Resonance Raman spectroscopy

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Energy level diagram showing relationship between Rayleigh, Raman, and resonance Raman scattering and fluorescence. Energy levels in Raman spectroscopy.jpg
Energy level diagram showing relationship between Rayleigh, Raman, and resonance Raman scattering and fluorescence.

Resonance Raman spectroscopy (RR spectroscopy or RRS) is a variant of Raman spectroscopy in which the incident photon energy is close in energy to an electronic transition of a compound or material under examination. [1] This similarity in energy (resonance) leads to greatly increased intensity of the Raman scattering of certain vibrational modes, compared to ordinary Raman spectroscopy.

Contents

Resonance Raman spectroscopy has much greater sensitivity than non-resonance Raman spectroscopy, allowing for the analysis of compounds with inherently weak Raman scattering intensities, or at very low concentrations. [2] [3] It also selectively enhances only certain molecular vibrations (those of the chemical group undergoing the electronic transition), which simplifies spectra. [3] For large molecules such as proteins, this selectivity helps to identify vibrational modes of specific parts of the molecule or protein, such as the heme unit within myoglobin. [4] Resonance Raman spectroscopy has been used in the characterization of inorganic compounds and complexes, [5] proteins, [6] [7] nucleic acids, [8] pigments, [8] and in archaeology and art history. [8]

Theory

In Raman scattering, photons collide with a sample and are scattered with a difference in energy: The scattered photons may be higher or lower in energy (have a shorter or longer wavelength) than the incident photons. This difference in energy is caused by excitation of the sample to a higher or lower vibrational energy level: if the sample was initially in an excited vibrational state, the scattered photon may be higher in energy than the incident photon (anti-Stokes Raman scattering). Otherwise, the scattered photon has a lower module of energy than the incoming photon (Stokes Raman scattering). Among the two phenomena, Stokes shift and anti-Stokes shift, the former is the most likely to occur. As a consequence, the relative intensity of Raman spectra acquired in Stokes mode is more intense than the other. For most materials, Raman scattering is extremely weak compared to Rayleigh scattering, in which light is scattered without loss of energy. [9] Raman-scattered light, which contains information about vibrational transitions, is therefore difficult to observe for many substances.

Resonance Raman spectroscopy takes advantage of an increase in the intensity of Raman scattering when the incident photons match the energy of an electronic transition. If the energy of the photon striking the sample is equal or close to that of an electronic transition in the sample, certain Raman-active vibrational modes—those producing nuclear displacement in the same direction as the electronic transition [10] —will exhibit greatly enhanced scattering, up to 106-fold compared to nonresonance Raman. [3] For totally symmetric modes, this increased scattering intensity results from so-called A-term or Franck-Condon scattering, due to the nonzero Franck-Condon overlaps between ground and excited states. Nontotally symmetric modes may also be enhanced by B-term or Herzberg-Teller scattering, if the symmetry of the mode is contained in the direct product of the two electronic state symmetries. [11] Resonance enhancement is most apparent in the case of π-π* transitions and least for metal centered (d–d) transitions. [5] Like ordinary Raman spectroscopy, RRS observes vibrational transitions producing a nonzero change in the polarizability of the molecule or material being studied.

Resonance Raman scattering differs from fluorescence in that it occurs without vibrational relaxation during the lifetime of the excited electronic state. It thus exhibits much narrower line widths than fluorescence. [11] However, fluorescence and resonance Raman scattering co-occur in many materials, and interference from fluorescence may complicate the collection of resonance Raman spectra. [3]

Variants

Typically, resonance Raman spectroscopy is performed in the same manner as ordinary Raman spectroscopy, using a single laser light source to excite the sample. The difference is the choice of the laser wavelength, which must be selected to match the energy of an electronic transition in the sample. A tunable laser is thus often used for resonance Raman spectroscopy, since a single laser can be used to generate many possible excitation wavelengths to match different samples. [8] By using multiple lasers, pulsed lasers, and/or certain sample preparation techniques, a range of more sophisticated variants of RRS can be performed, including:

Applications

Resonance (top) and nonresonance (bottom) Raman spectra of MoS2 on silicon. Note that excitation at 633 nm, near an electronic transition, causes appearance of bands that are too faint to be visible with excitation at 532 nm. Figure courtesy of David Tuschel. MSWFig6.webp
Resonance (top) and nonresonance (bottom) Raman spectra of MoS2 on silicon. Note that excitation at 633 nm, near an electronic transition, causes appearance of bands that are too faint to be visible with excitation at 532 nm. Figure courtesy of David Tuschel.

