Surface-enhanced Raman spectroscopy

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Raman spectrum of liquid 2-mercaptoethanol (below) and SERS spectrum of 2-mercaptoethanol monolayer formed on roughened silver (above). Spectra are scaled and shifted for clarity. A difference in selection rules is visible: Some bands appear only in the bulk-phase Raman spectrum or only in the SERS spectrum.

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.

History

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 4000 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.

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 differ substantially, distinguishing them experimentally has not been straightforward. The electromagnetic theory proposes the excitation of localized surface plasmons, while the chemical theory proposes the formation of charge-transfer complexes. The chemical theory applies only for species that have formed a chemical bond with the surface, so it cannot explain the observed signal enhancement in all cases, whereas the electromagnetic theory can apply even in those cases where the specimen is physisorbed only to the surface. It has been shown recently that SERS enhancement can occur even when an excited molecule is relatively far apart from the surface which hosts metallic nanoparticles enabling surface plasmon phenomena. This observation provides a strong support for the electromagnetic theory of SERS. Research in 2015 on a more powerful extension of the SERS technique called SLIPSERS (Slippery Liquid-Infused Porous SERS) has further supported the EM theory.

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. 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 that 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 ω_p, 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.

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 different enhancement mechanism that does not involve surface plasmons has been described. 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.

Surfaces

While SERS can be performed in colloidal solutions, today the most common method for performing SERS measurements is by depositing a liquid sample onto a silicon or glass surface with a nanostructured noble metal surface. While 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 or porous silicon as a support. Applying a thin film of silver onto wafers of silicon, through a day-long immersion in a saturated solution of silver nitrate in n-octanol, is a widely accepted method to prepare a surface-enhanced Raman scattering substrate. The most common metals used for plasmonic surfaces are silver and gold, however aluminum has recently been explored as an alternative plasmonic material, because its plasmon band is in the UV region, contrary to silver and gold. Hence, there is great interest in using aluminum for UV SERS. It has, however, surprisingly also been shown to have a large enhancement in the infrared, which is not fully understood. In the current decade, it has been recognized that the cost of SERS substrates must be reduced in order to become a commonly used analytical chemistry measurement technique. To meet this need, plasmonic paper has experienced widespread attention in the field, with highly sensitive SERS substrates being formed through approaches such as soaking, in-situ synthesis, screen printing and inkjet printing.

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 that 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 that 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.

Applications

SERS substrates prepared with silver nano rods are used to detect the presence of low abundance biomolecules, and can therefore detect proteins in body fluids. This technology has been utilized to detect urea and blood plasma label free in human serum and may become the next generation in cancer detection and screening. The ability to analyze the composition of a mixture on the nano scale makes the use of SERS substrates beneficial for environmental analysis, pharmaceuticals, material sciences, art and archeological research, forensic science, drug detection, food quality analysis, and single algal cell detection.

Oligonucleotide targeting

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.

Selection rules

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.