The first ever 3D "plasmon ruler" has been unveiled by researchers in the US, Germany and France. Until now, such nanoscale measuring devices were limited to measuring distances in just 1D, which meant that they could not be used to monitor 3D processes in biological and soft matter. The new sensor could prove useful for monitoring structural changes in biological samples, such as protein folding and DNA interactions.

Metals can absorb light by creating plasmons, which are particle-like collective excitations of conduction electrons at a metallic surface. A 1D plasmon ruler exploits the fact that the plasmon resonances of two metallic nanoparticles couple with each other when they are close together. The spectrum of light associated with the plasmons is strongly shifted toward the blue or red depending on how close or far apart the nanoparticles are to each other.

For example, in previous studies two gold nanoparticles were connected together via a single strand of DNA. When complementary double-stranded DNA was then added, researchers observed a significant blueshift in the light spectrum of the plasmon resonances. Since double-stranded DNA is much stronger than single stranded, the nanoparticles are pushed apart – that is, the distance between them becomes larger. By continuously monitoring the spectrum of the gold particles, the dynamics of the DNA "hybridization" could be recorded.

Stack of gold nanorods

Now, Laura Na Liu of the Lawrence Berkeley National Laboratory and colleagues at the University of Stuttgart and the University Blaise Pascal in Aubière have extended this concept so that it works in 3D. In their new plasmon ruler, the researchers employed a stack of five gold nanorods arranged in a "H" shape with the central rod acting as the horizontal bar of the H (see image). The other two pairs of rods were chosen so that they acted as quadrupolar "antennas" for visible lightwaves. When biological molecules are attached to the structure, the central rod or quadrupole antennas move relative to each other, which results in a shift of the plasmon resonances of the system that can be measured, just like the 1D ruler. The researchers fabricated their set-up using high-precision electron-beam lithography and layer-by-layer stacking nanotechniques.

"Compared with its 1D counterpart, our ruler offers additional degrees of freedom – such as rotating, twisting and tilting – to detect the dynamic behaviour of bioentities," Liu told physicsworld.com.

New generation of plasmon rulers

According to the researchers, the concept can be applied to single metallic nanocrystals joined together by oligonucleotides or peptides. This could lead to a new generation of plasmon rulers capable of monitoring events occurring during a wide variety of macromolecular transformations in 3D. Such transformations include DNA interacting with enzymes or proteins, protein folding and the dynamics of peptide motion, and the elastic vibrations of cells membranes in situ and in vivo, to name but a few.

"Metallic nanoparticles of different sizes could also be attached at different positions on DNA or proteins and each metallic element may move individually or collectively in three dimensions," explains Liu.

The team now hopes to make 3D plasmon rulers using biochemical linkers. The concept might even be extended to even more complicated plasmon structures, according to Carsten Sönnichsen of the Johannes Gutenberg University of Mainz in Germany.

The research is described in Science 332 1407.