Lignin Nanoparticles from Agro-Waste as Bio-Reducers and Metal Concentrators in Functional Nanofibrous Sensors

Lignin, an abundant biopolymer recovered from agro-forestry waste streams, exhibits intrinsic redox and chelating properties that enable the reduction of metal salts into nanoparticles (NPs) and the adsorption/stabilisation of the resulting nanostructures. These features make lignin-derived nanoparticles (LNPs) an attractive, low-cost functional component for hybrid nanomaterials and sensing platforms. In this work, we investigate electrospun nanofibres functionalised with LNPs as active concentrators of gold nanoparticles (AuNPs). LNPs extracted from agro-forestry by-products were embedded into polymer nanofibres, providing phenolic and quinone-like groups capable of reducing Au(III) precursors and immobilising the resulting AuNPs. By adjusting the pH of the gold salt solution and the chemical environment around the LNPs, we achieve selective, rational decoration of the nanofibres with AuNPs without external reducing agents. This strategy also enables the recovery of AuNPs from RAEE-derived (electronic-waste) leachates, offering a sustainable route for metal reclamation and upcycling into functional materials. SEM/TEM analysis confirms controlled AuNP nucleation and growth on the fibre surface, while electrical measurements show that AuNP loading increases nanofibre conductivity of about three orders of magnitude through improved charge-transport pathways. Preliminary gas-sensing tests toward polar VOCs reveal enhanced signal amplitude and tuneable selectivity, attributed to synergistic interactions between lignin functional groups, AuNP surface chemistry and the polymer matrix. Overall, this study demonstrates a bio-based, low-cost and sustainable materials platform in which lignin nanoparticles act simultaneously as redox agents, metal concentrators and functional dopants, enabling new hybrid nanofibrous architectures for sensing and resource recovery.