(725d) Resolving Subdiffraction Limit Distances Using Plasmon Coupling Microscopy

Discrete assemblies of noble metal nanoparticles enable new imaging and improved sensing applications. The distance dependent interactions between individual noble metal nanoparticles can be used to engineer discrete plasmonic nanostructures for applications in microscopy, sensing and imaging. For instance, DNA or RNA tethered noble metal nanoparticles (so called plasmon rulers) are active nanostructures which can indicate nanoscale distance changes through shifts in their resonance wavelength. Plasmon coupling measurements are not restricted to monitoring end-to-end distances of biopolymers in plasmon rulers. Instead, plasmon coupling can also resolve subdiffraction limit contacts between individual nanoparticle labeled surface receptors that perform a lateral diffusion on a cell surface. If two nanoparticles approach each other close enough for their plasmons to couple, the resonance wavelength of the dimer red-shifts. This effect is utilized in plasmon coupling microscopy (PCM). Individual, independent particles can be tracked with high spatial and temporal resolution in PCM. If two particles colocalize and can no longer be discerned optically, PCM monitors changes in the plasmon resonance wavelength to detect direct contacts between individual particles. Spectral shifts in the plasmon resonance of diffusing nanoparticles during optical colocalization can be detected in an optical widefield microscope using a ratiometric imaging approach. The sample is illuminated with unpolarized whitelight in conventional darkfield microscopy, and the detected image is split into two monochromatic images. Spectral shifts in the resonance wavelengths of two colocalized particles can then be detected as a change in the ratio of the dimer image in the two channels. The technology enables, for instance, to resolve close contacts between nanoparticle labeled cell surface receptors.