Fluorescent DNA-stabilized silver nanoclusters contain both cationic and neutral silver atoms. The absorbance spectra of compositionally pure solutions follow the trend expected for rod-shaped silver clusters, consistent with the polarized emission measured from individual nanoclusters. The data suggest a rod-like assembly of silver atoms, with silver cations mediating attachment to the bases.

We develop approaches to hold fluorescent silver clusters composed of only 10-20 atoms in nanoscale proximity, while retaining the individual structure of each cluster. This is accomplished using DNA clamp assemblies that incorporate a 10 atom silver cluster and a 15 or 16 atom silver cluster. Thermally modulated fluorescence resonance energy transfer (FRET) verifies assembly formation. Comparison to Förster theory, using measured spectral overlaps, indicates that the DNA clamps hold clusters within roughly 5 to 6 nm separations, in the range of the finest resolutions achievable on DNA scaffolds. The absence of spectral shifts in dual-cluster FRET pairs, relative to the individual clusters, shows that select few-atom silver clusters of different sizes are sufficiently stable to retain structural integrity within a single nanoscale DNA construct. The spectral stability of the cluster persists in a FRET pair with an organic dye molecule, in contrast to the blue-shifted emission of the dye.

We report on the first single emitter fluorescence spectra of DNA-stabilized silver clusters (Ag:DNAs) at ambient and cryogenic temperatures. While Ag:DNAs have received much attention recently due to their sequence-tunable emission wavelengths, the nature of the optical transitions (molecule-like or collective, cluster-like) is an open question. By removing the ensemble broadening present in previous spectroscopic studies, we probe the line widths Γ and brightness of individual Ag:DNA emitters. A roughly 5-fold increase in brightness from 295 to 1.7 K is accompanied by a factor of 2 decrease in Γ. The symmetric emission line shape, its insensitivity to embedding medium, and the independence of emission wavelength on excitation energy together indicate that the measured Γ represents the homogeneous line width, while the large, ∼100 meV values of Γ suggest rapid dephasing of a collective excited state of the silver cluster.

Hybrid nanostructures, in which a known number of quantum emitters are strongly coupled to a plasmonic resonator, should feature optical properties at room temperature such as few-photon nonlinearities or coherent superradiant emission. We demonstrate here that this coupling regime can only be reached with dimers of gold nanoparticles in stringent experimental conditions, when the interparticle spacing falls below 2 nm. Using a short transverse DNA double-strand, we introduce 5 dye molecules in the gap between two 40 nm gold particles and actively decrease its length down to sub-2 nm values by screening electrostatic repulsion between the particles at high ionic strengths. Single-nanostructure scattering spectroscopy then evidences the observation of a strong-coupling regime in excellent agreement with electrodynamic simulations. Furthermore, we highlight the influence of the planar facets of polycrystalline gold nanoparticles on the probability of observing strongly coupled hybrid nanostructures.