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Au-Ag core-shell (Au@Ag) nanobars could be synthesized from gold nanorod (NR) seeds with cysteine additives by a two-step process of reaction temperatures. The lateral sides of gold NRs surrounded by surfactant bilayers render cysteine additives binding on both ends of the NRs, and restricted silver deposition to their lateral sides at room temperature. Further, silver deposition can take place at first on the pre-formed silver layers on the lateral sides at higher temperatures and finally resulted in the formation of Au@Ag nanobars in which gold NRs are in the corner positions of the nanobars and their longitudinal axes parallel to the longer sides of the nanobars. [ABSTRACT FROM AUTHOR]

Highlights: [•] We report a simple and effective method to synthesize gold nanodumbbells (NDs). [•] Gold NDs are used to synthesize gold–silver core–shell nanoparticles (Au@Ag NPs). [•] Au@Ag NPs with bar and triangular bipyramid shapes were obtained at high temperature. [•] Gold NDs in Au@Ag nanobars are in the center position of bar NPs. [•] The longer end of Au ND is parallel to the shorter edge of triangular bipyramid NP. [ABSTRACT FROM AUTHOR]