Long- and Short-Ranged Chiral Interactions in DNA Assembled Plasmonic Chains

Abstract

Molecular chirality plays a crucial role in innumerable biological processes. The chirality of a molecule can typically be identified by its characteristic optical response, the circular dichroism (CD). CD signals have thus long been used to identify the state of molecules or to follow dynamic protein configurations. In recent years, the focus has moved towards plasmonic nanostructures, as they show potential for applications ranging from pathogen sensing to novel optical materials. The plasmonic coupling of the individual elements of such chiral metallic structures is a crucial prerequisite to obtain sizeable CD signals. We here identified and implemented various coupling entities - chiral and achiral - to obtain chiral transfer over distances close to 100 nm. The coupling is realized by an achiral nanosphere situated between a pair of gold nanorods that are arranged far apart but in a chiral fashion. We synthesized these structures with nanometer precision using DNA origami and obtained sample homogeneity that allowed us to directly demonstrate efficient chiral energy transfer between the distant nanorods. The transmitter particle causes a strong enhancement in amplitude of the CD response, the emergence of an additional chiral feature at the resonance frequency of the nanosphere, and a redshift of the longitudinal plasmonic resonance frequency of the nanorods. Numerical simulations closely match our experimental observations and give insights in the intricate behavior of chiral optical fields and the transfer of plasmons in complex architectures.