Nanoscale Polarization Control for Single Molecule Detection: Circular and Trochoidal Dichroism

当一个物体不能叠加在其镜像上时，它就是手性的。我们的左手和右手是不可叠加的，因此是手性物体。手性存在于整个自然界。DNA螺旋的扭转赋予了它一种特定的利手，这由扭转方向决定。光波在太空中传播时，可以使其呈螺旋状。这种特殊形式的光--称为圆偏振光--有一种特定的利手，这取决于扭曲是顺时针还是逆时针。当手性物体的左手相匹配时，圆偏振光与手性物体的相互作用更有利。因此，手性分子要么吸收更多的左旋或右旋圆偏振光。这种差异被称为圆二向色性，它给了人们对分子的利手性和几何学的重要见解。然而，圆二向色性信号通常很弱。在化学系大分子、超分子和纳米化学以及化学测量和成像项目的支持下，莱斯大学的斯蒂芬·林克教授正在研究放置在两个金属纳米颗粒之间的单个蛋白质，这些纳米颗粒聚集光并放大圆二向色性信号。与他的学生合作，林克教授正在应用先进的组装方法来控制蛋白质的放置，并使用复杂的光谱来研究潜在的增强机制，这一点仍然知之甚少。他们的发现可能对了解阿尔茨海默氏症和帕金森氏症等神经退行性疾病具有深远的适用性。此外，该项目融合了多个学科的概念，为博士后研究人员、研究生、本科生和高中生提供了一个有吸引力的培训平台。为了利用定义的纳米级组装和电场操作来检测单手性分子及其结构动力学，Link实验室正在应用DNA折纸方法精确控制纳米级几何图形，并结合为确定单个分子-等离子复合体的圆二色信号而开发的独特光谱功能。电磁模拟和电子显微镜有助于区分等离子体纳米结构本身产生的光学活性和感兴趣的手性分子。这些研究正在推进对等离子体偶联圆二向色性的机理的理解，并允许人们利用圆二向色性来创建用于传感单个蛋白质的分子结构的底物。该项目还在研究利用全内反射产生的表面限制波的摆线二向色性。摆线面场沿着光的传播方向有电分量，并且只存在于界面上。林克实验室正在探索哪些纳米结构和分子几何结构对摆线线场特别敏感，当这种场的侧翻运动的惯用手被切换时。林克教授还参与了莱斯大学的公民科学家计划，使他能够教育K-12学生纳米技术，并激励他们追求科学事业。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

An object is chiral when it cannot be superimposed on its mirror image. Our left and right hands are non-superimposable and are therefore chiral objects. Chirality exists throughout nature. The twist of a DNA helix gives it a specific handedness, determined by the twist direction. Light waves can be made to spiral as they travel through space. This special form of light -- called circularly polarized light -- has a specific handedness depending on whether the twist is clock-wise or counter clock-wise. Circularly polarized light interacts more favorably with chiral objects when their handedness match. Chiral molecules therefore either absorb more left-handed or right-handed circularly polarized light. The difference is known as circular dichroism, which gives important insights into the handedness and geometry of molecules. However, the circular dichroism signal is usually weak. With support from the Macromolecular, Supramolecular and Nanochemistry and Chemical Measurement and Imaging programs in the Division of Chemistry, Professor Stephan Link from Rice University is studying individual proteins placed in the gap between two metal nanoparticles, which concentrate the light and amplify the circular dichroism signal. Working with his students, Professor Link is applying advanced assembly methods to control the protein placement and using sophisticated spectroscopies to study the underlying enhancement mechanism, which is still poorly understood. Their discoveries could have far-reaching applicability to understanding neurodegenerative diseases, such as Alzheimer's and Parkinson's. In addition, the project incorporates concepts from multiple disciplines forming an attractive training platform for post-doctoral researchers, graduate, undergraduate, and high school students. To exploit defined nanoscale assembly and electric field manipulation for the detection of single chiral molecules and their structural dynamics, the Link lab is applying DNA origami methods for precise control over nanoscale geometry in combination with unique spectroscopic capabilities developed for determining the circular dichroism signals of single molecular-plasmonic complexes. Electromagnetic simulations and electron microscopy aid in the distinction between optical activity arising from the plasmonic nanostructure itself vs. the chiral molecule of interest. These studies are advancing the mechanistic understanding about plasmon-coupled circular dichroism and allow one to create substrates for sensing molecular structure of a single protein using circular dichroism. The project is also studying trochoidal dichroism using surface confined waves created by total internal reflection. Trochoidal fields have electric components along the propagation direction of light and only exist at an interface. The Link lab is exploring which nanostructure and molecular geometries are sensitive specifically to trochoidal fields when the handedness of the cartwheeling motion of such fields is switched. Professor Link is also participating in Rice University's Civic Scientist Program, allowing him to educate K-12 students about nanotechnology and inspire them to pursue scientific careers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.