8.5.3 Surface-enhanced Raman spectroscopy
Raman scattering is normally a very weak effect. Under normal conditions, the intensities of the Raman scattered peaks in a spectrum are
∼0.001% of the incident light. Therefore, if a Raman spectrum is desired
for an analyte with a relatively small concentration (such as for a thin film
deposited on a surface), some method of signal enhancement must be
employed in order to gather any useful information.
One method that is commonly used to enhance the Raman signal is
adsorption of the analyte onto a rough metal surface or colloidal particle to
obtain the Raman spectrum from the adsorbed species. Most often silver is
used, but gold and copper have also been observed to produce similar
effects. This type of signal enhancement is called surface-enhanced
Raman spectroscopy (SERS). SERS is a convenient method that allows
amplification of the Raman signal while simultaneously probing the
region near a metallic surface, thus allowing Raman spectra to be obtained
for low quantities of a substance that are adsorbed to a metallic surface.
The exact mechanism by which adsorption to a metallic surface produces
enhancement of the Raman signal is not fully understood. Two predominant theories have emerged to explain the effect: the electromagnetic enhancement theory and the chemical enhancement theory. Each
theory is able to explain some observations, but not others.
The electromagnetic theory claims that the generation of surface plasmons
by the incident light is the means by which Raman scattering is enhanced.
As may be recalled from our discussion of SPR (Section 8.4), a surface
plasmon is a collective oscillation of the electrons at the surface of a conducting metal (typically gold or silver) that is induced when light is shone
on its surface. The generation of surface plasmons greatly enhances the
electromagnetic field near the metallic surface and therefore would be
expected to enhance the strength of the Raman scattered light. This, then,
is the fundamental idea behind the electromagnetic enhancement theory.
The incident light produces surface plasmons, which enhance the electromagnetic field near the surface, which in turn produces a stronger
Raman signal.
The chemical enhancement theory appeals to a charge-transfer mechanism to explain the enhancement of Raman signal upon adsorption to a
metallic surface. The charge-transfer mechanism essentially states that
new electronic states are made available to the molecule due to its interaction with the metallic surface. These new allowable electronic states
SURFACE-SENSITIVE SPECTROSCOPIC METHODS 293
Raman scattering is normally a very weak effect. Under normal conditions, the intensities of the Raman scattered peaks in a spectrum are
∼0.001% of the incident light. Therefore, if a Raman spectrum is desired
for an analyte with a relatively small concentration (such as for a thin film
deposited on a surface), some method of signal enhancement must be
employed in order to gather any useful information.
One method that is commonly used to enhance the Raman signal is
adsorption of the analyte onto a rough metal surface or colloidal particle to
obtain the Raman spectrum from the adsorbed species. Most often silver is
used, but gold and copper have also been observed to produce similar
effects. This type of signal enhancement is called surface-enhanced
Raman spectroscopy (SERS). SERS is a convenient method that allows
amplification of the Raman signal while simultaneously probing the
region near a metallic surface, thus allowing Raman spectra to be obtained
for low quantities of a substance that are adsorbed to a metallic surface.
The exact mechanism by which adsorption to a metallic surface produces
enhancement of the Raman signal is not fully understood. Two predominant theories have emerged to explain the effect: the electromagnetic enhancement theory and the chemical enhancement theory. Each
theory is able to explain some observations, but not others.
The electromagnetic theory claims that the generation of surface plasmons
by the incident light is the means by which Raman scattering is enhanced.
As may be recalled from our discussion of SPR (Section 8.4), a surface
plasmon is a collective oscillation of the electrons at the surface of a conducting metal (typically gold or silver) that is induced when light is shone
on its surface. The generation of surface plasmons greatly enhances the
electromagnetic field near the metallic surface and therefore would be
expected to enhance the strength of the Raman scattered light. This, then,
is the fundamental idea behind the electromagnetic enhancement theory.
The incident light produces surface plasmons, which enhance the electromagnetic field near the surface, which in turn produces a stronger
Raman signal.
The chemical enhancement theory appeals to a charge-transfer mechanism to explain the enhancement of Raman signal upon adsorption to a
metallic surface. The charge-transfer mechanism essentially states that
new electronic states are made available to the molecule due to its interaction with the metallic surface. These new allowable electronic states
SURFACE-SENSITIVE SPECTROSCOPIC METHODS 293