Observation of coupling modes of surface plasmon resonance and local plasmon resonence and their applications to sensors

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The red color of gold colloid suspension is caused by the resonant oscillation of the free electrons in the gold nanoparticles with the incident electric field at 〜520 nm wavelength, The resonance is called surface plasmon resonance or sometimes local plasmon resonance. When the gold nanoparticle approach to a metal substrate, multipoles, such as a quadruple, are excited in the particle besides a dipole, due to the image dipole induced in the substrate. This causes a shift of the resonance wavelength toward longer wavelength. In research this substrate effect in the local plasmon resonance was investigated. In experiments slide glass on which a 25-nm gold film was deposited was used for a metal substrate. Gold nanoparticles were immobilized onto the substrate by a self-assembled monolayer of aminoethanethiol (AET) deposited onto a gold coated glass substrate. The diameter of the gold nanoparticles deposited were less than 100 nm. Resonance wavelengths obtained from the measured absorpt … More ion spectra of the gold nanoparticles were compared with its theoretical values. From these comparison it was found that the thickness of the AET monolayer is 0.3-0.4 nm. The coupling modes in particle-particle systems were also observed. For this observation a new technique for fabricating gold nanoparticle multilayers using a layer-by-layer deposition method. Poly (ethyleneimine) and poly (vinylsulfate) were used as a cationic polymer and an anionic polymer and gold nanoparticles were sandwiched between these films. In the gold nanoparticle multilayers fabricated with this technique is equal to the thickness of the sum of both polymer layers. The resonance wavelength shifted toward longer wavelength were observed as the particle size increased. When this nanoparticle multilayers were immersed into ethanol-water mixtures, the resonance wavelength shifted toward shorter wavelength as the water concentration increased. This is caused by the swelling of the polymer films between the gold particles. Through these works, it was confirmed that the gap between particle and substrate of particle and particle can be measured with subnanometer resolution. Less