Measuring molecules

Imaging peering into a molecule and observing in real-time the arrangement of the particles within - that's something American and German researchers are now capable of doing. With their new 'plasmon ruler,' they're able to make unprecedented 3D measurements at the nanometer scale.

The technique involves embedding noble metal nanoparticles like gold or silver into a molecule and using them as a ruler. Because gold, for instance, is a biologically harmless element, the technique does not kill the living molecules it's being used to study.

A new paper detailing the technique was published on Friday in the journal, Science.

According to one of the paper's authors, Harald Giessen of the University of Stuttgart, the technique has the potential to further the study of what exact properties of DNA make biological life viable.

"The point where you distinguish living matter from dead matter is encoded in proteins," he told Deutsche Welle. "We know what's in the proteins, but to this day it has not been possible to create life in a test tube. Life isn't just that matter - that protein - but it's also the structure of the protein and how it's folded. Understanding protein folding is a big part of understanding what life is."

Giessen added that so far the scientists have only proven that the technique works. They still have to "attach it to biological entities" like proteins or DNA in order to make real-time measurements.

The plasmon ruler functions in real-time with processes lasting seconds or "possibly milliseconds," according to Giessen.

"If it's faster than milliseconds, then we really have to develop the spectroscopic method to take scattering spectra on timescales that are less than milliseconds," he said.

Two new dimensions

Plasmons are electronic waves generated as light passing through the confines of nanoparticles or structures made of noble metals like gold or silver, according to the researchers.

Paul Alivisatos, the senior author of the paper and director of the Lawrence Berkeley National Laboratory in the United States, said "light scattering" can be used to determine the distance between two nanoparticles.

"Two noble metallic nanoparticles in close proximity will couple with each other through their plasmon resonances to generate a light-scattering spectrum that depends strongly on the distance between the two nanoparticles," he said in a statement.

While this light-scattering effect has been used to make one-dimensional measurements in the past, the scientists' new technique represents its first 3D application.

Carsten Sönnichsen, a professor at the University of Mainz, worked with Alivisatos' team to develop the initial one-dimensional plasmon ruler in 2005. He said that this new report, which he did not participate in, represents "a big step in the development of plasmonic rulers by showing a route to molecular sensing on a much more complex scheme than before."

"It will be the challenge for the next years to develop a chemical way to make these structures by self-assembly with molecules rather than with top-down lithography as in the current study," he told Deutsche Welle in an email. "However, progress in this field is rapid."

Anatoly Zayats, head of experimental biophysics and nanotechnology at King's College in London, described the project as a "huge technological endeavour," and added that it "opens up new capabilities to monitor in real time movement of large biological molecules, for example."

"The technical realization in this work is fantastic," he told Deutsche Welle in an e-mail. "I am sure it will create a lot of follow up work. The applications in biochemical and bio-medical sciences are most likely and have potential to produce immense impact."

Author: Gerhard Schneibel
Editor: Cyrus Farivar