(283e) Gold on Fractal Nanoparticles As Highly Active Surface-Enhanced Raman Scattering Substrates

Surface-enhanced Raman scattering (SERS) is an ultra-sensitive and powerful spectroscopic technique for molecular sensing and detection. The large enhancement of weak Raman signals arises from electromagnetic mechanism (EM) and chemical mechanism (CM). The EM which is the dominant contributor to SERS process, originates from excitation of localized surface plasmons (LSP) at or near gaps, crevices, or sharp tips of plasmonic materials. Whereas, CM is based on charge-transfer between physically or chemically adsorbed analytes and substrate surface. Both enhancement mechanisms require the analyte to be near the optimized plasmonic surfaces. In this work, unique gold nanostructures have been synthesized on para-amino benzoic acid terminated carbon black and fumed silica nanoparticles by using a cationic polyelectrolyte templating technique. This method involves using a cationic polyelectrolyte to provide enough positive charge density to concentrate gold ions concentration at the surface of the template. Fractal particles coated with poly-L-lysine (PLL) were suspended in a tetrachloroauric acid (HAuCl₄) solution. PLL enriched the interface with AuCl₄^- anions and ascorbic acid was added as the reducing agent, forming a thin layer of gold (Au) on the fractal nanoparticles. The absorbance spectrum and morphology of the Au nanoparticles was tuned by controlling the HAuCl₄ concentration. SERS enhancement of the gold-coated carbon black and fumed silica has also been successfully demonstrated and compared for the detection of 4-nitrobenzenethiol(4-NBT) as well as pyrene as a model polycyclic aromatic hydrocarbon (PAH). The cusps on the fractal particles coupled with the spiky edges of the gold layer significantly enhance SERS signals. Synthetized Au nanostructure were then immobilized on the surface of graphene nanoplatelets (GNPs)/carbon nanotubes(CNTs) composite through vacuum filtration method. Carbon nanomaterials such as GNPs and CNTs and their composites, known as effective absorbents for aromatic molecules, can enhance Raman signals through trapping the organic molecules for the Au surface. Our results suggest that Au nanostructure on fractal nanoparticles compounded with graphene/carbon nanotube composites are new, highly sensitive substrates for sensing and water purification.