloidal stability of the aqueous dispersions. Scanning electron microscopy images show the round shape and confirm the sizes of the produced LNPs.

This environmentally friendly method shows how to transform lignin into uniform, and stable nanoparticles using only ethanol and water. The resulting LNPs are well suited for applications that require precise reproducibility and sustainability.