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

Abstract

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 10 6 for monolayers of 4-aminothiophenol.

Electrodeposited Gold Nanodaggers on Conductive Metal Oxide Films Provide Substrates for Dual-Modality Surface Sensitive Vibrational Spectroscopy

“…These crystallographic surfaces of gold support a potential-dependent horizontal to vertical orientation change in adsorbed pyridine, 4-dimethylaminopyridine, and 4-methoxypyridine, and it is somewhat surprising that the surface enhanced vibrational studies described above clearly indicate that the AuNDs used herein provide the expected response of polycrystalline gold rather than that of Au {100} or Au {111} surfaces. Lead underpotential deposition testing − of the AuNDs (see Supporting Information) revealed that the extent of low energy crystal faceting on these AuNDs was lower than that previously reported . It is noteworthy that a pulsed potential step rather than a potential sweep and hold was used to make the AuNDs herein, and although the overall morphology as observed by SEM seems remarkably similar, the modified deposition and growth conditions likely make a significant difference to the crystallography of the electrodeposited structures.…”

“…Previous electrochemical studies have shown that under potential sweep and hold conditions, a stable Au­(I)-MOP complex is formed from an initial two electron reduction of the starting Au­(III) complex . Like other reports of electrodeposition of Au­(I) species, − Simon et al showed that nanodaggers are preferentially {111} or (100} textured.…”

“…The thin ITO films were metalized by electrodepositing Au from a deaerated solution of 0.1 M 4-methoxypyridine (MOP), 0.1 M NaCl, and 0.25 mM KAuCl 4 using a pulsed electrodeposition process (see Supporting Information for details). The presence of a high concentration of MOP is critical for the formation of the gold nanodaggers during the electrodeposition . Residual absorbed MOP was removed using the process described in the Supporting Information.…”

Electroless Plating of Metal Nanomaterials

“…This reaction exemplifies lessons to be learnt from viewing electroless plating and related approaches such as colloidal nanoparticle syntheses [112] from a unified perspective. The fact that DMAP also can be used for electrodepositing anisotropic Au nanostructures further strengthens this point [113] …”

“…The fact that DMAP also can be used for electrodepositing anisotropic Au nanostructures further strengthens this point. [113] Electroless plating contrasts colloidal syntheses in that it does not rely on dedicated capping agents: The deposited nanostructures are growing on surfaces and thus are shielded against aggregation by their immobilization, at least to some extent. One exception is the intermediate case of plating finely dispersed substrates, in which capping agents can be required to prevent clumping.…”