In the present study, we demonstrate the precise tuning of surface plasmon resonance over the full visible range by compositional variation of the nanoparticles. The addition of sulfide ions into the Au@Ag core-shell nanocubes generates stable Au@Ag/Ag(2)S core-shell nanoparticles at room temperature, and the plasmon extinction maximum shifts to the longer wavelength covering the entire visible range of 500-750 nm. Based on the optical property, the Au@Ag core-shell nanocubes are employed as a colorimetric sensing framework for sulfide detection in water. The detection limit is measured to be 10 ppb by UV-vis spectroscopy and 200 ppb by naked eyes. Such nanoparticles would be useful for decoration and sensing purposes, due to their precise color tunability and high stability.

The galvanic replacement reaction has recently been established as a standard protocol to create complex hollow structures with various compositions and morphologies. In the present study, the structural evolution of Ag nanocubes with Au precursors is monitored at the single-particle level by means of ex situ and in situ characterization tools. We explore two important features distinct from previous observations. First, the peak maximum of localized surface plasmon resonance (LSPR) spectra abruptly shifts at the initial stage and reaches a steady wavelength of ∼600 nm; however, the structure continuously evolves to yield a nanobox even during the late stages of the reaction. This steady wavelength results from a balance of the LSPR between the red-shift by the growth of the inner cavity and the blue-shift by the deposition of Au on the interior, as confirmed by theoretical simulations. Second, the change in morphology at different temperatures is first analyzed by both ex situ and in situ monitoring methods. The reaction at 25°C forms granules on the surface, whereas the reaction at 60°C provides flat and even surfaces of the hollow structures due to the large diffusion rate of Ag atoms in Au at a higher temperature. These plasmon-based monitoring techniques have great potentials to investigate various heterogeneous reaction mechanisms at the single-particle level. ■ INTRODUCTIONFor the past several decades, metal nanostructures have been extensively studied due to their unique physical and chemical properties and versatile applications in many areas. To control the properties of metal nanostructures so as to make them suitable for specific applications, the creation of complex nanostructures with multiple components and high-dimensional morphologies has been attempted, such as core−shell, 1,2 hollow, 3,4 and branched nanoparticles. 5,6 For the fabrication of these nanostructures, solid-state chemical reactions on the nanoscale are commonly applied, including galvanic replacement, 7,8 void formation via the Kirkendall effect, 9 and cationic and anionic exchanges 10 as well as the induction of kinetic growth during the synthesis step. In particular, galvanic replacement has been established as a standard method to make complex hollow nanostructures with a variety of compositions and morphologies.Galvanic replacement is basically a redox process between two metals with distinct reduction potentials. Oxidation occurs on the metal with a low reduction potential, and the reduction and deposition concomitantly occur on the other metal with a high reduction potential. The replacement reaction is known as a major cause of the corrosion of metal surfaces in the bulk form, and Xia et al. revitalized it as a simple and versatile route for the generation of metal hollow nanostructures. 11 The formation mechanism of this reaction was elucidated through a change in the morphology as observed in TEM images along the reaction progress. These outcomes included the generation of a specific spot, a continuous hollow formation, mor...