Surface Enhanced Raman Spectroscopy

Surface Enhanced Raman Spectroscopy, or Surface Enhanced Raman Scattering, often abbreviated SERS, is a surface sensitive technique that results in the enhancement of Raman scattering by molecules adsorbed on rough metal surfaces. The enhancement factor can be as much as 10¹⁴-10¹⁵, which allows the technique to be sensitive enough to detect single molecules.^[1]

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History

SERS from pyridine adsorbed on electrochemically roughened silver was produced by Martin Fleischman and coworkers in 1974; ^[2] they justified the large signal that they saw simply as a matter of the number of molecules that were scattering on the surface and did not recognize that there was a major enhancement effect. In 1977, two groups independently noted that the concentration of scattering species could not account for the enhanced signal and each proposed a mechanism for the observed enhancement, which still constitute the underlying principals for the modern theories of the SERS effect. Jeanmaire and Van Duyne ^[3] proposed an electromagnetic effect, while Albrecht and Creighton ^[4] proposed a charge-transfer effect.

Mechanisms

The exact mechanism of the enhancement effect of SERS is still a matter of debate in the literature. There are two primary theories and while their mechanisms are substantially different from each other, distinguishing them experimentally has not been straightforward. The electromagnetic theory relies upon the excitation of localized surface plasmons, while the chemical theory rationalizes the effect through the formation of charge-transfer complexes. The chemical theory only applies for species which have formed a bond with the surface, so it clearly cannot explain the observed signal enhancement in all cases, while the electromagnetic theory can apply even in those cases where the specimen is only physisorbed to the surface.

Electromagnetic Theory

The increase in intensity of the Raman signal for adsorbates on particular surfaces occurs because of an enhancement in the electric field provided by the surface. When the incident light in the experiment strikes the surface, localized surface plasmons are excited. The field enhancement is greatest when the plasmon frequency, ω_p, is in resonance with the radiation. Furthermore, in order for scattering to occur, the plasmon oscillations must be perpendicular to the surface; if they are in-plane with the surface, no scattering will occur. It is because of this requirement that roughened surfaces or arrangements of nanoparticles are typically employed in SERS experiments as these surfaces provide an area on which these localized collective oscillations can take place.^[5]

The light incident on the surface can excite a variety of phenomena in the surface, yet the complexity of this situation can be minimized by surfaces with features much smaller than the wavelength of the light, as only the dipolar contribution will be recognized by the system. The dipolar term contributes to the plasmon oscillations, which leads to the enhancement. The reason that the SERS effect is so pronounced is because the field enhancement occurs twice. Initially, the field enhancement magnifies the intensity of incident light which will excite the Raman modes of the molecule being studied, therefore increasing the signal of the Raman scattering. The Raman signal is then further magnified by the surface by the same mechanism as the incident light was, resulting in a greater increase in the total output signal of the experiment. At each stage the electric field is enhanced as E², for a total enhancement of E⁴.^[6]

The enhancement is not equal for all frequencies. For those frequencies for which the Raman signal is only slightly shifted from the incident light, both the incident laser light and the Raman signal can be near resonance with the plasmon frequency, leading to the E⁴ enhancement. When the frequency shift is large, the incident light and the Raman signal cannot both be on resonance with ω_p, thus the enhancement at both stages cannot be maximal.^[7]

The choice of surface metal is also dictated by the plasmon resonance frequency. Visible and near-infrared radiation (NIR) is used to excite Raman modes. Silver and gold are typical metals for SERS experiments because their plasmon resonance frequencies fall within these wavelength ranges, providing maximal enhancement for visible and NIR light. Copper is another metal whose absorption spectrum falls within the range acceptable for SERS experiments.^[8]

Chemical Theory

While the electromagnetic theory of enhancement can be applied regardless of the molecule being studied, it does not fully explain the magnitude of the enhancement observed in many systems. For many molecules, often those with a lone pair of electrons, in which the molecules can bond to the surface, a distinctly different mechanism of enhancement has been described which does not involved surface plasmons. This chemical mechanism involves charge transfer between the chemisorbed species and the metal surface. The chemical mechanism only applies in specific cases and probably occurs in concert with the electromagnetic mechanism.^[9]

The HOMO to LUMO transition for many molecules requires much more energy than the infrared or visible light typically involved in Raman experiments. When the HOMO and LUMO of the adsorbate fall symmetrically about the Fermi level of the metal surface, light of half the energy can be employed to make the transition, where the metal acts as a charge-transfer intermediate.^[10] Thus, a spectroscopic transition that might normally take place in the UV can be excited by visible light.^[11]

Surfaces

SERS is sensitive to the surface on which the experiment is taking place. The first experiments, and some modern experiments, took place on electrochemically roughened silver.^[12] Now surfaces are often prepared using a distribution of metal nanoparticles on the surface.^[13]

The shape and size of the metal nanoparticles strongly affects the strength of the enhancement because these factors influence the ratio of absorption and scattering events.^[14] There is an ideal size for these particles—not just any small particles will have the same impact on the Raman intensity—as well as an idea surface thickness for each experiment.^[15] Particles which are too large allow the excitation of multipoles, which are nonradiative. As only the dipole transition leads to Raman scattering, the higher-order transitions will cause a decrease in the overall efficiency of the enhancement. Particles which are too small, however, lose their electrical conductance and cannot enhance the field. Furthermore, when the particle size approaches a few atoms, the definition of a plasmon does not hold, as there must be a large collection of electrons to oscillate together.^[16]

Selection Rules

The name surface enhanced Raman spectroscopy implies that it provides the same information that traditional Raman spectroscopy does, simply with a greatly enhanced signal. While the spectra of most SERS experiments are very similar to the non-surface enhanced spectra, there are often differences in the number of modes present. Additional modes not found in the traditional Raman spectrum can be present in the SERS spectrum, while other modes can disappear.