Because of its selectivity and sensitivity, resonance Raman spectroscopy is typically used to study molecular vibrations in compounds that would have very weak and/or complex Raman spectra in the absence of resonance enhancement. Like ordinary Raman spectroscopy, resonance Raman is compatible with samples in water, which has a very weak scattering intensity and little contribution to spectra. However, the need for an excitation laser with a wavelength matching that of an electronic transition in the analyte of interest somewhat limits the applicability of the method. [8]

Pigments and Dyes

Dyes and pigments, all of which exhibit electronic transitions in the visible part of the electromagnetic spectrum, were among the first substances to be studied by resonance Raman spectroscopy. Resonance Raman spectra of beta-carotene and lycopene in intact plant samples were reported in 1970. [8] Since then, the method has been used to noninvasively measure levels of these nutrients in human skin. [19] The resonance Raman spectra of other polyene pigments, such as spheroidene and retinal, have been used to identify differences in chromophore conformation in photoactive proteins. [20] [21] Resonance Raman spectroscopy has been used in archaeology to identify dyes and pigments in cultural artifacts, and the ability of RRS to distinguish different modern inks and dyes has found application in forensic science. [8]

Proteins

Proteins have been widely examined by resonance Raman spectroscopy. Protein-bound cofactors that absorb in the visible wavelength range, such as heme, flavins, or transition metal complexes, can be examined by RRS with minimal spectral overlap from the rest of the molecule. [7] [22] This method has been used to examine gas binding in hemeproteins [23] and the catalytic cycle of various enzymes. [24] Using ultraviolet laser excitation, it is possible to selectively excite the sidechains of aromatic amino acids (phenylalanine, tyrosine, and tryptophan) to deduce the local environment and hydrogen-bonding interactions by these residues. [25] With shorter-wavelength ("deep") ultraviolet excitation, it is also possible to excite the peptide bonds of a protein in order to examine secondary structure. Protein folding and denaturation have been examined using deep-UV resonance Raman spectroscopy of the polypeptide backbone, with excitation wavelengths shorter than 200 nm. [25]

Nucleic acids and viruses

Resonance Raman spectroscopy with ultraviolet excitation can be used to examine the chemistry, structure, and intermolecular interactions of nucleic acids, specifically the bases. Interactions between nucleic acids and DNA-binding compounds such as drugs can be examined by selectively exciting either the nucleobases or the drug itself. [8] The resonance Raman spectra of DNA can be used to identify bacterial DNA in living cells, and to quantitate DNA under different culture conditions, and even to distinguish different bacterial species. [8] Viruses have also been studied using UV resonance Raman spectroscopy; the method has the capability to separately interrogate the structure of the nucleic acid or capsid protein components of the virus, through the choice of the appropriate excitation wavelength. [26]

Nanomaterials

Resonance Raman spectroscopy has also been used to characterize the structure and photophysical properties of nanoparticles. Using lasers tuned to the visible and near-infrared electronic transitions of carbon nanotubes, it is possible to enhance structure-sensitive vibrational bands of the nanotubes. [8] Nanowires of inorganic semiconductor materials including gallium phosphide and carbon-encapsulated mercury telluride have also been shown to exhibit resonance Raman spectra with visible excitation light. [27] [28]

See also

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<span class="mw-page-title-main">Spatially offset Raman spectroscopy</span>

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<span class="mw-page-title-main">Raman microscope</span> Laser microscope used for Raman spectroscopy

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The technique of vibrational analysis with scanning probe microscopy allows probing vibrational properties of materials at the submicrometer scale, and even of individual molecules. This is accomplished by integrating scanning probe microscopy (SPM) and vibrational spectroscopy. This combination allows for much higher spatial resolution than can be achieved with conventional Raman/FTIR instrumentation. The technique is also nondestructive, requires non-extensive sample preparation, and provides more contrast such as intensity contrast, polarization contrast and wavelength contrast, as well as providing specific chemical information and topography images simultaneously.

