Theory: Increasing Vibrations Makes SERS More Powerful

Surface-enhanced Raman scattering (SERS) could become an even more powerful tool for probing single molecules if those molecules can be made to vibrate more, a theoretical model suggests.

SERS is used to identify individual molecules based on their spectral "fingerprints," which are determined by the way light is shifted by the vibrations of atoms inside the molecule. This shift — Raman scattering — is amplified by nanostructured surfaces that confine light in the form of plasmons.

An illustration of light-mediated detection of a molecule. Courtesy of N. Antille/EPFL.

While this amplification is significant, SERS is still limited because molecules tend to vibrate weakly at room temperature.

Now researchers at the Swiss Federal Institute of Technology in Lausanne (EPFL) have proposed a way to increase molecular vibrations that would amplify Raman emissions even further. The technique also involves plasmons — specifically, exciting them with a laser that causes them to exert mechanical force on the molecules being probed.

In the theoretical model, as the light-force amplifies the vibrations of the molecule, the interaction between the molecule and the confined laser light grows stronger as well. This would dramatically increase the SERS signal well beyond what can be reached by current methods.

"Our work offers specific guidelines for designing more efficient metallic nanostructures and excitation schemes for SERS," said doctoral assistant Philippe Roelli. "It can push the limits of the technique in sensitivity and resolution."

Funding came from the European Research Committee, National Centre of Competence in Research of Quantum Engineering, Swiss National Science Foundation, Curie Institute and Max Planck-EPFL Center for Molecular Nanoscience and Technology.

The research was published in Nature Nanotechnology (doi: 10.1038/nnano.2015.264).