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

William Marsh Rice University, Houston TX

Investigators

Abstract

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.