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

圆二色性（CD）是分子手性区域对圆偏振光的差异吸收。CD提供了分子构象的重要信息，并已大量用于生物医学和药物样品的定性分析。然而，由于光的波长和分子手性结构域的尺寸不匹配，CD通常非常小并且对于定量分析不够灵敏。在过去的十年中，等离子体纳米结构已经被证明在雕刻光学近场以增强手性光-物质相互作用方面非常强大。另一方面，倏逝波腔衰荡光谱技术（EW-CRDS）利用腔内多次反射来增强弱吸收，已经成为一种成熟的、可实现的方法。因此，等离子体激元和EW-CRDS的成功组合有望大大提高手性分析的灵敏度。在这个提议中，我们计划将纳米等离子体激元和EW-CRDS的增强效应联合收割机结合起来，以提高凝聚相手性样品CD分析的灵敏度。目的是开发一种新型、灵敏的等离子体激元EW-CRDS手性定量分析装置。到目前为止，直接CD分析与EW-CRDS还没有被证明，由于困难，以保持在EW-CRDS腔的光的圆偏振。典型的EW-CRDS以线性偏振光学模式操作。因此，光场不能区分手性域的不同手性。为了解决这个问题，我们建议使用合理设计的等离子体纳米结构，局部地将线性偏振腔模式转换为手性分析的对映选择性局部场。等离子体纳米结构不仅解决了腔模的线偏振问题，而且提供了增强手性光-物质相互作用的机会。如果该项目成功，我们可以绕过线偏振光的限制，将EW-CRDS应用于直接和灵敏的CD分析。这将是首次使用EW-CRDS进行直接CD分析。在所提出的研究的成功，新的台式分析仪器的灵敏度和定量CD分析的基础上EW-CRDS可以预期。该项目的创新之处在于将联合收割机纳米等离子体与EW-CRDS相结合，使超灵敏的直接CD分析成为可能。这个项目不仅是有趣的基础研究，而且在生化分析和药物设计的真实的应用。

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.