Abstract
In this study, Au–Ag nanoboxes are converted into Au–Ag alloy nanocages by increasing the hole size. The extinction spectrum and the refractive index sensing characteristics of Au–Ag alloy nanocages with different geometric parameters are studied by using discrete dipole approximation method (DDA). With the increase of Au composition, the peak of local surface plasmon resonance (LSPR) shows approximately linear redshift and the sensitivity factor shows approximately linear decrease. The refractive index sensitivity can be effectively controlled by the Au–Ag ratio at large hole size because the hole and cavity surfaces distribute more environmental dielectric components. Therefore, increasing the hole size and decreasing the Au–Ag ratio can improve the refractive index sensitivity. These calculation results have also been verified experimentally. In order to illustrate the influence of alloy composition on the LSPR characteristics and the refractive index sensitivity, the local electric field distributions under different geometric parameters are plotted. We find that the electric field direction on the hole and cavity surfaces is controlled by the Au–Ag ratio and environmental dielectric constant. Moreover, the field vectors on the hole and cavity surfaces are formed by the superposition of the incident field, the electric field generated by the oscillating electrons on the outer surface, and the polarized field in the environmental dielectric constant.
Highlights
In recent years, the investigation on the optical properties of noble metal nanoparticles of Au [1], Ag [2], and Au-Ag alloy [3, 4] have greatly promoted the development of noble metal nanoparticles in biochemical spectral sensing [5], imaging [6,7,8], and various biomedical applications
From this comparison, decreasing the Au-Ag ratio and increasing the hole size can improve the refractive index sensitivity of the Au-Ag alloy nanocages, but the local surface plasmon resonance (LSPR) peak redshift with large hole size is much larger than that with low Au-Ag ratio, which indicates that hole size can better control the refractive index sensitivity
The plasmon extinction spectrum of Au-Ag alloy nanocages with different AuAg ratios and different hole sizes and the refractive index sensitivity of Au-Ag alloy nanocages are theoretically studied by discrete dipole approximation method (DDA)
Summary
The investigation on the optical properties of noble metal nanoparticles of Au [1], Ag [2], and Au-Ag alloy [3, 4] have greatly promoted the development of noble metal nanoparticles in biochemical spectral sensing [5], imaging [6,7,8], and various biomedical applications. At different stages of galvanic substitution, Au-Ag ratio, hole size, wall thickness, and wall length [34, 35] are all important factors affecting the LSPR peak of nanocages, and the refractive index sensitivity of nanoparticles is related to the LSPR offset [12, 36]. Because the inner hollow cavity is connected to the surrounding dielectric, the environmental dielectric constant inside and outside the Au-Ag alloy nanocages are set to be the same during the simulation and calculation of the refractive index sensitivity(denoted as ε) In this model, the minimum number of dipole points of the Au-Ag alloy nanocages is 13 900, which can ensure the accuracy and convergence of the DDA calculation results. The Au-Ag ratio and hole size D are variable. (a) High Ag composition, large hole size. (b) There is as much Au composition as Ag composition, small hole size
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