Licentiate seminar: Taoran Xu – Lignin Nanoparticles: Formation Mechanisms and UV Shielding Potential

The research presented in this thesis focuses on the valorization of kraft lignin through nanoparticle engineering, with the aim of developing high-value applications for this abundant but underutilized biopolymer. The work integrates two key aspects: (i) the effect of lignin esterification on nanoparticle self-assembly and (ii) the functional application of unmodified lignin nanoparticles (LNPs) in sunscreen formulations.

The first study investigates the self-assembly behavior of lignin nanoparticles through controlled esterification of industrial kraft lignin. By introducing hydrophobic ester groups of varying chain lengths, the research examines the structural and interfacial properties that influence nanoparticle formation. Comprehensive characterization using FTIR, 31P NMR, HSQC, SEC, TEM, and DLS confirms that esterification modulates lignin self-assembly behaviors,allowing for precise control over LNP morphology and surface properties. A refined self-assembly mechanism is proposed to elucidate the competing molecular interactions governing nanoparticle formation, highlighting the role of lignin’s intrinsic heterogeneity in determining aggregation behavior.Building upon these findings, the second study extends the application of engineered LNPs by incorporating them into sunscreen formulations as biobased UV-protective additives. The study demonstrates that LNPs derived from both spruce and eucalyptus kraft lignin enhance UV shielding across the UVB and UVA spectra, with spruce-derived nanoparticles providing superior broad-spectrum coverage. These formulations were further assessed for stability, color, and rheological properties, confirming that LNPs contribute to functional improvements in sunscreen emulsions. The integration of LNPs into cosmetic formulations underscores their potential as sustainable alternatives to conventional UV filters, bridging fundamental nanoparticle research with real-world applications.

Through this research, lignin valorization is advanced by filling the fundamental knowledge gap between structural modification, self-assembly dynamics, and material functionality. The findings contribute to the development of lignin-based nanomaterials for eco-friendly applications, demonstrating how precise chemical modifications can enhance both fundamental understanding and industrial relevance of lignin nanoparticles.