Self-assembly of noble metallic spherical aggregates from monodisperse nanoparticles: their synthesis and pronounced SERS and catalytic properties

Here, an oil-in-water emulsion method was used to assemble monodisperse noble metallic nanoparticles (NPs) in the oil phase into aggregates in the aqueous phase. The original size of the NPs is an essential factor in determining the final morphology: beyond the critical size, spherical aggregates with the original NPs can be formed; otherwise, complex aggregates with coarsing NPs come into being. The aggregation level of NPs and consequent properties of the as-constructed spherical assemblies can be optimized through tuning the evaporation rate of the organic solvent. The fabricated assemblies can serve as highly sensitive surface-enhanced Raman scattering (SERS) platforms for the detection of chemical or biological molecules, showing significantly high SERS activity toward rhodamine 6G (R6G) dye and 4-aminothiophenol (4-ATP) molecules. In addition, we further demonstrate the use of these highly sensitive SERS-active substrates to identify melamine at 1 x 10(-7) M to insure food safety and biosecurity. Furthermore, the catalytic reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in the presence of excess NaBH4 was also investigated, in which the as-prepared silver assemblies exhibited excellent catalytic activities due to their special 3D architecture and large number of active catalytic sites.