Optical Contribution of Graphene in Enhanced sensitivity of graphene-gold coupled surface plasmon resonance sensing

Abstract

Surface plasmons at metal/dielectric interface can resonate with the incident light depending on the incident angle, wavelength of the light or the refractive index (RI) of the medium. These characteristics have been utilized as a basis of surface plasmon resonance (SPR) sensors, providing label-free and real-time sensing format. However, the sensitivity of SPR sensors still needs to be improved to meet the requirements for the small molecule sensing. In order to enhance the performance, plasmonic nanomaterials have been introduced on SPR sensor chip. Dirac fermions in graphene can behave like photons showing linear dispersion relation. In this regard, graphene can be utilized as an alternative plasmonic material to conventional metal nanostructure. Herein, we employed graphene from different preparation methods; graphene oxide (GO) using Hummer???s method, graphene from chemical vapor deposition (CVD) and N-doped reduced graphene oxide were incorporated on top of Au film of 50 nm thickness with the aim to increase electric field through coupling of graphene plasmons with propagating SPs from Au film which was located in Kretschmann configuration-based surface plasmon resonance spectroscopy. Thickness, reduction state and nitrogen doping state of graphene were systematically controlled and RI sensing was conducted. Au substrates with CVD graphene bilayers showed highest RI sensitivity (RIS) compared to bare Au film or Au film with other types of graphene and the figure of merit of Au/graphene substrates was not deteriorated due to the extremely thin graphene layer. Immuno-sensing was demonstrated with the mass sensitivity of 1430 pg/mm2, 3.3 times higher than that of bare Au film. Moreover, the optical contribution of graphene for the overall sensitivity enhancement mechanism could be confirmed by studying RIS. Therefore, graphene adlayers could amplify electric field and biomolecular adsorption which was experimentally proved by monitoring RIS and immunoassay