Lignin-Based Copper Nanoparticles for Green and Flexible Electronics

Copper nanoparticles offer a cost-effective and sustainable alternative to silver-based materials for conductive inks in printed and flexible electronics. However, their practical application is often hindered by the need for inert atmospheres and environmentally hazardous reducing agents, but most critically by their rapid postsynthesis oxidation. In this work, we report a green, scalable synthesis of oxidation-resistant metallic Cu nanoparticles via a microwave-assisted polyol method using sodium hypophosphite as a nontoxic reducing agent and lignin as a capping and stabilizing agent. Nanoparticle formation occurs during a final microwave irradiation step as short as 5 min, following a brief homogenization stage, resulting in moderately polydisperse particles with average diameters around 150 nm and without the need for an inert atmosphere. The lignin coating significantly enhances the stability of the particles, maintaining their metallic state for over 120 days in ethanol and 60 days in air. Structural and compositional analyses confirm the effectiveness of lignin in preventing surface oxidation, while electrical conductivity tests show promising values (up to 3.83 × 106 S/m, corresponding to a resistivity of 26.1 μΩ·cm), outperforming commercial references. These results demonstrate the potential of lignin-stabilized copper nanoparticles as eco-friendly conductive fillers for next-generation green and flexible electronics.