Plasmonic metallic nanoparticles (NPs) have been explored extensively in terms of their fundamental and practical applications because of their excellent properties. Ag provides high enhanced-Raman activity, Au provides excellent chemical stability, while Pt is one of the most efficient components for catalysts. It inevitably leads to the necessary combination of these elements in a single NP system. The synthetic approach for well-defined nanostructures, however, still has many challenges. One example is the synthesis of uniform Ag@Au core@shell NPs because the Ag core is easily etched away due to the galvanic replacement reaction which causes gaps in the Au shell or hollow sections at the interface of the core and the shell.<br/>In this report, Au@Ag core@shell NPs were successfully synthesized with uniform morphology and tunable size, composition and plasmonic property [1]. In addition, Au@Ag core-shell NPs were demonstrated to have better chemical stability than pure Ag NPs and very high SERS activity [2]. Moreover, Ag in the shell also could suppress the galvanic replacement reaction allowing the formation of double shell (Au@Ag)@Au NPs without any defects or gaps in the structure. All these attractive aspects arise from the charge-transfer effect which has been shown to occur in the Au-Ag system. In this study, the intermediate Ag shell thickness is limited to the range where the charge transfer effect takes place, allowing the ability to create Au@Ag@Au NPs without significant alloying or defects in the structure [3].<br/>By extending this phenomenon to other NP systems, insight can be gained into how to manipulate the particle structure and composition towards the desired characteristics. With this goal in mind, a series of different structures, such as Pt@Ag core@shell NPs [4] and Au@Ag@Pt core@multi-shell NPs [5] were created. The results demonstrate that the charge-transfer effect also occurred in other systems, thus it can be extended to a wide range of heterostructure NPs and provides insight into how to exploit this phenomenon to create silver-based sensing probes with enhanced robustness, high optical/plasmonic activity and plasmonic characteristics for a targeted application.

Aqueous syntheses of multi-metallic nanostructures utilizing charge-transfer effect

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ACS Fall 2022

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