The programmed assembly of gold nanoparticles on DNA templates allows the design of nanostructures with optical properties that directly depend on the morphology of a biochemical scaffold. Indeed, the nanometer-scale sensitivity of plasmon coupling allows the translation of minute conformational changes into macroscopic optical signals. In particular, we showed that it is possible to design DNA-linked gold nanoparticle dimers that act as nanoscale actuators: nanostructures that change conformation under an external stimulus such as the hybridization of a specific DNA strand (L. Lermusiaux et al, ACS Nano 6, 10992 (2012)). Importantly, we are able to monitor, on a simple color camera, the conformational changes of a single DNA-assembled gold particle dimer as its gap is reduced from 20 nm to 1 nm when varying the local ionic strength (L. Lermusiaux et al, ACS Nano 9, 978 (2015)).
We will also discuss recent results on the use of 3D DNA origamis as scaffolds for the assembly of plasmonic nanostructures, demonstrating that the conformation of the origami can be correlated to single nanostructure spectroscopy measurements. The flexibility of these biochemical templates opens exciting perspectives for the optical sensing of specific physicochemical stimuli that actively modify their 3D conformation, such as short biomolecules, specific cations or organic trace elements in water.
The nanometer-scale sensitivity of plasmon coupling allows the translation of minute morphological changes in nanostructures into macroscopic optical signals. In particular, single nanostructure scattering spectroscopy provides a direct estimation of interparticle distances in gold nanoparticle (AuNP) dimers linked by a short DNA double-strand [M. P. Busson et al, Nano Lett. 11, 5060 (2011)]. We demonstrate here that this spectroscopic information can be inferred from simple widefield measurements on a calibrated color camera [L. Lermusiaux et al, ACS Nano 9, 978 (2015)]. This allows us to analyze the influence of electrostatic and steric interparticle interactions on the morphology of DNA-templated AuNP groupings. Furthermore, polarization-resolved measurements on a color CCD provide a parallel imaging of AuNP dimer orientations. We apply this spectroscopic characterization to identify dimers featuring two different conformations of the same DNA template. In practice, the biomolecular scaffold contains a hairpin-loop that opens after hybridization to a specific DNA sequence and increases the interparticle distance [L. Lermusiaux et al, ACS Nano 6, 10992 (2012)]. These results open exciting perspectives for the parallel sensing of single specific DNA strands using plasmon rulers. We discuss the limits of this approach in terms of the physicochemical stability and reactivity of these nanostructures and demonstrate the importance of engineering the AuNP surface chemistry, in particular using amphiphilic ligands [L. Lermusiaux and S. Bidault, Small (2015), in press].
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