Optical techniques are widely used tools to investigate biological species. Raman spectroscopy is a powerful analytical method to probe the molecular signature of cellular biomolecules. It can characterize biological specimens at high special resolution in a non-destructive and non-invasive manner.
For probing the structure fingerprints of biomolecules, Raman spectroscopy utilizes a monochromatic light, typically generated by a laser source in the mid-ultraviolet (mid-UV, 200-400 nm), visible (400-700 nm) or near -infrared (NIR, 0.7-1.1 µm) regions. Upon excitation of sample by an incident laser, two scattering phenomena can occur. Rayleigh scattering which causes by elastic scattering and Raman scattering which causes by inelastic scattering.
Raman scattering vs Rayleigh scattering
Rayleigh scattering does not yield any specific information about the molecule because it produces photons of the same energy, and thus wavelength and frequency as incident photons. Therefore, there is no transfer of energy from the incident photons and Raman spectroscopy detects no Raman shift.
Alternatively, the light might be scattered at a range of different wavelengths, and hence frequencies which referred to as Raman scattering (or inelastic scattering). Raman scattering is an extremely rare phenomenon with only 1 in incident photons undergoing inelastic scattering. The inelastic scattered photons will have either higher energy which referred to as anti-Stokes Raman scattering and lower energy which referred to Stokes Raman Scattering due to the interaction of incident photons with vibrational modes associated with chemical bonds. Raman spectroscope measures the energy of Raman scattered photons as a difference of wavelength compared to excitation wavelength. A Raman spectrum is a plot of the photon intensity in arbitrary units (a.u. or counts) as a function of Raman Shift which is plotted in wavenumber, (1/wavelength).
An example of Raman spectrum
Fortis Gaba, William J. Tipping , Mark Salji, Karen Faulds, Duncan Graham and Hing Y. Leung. Raman Spectroscopy in Prostate Cancer: Techniques,
Applications and Advancements.