Single-Particle Spectroscopic Study on Fluorescence Enhancement by Plasmon Coupled Gold Nanorod Dimers Assembled on DNA Origami

Metal-enhanced fluorescence has attracted much attention due to its scientific importance and lots of potential applications. Plasmon coupled metal nanoparticles have been demonstrated to further improve the enhancement effects. Conventional studies of metal-enhanced fluorescence on the bulk systems are complicated by the ensemble average effects over many critical factors with large variations. Here, fluorescence enhancement of ATTO-655 by a plasmon coupled gold nanorod dimer fixed on a DNA origami nanobreadboard was studied on the single-particle level. A series of gold nanorod dimers with linear orientation and different gap distances ranging from 6.1 to 26.0 nm were investigated to explore the plasmon coupling effect on fluorescence enhancement. The results show that the dimer with the smallest gap (6.1 nm) gives the highest enhancement (470-fold), and the enhancement gradually decreases as the gap distance increases and eventually approaches that from a monomer (120-fold). This trend is consistent with the numerical calculation results. This study indicates that plasmon coupling in gold nanorod dimers offers further increased excitation efficiency to achieve large fluorescence enhancement

Geometry-Dependent Plasmonic Tunability and Photothermal Characteristics of Multibranched Gold Nanoantennas

Plasmon resonances of anisotropic multibranched nanostructures are governed by their geometry, allowing morphology-directed selective manipulation of the optical properties. In this work, we have synthesized multibranched gold nanoantennas (MGNs) of variable geometry by a one-step seedless approach using 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) as a capping and reducing agent. This approach enables us to modulate the MGNs’ geometry by controlling three different parameters: concentration of HEPES, concentration of Au³⁺, and pH of HEPES buffer. By altering the MGNs morphology with minimal increase in the overall dimensions, the plasmon resonances were tuned from the visible to the near-infrared. The MGNs plasmon resonances demonstrated a nonintuitive blue-shift when pH > p<i>K</i>_a of HEPES which we attributed to emergence of charge transfer oscillations formed when MGNs cluster to dimers and trimers. Further, due to the presence of multiple sharp protrusions, the MGNs demonstrated a refractive index sensitivity of 373 nm/RIU, which is relatively high for this class of branched nanostructures of similar size. Finally, the sharp protrusions of MGNs also give rise to intense photothermal efficiencies; ∼53 °C was achieved within 5 min of laser illumination, demonstrating the efficacy of MGNs in therapeutic applications. By modulating the mass density of MGNs, the laser flux, and time of illumination, we provide a detailed analysis of the photothermal characteristics of MGNs