Surface-enhanced Raman spectroscopy
Surface-enhanced Raman spectroscopy or surface-enhanced Raman scattering (SERS) is a surface-sensitive technique that enhances Raman scattering by molecules adsorbed on rough metal surfaces or by nanostructures such as plasmonic-magnetic silica nanotubes.  The enhancement factor can be as much as 1010 to 1011, which means the technique may detect single molecules.
SERS from pyridine adsorbed on electrochemically roughened silver was first observed by Martin Fleischmann, Patrick J. Hendra and A. James McQuillan at the Department of Chemistry at the University of Southampton, Southampton, UK in 1973. This initial publication has been cited over 2400 times. The 40th Anniversary of the first observation of the SERS effect has been marked by the Royal Society of Chemistry by the award of a National Chemical Landmark plaque to the University of Southampton. 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. Their theories are still accepted as explaining the SERS effect. Jeanmaire and Van Duyne proposed an electromagnetic effect, while Albrecht and Creighton proposed a charge-transfer effect. Rufus Ritchie, of Oak Ridge National Laboratory's Health Sciences Research Division, predicted the existence of the surface plasmon.
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 differ substantially, distinguishing them experimentally has not been straightforward. The electromagnetic theory posits the excitation of localized surface plasmons, while the chemical theory proposes the formation of charge-transfer complexes. The chemical theory only applies for species which have formed a chemical bond with the surface, so it 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.
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, ωp, is in resonance with the radiation. 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 occur.
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 SERS effect is so pronounced because the field enhancement occurs twice. First, 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 due to the same mechanism which excited the incident light, resulting in a greater increase in the total output. At each stage the electric field is enhanced as E2, for a total enhancement of E4.
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 ωp, thus the enhancement at both stages cannot be maximal.
The choice of surface metal is also dictated by the plasmon resonance frequency. Visible and near-infrared radiation (NIR) are 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's absorption spectrum also falls within the range acceptable for SERS experiments. Platinum and palladium nanostructures also display plasmon resonance within visible and NIR frequencies.
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 different enhancement mechanism has been described which does not involve 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.
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. Thus a spectroscopic transition that might normally take place in the UV can be excited by visible light.
SERS is sensitive to the surface on which the experiment is taking place. The first experiments were performed on electrochemically roughened silver. Now surfaces are often prepared using a distribution of metal nanoparticles on the surface as well as using lithography.
The shape and size of the metal nanoparticles strongly affect the strength of the enhancement because these factors influence the ratio of absorption and scattering events. There is an ideal size for these particles, and an ideal surface thickness for each experiment. 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 lose their electrical conductance and cannot enhance the field. 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. An ideal SERS substrate must possess high uniformity and high field enhancement. Such substrates can be fabricated on a wafer scale and label-free superresolution microscopy has also been demonstrated using the fluctuations of surface enhanced Raman scattering signal on such highly uniform, high performance plasmonic metasurfaces. 
SERS can be used to target specific DNA and RNA sequences using a combination of gold and silver nanoparticles and Raman-active dyes, such as Cy3. Specific single nucleotide polymorphisms (SNP) can be identified using this technique. The gold nanoparticles facilitate the formation of a silver coating on the dye-labeled regions of DNA or RNA, allowing SERS to be performed. This has several potential applications: for example Cao et al. report that gene sequences for HIV, Ebola, Hepatitis, and Bacillus Anthracis can be uniquely identified using this technique. Each spectrum was specific, which is advantageous over fluorescence detection; some fluorescent markers overlap and interfere with other gene markers. The advantage of this technique to identify gene sequences is that several Raman dyes are commercially available, which could lead to the development of non-overlapping probes for gene detection.
The term 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 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.
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 of the free molecule can appear in the SERS spectrum.
A molecule's symmetry 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 how the symmetry is modified.
- Xu, X., Li, H., Hasan, D., Ruoff, R. S., Wang, A. X. and Fan, D. L. (2013), Near-Field Enhanced Plasmonic-Magnetic Bifunctional Nanotubes for Single Cell Bioanalysis. Adv. Funct. Mater.. doi: 10.1002/adfm.201203822
- Blackie, Evan J.; Le Ru, Eric C.; Etchegoin, Pablo G. (2009). "Single-Molecule Surface-Enhanced Raman Spectroscopy of Nonresonant Molecules". J. Am. Chem. Soc. 131 (40): 14466–14472. doi:10.1021/ja905319w. PMID 19807188.
- Blackie, Evan J.; Le Ru, Eric C.; Meyer, Matthias; Etchegoin, Pablo G. (2007). "Surface Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study". J. Phys. Chem. C 111 (37): 13794–13803. doi:10.1021/jp0687908.
- Nie, S; Emory, SR (1997). "Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering". Science 275 (5303): 1102–6. doi:10.1126/science.275.5303.1102. PMID 9027306.
- Le Ru, Eric C.; Meyer, Matthias; Etchegoin, Pablo G. (2006). "Proof of Single-Molecule Sensitivity in Surface Enhanced Raman Scattering (SERS) by Means of a Two-Analyte Technique". J. Phys. Chem. B 110 (4): 1944–1948. doi:10.1021/jp054732v. PMID 16471765.
