Laser-Generated Bimetallic Ag-Au and Ag-Cu Core-Shell Nanoparticles for Refractive Index Sensing

Localized surface plasmon resonance (LSPR) wavelength of Ag, Au, and Cu nanoparticles (NPs) falls in visible region and is highly sensitive to size, shape, and surrounding medium. Refractive index sensitivity (RIS) and figure-of-merit (FOM) of Ag, Au, and Cu are analyzed for different particle sizes using the quasi-static Mie theory. The simulation results reveal that RIS and FOM of Ag NPs are higher than Au and Cu NPs. Bimetallic Ag-Au and Ag-Cu core-shell NPs exhibit two resonance peaks, corresponding to hybridization of core and nanoshell plasmon modes, are investigated for simultaneous sensing in two widely separated wavelength regions. A sequential laser ablation method is used to generate bimetallic Ag-Au and Ag-Cu core-shell NPs in liquid medium, and their LSPR peak shift and broadening are monitored in different refractive index liquids. Laser-generated Ag-Au NPs with Au shell of 1-2 nm show optimum RIS and FOM in lower-wavelength Ag plasmon channel. The Au shell not only improves the chemical stability of Ag NPs but also increases the index sensitivity at an optimum thickness. Further, in higher-wavelength Au plasmon channel, both RIS and FOM increase with shell thickness, but their values are lower than those in Ag plasmon channel.