(673g) Chirality Switch in Gold Nanorod Dimers Bridged By dsDNA

Gold nanorods are widely utilized as colloidal building blocks for fabricating plasmonic nanostructures whose chiroptical activity depends sensitively on their interparticle orientation and separation distances. Recent experiments successfully self-assembled pegylated gold nanorods into chiral dimers that are bridged by a single piece of double-stranded DNA (dsDNA) [1]. Interestingly, the chirality and the dihedral angle between the nanorods depended on whether they are in an intracellular or extracellular environment. While controlling the assembled structure of the nanorods is crucial for their application as plasmonic materials, the mechanism behind this chirality switch and the factors determining the observed dihedral angles are not well understood. Here, we investigate the underlying physics governing the interparticle configuration of the gold nanorod dimers through a theoretical model that describes the various interactions of the dimers as a function of their interparticle distance and angle. The results demonstrate that the competition between repulsive forces – from grafted polymers and electrostatic interactions – and attractive interactions – from depletion and van der Waals interactions – determine the separation distance and the dihedral angle of the dimers, while the mechanical properties of dsDNA, especially its twist-stretch coupling, lead to the observed chirality switch of the dimers. Predictions of the interparticle configurations of the dimers are made as functions of the pH and ionic concentration of the solvent as well as the degree of pegylation, aspect ratio of the nanorods, and the length of the dsDNA bridge. Our results demonstrate that the nanorod dimers can be used as nanoscopic optical sensors that are strongly responsive to changes in the solvent conditions. We expect that these findings will provide design guidelines for engineering gold nanorod dimers for applications in biosensing, chiral catalysis, and photodynamic therapies.

[1] Sun et al., Nature communications 2017, 8.1, 1-10