Prospective life cycle assessment of emerging silver nanoparticle synthesis methods: One-pot polyethyleneimine chemical reduction and biological reductions

Safer and cleaner synthesis methods of silver nanoparticles (AgNPs) are developed through experiments with simplified reactions and minimal toxic reagents. However, their efficacy in environmental improvement must be assessed from a cradle-to-gate system perspective to support meaningful decision-making. Previous life cycle assessment (LCA) studies on AgNPs often overemphasize the contribution of Ag sourcing on the synthesis, rendering the development of alternative synthesis methods insignificant. In this study, we examined three emerging synthesis methods of AgNPs: (a) a simple one-pot method with polyethyleneimine as the sole additive, (b) a bio-based method with a reducing agent originating from leaf extract, and (c) a bio-based method that utilized bacterial activity for silver reduction. To evaluate their environmental performance, and to clarify the influence of factors beyond silver sourcing, we conducted a prospective LCA for lab-scale and industrial-scale production. The resulting global warming potential was 554-783 kg CO2e/kg AgNPs at the lab scale and 472-534 kg CO2e/kg AgNPs at the industrial scale, outperforming conventional chemical reduction methods. We further investigated the potential performance changes by shifting from primary to secondary silver sources and by increasing the use of renewable energy in future societies. Scenario analyses revealed that transitioning to secondary silver sources could further improve environmental performance. Consequently, the choice of synthesis methods becomes significant, as it directly influences both electricity and reagent input. This study demonstrates an LCA framework that assessed current and prospective syntheses of AgNPs to support the research and development toward cleaner AgNPs in an expanding industry.