Enhanced circular dichroism and biosensing application of planar chiral nanostructure by covering graphene nanobelts

Circular dichroism (CD) of plasmonic nanostructures has broad application prospects, including negative refraction, chemical analysis, and biosensing. Compared with three-dimensional chiral structures, planar chiral structures with easy preparation show a weak CD effect. In this study, graphene nanobelts (GNs) were introduced into planar chiral structures composed of the IL-shaped chiral nanostructures (ILCNs) to enhance the effect of CD. The simulation results showed that the enhancement of CD was obtained from the plasmonic coupling between ILCNs and GNs. It led to different enhancements of the absorption under different circularly polarized lights. The bonding mode, anti-bonding mode, and trapping mode were used to interpret the coupling between the nanorods in ILCNs/GNs. The electromagnetic loss distributions of ILCNs/GNs showed that the electromagnetic loss was transferred from the metal part to the graphene part. The CD spectra of ILCNs with different graphene nanostructures revealed that the enhancements of absorption and CD mostly depended on the graphene belt located in the gaps of ILCNs. Meanwhile, the CD effect strongly depended on the geometric parameters of ILCNs/GNs. Besides, the CD spectra for chiral molecules could be enhanced by ILCNs/GNs under a different handedness. The maximum enhancement factor was 3200 times in the near-infrared band. These results might help explore the mechanism of CD enhancement and promote the application of the planar chiral structure in biosensing.