Gold nanoparticle-based core-shell and hollow spheres and ordered assemblies thereof

We report on the preparation and characterization of gold nanoparticle (Au-NP)-based core-shell particles and hollow spheres and of ordered structures obtained from their self-assembly. Coated particles, comprising polystyrene (PS) cores and polyelectrolyte (PE) or Au-NP/PE shells, were prepared by the layer-by-layer technique, that is, via the sequential adsorption of oppositely charged PEs onto PS cores, or by the infiltration of gold nanoparticles into preassembled PE multilayers onto PS particles, respectively. The thickness of the gold nanoparticle-based coatings was varied by altering the number of preassembled PE layers into which gold nanoparticles were infiltrated and by additional electroless gold plating. Hollow spheres were obtained from the core-shell particles by either calcination or by dissolution of the PS core by exposure to tetrahydrofuran. The coated colloids were used as building blocks for the construction of ordered particle assemblies. Two different approaches were employed to form the ordered particle structures. The first involved the gravity sedimentation of Au-NP/PE-coated PS spheres, while the second method entailed the formation of ordered assemblies of PE-coated PS spheres, followed by infiltration with gold nanoparticles. 3D macroporous ("hollow") ordered structures were subsequently prepared from the ordered core-shell particle assemblies by removal of the PS cores. UV-visible absorption measurements of core-shell and hollow Au-NP/PE microspheres, and 3D ordered assemblies of these particles, show red-shifted plasmon resonance bands relative to the gold nanoparticles in solution. Near-IR reflectance spectra of the 3D ordered structures of hollow Au-NP/PE microspheres show Bragg reflectance peaks that are blue-shifted relative to the corresponding structures prior to dissolution of the core. The strategy described opens a new avenue to the production of particle assemblies derived from core-shell and hollow colloids, which may provide new opportunities for their application in areas such as photonics, photoelectronics, and catalysis.