Highly Stable Surface-Enhanced Raman Spectroscopy Substrates Using Few-Layer Graphene on Silver Nanoparticles

Graphene can be effectively applied as an ultrathin barrier for fluids, gases, and atoms based on its excellent impermeability. In this work, few-layer graphene was encapsulated on silver (Ag) nanoparticles for the fabrication of highly stable surface-enhanced Raman scattering (SERS) substrates, which has strong resistance to oxidation of the Ag nanoparticles. The few-layer graphene can be successfully grown on the surface of the Ag nanoparticles through a simple heating process. To prevent the agglomeration of the Ag nanoparticles in the fabrication process, poly(methyl methacrylate) (PMMA) layers were used as a solid carbon source instead of methane (CH₄) gas generally used as a carbon source for the synthesis of graphene. X-ray diffraction (XRD) spectra of the few-layer graphene-encapsulated Ag nanoparticles indicate that the few-layer graphene can protect the Ag nanoparticles from surface oxidation after intensive annealing processes in ambient conditions, giving the highly stable SERS substrates. The Raman spectra of Rhodamine 6G (R6G) deposited on the stable SERS substrates exhibit maintenance of the Raman signal intensity despite the annealing process in air. The facile approach to fabricate the few-layer graphene-encapsulated Ag nanoparticles can be effectively useful for various applications in chemical and biological sensors by providing the highly stable SERS substrates.