Role of the Deep Eutectic Solvent Reline in the Synthesis of Gold Nanoparticles

This work presents a mechanistic understanding of thesynthesisof small (<3 nm) gold nanoparticles in a nontoxic, eco-friendly,and biodegradable eutectic mixture of choline chloride and urea (reline)without the addition of external reducing or stabilization agents.Reline acts as a reducing agent by releasing ammonia (via urea hydrolysis), forming gold nanoparticles even at trace ammoniaconcentration levels. Reline also affects the speciation of the goldprecursor forming gold chloro-complexes, stabilizing Au+ species, leading to an easier reduction and avoiding the otherwisefast disproportionation reaction. Such a capability is however lostin the presence of large amounts of water, where water replaces thechloride ligands in the precursor speciation. In addition, relineacts as a weak stabilizing agent, leading to small particles (<3nm) and narrow distributions although agglomerates quickly form. Suchproperties are maintained in the presence of water, indicating thatit is linked to the urea stabilization rather than the hydrogen-bondingnetwork. This work has important implications in the field of greensynthesis of nanoparticles with small sizes, especially for biomedicaland health care applications, due to the nontoxic nature of the componentsof deep eutectic solvents in contrast to the conventional routes. Nontoxic, eco-friendly, and biodegradableeutectic mixturescan replace external reducing and stabilization agents in the synthesisof metal nanoparticles.