Nanoplasmonic structures designed for trace analyte detection by surface enhanced Raman spectroscopy typically require sophisticated nanofabrication techniques. An alternative to fabricating such arrays is to rely on self-assembly of nanoparticles at liquid-liquid or liquid-air interfaces into close-packed arrays.The density of the arrays can be fine tuned by modifying the nanoparticle functionality, pH of the solution, and salt concentration. Importantly, these arrays are robust, "self-healing", reproducible, and extremely easy to handle. Herein, we report on the use of such platforms made of Au nanoparticles for detection of (multi)analytes from the aqueous, organic, or air phases. The total interfacial area of the Au array, at the liquid-liquid interface, is approximately 25 mm 2 , making this platform ideal for small volume samples, low concentrations, and trace analytes. Importantly, the ease of assembly and rapid 2 detection makes this platform ideal for in-field sample testing of toxins such as explosives, drugs, or other hazardous chemicals.3

An emitter in the vicinity of a metal nanostructure is quenched by its decay through non-radiative channels, leading to the belief in a zone of inactivity for emitters placed within <10nm of a plasmonic nanostructure. Here we demonstrate that in tightly-coupled plasmonic resonators forming nanocavities "quenching is quenched" due to plasmon mixing. Unlike isolated nanoparticles, plasmonic nanocavities show mode hybridization which massively enhances emitter excitation and decay via radiative channels. This creates ideal conditions for realizing single-molecule strong-coupling with plasmons, evident in dynamic Rabi-oscillations and experimentally confirmed by laterally dependent emitter placement through DNA-origami.The lifetime of an excited atomic state is determined by the inherent properties of the atom and its environment, first theoretically suggested by Purcell [1] followed by experimental demonstration [2]. Subsequent experiments further verified this by placing atomic emitters within various optical-field-enhancing geometries [3][4][5]. Plasmonic structures have the ability to massively enhance electromagnetic fields, and therefore dramatically alter the excitation rate of an emitter [6]. However, it is well known that placing an emitter close to a plasmonic structure (< 10nm), quenches its fluorescence [7][8][9]. Analysis by Anger et al. [6] showed this is due to the coupling of the emitter to non-radiative higher-order plasmonic modes that dissipate its energy. This 'zone of inactivity' was previously believed to quench all quantum emitters. However, recent advancements have shown that an emitter's emission rate can be enhanced with plasmonic nano-antennas [10][11][12][13][14][15][16][17].Generally a single emitter placed into near-contact with an optical antenna gives larger fluorescence since the antenna efficiently converts far-field radiation into a localized field and vice versa [10,12,13,18]. This was recently demonstrated by Hoang et al. [17] who showed that a quantum dot in a 12nm nano-gap exhibits ultrafast spontaneous emission. What however remains unclear is if this enhanced emission is strong enough to allow for single emitter strong coupling.In this Letter, we demonstrate and explain why quenching is substantially suppressed in plasmonic nanocavities, to such a degree that facilitates lightmatter strong-coupling of single-molecules, even at roomtemperature, as we recently demonstrated experimentally [19]. This is due to: (i) the dramatic increase in the emitter excitation (similar to plasmonic antennas), and (ii) the changed nature of higher-order modes that acquire a radiative component, and therefore increase the quantum yield of the emitter. Modes in plasmonic nanocavities are not a simple superposition of modes from the isolated structures, but instead are hybridplasmonic states [20][21][22][23][24]. Hence, higher-order modes that are dark for an isolated spherical nanoparticle, radiate efficiently for tightly-coupled plasmonic structures [25], significantly reducing the non-radiative...

