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.

The predictable nature of deoxyribonucleic acid (DNA) interactions enables assembly of DNA into almost any arbitrary shape with programmable features of nanometer precision. The recent progress of DNA nanotechnology has allowed production of an even wider gamut of possible shapes with high-yield and error-free assembly processes. Most of these structures are, however, limited in size to a nanometer scale. To overcome this limitation, a plethora of studies has been carried out to form larger structures using DNA assemblies as building blocks or tiles. Therefore, DNA tiles have become one of the most widely used building blocks for engineering large, intricate structures with nanometer precision. To create even larger assemblies with highly organized patterns, scientists have developed a variety of structural design principles and assembly methods. This review first summarizes currently available DNA tile toolboxes and the basic principles of lattice formation and hierarchical self-assembly using DNA tiles. Special emphasis is given to the forces involved in the assembly process in liquid-liquid and at solid-liquid interfaces, and how to master them to reach the optimum balance between the involved interactions for successful self-assembly. In addition, we focus on the recent approaches that have shown great potential for the controlled immobilization and positioning of DNA nanostructures on different surfaces. The ability to position DNA objects in a controllable manner on technologically relevant surfaces is one step forward towards the integration of DNA-based materials into nanoelectronic and sensor devices.

hybrid materials at truly nanoscale dimensions, enabling functionality at enormously high spatial densities. [ 11,12 ] Given their nanometer size scale, such clusterbased materials may also combine desirable properties usually associated with the molecular regime, such as high fl uorescence quantum yields and large Stokes shifts, with emergent near-fi eld interactions arising from the polarizability of free electron systems, as currently exploited in surface-enhanced Raman spectroscopy [ 13 ] (SERS), and plasmonic coupling schemes with metal nanoparticles. [ 12 ] Despite the numerous Ag:DNA [ 3,[14][15][16][17][18][19] studies, little is known about the mechanism by which fl uorescent excitation and emission occur. It was previously established in the literature on small metal clusters that the excitation energies are considerably up-shifted from the particle-in-box energies, due to Coulomb interactions that lead to collective, phased oscillation of the cluster's valence electrons. [ 20,21 ] Recent experimental works [ 22,23 ] indicate that fl uorescent Ag:DNA contains a neutral silver core that is surrounded by base-bound silver ions. This arrangement is analogous to the known structure of gold clusters that are stabilized by small organic ligand molecules, [24][25][26] which possess both transitions associated with the gold core, and transitions involving charge transfer between the gold core and the surrounding ligands plus their directly attached gold atoms. [ 25,26 ] For Ag:DNAs, the specifi c mode of cluster-DNA binding is unknown, but given the existence of a neutral silver core one might expect both core-centered transitions and charge transfer transitions between the core and baseattached silver ions.In the case of such silver core-ligand transitions, small variations in the specifi c conformation of the DNA might be expected to affect directions of transition dipole moments. Thus it is of great interest to study individual Ag:DNAs under conditions that minimize environmental fl uctuations, using techniques that can probe such orientational effects.In this paper, we present single-cluster optical studies of Ag:DNAs that investigate both their response as a function of the polarization of the excitation light and the polarization of the light they emit. Single particle polarization studies have previously been used to investigate individual fl uorescent molecules, providing insight into the orientation of the emitters in the surrounding medium [ 27,28 ] as well as photophysical events of single molecules, such as rotational jumps of a single Polarization-resolved excitation and emission measurements on individual 10-15 atom silver clusters formed on DNA are presented. The emission is highly linearly polarized, typically around 90% for all emitters, whereas the polarization dependence of the excitation strongly varies from emitter to emitter. These observations support the hypothesis that the luminescence arises from collective electron oscillations along rod-shaped silver clusters, whereas the excitation ...

Strong coupling between molecules and confined light modes of optical cavities to form polaritons can alter photochemistry, but the origin of this effect remains largely unknown. While theoretical models suggest a suppression of photochemistry due to the formation of new polaritonic potential energy surfaces, many of these models do not account for the energetic disorder among the molecules, which is unavoidable at ambient conditions. Here, we combine experiments and simulations to show that for an ultra-fast photochemical reaction such thermal disorder prevents the modification of the potential energy surface and that suppression is due to radiative decay of the lossy cavity modes. We demonstrate that by increasing the coupling strength we can reduce such losses and enhance reactivity of the strongly coupled system, in contrast to the theoretical paradigm, which would predict stronger suppression. We also show that the excitation spectrum under strong coupling is a product of the excitation spectrum of the ”bare” molecules and the absorption spectrum of the molecule-cavity system, suggesting that polaritons can act as gateways for channeling an excitation into a molecule, which then reacts ”normally”. Our results therefore imply that strong coupling provides a means to tune the action spectrum of a molecule, rather than to change the reaction.

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