Circular dichroism measures the differential interaction of matter with left-handed and right-handed circularly polarized light. Circular dichroism spectroscopy is essential for quantifying protein secondary structure composition, and has more recently been employed to probe chiral nanomaterials, which can have extraordinary strong circular dichroism effects compared to their molecular counterparts. Here we investigated the circular differential scattering of a simple chiral plasmonic system, a twisted side-by-side Au nanorod dimer, using single-particle circular dichroism spectroscopy complimented with electromagnetic simulations. This approach enabled us to quantify the effects of structural symmetry breaking, namely size-mismatch between the constituent Au nanorods and large twist angles, on the resulting circular dichroism scattering spectrum. Our results demonstrate that, if only scattering is considered as measured by dark-field spectroscopy, a homo-dimer of Au nanorods with similar sizes produces a circular differential scattering lineshape that is different from the bisignate response of the corresponding conventional CD spectrum, which measures extinction for this nanorod dimer sample, i.e. the sum of scattering and absorption. On the other hand, symmetry breaking in a hetero-dimer with Au nanorods with different sizes yields a bisignate circular differential scattering lineshape. In addition, we provide a general method for correcting linear dichroism artifacts arising from slightly elliptically polarized light in a typical dark-field microscope, as is necessary especially when measuring highly anisotropic nanostructures, such as side-by-side nanorods. This work lays the foundation for understanding absorption and scattering contributions to the CD lineshape of single nanostructures free from ensemble-averaging, especially important for self-assembled chiral nanostructures that usually exist as both enantiomers.

Circular differential scattering of single chiral self-assembled gold nanorod dimers

251st National Meeting (2016)

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