Plasmonic coupling dynamics of silver nanoparticles in an optical trap
30.11.2015 Blattmann M, Rohrbach A. Nano Lett. 2015;15(12):7816-21. Optical tweezers have proven to be the optimal tool for the contactless manipulation of nanoscaled particles embedded in fluid, often biological environments. Similar to molecular assembly in biology, metal nanoparticles diffusing inside an optical trap can find their best interaction state with other particles by thermal noise probing each other, thereby allowing the self-organization into more complex structures. In this nanoletter we demonstrate how particle binding and plasmonic coupling similar to FRET between optically trapped 80nm silver spheres can be either spontaneous or induced by another particle through confinement of diffusion. Plasmonic coupling is measured by a red shift of white excitation light, which is backscattered from the particles and recorded by a spectrometer at 30 Hz. In addition, we use three-dimensional back focal plane interferometry, where forward scattered light from the trapping laser is measured at 1 MHz. By this combination of plasmonic and interferometric sensing we are able to identify the ultra-fast entry and exit of particles, their displacements inside the optical trap and the forming of dimers, trimers or tetramers. This technique allows to investigate biochemical interactions between surface-coated nano-particles in great detail. 