Seedless, copper-induced synthesis of stable Ag/Cu bimetallic nanoparticles and their optical properties

Abstract

In this study, we demonstrate a sensitive and selective method for the seedless synthesis of an Ag@Cu bimetallic nano-structured material based on the competitive coordination chemistry of cysteine with Cu²⁺ and Ag⁺. The subsequent addition of Ag⁺ to the cysteine–Cu²⁺, leads to the formation of a perfectly transparent orange–yellow color at 425 nm in the UV-visible region after ca. 3 h of mixing time. These changes are ascribed to the formation of Ag@Cu bimetallic nanoparticles. Visual observations and transmission electron microscope data indicate that the reaction mixture containing different orders of reactants (cysteine + Ag⁺, cysteine + Cu²⁺, Ag⁺ + Cu²⁺ + cysteine, and cysteine + Cu²⁺ + Ag⁺) have different colors as well as different morphologies. The reaction proceeds through the reduction of Ag⁺ ions into Ag⁰ by the side chain HS-moiety of the cysteine–Cu²⁺ complex. The resulting cystine–Cu²⁺ complex is subsequently adsorbed onto the surface of Ag⁰. The reduction of Cu²⁺ occurred on the surface of the Ag⁰ by under potential deposition. On the basis of this, the formation of Ag@Cu bimetallic nanoparticles can be visualized with the naked eye through the colorless-to-orange–yellow color change. Cysteine could not reduce the Cu²⁺ ions into metallic copper under normal conditions because Cu²⁺ has a strong affinity towards coordination through the thiol moiety.