The plasmonic ruler goes 3D!

Noble metal nanoparticles have distinct optical properties arising from localized surface plasmon resonance (LSPR). It is wellestablished that a LSPR spectrum depends strongly on the size and shape of the nanoparticles as well as on the distance between the nanoparticles. The precise measurement of the interparticle distance-dependent LSPR spectrum has been the fundamental principle of many analytical methods in biology. These types of bioassays can be done with a large ensemble of particles either in homogenous solutions (i.e. colorimetric assays) or on substrates. Alternatively, the assay can be carried out through individual particles (i.e. single-particle-based LSPR). The single-particle-based LSPR technique has the advantages of high detection sensitivity, a good S/N ratio, low sample consumption, and multiplexing potential. Under a dark-field microscope, individual metal nanoparticles can easily be observed as they scatter light intensely and do not blink or photobleach. When two nanoparticles are in close proximity, their plasmon resonances couple with each other and generate a light-scattering spectrum depending strongly on the interparticle distance. This effect has been used to create one-dimensional plasmon rulers, that is, two nanoparticles linked with (bio)chemical linkers. Under high-resolution dark-field imaging spectroscopy, distinct spectral shifts triggered by biomolecular binding events and/or biological processes can be measured based on their modulation of the linkers and in turn the distances of two individual plasmonic particles. This one-dimensional plasmonic ruler has been successfully used to detect DNA conformational change, DNA bending and RNA cleavage. Recently, writing in Science, Na Liu, Mario Hentschel, Thomas Weiss, A. Paul Alivisatos, and Harald Giessen have reported a more powerful plasmonic ruler, a 3D plasmonic ruler, where multiple plasmonic nanoparticles are put in a spatial arrangement. This 3D plasmonic ruler in combination with high-resolution plasmon spectroscopy and plasmon-induced transparency as well as high-order resonances offers a blueprint for optically determining the structural dynamics of single 3D entities. The emergence of this pioneering work is a result of the advances in nanofabrication techniques, theoretical simulation, and high-resolution spectroscopy techniques. Using high-precision electron-beam-based top-down fabrication techniques and layer-by-layer stacking nanotechnologies, the researchers fabricated a 3D plasmonic structure containing five gold nanorods (Figure 1). In their design, a nanorod is stacked between two pairs of parallel nanorods. The two pairs of parallel rods