Abstract:

The preparation of plasmonic metal-based substrates has been a hot research topic during the past decades in the area of surface-enhanced Raman spectroscopy (SERS). The localized surface plasmon resonance effect of plasmonic metal nanostructures enhances the electromagnetic field for SERS analysis, thereby making SERS an extremely sensitive detection technique. However, commonly developed plasmonic metal substrates exhibit poor stability and reproducibility. Since the separation of graphene from graphite, graphene has been widely used in various fields because of its unique physical, chemical, electronic, and optical properties. In the field of SERS, graphene has been used for graphene-enhanced Raman scattering, which makes use of the chemical enhancement mechanism in SERS. In addition, it has capabilities of surface molecular enrichment, quenching fluorescence, surface homogenization, and has strong chemical stability. Due to these characteristics of graphene, SERS substrates based on graphene-metal nanocapsules have attracted the attention of researchers. In this work, a small size gold-graphitic nanocapsules (Au@G) was prepared by chemical vapor deposition (CVD). The material exhibits a core-shell structure consisting of a graphitized carbon layer coated on Au nanoparticles (Au NPs). The Au NP core of the Au@G provides a major enhancement factor for Raman analysis, and the external graphitized carbon shell ensures strong chemical stability of the material. The Au@G exhibits a uniform particle size with diameter ~17 nm. In order to control the size of the Au@G, tetraethyl orthosilicate (TEOS) and tetraethylorthotrimethylammonium bromide were used as the raw material and template, respectively, a 45 nm-thick layer of mesoporous silica was coated on the synthesized Au NPs. The presence of the mesoporous silica capping layer prevented aggregation and particle size growth of the Au NPs during high-temperature CVD. At the same time, we studied the effect of TEOS concentration on the growth of the graphitized carbon layer during CVD. The results revealed that a decrease of the TEOS concentration is conducive for obtaining a high graphitic Au@G, and the concentration of TEOS does not affect the particle size of the Au@G. Raman detection of crystal violet molecules using Au@G demonstrated the latter's good Raman enhancement effect. The Au@G prepared by high-temperature CVD exhibits a clean surface with no impurities. It is an SERS substrate with both physical and chemical enhancement. The unique Raman spectral peaks and small size of Au@G ensure its great potential for use in the fields of molecular detection and cell imaging analysis.

Key words: Gold-graphitic nanocapsues, Chemical vapor deposition, Surface-enhanced Raman spectroscopy, Mesoporous silica, Au nanoparticles