Science 332, 1407–1410 (2011)

The plasmon resonance wavelength of metal nanoparticles such as gold is affected by changes in the particle's immediate environment, and these changes can be measured using scattering or absorption spectroscopy. Moreover, two nanoparticles placed in close proximity can exhibit a shift in resonance wavelength that depends on the separation between them. This effect has been used to make plasmon rulers, which measure nanoscale distances in one dimension and have been employed to study DNA hybridization. Laura Na Liu and colleagues have now created a three-dimensional plasmon ruler from a stack of five gold nanorods.

The researchers — who are based at the University of California, Berkeley, Lawrence Berkeley National Laboratory, the University of Stuttgart and the University Blaise Pascal — used electron-beam lithography and layer-by-layer stacking techniques to create nanorod structures in which one nanorod was perpendicularly stacked between two parallel rod pairs. Strong coupling between the sandwiched nanorod and the two nanorod pairs permits the excitation of two sharp quadrupolar resonances in the broad dipolar resonance profile. This allows high-resolution plasmon spectroscopy to be carried out, and for the exact position of the middle nanorod with respect to the others to be detected.

Liu and colleagues suggest that the approach could also be applied to metal nanoparticles joined together by oligonucleotides or peptides, and that 3D plasmon rulers could be used to study biological processes in the future.