Gold-copper oxide core-shell plasmonic nanoparticles: the effect of pH on shell stability and mechanistic insights into shell formation.

Plasmonic nanoparticles play an important role in applications for chemical sensing, catalysis, medicine, and biosensing. The localized surface plasmon resonance (LSPR) of a nanoparticle is determined by factors such as size, shape, and the local dielectric environment. Here, we report a simple colloidal synthesis method to create core-shell plasmonic nanoparticles with a gold core and a copper oxide (CuO) shell. The gold cores are particles of various shapes and sizes, including nanorods, nanobipyramids, and nanoshells, and the CuO shell is on the order of 30-40 nm thick. The growth of the oxide shell red shifts the plasmonic absorption of the gold core particle by up to 250 nanometers, resulting in a particle that can absorb into the near-infrared (NIR). Additionally, we report the unique ability to immediately remove and regrow the oxide shell by simple changes to the solution pH. We demonstrate the repeated dissolution and nucleation of the oxide shell through the addition of an acid and a base, respectively. The process is confirmed by characterization using Ultraviolet-Visible-Near-IR (UV-Vis-NIR) spectroscopy and electron microscopy of the particles. After several iterations of this process, we report the formation of large CuO spheres, where CuO nucleation on other CuO particles is favored over the gold nanoparticles. In addition, we provide insight into the role of ligands in shell formation.