Investigation on the Single-Molecule Sensing of Bimetallic Plasmonic Nanoparticle Using Modified Discrete Dipole Approximation Method

Different spherical bimetallic nanoparticles (NPs) of plasmonic metals gold (Au), silver (Ag), and copper (Cu) are investigated for single-molecule sensing using the modified discrete dipole approximation (DDA) method. Two core-shell and random distribution models of bimetallic structure are investigated. It is observed that the core-shell bimetallic structure, whose core is Au or Cu and whose shell is Ag, leads to a critical state in which the direction of the plasmonic oscillations of the nanoparticle changes drastically with the addition of molecules around the NP. It leads to a significant peak shift, which is not observed for a monometallic plasmonic NP. A detailed examination of the electric nearfield intensities distribution shows that in the mentioned critical state, the addition of the guest molecule leads to the change of radial dipolar oscillations to dipolar oscillations perpendicular to the direction of the incident wave electric field and a significant redshift of the peak of the spectrum. Such property can be used to optimize the sensing of plasmonic nanostructures.