One of the few ways to differentiate between molecular enantiomers are chiroptical spectroscopies like circular dichroism or circularly polarized luminescence. However, these types of spectroscopy are often limited to high analyte concentrations due to the mismatch between the size of a molecule and a propagating electromagnetic wavelength. Previous studies have used plasmonic nanoparticles to compress the excitation field and dramatically increase ensemble-averaged chiroptical signals. However, the diffraction limit of light microscopy makes it difficult to measure how the signal enhancement depends on a molecule’s position relative to a nanoparticle. We achieved super-resolution microscopy and spatially resolved the circular polarization of molecules near two-dimensional chiral nanoparticles on the nanometer scale using Points Accumulation for Imaging in Nanoscale Topography (PAINT) with an imaging polarimeter. First, we demonstrated with full-field electromagnetic simulations that the optical chirality of the nanoparticle near-field exceeds that of a circularly polarized propagating wave. Second, we built and characterized a polarimeter compatible with a wide-field microscope to measure the circularly polarized fluorescence of dyes near these particles. Finally, we probed the near-field emission enhancement of free and DNA-bound cyanine dyes near chiral and achiral particles one molecule at a time using PAINT. At 20-nm resolution, we observed spatial heterogeneities in the emission enhancement, which we correlate to the optical chirality of the near-field surrounding the chiral nanoparticles. Our results outline a method for spatially mapping the chiral near-fields of plasmonic substrates and point to the promise of performing chiroptical spectroscopy at the single-molecule level.

Near-field optical chirality of two-dimensional chiral plasmonic nanoparticles revealed by super-resolution microscopy

Fall 2022 abstracts, posters, and presentations will launch in mid-September.

ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.