Iodide-mediated reduction of AuCl₄^– and a new green route for the synthesis of single crystalline au nanostructures with pronounced electrocatalytic activity

Abstract

The surfactant-assisted seed-mediated growth approach has been widely used for the synthesis of anisotropic Au nanoparticles. Interestingly, it has been shown that the iodide impurities present in the surfactant ions can control the shape and surface structure of Au nanoparticles. In this article, we describe a different role of iodide ions in the surfactant-free synthesis of single crystalline polyhedral Au nanostructures and their electrocatalytic activity. This new green route for the synthesis of Au nanostructures involves the reduction of Au(III) complex by iodide ions to a metastable Au(I) complex and the subsequent disproportionation of Au(I) at room temperature to metallic Au. The quasispherical Au nanoparticles produced at the initial stage of the reaction autocatalyze the disproportionation and produce Au nanoparticles of polyhedral shape with an average size of 45 nm. The concentration of iodide ions play a crucial role in controlling the shape and surface structure of the nanoparticles. The surface of the nanoparticles obtained at low concentration (10 μM) is exclusively (111) plane. The unreduced Au(III) ions stabilize the Au nanoparticles. Aggregated nanoparticles with irregular shape were obtained at a high concentration of iodide ions. The presence of either silver ions or naked Au nanoseeds facilitate/catalyze the disproportionation of Au(I) complex; significant enhancement in the growth of Au nanoparticle was observed in the presence of silver ions. Silver ions induce the growth of dendritic Au nanostructures. The dendritic nanostructures have multiple arms with an average size of 190 nm. The naked Au nanoseeds catalyze the disproportionation and favor the growth of twined Au nanoparticles with size ranging from 50 to 85 nm. The disproportionation of Au(I) does not occur in the presence of coordinating ligands. The coordinating ligands completely inhibit the disproportionation reaction. The electrochemical characteristics of the single crystalline nanoparticles was examined with the surface-sensitive electrode reaction of As(III). The single crystalline Au nanoparticles show excellent electrocatalytic activity toward the detection of As(III). The nanoparticle-based electrode could detect as low as 10 ppb of As(III), and it has very high sensitivity with respect to the polycrystalline Au and the electrode modified with other nanoparticles.