Stimulated Raman spectroscopy, also referred to as stimulated Raman scattering (SRS), is a form of spectroscopy employed in physics, chemistry, biology, and other fields. The basic mechanism resembles that of spontaneous Raman spectroscopy: a pump photon, of the angular frequency , which is scattered by a molecule has some small probability of inducing some vibrational transition, as opposed to inducing a simple Rayleigh transition. This makes the molecule emit a photon at a shifted frequency. However, SRS, as opposed to spontaneous Raman spectroscopy, is a third-order non-linear phenomenon involving a second photon—the Stokes photon of angular frequency —which stimulates a specific transition. When the difference in frequency between both photons resembles that of a specific vibrational transition the occurrence of this transition is resonantly enhanced. In SRS, the signal is equivalent to changes in the intensity of the pump and Stokes beams. The signals are typically rather low, of the order of a part in 10^5, thus calling for modulation-transfer techniques: one beam is modulated in amplitude, and the signal is detected on the other beam via a lock-in amplifier. Employing a pump laser beam of a constant frequency and a Stokes laser beam of a scanned frequency allows for unraveling the molecule's spectral fingerprint. This spectral fingerprint differs from those obtained by other spectroscopy methods, such as Rayleigh scattering, as the Raman transitions confer different exclusion rules than those that apply to Rayleigh transitions.

Micro-spatially offset Raman spectroscopy (micro-SORS) is an analytical technique developed in 2014 that combines SORS with microscopy. The technique derives its sublayer‐resolving properties from its parent technique SORS. The main difference between SORS and micro-SORS is the spatial resolution: while SORS is suited to the analysis of millimetric layers, micro-SORS is able to resolve thin, micrometric-scale layers. Similarly to SORS technique, micro-SORS is able to preferentially collect the Raman photons generated under the surface in turbid media. In this way, it is possible to reconstruct the chemical makeup of micrometric multi-layered turbid system in a non destructive way. Micro-SORS is particularly useful when dealing with precious or unique objects as for Cultural Heritage field and Forensic Science or in biomedical applications, where a non-destructive molecular characterization constitute a great advantage.

<span class="mw-page-title-main">Coherent Raman scattering microscopy</span> Multi-photon microscopy technique

Coherent Raman scattering (CRS) microscopy is a multi-photon microscopy technique based on Raman-active vibrational modes of molecules. The two major techniques in CRS microscopy are stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS). SRS and CARS were theoretically predicted and experimentally realized in the 1960s. In 1982 the first CARS microscope was demonstrated. In 1999, CARS microscopy using a collinear geometry and high numerical aperture objective were developed in Xiaoliang Sunney Xie's lab at Harvard University. This advancement made the technique more compatible with modern laser scanning microscopes. Since then, CRS's popularity in biomedical research started to grow. CRS is mainly used to image lipid, protein, and other bio-molecules in live or fixed cells or tissues without labeling or staining. CRS can also be used to image samples labeled with Raman tags, which can avoid interference from other molecules and normally allows for stronger CRS signals than would normally be obtained for common biomolecules. CRS also finds application in other fields, such as material science and environmental science.

Raman spectroelectrochemistry (Raman-SEC) is a technique that studies the inelastic scattering or Raman scattering of monochromatic light related to chemical compounds involved in an electrode process. This technique provides information about vibrational energy transitions of molecules, using a monochromatic light source, usually from a laser that belongs to the UV, Vis or NIR region. Raman spectroelectrochemistry provides specific information about structural changes, composition and orientation of the molecules on the electrode surface involved in an electrochemical reaction, being the Raman spectra registered a real fingerprint of the compounds.

References

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