- 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. Bibcode:1974CPL....26..163F. doi:10.1016/0009-2614(74)85388-1.
- 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. doi:10.1016/S0022-0728(77)80224-6.
- Albrecht, M. Grant; J. Alan Creighton (1977). "Anomalously Intense Raman Spectra of Pyridine at a Silver Electrode". Journal of the American Chemical Society 99 (15): 5215–5217. doi:10.1021/ja00457a071.
- "Technical Highlights. New Probe Detects Trace Pollutants in Groundwater". Oak Ridge National Laboratory Review 26 (2).
- Smith, E.; Dent, G., Modern Raman Spectroscopy: A Practical Approach. John Wiley and Sons: 2005 ISBN 0-471-49794-0
- Moskovits, M., Surface-Enhanced Raman Spectroscopy: a Brief Perspective. In Surface-Enhanced Raman Scattering – Physics and Applications, 2006; pp. 1–18 ISBN 3-540-33566-8
- Campion, Alan; Kambhampati, Patanjali (1998). "Surface-enhanced Raman scattering". Chemical Society Reviews 27 (4): 241. doi:10.1039/A827241Z.
- Creighton, J. Alan; Eadon, Desmond G. (1991). "Ultraviolet?visible absorption spectra of the colloidal metallic elements". Journal of the Chemical Society, Faraday Transactions 87 (24): 3881. doi:10.1039/FT9918703881.
- Langhammer, Christoph; Yuan, Zhe; Zorić, Igor; Kasemo, Bengt (2006). "Plasmonic Properties of Supported Pt and Pd Nanostructures". Nano Letters 6 (4): 833–838. Bibcode:2006NanoL...6..833L. doi:10.1021/nl060219x. PMID 16608293.
- Lombardi, John R.; Birke, Ronald L.; Lu, Tianhong; Xu, Jia (1986). "Charge-transfer theory of surface enhanced Raman spectroscopy: Herzberg–Teller contributions". The Journal of Chemical Physics 84 (8): 4174. Bibcode:1986JChPh..84.4174L. doi:10.1063/1.450037.
- Lombardi, J.R.; Birke, R.L. (2008). "A Unified Approach to Surface-Enhanced Raman Spectroscopy". Journal of Physical Chemistry C 112 (14): 5605–5617. doi:10.1021/jp800167v.
- Fleischmann, M.; Hendra, P.J.; McQuillan, A.J. (1974). "Raman spectra of pyridine adsorbed at a silver electrode". Chemical Physics Letters 26 (2): 163–166. Bibcode:1974CPL....26..163F. doi:10.1016/0009-2614(74)85388-1.
- Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.; Schultz, S. (2002). "Shape effects in plasmon resonance of individual colloidal silver nanoparticles". The Journal of Chemical Physics 116 (15): 6755. Bibcode:2002JChPh.116.6755M. doi:10.1063/1.1462610.
- Witlicki, Edward H. et al. (2011). "Molecular Logic Gates Using Surface-Enhanced Raman-Scattered Light". J. Am. Chem. Soc. 133 (19): 7288–7291. doi:10.1021/ja200992x.
- H.Lu et al.; Zhang, Haixi; Yu, Xia; Zeng, Shuwen; Yong, Ken-Tye; Ho, Ho-Pui (2011). "Seed-mediated Plasmon-driven Regrowth of Silver Nanodecahedrons (NDs)". Plasmonics 7 (1): 167–173. doi:10.1007/s11468-011-9290-8.
- Aroca, R., Surface-enhanced Vibrational Spectroscopy. John Wiley & Sons (2006) ISBN 0-471-60731-2
- Bao, Li-Li; Mahurin, Shannon M.; Liang, Cheng-Du; Dai, Sheng (2003). "Study of silver films over silica beads as a surface-enhanced Raman scattering (SERS) substrate for detection of benzoic acid". Journal of Raman Spectroscopy 34 (5): 394–398. Bibcode:2003JRSp...34..394B. doi:10.1002/jrs.993.
- Ayas, S. (2013). "Label-Free Nanometer-Resolution Imaging of Biological Architectures through Surface Enhanced Raman Scattering". Scientific Reports 3: 2624. doi:10.1038/srep02624
- Cao, Y. C.; Jin, R; Mirkin, CA (2002). "Nanoparticles with Raman Spectroscopic Fingerprints for DNA and RNA Detection". Science 297 (5586): 1536–1540. Bibcode:2002Sci...297.1536C. doi:10.1126/science.297.5586.1536. PMID 12202825.
- Moskovits, M.; Suh, J. S. (1984). "Surface selection rules for surface-enhanced Raman spectroscopy: calculations and application to the surface-enhanced Raman spectrum of phthalazine on silver". The Journal of Physical Chemistry 88 (23): 5526–5530. doi:10.1021/j150667a013.
- Brolo, A.G.; Jiang, Z.; Irish, D.E. (2003). "The orientation of 2,2′-bipyridine adsorbed at a SERS-active Au(111) electrode surface". Journal of Electroanalytical Chemistry 547 (2): 163–172. doi:10.1016/S0022-0728(03)00215-8.