Lignin is one of the most abundant aromatic polymers on Earth and has long been recognized as a promising biomass resource. However, due to its complex and heterogenous structure and resistance to degradation, its utilization has largely been limited to combustion for energy. To unlock its full potential, our research focused on the optical properties of lignin, aiming to control its luminescence intensity and emission wavelength by manipulating the local environment around chromophores.
Specifically, we genetically engineered poplar trees to overexpress the enzyme Feruloyl-CoA 6’-hydroxylase (F6’H1), which converts feruloyl-CoA—an intermediate in lignin biosynthesis—into scopoletin, a coumarin derivative with excellent luminescent properties. The resulting lignin incorporated scopoletin structures, leading to a red-shift in emission wavelength into the visible range and suppression of fluorescence quenching.
The engineered lignin maintained clear luminescence even in low-polarity solvents, indicating uniform distribution of scopoletin within the lignin molecule. Furthermore, the luminescence was preserved when the lignin was embedded in polymer matrices, and its intensity varied depending on the solvent and polymer interactions, highlighting the importance of material design.
Additionally, the lignin exhibited pH-responsive fluorescence, with intensity increasing under alkaline conditions and decreasing under acidic conditions. Reversible photo-dimerization of scopoletin was also observed upon UV irradiation, endowing the lignin with light-responsive properties for the first time. These features suggest potential applications in stimuli-responsive materials, such as shape-memory polymers, photo-switchable gels, fluorescent tags, and 3D printing materials.
This pioneering study demonstrates the feasibility of transforming underutilized biomass into high-performance optical materials through molecular design and genetic engineering. It represents a significant step toward the development of environmentally friendly, sustainable photo-functional materials and offers promising prospects for future innovations in materials science, environmental technology, and biotechnology.