Monitoring Simultaneous Distance and Orientation Changes in Discrete Dimers of DNA Linked Gold Nanoparticles.

Important optical properties of discrete pairs of DNA tethered gold nanoparticles, including their scattering cross-section and resonance wavelength, depend both on the dimer structure and the refractive index of their immediate environment. We show that far-field polarization microscopy aids the optical identification and interpretation of structural changes including hinge motions and nanoscale distance changes in individual assemblies. Grecco and Martinez have shown in their theoretical work that the interparticle separation dependent polarization anisotropy of discrete nanoparticle dimers enables nanoscale distance measurements (Optics Express 2006, 14, 8716 - 8721). Here we implement this approach experimentally and evaluate measured polarization anisotropies in the framework of a dipolar coupling model. We use polarization sensitive darkfield microscopy to resolve simultaneous distance and orientation changes during the compaction of discrete pairs of DNA tethered gold nanoparticles by fourth generation polyamidoamino (PAMAM) dendrimers. The relative contributions from interparticle separation and refractive index variations to changes in the light polarization and scattering intensity are quantified and compared.