Fortunately, expanding the wavelength to mid-infrared (MIR), the vibrational signals can be utilized to improve the dimensionality of molecular signals. However, the intrinsic VCD signal of molecules is very weak, around 3 magnitudes smaller than ECD signals. Hence, the potentiality of enhanced VCD spectroscopy is worth exploring.

In a new paper published in Light Science & Application, a team of scientists, led by Professor Chengkuo Lee from Center for Intelligent Sensors and MEMS (CISM), Department of Electrical and Computer Engineering, National University of Singapore, Singapore, has developed an infrared chiral plasmonic metamaterials (IRCPMs) based on perpendicular positioned nanorods with a metal-insulator-metal (MIM) structure as an enhanced VCD sensing platform. As gold on the bottom functions as a reflector, the incident light is either absorbed by the nanostructures or reflected to free space. Hence, by reading the reflection spectrum only, the absorption spectrum can be calculated accordingly.

The authors applied temporal coupled-mode theory to explain and optimize the chiral metamaterials and concluded that the two most significant factors that influence the VCD response are the near-field coupling strength and the loss ratios. Based on these calculation and simulation results, the authors broke both the in-plane and out-of-plane symmetries by varying the thickness and position of the nanorods, and experimentally validated their conclusion.

The optimized chiral metamaterial was then used for sensing demonstration. The authors coated protein samples that contain secondary structures onto the chiral metamaterials, and successfully obtained enhanced VCD signals of the amide I vibration peaks. They also measured the sensing performance of different concentrations and different mixture ratios. These results help quantify the limit of detection and sensitivity of this technology.