Dark-field optical microscopy allows the visualization of individual noble metal nanoparticles down to sizes around 20nm due to the excitation of plasmons. The color or resonance wavelength of such plasmonic nanoparticles depends on geometry, orientation, proximity to other particles and the refractive index of the environment, making them sensitive sensors for the nano-scale processes.

We report here on recent progress in our group among the three requirements for plasmonic nanosensors applications: parallel single particle dark-field spectroscopy [1], relevant biological functionalization, [2] and production of optimized plasmonic nanoparticles [3]. First, we developed a way to investigate many randomly deposited nanoparticles in parallel using a liquid crystal device as a spatially addressable electronic shutter. Secondly, we improved plasmon based biosensing by developing a membrane based functionalization (see figure). We coated the particles with a lipid bilayer which are easily bio-functionalized (many different headgroups are commercially available) and also block efficiently nonspecific binding. Testing this system by detecting Streptavidin binding to biotinylated lipids, we observe a spectral shift of 2.9 ± 1.8 nm. Finally, we produced the best known plasmonic structures by coating gold nanorods with a thin silver shell. The coating leads to a reduction of the single particle linewidth compared to uncoated gold rods at the same resonance wavelength.

[1]  Becker, Schubert, and Sönnichsen, “Gold Nanoparticle Growth Monitored in situ Using a Novel Fast Optical Single-Particle Spectroscopy Method”, Nano Letters 2007, 7, 1664

[2] Baciu, Becker, Janshoff, and Sönnichsen, “Protein-Membrane Interaction Probed by Single Plasmonic Nanoparticles“, Nano Letters, ASAP Article, DOI: 10.1021/nl080805l

[3] Becker, Zins, Jakab, et al. „Plasmonic Focusing Reduces Ensemble Linewidth of Silver-Coated Gold Nanorods”, Nano Letters, ASAP Article, DOI: 10.1021/nl080720k

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1. Enhanced thermal stability of gold and silver nanorods through thin surface layers
2. Growth of gold tips onto hyperbranched CdTe nanostructures