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We report a facile route to the synthesis of Ag@Au–Pt trimetallic nanocubes in which the Ag, Au, and Pt atoms are exposed at the corners, side faces, and edges, respectively. Our success relies on the use of Ag@Au nanocubes, with Ag 2 O patches at the corners and Au on the side faces and edges, as seeds for the site-selective deposition of Pt on the edges only in a reaction system containing ascorbic acid (H 2 Asc) and poly(vinylpyrrolidone). At an initial pH of 3.2, H 2 Asc can dissolve the Ag 2 O patches, exposing the Ag atoms at the corners of a nanocube. Upon the injection of the H 2 PtCl 6 precursor, the Pt atoms derived from the reduction by both H 2 Asc and Ag are preferentially deposited on the edges, leading to the formation of Ag@Au–Pt trimetallic nanocubes. We demonstrate the use of 2,6-dimethylphenyl isocyanide as a molecular probe to confirm and monitor the deposition of Pt atoms on the edges of nanocubes through surface-enhanced Raman scattering (SERS). We further explore the use of these bifunctional trimetallic nanoparticles with integrated plasmonic and catalytic properties for in situ SERS monitoring the reduction of 4-nitrothiophenol by NaBH 4 . Upon the removal of Ag via H 2 O 2 etching, the Ag@Au–Pt nanocubes evolve into trimetallic nanoboxes with a wall thickness of about 2 nm and well-defined openings at the corners. The trimetallic nanoboxes embrace plasmon resonance peaks in the near-infrared region with potential in biomedical applications.