Silver nanocrystals have fascinating optical properties known as localized surface plasmon resonance, which is essential to applications in sensing and imaging. For example, Ag nanocubes embrace surface-enhanced Raman scattering (SERS) properties with enhancement factors up to 106 at visible excitation wavelengths for highly sensitive detection of chemical or biological species. Unfortunately, elemental Ag is highly susceptible to oxidation under conditions that involve oxidants, halide ions, and acids. Such chemical instability often results in changes to the morphology of Ag nanocrystals, particularly at corners and edges with high energies, and ultimately compromise their performance. Additionally, the toxicity of the released Ag+ ions also limits the potential applications of Ag nanocrystals in a biological system. One potential solution to improve the chemical stability of elemental Ag is to form alloys with a more stable metal such as Au or protect the Ag nanocrystals with ultrathin Au shells. However, it is difficult to form Ag-Au alloys by reducing their precursors simultaneously in a solution phase due to their substantial difference in reactivity. It is also challenging to coat Ag nanocrystals with Au shells due to galvanic replacement. In this talk, I will report our recent developments in addressing these challenges, with a focus on the syntheses of Ag-Au bimetallic nanocrystals with greatly enhanced plasmonic properties and improved chemical stability for chemical and biological sensing and imaging.

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