Fabricating nanocavities in which optically active single quantum emitters are precisely positioned is crucial for building nanophotonic devices. Here we show that self-assembly based on robust DNA-origami constructs can precisely position single molecules laterally within sub-5 nm gaps between plasmonic substrates that support intense optical confinement. By placing single-molecules at the center of a nanocavity, we show modification of the plasmon cavity resonance before and after bleaching the chromophore and obtain enhancements of ≥4 × 103 with high quantum yield (≥50%). By varying the lateral position of the molecule in the gap, we directly map the spatial profile of the local density of optical states with a resolution of ±1.5 nm. Our approach introduces a straightforward noninvasive way to measure and quantify confined optical modes on the nanoscale.

Interactions between a single emitter and cavity provide the archetypical system for fundamental quantum electrodynamics. Here we show that a single molecule of Atto647 aligned using DNA origami interacts coherently with a sub-wavelength plasmonic nanocavity, approaching the cooperative regime even at room temperature. Power-dependent pulsed excitation reveals Rabi oscillations, arising from the coupling of the oscillating electric field between the ground and excited states. The observed single-molecule fluorescent emission is split into two modes resulting from anti-crossing with the plasmonic mode, indicating the molecule is strongly coupled to the cavity. The second-order correlation function of the photon emission statistics is found to be pump wavelength dependent, varying from g (2) (0) = 0.4 to 1.45, highlighting the influence of vibrational relaxation on the Jaynes-Cummings ladder. Our results show that cavity quantum electrodynamic effects can be observed in molecular systems at ambient conditions, opening significant potential for device applications.

We report on a simple, fast, and inexpensive method to study adsorption and desorption of metallic nanoparticles at a liquid/liquid interface. These interfaces provide an ideal platform for the formation of two-dimensional monolayers of nanoparticles, as they form spontaneously, cannot be broken, and are defectcorrecting, acting as 2D 'nanoparticle traps'. Such two-dimensional self-assembled nanoparticle arrays have a vast range of potential applications in displays, catalysis, plasmonic rulers, optoelectronics, sensors and detectors. Here, we show that 16 nm diameter gold nanoparticles can be controllably adsorbed to a water/1,2-dichloroethane interface, and we can direct the average inter-particle spacing at the interface over the range 6-35 nm. The particle density and average inter-particle spacing are experimentally assessed by measuring the optical plasmonic response of the nanoparticles in the bulk and at the interface, and by comparing the experimental data with existing theoretical results.Keywords: liquid-liquid interface, Plasmonic Ruler, Nanoparticles, Self Assembly, Centrifugation. Nanoparticle (NP) adsorption at liquid-liquid interfaces (LLI) is a well established phenomenon that was first reported independently more than a century ago by Ramsden and Pickering. 1, 2 More recently, plasmonic NPs at LLIs have been reported to possess novel "metal liquid like" properties 3 that have sparked a renewed interest. Driven by the growing need for cheap, fast and reproducible bottom-up assembly for nanotechnological applications, the unique optical, 4 magnetic, 5 electrical 6 and chemical 7 properties of such films has attracted intensive research in a rapidly growing field.NP assemblies at LLIs hold great promise in diverse fields ranging from electrovariable optics 8 to templates for hierarchical self assembly, 9 and plasmonic rulers. 10 One of the main benefits of localising NPs at a LLI for these applications is the aforementioned self-assembly. 11 Currently, most technological applications of nanoassemblies are based around solid-state fabrication. Although the control that the solid interface offers is unparalleled, it does have several drawbacks when compared to a LLI. One such drawback is topological defects -these can be introduced either during or post-manufacturing and are extremely difficult to correct. In fact it is often easier to fabricate a new device rather than attempting to repair defects. Conversely, a LLI system has the ability to self-correct without any external manipulation. 12 An additional drawback of a solid state system becomes clear when introducing the exciting concept of a plasmonic ruler. [13][14][15] The coupling between excitations of localized plasmons in NPs can allow for precise spatial information. This concept has been demonstrated between 2D arrays, 10 particle dimers 14 at solid interfaces and tethered NPs in bulk solution. 16 It has even been recently demonstrated to provide 3-dimensional structural information. 17 The use of plasmonic particles at the LLI sh...