Ag@Au Concave Cuboctahedra: A Unique Probe for Monitoring Au-Catalyzed Reduction and Oxidation Reactions by Surface-Enhanced Raman Spectroscopy

We report a facile synthesis of Ag@Au concave cuboctahedra by titrating aqueous HAuCl₄ into a suspension of Ag cuboctahedra in the presence of ascorbic acid (AA), NaOH, and poly­(vinylpyrrolidone) (PVP) at room temperature. Initially, the Au atoms derived from the reduction of Au³⁺ by AA are conformally deposited on the entire surface of a Ag cuboctahedron. Upon the formation of a complete Au shell, however, the subsequently formed Au atoms are preferentially deposited onto the Au{100} facets, resulting in the formation of a Ag@Au cuboctahedron with concave structures at the sites of {111} facets. The concave cuboctahedra embrace excellent SERS activity that is more than 70-fold stronger than that of the original Ag cuboctahedra at an excitation wavelength of 785 nm. The concave cuboctahedra also exhibit remarkable stability in the presence of an oxidant such as H₂O₂ because of the protection by a complete Au shell. These two unique attributes enable <i>in situ</i> SERS monitoring of the reduction of 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) by NaBH₄ through a 4,4′-dimercaptoazobenzene (<i>trans</i>-DMAB) intermediate and the subsequent oxidation of 4-ATP back to <i>trans</i>-DMAB upon the introduction of H₂O₂