Few-Layer Graphene-Encapsulated Metal Nanoparticles for Surface-Enhanced Raman Spectroscopy

Shell-isolated surface-enhanced Raman scattering (shell-isolated SERS), where the isolating shell prevents metal molecule interactions and improves the stability of the metal nanoparticles, has recently drawn a tremendous amount of attention. However, obtaining an ultrathin, seamless, and chemically stable isolating shell for shell-isolated SERS is still in its infancy. Graphene, with a high optical transparency and chemical inertness, is an ideal candidate to serve as an isolating shell. In the present work, graphene with a controlled number of layers is grown on the surface of metal nanoparticles via chemical vapor deposition, creating graphene-encapsulated metal nanoparticles (M@G, where M = Cu, Ag, and Au) suitable for shell-isolated SERS. Ultraviolet visible spectroscopy of Ag@G, Cu@G, and their corresponding nanoparticles indicates that graphene can prevent the surface oxidation of Ag and Cu nanoparticles after exposure to ambient air, giving a SERS-active substrate with a long lifetime. The Raman spectra of cobalt phthalocyanine and rhodamine 6G on M@G substrates show that Au@G can dramatically suppress photobleaching and fluorescence of the probe molecules, resulting in an enhanced Raman signal. Hence, M@G is a promising material for applications in chemical and biological detection because of its long-term stability and superior SERS performance.