Direct measurement of circular dichroism by evanescent wave cavity ring-down spectroscopy using rationally designed plasmonic structures

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Circular dichroism (CD) is the differential absorption of circularly polarized light by molecular chiral domains. CD provides important information of the molecular conformation and has been heavily used for qualitative analysis of biomedical and pharmaceutical samples. However, due to the mismatch of the wavelength of light and the size of molecular chiral domains, CD is typically very small and not sensitive enough for quantitative analysis. In the last decade, plasmonic nanostructures have been demonstrated very powerful in sculpting optical near fields to enhance chiral light-matter interaction. On the other hand, evanescent wave cavity-ring-down spectroscopy (EW-CRDS) has become a matured and realizable method to increase weak absorption by multiple reflection in the cavity. A successful combination of plasmonics and EW-CRDS is therefore expected to greatly improve the sensitivity of chiral analysis. In this proposal, we plan to combine the enhancement effect of nanoplasmonics and EW-CRDS to boost the sensitivity of CD analysis on condense phase chiral samples. The objective is to develop a novel and sensitive plasmonic EW-CRDS apparatus for quantitative chiral analysis. So far, direct CD analysis with EW-CRDS has not been demonstrated due to the difficulty to maintain the circular polarization of light in the cavity of EW-CRDS. Typical EW-CRDS operates with linearly polarized optical modes. Therefore, the optical field cannot discriminate different handedness of chiral domains. To address this issue, we propose using rationally designed plasmonic nanostructures to locally transform the linearly polarized cavity mode into enantioselective local field for chiral analysis. Plasmonic nanostructures not only solve the problem of linear polarization of cavity mode but also offer the opportunity to enhance chiral light-matter interaction. If this project is successful, we can bypass the limitation of linearly polarized light and apply EW-CRDS to direct and sensitive CD analysis. This would be the first demonstration of direct CD analysis using EW-CRDS. Upon the success of the proposed research, new desktop analytical instrument for sensitive and quantitative CD analysis based on EW-CRDS can be anticipated. The innovation of this project is to combine nanoplasmonics with EW-CRDS to make ultrasensitive direct CD analysis possible. This project is not only interesting for fundamental research but also for real applications in biochemical analysis and drug design.