Green synthesis of gold and silver nanoparticles: Challenges and opportunities

The synthesis of gold and silver nanoparticles was evaluated using green chemistry metrics, with an emphasis on process mass intensity (PMI). Opportunities for improving synthetic methods were identified based on these metrics. Solvent usage was identified as the biggest challenge among nanoparticle syntheses, including those otherwise deemed as green, with PMIs that were typically in the thousands. The synthesis of ligated metal nanoparticles by arrested precipitation was identified as the best opportunity for making impactful improvements, since they are the most industrially relevant but the least green. Although this review focuses on gold and silver nanoparticles, much of the discussion also pertains to inorganic nanomaterials synthesis in general.