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Figure 4. DNA origami for assembling NPs into chiral geometries.( a) Plasmonic helices created by arranging gold NPs on DNA origami( scale bar in SEM image, 50 nm) and corresponding theoretical( grey) and experimental( black) CD spectra.( b, c) Plasmonic cross nanostructures made of two gold nanorods located at opposite sides of the template can be made reconfigurable driven by changes in illumination wavelength( b) or pH( c). Figures adapted from [ 9 – 11 ].
More complex NP arrangements can extend this concept from intensity enhancement to directional emission. DNA origami has been used to assemble ultracompact meta-emitters composed of a plasmonic trimer antenna driven by a single dye molecule. By placing the emitter in one of the trimer gaps, both electric and magnetic dipole modes of the antenna can be excited. Their interference fulfils a Kerker-type condition at the fluorophore emission wavelength, leading to preferential emission in one direction. [ 4 ] This example shows that DNA origami can be used not only to increase brightness, but also to engineer the radiation pattern of a single emitter.
The approach is not restricted to noble-metal particles. Recent functionalization strategies combining click chemistry with freeze – thaw protocols have expanded the range of NPs that can be incorporated into DNA origami templates. [ 5 ] These include silicon and silica particles, oxides, polymers, core – shell structures, and metallic nanostructures. High-index dielectric NPs, such as silicon, are especially attractive in this context. Compared with metals, they exhibit much lower ohmic losses and can support both electric and magnetic resonances. Their incorporation into DNA origami templates therefore opens the possibility of studying dielectric nanoantennas with the same single-emitter control previously achieved for plasmonic systems. This strategy has recently been used to investigate the distance-dependent interaction between an individual fluorophore and a single silicon NP. [ 6 ] Unlike metallic antennas, where very small emitter – metal separations can lead to strong non-radiative quenching, [ 7 ] dielectric particles can modify the fluorescence decay rate while preserving a high fluorescence quantum yield, even at short distances. These experiments illustrate how DNA origami can serve not only as a fabrication method, but also as a controlled platform for comparing different antenna materials and geometries at the single-molecule level.
CHIRAL PLASMONIC SYSTEMS Chirality describes an object that cannot be superimposed on its mirror image. When circularly polarized light interacts with a chiral structure, the two polarization states are not absorbed equally, giving rise to chiroptical activity. The most common manifestation is
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