Synthesis of Chiral Au-Ag Core-Shell Nanoparticles and Their Localized Surface Plasmon Resonance Properties

Abstract

Noble metal nanoparticles (NPs) such as silver (Ag), and gold (Au) exhibit localized surface plasmon resonance (LSPR) peaks in the visible light region, and their optical properties can be tuned by their size and shape. Advances in colloid chemistry enable the introduction of structural chirality to these plasmonic NPs. LSPRs of chiral metal NPs are responsive to circularly polarized light (CPL), giving them chiroptical properties. For example, Au nanoparticles prepared with L- or D-cysteines as a ligand showed almost the same shape of extinction spectra, but their CD spectra showed opposite peaks in the LSPR wavelength region, depending on the handedness of ligands. [1] Metal nanoparticles having chiral structures are expected to be applied to high-sensitivity chiral sensing. While Au nanoparticles have been extensively explored due to their high chemical stability, chiral Ag nanostructures have not been investigated in spite of the potential for a stronger LSPR-induced electric field. In this study, we report the development of a novel colloidal method to synthesize chiral Au-Ag core-shell nanoparticles via electrochemical deposition of Ag layer onto the surface of pre-synthesized chiral Au nanoparticles. Chiral Au NPs were prepared according to a previous report using L-cysteine as the ligand. [1] Thus-prepared NPs were loaded on a glassy carbon (GC) electrode. A Cu monolayer was deposited on Au NPs by the underpotential deposition (UPD) in an aqueous solution containing 1.0 mmol dm -3 CuSO 4 . The Cu UPD layer was replaced with Ag through galvanic replacement by immersing the electrode with the Cu-UPD Au NPs in a 10 mmol dm -3 AgNO 3 aqueous solution. The amount of Ag deposition on individual Au NPs was controlled by repeating the procedures of Cu UPD and Ag replacement. Cyclic voltammograms were measured on GC electrodes loaded with chiral Au NPs in a CuSO 4 solution. With a negative sweep of the electrode potential, a cathodic current peak originating from the Cu UPD was observed at ca. +0.5 V vs. RHE and a current assignable to bulk Cu deposition appeared at the potential more negative than +0.25 V vs. RHE. In contrast, a potential sweep to positive direction generated an anodic current corresponding to the oxidation of the Cu layer. Based on these results, we determined the potential of Cu UPD on chiral Au NPs. The average diameter of chiral Au NPs prepared with L-cysteine was 117 nm, and a twisted surface was confirmed by SEM measurements. Even after three cycles of the above-mentioned Ag deposition procedures, no clear change in the shape of chiral Au NPs was observed. However, the average size of particles increased to 123 nm after these cycles, indicating the deposition of Ag shell layer of ca. 1 nm thickness on Au NPs per cycle. This study successfully demonstrates the preparation of chiral Au-Ag core-shell NPs through electrochemical methods involving Cu UPD on Au NPs and subsequent galvanic replacement with Ag. References (1) K. T. Nam, et al, Nat. Commun, 11 , 263 (2020).