Optical nanoantennas are known to focus freely propagating light and reversely to mediate the emission of a light source located at the nanoantenna hotspot. These effects were previously exploited for fluorescence enhancement and single-molecule detection at elevated concentrations. We present a new generation of self-assembled DNA origami based optical nanoantennas with improved robustness, reduced interparticle distance, and optimized quantum-yield improvement to achieve more than 5000-fold fluorescence enhancement and single-molecule detection at 25 μM background fluorophore concentration. Besides outperforming lithographic optical antennas, DNA origami nanoantennas are additionally capable of incorporating single emitters or biomolecular assays at the antenna hotspot.

Plasmonic structures are known to affect the fluorescence properties of dyes placed in close proximity. This effect has been exploited in combination with single-molecule techniques for several applications in the field of biosensing. Among these plasmonic structures, top-down zero-mode waveguides stand out due to their broadband capabilities. In contrast, optical antennas based on gold nanostructures exhibit fluorescence enhancement on a narrow fraction of the visible spectrum typically restricted to the red to near-infrared region. In this contribution, we exploit the DNA origami technique to self-assemble optical antennas based on large (80 nm) silver nanoparticles. We have studied the performance of these antennas with far- and near-field simulations and characterized them experimentally with single-molecule fluorescence measurements. We demonstrate that silver-based optical antennas can yield a fluorescence enhancement of more than 2 orders of magnitude throughout the visible spectral range for high intrinsic quantum yield dyes. Additionally, a comparison between the performance of gold and silver-based antennas is included. The results indicate that silver-based antennas strongly outperform their gold counterparts in the blue and green ranges and exhibit marginal differences in the red range. These characteristics render silver-based optical antennas ready for applications involving several fluorescently labeled species across the visible spectrum.

Electric and magnetic optical fields carry the same amount of energy. Nevertheless, the efficiency with which matter interacts with electric optical fields is commonly accepted to be at least 4 orders of magnitude higher than with magnetic optical fields. Here, we experimentally demonstrate that properly designed photonic nanoantennas can selectively manipulate the magnetic versus electric emission of luminescent nanocrystals. In particular, we show selective enhancement of magnetic emission from trivalent europium-doped nanoparticles in the vicinity of a nanoantenna tailored to exhibit a magnetic resonance. Specifically, by controlling the spatial coupling between emitters and an individual nanoresonator located at the edge of a near-field optical scanning tip, we record with nanoscale precision local distributions of both magnetic and electric radiative local densities of states (LDOS). The map of the radiative LDOS reveals the modification of both the magnetic and electric quantum environments induced by the presence of the nanoantenna. This manipulation and enhancement of magnetic light-matter interaction by means of nanoantennas opens up new possibilities for the research fields of optoelectronics, chiral optics, nonlinear and nano-optics, spintronics, and metamaterials, among others.

Optical antennas are nanostructures designed to manipulate light–matter interactions by interfacing propagating light with localized optical fields. In recent years, numerous devices have been realized to efficiently tailor the absorption and/or emission rates of fluorophores. By contrast, modifying the spatial characteristics of their radiation fields remains challenging. Successful phased array nanoantenna designs have required the organization of several elements over a footprint comparable to the operating wavelength. Here, we report unidirectional emission of a single fluorophore using an ultracompact optical antenna. The design consists of two side-by-side gold nanorods self-assembled via DNA origami, which also controls the positioning of the single-fluorophore. Our results show that when a single fluorescent molecule is positioned at the tip of one nanorod and emits at a frequency capable of driving the antenna in the antiphase mode, unidirectional emission with a forward to backward ratio of up to 9.9 dB can be achieved.

