Bifunctional Ag@SiO₂/Au Nanoparticles for Probing Sequential Catalytic Reactions by Surface‐Enhanced Raman Spectroscopy

Abstract We report the synthesis of bifunctional Ag@SiO₂/Au nanoparticles with an “islands in the sea” configuration by titrating HAuCl₄ solution into an aqueous suspension of Ag@SiO₂ core–shell nanocubes in the presence of NaOH, ascorbic acid, and poly(vinyl pyrrolidone) at pH 11.9. The NaOH plays an essential role in generating small pores in the SiO₂ shell in situ, followed by the epitaxial deposition of Au from the Ag surface through the pores, leading to the formation of Au islands (6–12 nm in size) immersed in a SiO₂ sea. By controlling the amount of HAuCl₄ titrated into the reaction system, the Au islands can be made to pass through and protrude from the SiO₂ shell, embracing catalytic activity toward the reduction of 4‐nitrophenol to 4‐aminophenol by NaBH₄. While the Ag in the core provides a strong surface‐enhanced Raman scattering activity, the SiO₂ sea helps maintain the Au component as compact, isolated, and stabilized islands. The Ag@SiO₂/Au nanoparticles can serve as a bifunctional probe to monitor the stepwise Au‐catalyzed reduction of 4‐nitrothiophenol to 4‐aminothiophenol by NaBH₄ and Ag‐catalyzed oxidation of 4‐aminothiophenol to trans‐4,4′‐dimercaptoazobenzene by the O₂ from air in the same reaction system.