(Invited) Magneto-Optical and Plasmonic Composite Particles Synthesized By Deposition of Au Nanoparticles on Bismuth-Substituted Yttrium-Iron Garnet Core
Authors: Kitamoto Yoshitaka , Ishii Yusuke 
Published: February 2020

Although immunochromatograph using nanoparticles has some merits such as an ease of use and a quick response, it has some issues such as a poor sensitivity and a lack of quantitativity. To overcome the issues, we propose a new immunoassay method using a magneto-optical (MO) effect based on nanoparticle technologies. It is expected that the MO immunoassay leads to a higher sensitivity and a quantitative measurement. Nanoparticles with a high MO effect are required for this technique. Bismuth-substituted yttrium-iron garnet (Bi-YIG) is one of the candidates, which exhibits a large Faraday effect. To enhance the Faraday effect for highly sensitive MO immunoassay, surface plasmon resonance (SPR) effect is employed in the preset study. Au nanoparticles have an absorption peak based on the SPR effect in the wavelength range of 500-600 nm by optimizing their size and interparticle distance. Since Bi-YIG has a peak in its MO spectrum at around 550 nm, the SPR absorption will enhance the MO effect, obtaining a maximum Faraday rotation in the range.
In the present study, we fabricated MO and plasmonic particles composed of a Bi-YIG particle as a core and Au nanoparticles deposited on the core. First, Bi-YIG particles of 300-600 nm in size as the core were synthesized by coprecipitation and heat-treatment for crystallization, followed by milling to optimize the size. Then, Au nanoparticles were deposited on the Bi-YIG particles by a polyol reduction method using formaldehyde as a reducing agent. Morphological properties such as size and interparticle distance of the Au nanoparticles manipulated by the concentrations of Au ion and formaldehyde for their deposition are important in the SPR effect, resulting in the enhancement of the magneto-optical effect. The specimens were characterized by transmission electron microscopy, X-ray diffraction (XRD), optical absorption spectroscopy, and magneto-optical evaluations in a transmission mode (Faraday rotation).
The morphology of Au nanoparticles on Bi-YIG particles depended on the deposition conditions. The clear SPR absorption and the MO enhancement were obtained in the composites with a larger amount of Au nanoparticles deposited on Bi-YIG particles as shown in Fig. 1. The enhancement of the Faraday rotation was observed in the range of 500-700 nm (Fig. 1(a)), corresponding to the wavelength range where the clear SPR absorption was observed (Fig. 1(b)). The enhancement of the Faraday rotation reached 1.54 times in maximum at around 550 nm of wavelength. The dependence of optical and magneto-optical properties as a function of Au concentration was attributed to the morphology of the Au nanoparticles deposited on Bi-YIG particles. The coverage of Au nanoparticles on Bi-YIG strongly influenced the enhancement of the Faraday rotation based on the SPR effect. Although the quantity of the deposited Au nanoparticles confirmed by XRD was increased, the MO enhancement was not confirmed in the case that large Au nanoparticles were deposited and the coverage was low on the Bi-YIG surface.

Figure 1