Ultrafast gelation of multifunctional lignin hydrogels enhanced by Fe3+ and Al3+ ions: improved antioxidant, antibacterial, and mechanical properties

Traditional lignin-silver nanoparticle hydrogels are often limited by slow gelation (> 3 h), weak mechanical properties, and limited antibacterial efficacy. In this study, we introduce a novel approach by incorporating metal ions (Fe 3+ and Al 3+ ) into lignin-silver nanoparticle (AgNP) composites embedded in an acrylic acid (AA) matrix. This modification significantly enhances the hydrogel's multifunctionality. The presence of metal ions accelerates gelation, enabling rapid formation of highly crosslinked structures within seconds, a notable improvement over conventional lignin-based hydrogels, which typically exhibit slower gelation. Detailed characterization using SEM, FTIR, and XPS analyses reveals that metal ions interact with lignin moieties, inducing redox reactions and coordinating with functional groups, such as phenolic hydroxyl and carboxyl groups. The Fe 3+ -based hydrogel demonstrates potent antibacterial activity, driven by reactive oxygen species (ROS) generation (MIC = 1 mg/mL and MBC = 2 mg/mL against Escherichia coli and Staphylococcus aureus , respectively), while the Al 3+ -based hydrogel shows enhanced mechanical resilience, maintaining 95% recovery after compression. Compared to conventional lignin@AgNPs-AA hydrogels, both hydrogels exhibit remarkable multifunctional properties, including 2.5 × higher compressive strength, 90% DPPH radical scavenging activity, and improved stability. Mechanistic studies highlight distinct antibacterial pathways: Fe 3+ promotes oxidative stress through Fenton reactions, while Al 3 ⁺ disrupts bacterial membranes. This work establishes a rapid, metal-ion-catalyzed approach for the development of lignin-based hydrogels, addressing previous limitations and offering promising applications in wound healing, sensors, and environmental remediation. Graphical abstract