Reactivity of (+)-Catechin with Copper(II) Ions: The Green Synthesis of Size-Controlled Sub-10 nm Copper Nanoparticles

The synthesis of metal nanoparticles via greener and sustainable methods has gained a great attention because of the use of natural products which are nontoxic and environmentally benign. Though many efforts have been made to prepare nanostructures employing plant extracts rich in polyphenols, their role in the formation of nanoparticles has not been fully investigated. This paper demonstrates a route of size-controllable synthesis of copper nanoparticles (CuNPs) using (+)-catechin solution and the chemistry behind the reaction. Altering pH of process parameters allowed for the formation of nanoparticles with sizes ranging from 3 to 40 nm evaluated by high-resolution transmission electron microscopy. The results revealed that the higher the pH, the smaller and the more stable nanoparticles are obtained. Higher concentrations of copper(II) ions [Cu(II)] created larger nanostructures which tended to aggregate. Stable, monodisperse CuNPs with a diameter of about 3 nm were obtained for a (+)-catechin to a Cu(II) molar ratio of 2:1 and pH 11. HPLC analysis showed that (+)-catechin solution at pH 11 contains procyanidins as major phenolic components, and their type and content depend on the presence of copper(II) ions. The proposed mechanism of CuNP synthesis reaction involves pH-dependent oxidation of (+)-catechin and simultaneous reduction of Cu(II).