PERSPECTIVES
FOLDING DNA
range of materials, sizes, and shapes, often with high crystallinity and narrow size distributions. These properties are particularly important for improving the optical response and reproducibility of optical antennas. Finally, both the antenna elements and single-photon emitters can be positioned with nanometer-scale precision and controlled stoichiometry within three-dimensional nanoscale geometries. Together, these advantages have made it possible to revisit fundamental light-matter interaction phenomena at the single-molecule level.
NANOANTENNAS FOR EMISSION MANIPULATION Optical antennas modify the emission of nearby molecules or quantum dots by reshaping their local photonic environment. The resulting coupling depends not only on spectral overlap, but also on the emitter – antenna distance and on the orientation of the emitter transition dipole with respect to the local field. These geometrical parameters are difficult to control with conventional lithography. In practice, lithographic antennas are usually fabricated first, and emitters are then deposited or selected afterwards. This often leads to large sample-to-sample variations, requiring the screening of many devices to identify favorable configurations. Elaborated approaches deposit multiple emitters
Figure 2. Sketch of the DNA origami technique, where short ssDNA staple strands( colored) fold a long scaffold( gray). A fluorophore( green sphere) can be directly covalently attached to one of the ssDNA staples, so that its position on the final structure is controlled with nanometric accuracy. Incorporation of colloidal NPs is achieved via DNA-functionalization of their surface and subsequent hybridization with complementary ssDNA extensions in the DNA origami design.
and determine their orientation prior to nanostructure fabrication, a technique that remains difficult to scale.
DNA origami addresses this dual challenge by combining, within the same nanoscale template, control over the emitter and assembly of the antenna. Specific staple strands can be modified to place a single fluorophore at a defined position, and, in suitable designs, its orientation can also be constrained with respect to the antenna axis. This is crucial because efficient coupling depends not only on the distance between the emitter and the antenna, but also on the alignment of the molecular transition dipole with the local optical field and the antenna. Beyond emitter placement, DNA origami also enables the self-assembly of multiple NPs into well-defined geometries, allowing a wide variety of nanoantenna architectures to be fabricated. This combination of positional and orientational control, together with programmable NP assembly has made DNA origami a powerful platform to study how optical antennas influence single-molecule emission, from enhanced photostability and fluorescence enhancement to directional control of the emitted light.
The gold NP dimer is the simplest and most widely used example. Two particles are assembled with a well-defined nanometer gap, creating a plasmonic hotspot between them. DNA origami can position a single fluorophore in this gap, where the optical near field is strongest, while also controlling its orientation relative to the dimer axis. Such structures reduce geometrical heterogeneity and provide a direct experimental platform to compare single-molecule measurements with electromagnetic simulations. [ 3 ]
Figure 3. DNA origami assembled nanoantennas for emission manipulation.( a) Dimer of two gold spheres with an oriented single dye at the hotspot. The zoom highlights the gap where a doubly linked fluorophore( symbolized by a red double arrow) was positioned either perpendicular or parallel to the two NPs forming the dimer. Fluorescence transients show the difference in intensity thanks to the optimized orientation.( b) Unidirectional emission from a trimer nanoantenna assembled by DNA origami that places a single fluorophore( red spot) between two NPs. The simulated radiation pattern evidences the directionality obtained.( c) Sketch of a rectangular DNA origami structure with a single gold NP and fluorophore employed to study the distance dependence energy transfer between both. As the distance gets smaller, a reduction in fluorescence intensity is observed.( d) Same study using a silicon NP. In this case, the intensity remains constant regardless of the distance, while the emission gets accelerated in close proximity. Figures adapted from [ 3,4,6,7 ].
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