Three-dimensional periodic structures of gold nanoclusters in the interstices of sub-100 nm polymer particles toward surface-enhanced Raman scattering

Abstract

Regularly ordered polymer nanoparticle (PNP) assemblies incorporating gold nanoparticle (Au NP) clusters into the PNP interstices were fabricated by a simultaneous deposition of PNPs and Au NPs on a glass substrate. Monodisperse PNPs with an average size of 66 nm were employed as a template in the co-assembly to create the sub-100 nm periodic Au nanostructures on the substrate. First, mono-layering of PNP array with incorporation of 14 nm Au NPs was performed by a drop-casting to examine the number ratio of Au NPs to PNPs for multi-layering. Absorption spectra of the mono-layered co-assemblies of PNPs and Au NPs were employed to characterize the clustered state of Au NPs in the interstices of mono-layered PNPs. The number ratio suitable for homogeneous incorporation of Au NPs clustered in the interstice was found to be ranged from 6 to 8 in the characterization. Then, multi-layered co-assemblies of PNPs and clustered Au NPs were fabricated by a vertical deposition method with the Au NP number ratio of 8 to PNPs. Lifting rate of the substrate on which the PNPs were deposited was varied in the vertical deposition method to tune the film thickness of NP co-assembly. A decrease in the lifting rate to 1 μm/s could thicken the film to 0.71 μm corresponding to 13 layers of PNPs, resulting in the fabrication of periodic structures of Au NP clusters with a high packing density. Signal-to-noise ratio in the Raman measurement using p-mercaptobenzoic acid as a target molecule was successfully enhanced by multi-layering of the co-assembly, indicating that Au NP clusters were homogeneously incorporated into the interstices of PNPs in the co-assemblies.