Raman spectroscopy

Raman spectroscopy is based on inelastic scattering of incident monochromatic photons on molecular vibrations which depend on atomic masses of the atoms constituting the molecule, chemical bonds between atoms, molecular structure and conformation, as well as interactions with the environment. The Raman spectrum of each compound represents its specific molecular fingerprint and can be used for its identification and spatial localization within more complex contexts of other chemical species. Raman spectroscopy is well suited for analysis of biological samples containing water due to relatively weak interference of the water signal. Because of the linear relationship between Raman intensity and molecule concentration, Raman spectroscopy is particularly suited for quantitative analysis.

Confocal Raman microscopy

Confocal Raman microscopy is a contactless, non-invasive and often non-destructive method combining Raman spectroscopy and confocal microscopy at a spatial resolution of a few μm3. From micro Raman (micro-RS) spectra scanned point-by-point over the specimen, 2D or 3D Raman images (so-called Raman chemical maps) can be constructed to visualize the chemical as well as isotopic composition of the scanned object. Raman measurement can be performed inside living cells and does not require special preparation or staining of the specimen. Various isotopically substituted molecular species can be identified due to characteristic frequency downshifts of their Raman bands caused by heavier isotopes. Raman spectroscopy and micro-RS are fully compatible with stable isotope labeling and stable isotope probing strategies.

Surface enhanced Raman spectroscopy (SERS)

According to the name, the enhancement of signal of Raman scattering (as much as 1010) occurs at molecules adsorbed on a surface of either noble metals or special nanostructures. The effect on scattering is caused by electromagnetic and/or chemical enhancement. The signal magnification may allow detection of single molecules.

Tip enhanced Raman spectroscopy (TERS)

Basically a combination of surface enhanced Raman spectroscopy (SERS) together with atomic force microscopy (AFM) getting the best of each. Laser beam is focused at the golden tip of the AFM pin, exciting enhanced Raman scattering in resolution outperforming optical microscopy by orders of magnitude. Again, similarly as for SERS, the signal magnification may allow detection of single molecules.

Coherent anti-Stokes Raman spectroscopy (CARS)

This vibrational contrast technique enhances the vibrational signal compared to spontaneous Raman scattering. Originally described as three-wave mixing it employs three lasers: pump beam and Stokes beam of the frequency difference in resonance with sample contents and the probe beam. Resonance occurs at a given frequency of the absorbed radiation that matches the vibrational frequency of the covalent bonds of the molecule.

Optical (laser) tweezer

Optical tweezer is a tightly focused beam of light (laser) capable of holding microscopic particles such as bacteria, cells etc., depending on the proportion of refractive index of the particle compared to the surrounding media.

Vibrational spectroscopy

Apart from Raman spectroscopy, there is a kind of complementary method of (near-) infrared spectroscopy to be accounted for among methods of vibrational spectroscopy. There are many differences between both methods: different excitation wavelengths of the excitation light source (visible vs infrared) due to which we may detect (by different types of detectors) either the change in polarizability of molecules or change in dipole moment. Due to strong infra-red absorption of water, IR spectroscopy is not convenient for analyses of dissolved chemicals or biological samples unless dried. But compared to otherwise preferentially used Raman spectroscopy, it has a great advantage in measuring (auto-)fluorescent samples strongly interfering with Raman signal.