Model-Based Insight into Single-Molecule Plasmonic Mislocalization

Noble metal nanoparticles act as visible-wavelength antennas that reshape the emission of nearby dye molecules. Because single-molecule imaging overcomes ensemble averaging, it is uniquely suited for understanding these plasmonic effects. We have engineered single-particle assemblies consisting of a gold nanodisk surrounded by ATTO590 dye molecules attached by double-stranded DNA linkers. These assemblies provide a controlled geometry that places plasmon-coupled dye molecules at specific radial positions in the plane of the nanodisk and provides the ground truth actual dye molecule position for further analysis. We modeled the molecule-nanoantenna interaction as two coupled dipoles and propagated their interfering fields through a diffraction-limited microscope to generate a coupled-dipole image that accurately recreates the distorted PSFs seen in numerical simulations of plasmon-enhanced single-molecule fluorescence. We fit simulated and experimental data to the model function as well as to a standard Gaussian function and evaluated the advantages of the model approach. Overall, we find that the image model can better recapture certain aspects of the dye molecule mislocalization, and we propose that remaining gaps can be addressed by integrating higher-order plasmon multipole effects into the nanodisk response as well as by simultaneously using both image and spectral information.