Peptide Controlled Assembly of Palladium Nanoparticles on High-Aspect-Ratio Gold Nanorods

The use of gold nanorods as a template for bimetallic applications, including plasmonics, imaging, and phototherapy, is widely studied due to the unique optical properties of the gold materials. Generally, gold nanorods prepared with a Ag-assisted method are used, with a typical aspect ratio (length vs width) around 3, resulting in surface plasmons resonating within the UV–visible range. Due to these unique properties, gold nanorods are often combined with a catalytically active second metal to increase their range of applicability. The use of higher-aspect-ratio gold nanorods as a bimetallic template is an attractive alternative to the typical method as the longitudinal plasmon of these materials can be adjusted into the near-infrared (NIR) range. This adjustment expands the applications of bimetallic Au nanorods in the second-NIR biological window, which spans 1000–1700 nm. In this work, a materials-binding peptide, CPd4, is used to control the size and shape formation of palladium nanoparticles onto the surface of high-aspect-ratio gold nanorods. Through variation of the solvent used when attaching the peptide, the concentration of CPd4 retained on the nanorods can be manipulated. Higher concentrations of peptide, once palladium was added, were seen to result in smaller cubelike nanoparticles on the surface, whereas lower concentrations of peptide yielded larger spherical palladium nanoparticles. The ability to tune the size and shape of palladium nanorods on the gold nanorod surface allows for optimization of catalytic activity with applications in the NIR range.