Copper nanoparticles (Cu-NPs) have a wide range of applications as catalysts, in scientific fields as diverse as drug discovery and materials science. The natural abundance of copper, and its relatively low cost, makes it a viable alternative to catalysts made from rare and expensive precious metals, such as platinum and palladium. However, the synthesis of Cu-NPs usually involves high temperatures and toxic solvents. Additionally, Cu-NPs produced via conventional synthesis tend to agglomerate and oxidise, and require the use of inorganic chemicals to maintain their catalytic activity. New research, published in Small, details proof-of-concept experiments demonstrating that the metal-reducing bacterium Shewanella oneidensis offers a greener route to Cu-NP synthesis, and the potential to reclaim copper from wastewater streams.