Insights from a nanoparticle minuet: two-dimensional membrane profiling through silver plasmon ruler tracking.

Individual pairs of polymer-tethered silver nanoparticles, so-called silver plasmon rulers, enable distance and orientation measurements on the nanoscale. The reduced linear dichroism and the spectrum of the light scattered from individual plasmon rulers encode information about their orientation and average interparticle separation, respectively. We took advantage of the gain in information silver plasmon rulers offer as probes in optical tracking and analyzed the translational and rotational motions as well as the extension of individual silver plasmon rulers diffusing on the plasma membrane of lysed HeLa cells. Consistent with a compartmentalization of the cell surface on the length scales of the plasmon rulers, most rulers were either immobilized or performed a confined lateral diffusion. Structural details of a plasmon ruler's confinement region became accessible utilizing the orientation and interparticle separation dependent optical response of the plasmon rulers. This approach, which we refer to as polarization-resolved plasmon coupling microscopy, enabled a detailed structural characterization of individual membrane compartments and provided a quantitative metrics to characterize the structural lateral heterogeneity of cell membranes on submicrometer length scales. In combination with adequate tracking methods, the "dance" performed by membrane confined dimers of flexibly linked noble metal nanoparticles revealed deep insight into the underlying membrane morphology.