Extrinsic chiral plasmonic sensors based on a nanohole array.

Determining the stereochemical structures of biomolecules is very important for biomedicine, bioscience, and food safety due to the distinct biological functions of enantiomers. Chiral plasmonic sensors (CPSs) have demonstrated their ultra-sensitive detection ability to a few picogram levels due to the excited superchiral electromagnetic field near metasurfaces, which can greatly enhance the interaction between lights and chiral biomolecules. However, the reported CPSs usually rely on the intrinsic chiroptical effects that require at least two samples to determine the molecular structures, which decrease the detection precision and improve the detection cost. In this study, we theoretically and experimentally demonstrate one extrinsic CPS based on the nanohole array (NHA). Through covering the nanohole array with a chiral medium, the measured negative and positive g-factors increase simultaneously for L-phenylalanine but decrease for D-phenylalanine. These regular signals give clear criteria to determine the molecular structures, and the detection sensitivity can be enhanced for about 1 × 10 times when compared with traditional methods. The in-depth research reveals that this enhancement comes from the excited superchiral field near nanoholes and highly depends on both the real and imaging parts of the Pasteur constant. These findings offer one new, to the best of our knowledge, approach toward high-performance CPSs.