Characterization of localized surface plasmon resonance transducers produced from Au25 nanoparticle multilayers

This article reports the preparation of gold plasmonic transducers using a nanoparticle self-assembly/heating method and the characterization of the films using scattering-type scanning near-field optical microscopy (s-SNOM). Nanoparticle-polymer multilayer films were prepared by the layer-by-layer assembly on glass slides by alternating exposures to monodisperse Au25 nanoparticles and ionic polymer linkers. Thermal evaporation of organic matters from the nanoparticle-polymer multilayer films at 600 °C allowed the nanoparticles to coalescence and form nanostructured films. Characterization of the nanostructured films generated from Au25 nanoparticles using atomic force microscopy (AFM) showed that the films have rounded, small, island-like morphologies (d: 30–50 nm) with a pit in the center of many islands. However, further characterizations with s-SNOM revealed that the produced nanoislands contain a single gold cluster in a pit surrounded by donut-shaped dielectric species. Formation of such a structure is thought to be resulted from the embedding of gold clusters under the reorganized polysiloxane binder coatings and glass surfaces during heat treatment of the Au25 nanoparticle multilayer films. The nanostructured films displayed strong surface plasmon resonance bands in UV–vis spectra with a peak absorbance occurring at ∼545–550 nm. The optical sensing capability of the films was examined using d-glucose-functionalized gold island films with the interaction of Concanavalin A (ConA). The result showed that the adsorption of ConA on island films causes a large change in the LSPR band intensity.