Role of Au(I) Intermediates in the Electrochemical Formation of Highly Anisotropic Gold Nanostructures with Near-IR SERS Applications

In the presence of certain stabilizing ligands, such as pyridine derivatives, the reduction of Au­(III) ions has been speculated to generate Au­(I) intermediates that may play a key role in nanoparticle growth. Herein, the electrochemical behavior of Au­(III) in the presence of 4-methoxypyridine, Py, is reported in aqueous electrolytes. Voltammetric analysis reveals that a spontaneously formed Au­(III)–Py complex undergoes a two-step reduction process. The first reduction involves the transfer of two electrons and produces a Au­(I) species. A more cathodic one-electron transfer results in electrodeposited gold. Sustained generation of the Au­(I)–Py intermediate species produced from the first reduction step leads to disproportionation and the formation of aggregated nanoparticle meshes that loosely adhere to the ITO electrode. Conversely, application of more negative potentials leads to the formation of highly anisotropic nanodaggers from the electrodeposition of the Au­(I) species. The shape-directing properties of Py adsorbed on the nucleated gold result in preferential ⟨111⟩ growth. The length scale of the deposited dagger-like shapes is dependent on deposition potential and deposited charge, and arms extending several hundred nanometers are reported. Optical characterizations show extinction extending well into the near-infrared region, which is attributed to localized surface plasmonic resonances. Near-IR Raman sensing applications are demonstrated using FT-Raman with 1064 nm excitation. The nanodaggers provide SERS enhancement factors greater than 10⁶ for monolayers of 4-aminothiophenol