Standard fluorescence microscopy relies on filter-based detection of emitted photons after fluorophore excitation at the appropriate wavelength. Although of enormous utility to the biological community, the implementation of approaches for simultaneous multicolor fluorescence imaging is commonly challenged by the large spectral overlap between different fluorophores. Here, we describe an alternative multicolor fluorescence imaging methodology that exclusively relies on the absorption spectra of the fluorophores instead of their fluorescence emissions. The method is based on multiplexing optical excitation signals in the frequency domain and using single color-blind detection. Because the spectral information is fully encoded during excitation, the method requires minimal spectral filtering on detection. This enables the simultaneous identification of multiple color channels in a single measurement with only one color-blind detector. We demonstrate simultaneous three-color confocal imaging of individual molecules and of four-target imaging on cells with excellent discrimination. Moreover, we have implemented a non-negative matrix factorization algorithm for spectral unmixing to extend the number of color targets that can be discriminated in a single measurement. Using this algorithm, we resolve six spectrally and spatially overlapping fluorophores on fixed cells using four excitation wavelengths. The methodology is fully compatible with live imaging of biological samples and can be easily extended to other imaging modalities, including super-resolution microscopy, making simultaneous multicolor imaging more accessible to the biological research community.

AbstractFörster Resonance Energy Transfer (FRET) is a widely applied technique in biology to accurately measure intra- and inter-molecular interactions at the nanometre scale. FRET is based on near-field energy transfer from an excited donor to a ground state acceptor emitter. Photonic nanoantennas have been shown to modify the rate, efficiency and extent of FRET, a process that is highly dependent on the near-field gradient of the antenna field as felt by the emitters, and thus, on their relative distance. However, most of the experiments reported to date focus on fixed antennas where the emitters are either immobilized or diffusing in solution, so that the distance between the antenna and the emitters cannot be manipulated. Here, we use scanning photonic nanoantenna probes to directly modulate the FRET efficiency between individual FRET pairs with an unprecedented nanometric lateral precision of 2 nm on the antenna position. We find that the antenna acts as an independent acceptor element, competing with the FRET pair acceptor. We directly map the competition between FRET and donor-antenna transfer as a function of the relative position between the antenna and the FRET donor-acceptor pair. The experimental data are well-described by FDTD simulations, confirming that the modulation of FRET efficiency is due to the spatially dependent coupling of the single FRET pair to the photonic antenna.

Controlling directionality of optical emitters is of utmost importance for their application in communication and biosensing devices. Metallic nanoantennas have been proven to affect both excitation and emission properties of nearby emitters, including the directionality of their emission. In this regard, optical directional nanoantennas based on a Yagi–Uda design have been demonstrated in the visible range. Despite this impressive proof of concept, their overall size (~λ2/4) and considerable number of elements represent obstacles for the exploitation of these antennas in nanophotonic applications and for their incorporation onto photonic chips. In order to address these challenges, we investigate an alternative design. In particular, we numerically study the performance of a recently demonstrated “ultracompact” optical antenna based on two parallel gold nanorods arranged as a side-to-side dimer. Our results confirm that the excitation of the antiphase mode of the antenna by a nanoemitter placed in its near-field can lead to directional emission. Furthermore, in order to verify the feasibility of this design and maximize the functionality, we study the effect on the directionality of several parameters, such as the shape of the nanorods, possible defects in the dimer assembly, and different positions and orientations of the nanoemitter. We conclude that this design is robust to structural variations, making it suitable for experimental upscaling.

At the plasma membrane level, extensive research has demonstrated that multiple molecules, such as proteins and lipids, interact in a dynamic fashion creating transient nanoscale compartments of functional activity. [6][7][8][9][10] Such findings were revealed owing to the recent development of different optical techniques aimed at improving both spatial and temporal resolution beyond that of conventional diffraction-limited optical methods. [10][11][12][13][14][15] Yet, monitoring dynamic multi-molecular interactions in living cells at the nanoscale remains challenging.With the advancement of single-molecule and super-resolution approaches, multiple techniques have been implemented aiming to reduce the illumination volume set by diffraction, thus enabling single-molecule dynamic studies at high labeling conditions in living cells on the nanoscale. For instance, stimulated emission depletion microscopy (STED) [10] and metallic nanoapertures [16][17][18] reduce the illumination area down to 50-200 nm in diameter. However, in the case of STED, both the high laser powers required and the increased photobleaching constitute significant drawbacks for its routine application in living cells. [19] Fluorescence cross-correlation spectroscopy moreover, although dual-color STED is nowadays widely used for superresolution imaging in fixed cells, [20] its extension to living cells