Nanostructures hybrides or-origamis d'ADN pour l'exaltation et la détection de molécules uniques

By leveraging the precise nanoscale control offered by DNA origamis, this work aims at optimizing the interaction between a plasmonic resonator and a single molecule, either to detect this molecule or to manipulate its photophysical properties.To develop a digital colorimetric biosensing platform, dynamic DNA origamis are used as nanoscale actuators on which hybrid plasmonic nanostructures are assembled. These nanostructures shift their morphology when interacting with specific targets, in particular DNA single strands, thanks to an active site in the origami that is sensitive to DNA strand displacement reactions. To translate these conformational changes into an optical signal for biosensing, we exploit the nanoscale dependence of plasmon coupling on the interparticle spacing between two gold nanospheres, which is modulated by allostery.Dark-field microscopy enables far-field monitoring of nanoscale distance changes in single gold dimers using a simple color camera. The morphology of the hybrid nanostructure is influenced by the geometry of the DNA origami but also by steric and electrostatic repulsions between the nanospheres. In practice, the conformation of the active site can only be translated into a macroscopic optical response at high local ionic strength, where these repulsions are screened.One-step colorimetric sensing of DNA single strands is achieved via either scattering spectroscopy or by monitoring the hue of single dimers in dark-field images, yielding similar responses. Kinetics and the limit of detection are investigated with target concentrations ranging from 100 nM to 100 pM. These results highlight the potential of colorimetric sensing of single DNA strands using a color camera.Furthermore, the field enhancements and confinements allowed by plasmonic resonators, associated with the nanoscale control offered by DNA origamis, are applied to enhance the interaction between light and a single fluorescent molecule at room temperature. At room temperature, the optical properties of single quantum emitters are strongly inhibited by electron-phonon coupling but accelerating spontaneous emission thanks to the Purcell effect in plasmonic resonators can overcome these limitations. A DNA origami was designed to host a single ATTO647N molecule in the nanoscale hotspot within a resonator made of dimers of plasmonic nanoparticles.Simulations showed that 60 nm gold nanospheres with spacings below 3 nm provide Purcell factors exceeding 105 when the dipole is oriented parallel to the dimer axis, while maintaining high emission yields. Field enhancements can be further increased by the lightning rod effect provided by the tips of plasmonic nanocubes. To assemble anisotropic particles with an orientational control, a DNA origami was designed to host two cubes in a tip-to-tip conformation by shape complementarity. The optical properties of these nanostructures are investigated by fluorescence lifetime spectroscopy, at the single emitter level, under different conditions of ionic strengths and refractive indices to minimize the interparticle spacings and maximize the spectral overlap between fluorescent molecule and plasmonic resonator. These measurements demonstrate a strong reduction in fluorescence lifetimes in the presence of gold nanosphere and nanocube dimers. Purcell factors in the range of 50-600 are achieved in water-based media, while in glycerol-based media, values exceeding 1000 are observed, reaching the limit of sensitivity of our setup. This approach highlights the potential of plasmonic nanostructures for studying coherent light-matter interactions at room temperature using fluorescent molecules as single quantum emitters.