Advancing Chiral Plasmonics Using Single-Molecule Localization Microscopy

Abstract

Fluorescence microscopy is an important tool for studying structures in living cells, but its use is limited to sizes greater than 300 nm due to the diffraction limit of visible light. Single-molecule localization microscopy overcomes this limitation by fitting the diffraction-limited image of a single molecule to a two-dimensional Gaussian function, enabling the study of subcellular biophysics and nanoscale devices. On the other hand, plasmonics is the study of the collective oscillation of free electrons in a metal. Plasmonics has enabled incredible control of light on the nanoscale. Plasmonic nanoparticles act as antennas that convert propagating visible light from the far field to localized electromagnetic fields. These nanoparticles can also act as antennas that couple near-field energy to the far field by increasing the radiative rate of nearby fluorescent molecules. The net effect is that plasmonic nanoparticles can increase the number of photons detected per unit of time from a fluorescent molecule. This plasmon-enhanced fluorescence has successfully been applied to increase the precision in single-molecule localization microscopy. The subfield of chiral plasmonics deals with the ability of these plasmonic nanoparticles to increase optical signals related to the molecular property of chirality—the geometric property of molecules that cannot be superimposed on their mirror image through rotations and translations. In addition to increasing the electric field intensity, nanoparticles can twist a circularly polarized propagating wave through a circle in an area much smaller than its wavelength, thus more efficiently coupling ultraviolet and visible light (hundreds of nanometers) to the length scale of chiral molecules (one angstrom − one nanometer). This subfield of chiral plasmonics has grown very fast in the last ten years because of its broad applicability to many research areas including biosensing, chemical catalysis, pharmacology, agricultural science, and information technology. The work in this thesis combines the resolution of single-molecule localization microscopy with the enhancement of chiral plasmonics. I collaborated with Saaj Chattopadhyay to design and build a polarization microscope capable of studying single-molecule chiral plasmonics. I then used this instrument for three major projects. I measured a strong emission dissymmetry from single nanohelicoids fabricated by collaborators in Dr. Nicholas Kotov’s lab and determined that their fabrication method was highly selective based on single-particle data. In a study of near-field interactions of chiral nanoparticle dimers with achiral fluorescent molecules, I measured an induced fluorescence dissymmetry in the achiral dye Cy5.5 of ~|0.5| from only 100 zmol sample (104 molecules). I collaborated with Saaj Chattopadhyay to show that the electromagnetic property of optical chirality represents regions of maximal redirection of left-circularly polarized and right-circularly polarized emission by correlating electromagnetic simulations with experimental data. I collaborated with Maciej Lipok and Saaj Chattopadhyay to apply the polarization microscope to the interactions of achiral nanoparticles bound to a chiral biopolymer, the amyloid fibril. We found that binding to amyloid can induce a circular dichroism signal from gold nanorods without micron-scale order and that small aggregates of a few (2-5) nanoparticles dominate this induced signal. This thesis demonstrates how single-molecule and single-particle techniques can uncover heterogeneities missed in bulk experiments to enable improved sensing of enantiomers, better selectivity for single-molecule imaging of chiral biomolecules, and more efficient light-emitting devices. More broadly, this thesis expands the list of properties we can observe at the nanoscale by applying single-molecule localization microscopy to chirality.