A hybrid system with highly enhanced graphene SERS for rapid and tag-free tumor cells detection

Abstract

By dint of unique physical/chemical properties and bio-compatibility, graphene can work as a building block for a SERS substrate and open up a unique platform for tumor cells detection with high sensitivity.Herein we demonstrate a facile system with highly enhanced surface enhanced Raman spectroscopy of graphene (G-SERS). The system consists of a reduced graphene oxide (rGO) sandwiched by silver and gold nanostructures. Due to the ultrasmall thickness of rGO, the inter-coupling between Ag and Au nanoparticles is precisely controlled and the local field enhancement has been improved to more than 70 times. Associated with the unique chemical mechanism of rGO, the hybrid system has been utilized to identify tumor cells without using any biomarkers. We believe this research will be important for the applications of rGO in cancer screening.Raman spectroscopy is a reliable technique to understand molecules information with the inelastic scattering process in Raman scattering. In practical applications, the Raman signals are relatively weak due to the small scattering cross-section of molecules. Surface-enhanced Raman scattering (SERS) can enhance the fingerprints of molecules and overcome the limitation of sensitivity in conventional Raman spectroscopy, especially in use of noble metal materials 1 . In SERS researches, electromagnetic mechanism (EM) and chemical mechanism (CM) are well accepted mechanisms. Typically, the enhancement factor from EM is roughly proportional to |E| 4 . Strong EM enhancement is usually obtained with electromagnetic "hot spot" with a strongly enhanced local field 2 . The enhancement factor from EM has been improved to be more than 10 8 . To further improve the enhancement factor, the coupling between localized surface plasmons (LSPs) has been considered 1,3-6 . However, the coupling of LSPs is critically sensitive to the separation distance between the LSPs. It has been found to increase dramatically with the decrease of separation distance 7,8 . Thus precisely control the separation distance to sub-nanometer scale is a key to further improve the enhancement factor of SERS. Compared with other materials, the thickness of monolayer graphene is well fixed at sub-nanometer level and can be very uniform over a large area. Therefore, combining rGO with the coupling of LSPs can be potentially used to further improve the sensitivity of SERS.CM is based on the charge transfer and the mixing of molecular orbitals with electronic states between molecules and substrate 9 . In addition to the improved coupling of LSPs, graphene has also been used to enhance SERS via CM due to the high affinity of probe molecules to graphene layer. Because of its smooth surface and atomic thickness with excellent optical transmission, graphene has been well accepted as a novel material for SERS substrate [10][11][12] , and has been widely applied in molecular selectivity 13,14 , dye molecule detection, labeled tumor cell detection [3][4][5]15 , biosensing 16 et al. Among the graphene family, reduced graphene oxide (rGO)...