Surface Enhanced Raman Spectroscopy
Encyclopedia
Raman scattering
Raman scattering or the Raman effect is the inelastic scattering of a photon. It was discovered by Sir Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids, and by Grigory Landsberg and Leonid Mandelstam in crystals....
by molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s adsorbed
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
on rough metal surfaces
Surface science
Surface science is the study of physical and chemical phenomena that occur at the interface of two phases, including solid–liquid interfaces, solid–gas interfaces, solid–vacuum interfaces, and liquid-gas interfaces. It includes the fields of surface chemistry and surface physics. Some related...
. The enhancement factor can be as much as 1010 to 1011, which means the technique may detect single molecules.
History
SERS from pyridinePyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
adsorbed on electrochemically roughened silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
was produced by Martin Fleischman and coworkers PJ Hendra and AJ McQuillan in 1974;
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. 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
Oak Ridge National Laboratory
Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...
'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 electromagneticElectromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
theory posits the excitation of localized surface plasmons
Surface plasmon resonance
The excitation of surface plasmons by light is denoted as a surface plasmon resonance for planar surfaces or localized surface plasmon resonance for nanometer-sized metallic structures....
, while the chemical theory proposes the formation of charge-transfer complexes. The chemical theory only applies for species which have formed a chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
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
Physisorption
Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption...
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 fieldElectric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
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
Plasmon
In physics, a plasmon is a quantum of plasma oscillation. The plasmon is a quasiparticle resulting from the quantization of plasma oscillations just as photons and phonons are quantizations of light and mechanical vibrations, respectively...
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 nanoparticle
Nanoparticle
In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. Particles are further classified according to size : in terms of diameter, coarse particles cover a range between 10,000 and 2,500 nanometers. Fine particles are sized...
s are typically employed in SERS experiments as these surfaces provide an area on which these localized collective oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
s 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
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
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
Normal mode
A normal mode of an oscillating system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation. The frequencies of the normal modes of a system are known as its natural frequencies or resonant frequencies...
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
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
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
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
and near-infrared radiation (NIR) are used to excite Raman modes. Silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
and gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
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 pairLone pair
In chemistry, a lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons...
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
Chemisorption
Chemisorption is a sub-class of adsorption, driven by a chemical reaction occurring at the exposed surface. A new chemical species is generated at the adsorbant surface...
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
HOMO/LUMO
HOMO and LUMO are acronyms for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. The energy difference between the HOMO and LUMO is termed the HOMO-LUMO gap...
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
Fermi energy
The Fermi energy is a concept in quantum mechanics usually referring to the energy of the highest occupied quantum state in a system of fermions at absolute zero temperature....
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
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.
Oligonucleotide targeting
SERS can be used to target specific DNADNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
and RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
sequences using a combination of gold and silver nanoparticles and Raman-active dyes, such as Cy3
Cyanine
Cyanine is a non-systematic name of a synthetic dye family belonging to polymethine group. Cyanines have many uses as fluorescent dyes, particularly in biomedical imaging...
. 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 symmetryMolecular symmetry
Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...
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
Infrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...
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.