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

当一个物体不能叠加在它的镜像上时，它就是手性的。我们的左手和右手是不重叠的，因此是手性物体。手性在自然界中普遍存在。DNA螺旋的扭曲使其具有特定的手性，这是由扭曲方向决定的。光波在空间中传播时可以形成螺旋形。这种特殊形式的光——被称为圆偏振光——具有特定的手性，这取决于扭曲是顺时针还是逆时针。当手性物体的旋向匹配时，圆偏振光与手性物体的相互作用更有利。因此，手性分子吸收更多的左旋或右旋圆偏振光。这种差异被称为圆二色性，它为分子的手性和几何形状提供了重要的见解。然而，圆二色性信号通常很弱。在化学系大分子、超分子、纳米化学和化学测量与成像项目的支持下，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，这些蛋白质可以集中光并放大圆二色性信号。林克教授与他的学生一起，运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。他们的发现可能对理解神经退行性疾病具有深远的适用性，比如阿尔茨海默氏症和帕金森氏症。此外，该项目融合了多学科的概念，形成了一个吸引博士后研究人员、研究生、本科生和高中生的培训平台。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折纸方法来精确控制纳米级几何形状，并结合独特的光谱能力来确定单分子等离子体复合物的圆二色性信号。电磁模拟和电子显微镜有助于区分由等离子体纳米结构本身引起的光学活性与感兴趣的手性分子。这些研究促进了对等离子体耦合圆二色性的机制理解，并允许人们利用圆二色性创建用于检测单个蛋白质分子结构的底物。该项目还利用由全内反射产生的表面受限波研究浪面二色性。摆线场沿光的传播方向具有电分量，并且只存在于界面上。林克实验室正在探索，当这些场的侧翻运动的手性被改变时，哪些纳米结构和分子几何结构对摆面场特别敏感。林克教授还参加了莱斯大学的公民科学家计划，让他教育K-12学生关于纳米技术，并激励他们追求科学事业。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.