The modes observed in any spectroscopic experiment are dictated by the symmetry of the molecules and are usually summarized by selection rules. When molecules are adsorbed to a surface, the symmetry of the system can change, slightly modifying the symmetry of the molecule, which can lead to differences in mode selection.^[17]

One common way in which selection rules are modified arises from the fact that many molecules that have a center of symmetry lose that feature when adsorbed to a surface. The loss of a center of symmetry eliminates the requirements of the mutual exclusion rule, which dictates that modes can only be either Raman or Infrared active. Thus, modes that would normally appear only in the infrared spectrum appear of the free molecule can appear in the SERS spectrum.^[18]

The symmetry of a molecule can be changed in different ways depending on the orientation in which the molecule is attached to the surface. In some experiments, it is possible to determine the orientation of adsorption to the surface from the SERS spectrum, as different modes will be present depending on the way in which the symmetry is modified.^[19][20]

References

1. ^ Nie, S.; Emory, S. R., Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, (5303), 1102-1106.
2. ^ Fleischmann, M.; PJ Hendra and AJ McQuillan (15 May 1974). "Raman Spectra of Pyridine Adsorbed at a Silver Electrode". Chemical Physics Letters 26 (2): 163-166.
3. ^ Jeanmaire, David L.; Richard P. van Duyne (1977). "Surface Raman Electrochemistry Part I. Heterocyclic, Aromatic and Aliphatic Amines Adsorbed on the Anodized Silver Electrode". Journal of Electroanalytical Chemistry 84: 1-20. Elsevier Sequouia S.A..
4. ^ Albrecht, M. Grant; J. Alan Creighton (1977). "Anomalously Intense Raman Spectra of Pyridine at a Silver Electrode". Journal of the American Chemical Society 99: 5215-5219.
5. ^ Smith, E.; Dent, G., Modern Raman Spectroscopy: A Practical Approach. John Wiley and Sons: 2005.
6. ^ Moskovits, M., Surface-Enhanced Raman Spectroscopy: a Brief Perspective. In Surface-Enhanced Raman Scattering – Physics and Applications, 2006; pp 1-18.
7. ^ Campion, A.; Kambhampati, P., Surface-enchanced Raman scattering. Chemical Society Reviews 1998, 27, 241-250.
8. ^ Creighton, J. A.; Eadon, D. G., Ultraviolet–visible absorption spectra of the colloidal metallic elements. J. Chem. Soc., Faraday Trans 1991, 87, 3881-3891.
9. ^ Lombardi, J. R.; Birke, R. L.; Lu, T.; Xu, J., Charge-transfer theory of surface enhanced Raman spectroscopy: Herzberg--Teller contributions. The Journal of Chemical Physics 1986, 84, (8), 4174-4180.
10. ^ Lombardi, J. R.; Birke, R. L.; Lu, T.; Xu, J., Charge-transfer theory of surface enhanced Raman spectroscopy: Herzberg--Teller contributions. The Journal of Chemical Physics 1986, 84, (8), 4174-4180.
11. ^ Campion, A.; Kambhampati, P., Surface-enchanced Raman scattering. Chemical Society Reviews 1998, 27, 241-250.
12. ^ Fleischmann, M.; Hendra, P. J.; McQuillan, A. J., Raman spectra of pyridine adsorbed at a silver electrode. Chemical Physics Letters 1974, 26, (2), 163-166.
13. ^ Mock, J. J., Shape effects in plasmon resonance of individual colloidal silver nanoparticles. The Journal of Chemical Physics 2002, 116, (15), 6755.
14. ^ Aroca, R., Surface-enhanced Vibrational Spectroscopy. UK John Wiley & Sons Ltd 2006.
15. ^ Mahurin, L.-L. B. S. M.; Dai, C.-D. L. S., Study of silver films over silica beads as a surface-enhanced Raman scattering (SERS) substrate for detection of benzoic acid. Journal of Raman Spectroscopy 2003, 34, (5), 394-398.
16. ^ Moskovits, M., Surface-Enhanced Raman Spectroscopy: a Brief Perspective. In Surface-Enhanced Raman Scattering – Physics and Applications, 2006; pp 1-18.
17. ^ Moskovits, M.; Suh, J. S., Surface selection rules for surface-enhanced Raman spectroscopy: calculations and application to the surface-enhanced Raman spectrum of phthalazine on silver. J. Phys. Chem. 1984, 88, (23), 5526-5530.
18. ^ Smith, E.; Dent, G., Modern Raman Spectroscopy: A Practical Approach. John Wiley and Sons: 2005.
19. ^ Brolo, A. G.; Jiang, Z.; Irish, D. E., The orientation of 2,2'-bipyridine adsorbed at a SERS-active Au(1 1 1) electrode surface. Journal of Electroanalytical Chemistry 2003, 547, (2), 163-172.
20. ^ Michota, A.; Bukowska, J., Surface-enhanced Raman scattering (SERS) of 4-mercaptobenzoic acid on silver and gold substrates. Journal of Raman Spectroscopy 2003, 34, (1), 21-25.