iSCAT Fluctuation Microscopy of Plasmon Rulers Assembled from 8 nm and 16 nm Gold Nanoparticles.

Dimers of noble metal nanoparticles (NPs) connected by a molecular tether represent Plasmon Rulers, whose scattering spectrum depends on the interparticle separation and orientation relative to the incident light polarization. Tethered NP motion leads to fluctuations of the Plasmon Ruler signal that contains information about the tether properties and its structural dynamics. Detection of Plasmon Rulers by interferometric scattering (iSCAT) microscopy facilitates a reduction of the NP size used in Plasmon Rulers when compared to darkfield microscopy, which is desirable to reduce the perturbation of molecular properties. It is, however, unclear how the Plasmon Ruler performance decreases with NP size and over what NP size range tethered NP motion induced iSCAT contrast fluctuations can still be detected. In this work we investigate the iSCAT contrast fluctuations associated with Plasmon Rulers assembled from 8 nm (8) and 16 nm (16) gold NPs (AuNPs). Specifically, we recorded iSCAT contrast fluctuations of 16/16, 16/8, and 8/8 Plasmon Rulers containing a DNA tether with an end-to-end length of 14.8 nm. These experiments reveal significantly different mean absolute deviation (MAD) distributions for all three Plasmon Rulers and a decrease in the contrast fluctuations in the order of 16/16 > 16/8 > 8/8. Nevertheless, even the smallest investigated 8/8 Plasmon Rulers still provide significantly increased levels of iSCAT signal fluctuations compared to monomers or Plasmon Rulers with collapsed DNA tether.