Spectroscopic and Microscopic Study of Silicon-Lignin Interaction: Effects on Plant Cell Walls and Industrial Potential

Silicon (Si) is one of the most abundant elements on Earth and plays a crucial role in plant biology. It is a beneficial element for plants, contributing to their mechanical support, enhancing grain yield, facilitating mineral nutrition, and bolstering stress response mechanisms. Si is present in plant cell walls (CWs), where it is covalently bound to polysaccharides and lignin. This incorporation strengthens CWs, increases mechanical resilience, and mitigates the effects of biotic and abiotic stresses. Despite its importance, the precise interaction between Si and the lignin formation process in CWs remains poorly understood. This study explores the interaction between Si and lignin model compounds during in vitro synthesis, with a particular focus on how Si concentrations modulate lignin polymerization dynamics over time. Fluorescence techniques (microscopy and spectroscopy), combined with FTIR spectroscopy and atomic force microscopy (AFM), proved effective in monitoring the effects of Si on molecular conformation and aggregation behavior. We investigated the interaction of SiO₂ (complexed with NH₄OH) with the peroxidase-catalyzed polymerization of a lignin monomer into a dehydrogenation polymer (DHP) as a model system. Three Si concentrations (0.1 mM, 0.6 mM, and 6 mM) were analyzed and compared with a pure DHP sample. Samples were monitored at three synthesis intervals: 1 minute, 4 hours, and 24 hours. Results demonstrate that lower Si concentrations promote the aggregation of lignin oligomers into larger particles, while higher concentrations increase oligomer repulsion, preventing particle growth. Fluorescence spectroscopy revealed that polymer fragments containing conjugated -C=C- and -C–C- bonds exhibit redshifts in the green region (500–520 nm), with the extent of the shift correlating to Si concentration. Fluorescence microscopy and AFM showed that samples with higher Si concentrations formed more compact structures, with increased rigidity attributed to Si binding. FTIR spectroscopy identified Si-specific bands (967 cm⁻¹ and 1093 cm⁻¹) in the final polymers, indicating Si’s integration into the lignin structure. Notably, Si did not form new intermolecular bonds but interacted with dimers formed during DHP synthesis, inhibiting the formation of larger lignin fragments. Bands above 1200 cm⁻¹ in the FTIR spectra of Si-containing samples became less prominent, suggesting a significant impact of Si on lignin chemistry. These findings provide valuable insights into the complex interaction between silicon (Si) and lignin, demonstrating Si’s role in restructuring lignin, shaping cell wall architecture, and improving stress tolerance—key aspects for advancing research in plant biology. Additionally, the exploration of lignin-silica composites offers significant potential, enabling the development of sustainable materials with diverse applications in industries such as construction, medicine, energy, environmental protection, and advanced technologies.