ABSTRACT
Several techniques based on Raman scattering have been developed to address the diverse needs of and situations in which biomedical samples can be found. To study biomolecules, spontaneous Raman spectroscopy is mostly used in the near-infrared region of the optical spectrum where laser damage and excitation of autofluorescence is minimal; however, spontaneous Raman scattering produces a very weak signal, oftentimes orders of magnitude weaker than its fluorescence counterpart. Several techniques have been developed to address this shortcoming, including enhanced Raman scattering (SERSa and TERSb) and coherent anti-Stokes Raman scattering (CARS). asurface-enhanced Raman scatteringb tip-enhanced Raman scattering
Nadia Djaker-Oudjhara
12.1 IntroductionThe SERS process involves the use of Raman-active molecules very close to a metal surface, so surface plasmons can very efficiently couple the energy of the incoming laser light with the molecules, thereby amplifying the signal up to 1014 times.1 Although the specific details leading to this huge amplification are still not entirely understood, Raman scattering even from single molecules has been observed, making SERS ideal for label-free molecular assays. The only limitations of SERS are that it does require the presence of a relatively large metal surface and that it decays rapidly with distance away from the surface. CARS amplifies Raman signals without adding any exogenous elements. A CARS signal is generated when two lasers are focused into the sample, one acting as the source, leading to Stokes-shifted Raman scattering, while the other one is tuned to the same frequency as a specific Raman peak so as to selectively scatter light at this selective Raman mode. The two lasers interact nonlinearly within their overlap volume, simultaneously exciting the characteristic chemical vibration and driving the amplification of the corresponding anti-Stokes peak. The resulting CARS signal is more than a thousand times stronger than the original Raman signal and can be used for selective imaging chemically without the use of fluorescent dyes.
