Probing DNA Stiffness through Optical Fluctuation Analysis of Plasmon Rulers.

The distance-dependent plasmon coupling between biopolymer tethered gold or silver nanoparticles forms the foundation for the so-called plasmon rulers. While conventional plasmon ruler applications focus on the detection of singular events in the far-field spectrum, we perform in this Letter a ratiometric analysis of the continuous spectral fluctuations arising from thermal interparticle separation variations in plasmon rulers confined to fluid lipid membranes. We characterized plasmon rulers with different DNA tethers and demonstrate the ability to detect and quantify differences in the plasmon ruler potential and tether stiffness. The influence of the nature of the tether (single-stranded versus double-stranded DNA) and the length of the tether is analyzed. The characterization of the continuous variation of the interparticle separation in individual plasmon rulers through optical fluctuation analysis provides additional information about the conformational flexibility of the tether molecule(s) located in the confinement of the deeply subdiffraction limit interparticle gap and enhances the versatility of plasmon rulers as a tool in Biophysics and Nanotechnology.