Just like human fingerprints – a primary human characteristic, the Raman scattering of a molecule also gives a “fingerprint” spectrum, i.e. Raman spectrum. The patterns of a Raman spectrum are characteristic of the particular molecule—no two molecules have the same pattern. Raman spectroscopy has been receiving more and more attention in the scientific community as a way to study the physical characteristics of materials.
What is a Raman spectrum?
A Raman spectrum is all about light. When light of a certain wavelength (usually a laser) interacts with any material, most of its photons are deflected off the material. This deflection is called elastic scattering, or Rayleigh scattering. The scattered photons have the same wavelength as the incident photons.
However, a very small amount (approximately one in a million) of the incident photons is inelastically scattered. These photons are inelastically scattered because they have either lost or gained some energy during their interaction with the material. In a molecule, atoms are not statically connected to each other. Instead, molecular bonds vibrate and rotate constantly. Inelastically scattered photons lose or gain energy when interacting with those molecular bonds. Thus, a Raman spectrum is a signature of many molecular bonds in a material. Indian scientist C.V. Raman first discovered the Raman scattering effect and won the Nobel Prize in Physics in 1930.
Many nanomaterials have strong identifying Raman spectra which provide information on both inter- and intra- molecular vibrational modes. Even for the same material, like single-walled carbon nanotubes, Raman spectral frequencies can vary according to the tube diameter and can be used to determine material quality.
One main shortcoming of the Raman scattering technique is that the signal is normally extremely weak (only one in a million photos are inelastically scattered!), so relatively high intensity lasers have to be used and expensive, highly sensitive detectors have to be used to measure this very weak scattered light.
Surface-enhanced Raman spectroscopy
Surface-enhanced Raman spectroscopy (SERS) is a well-known technique based on Raman spectroscopy and nanotechnology. It has been widely applied for obtaining Raman spectra of compounds at low concentrations, in some cases down to the single-molecule level.
About 40 years ago, it was discovered –originally by accident– that the molecules adsorbed to the surface of silver or gold nanoparticles exhibit a huge increase in their Raman signals, by a factor of thousands to millions. We now call this phenomenon surface-enhanced Raman scattering (SERS). This “surface enhancement” effect is greatest for silver and gold nanoparticles and is considerably smaller for other metal nanoparticles. In particular, it is largest for particles that have shapes with sharp edges or junctions (like prisms or aggregated particles) rather than spherical particles. The presence of a sharply curved metal surface and junctions among particles greatly amplifies the electric field of the laser light where the molecule is sitting.
In addition to its widespread usage in micro-trace analysis, SERS has another major advantage as a nanoparticle tracking technique: it can be performed with a modified microscope (such as the WITec alpha 500 in Dr. Vikesland’s group) allowing us to make Raman measurements inside cells and even in specific regions within cells. Using SERS signals, we can easily reconstruct a 2D or 3D Raman image of particle locations.
SERS tracking of gold nanoparticles (red spots in Raman images) in primary hepatic cells (Detzel 2012)