Nanotechnology-Enabled Sensors

246 Chapter 5: Characterization Techniques for Nanomaterials
light, the frequency is shifted up and it is referred to as an anti-Stokes
emission. 71-73 The energy of the scattered photon, E, is related to the energy
of the incident photon, E0 = hυ0, by:
E = hυ0 ± ΔEυ , (5.12)
Stokes: υ = υ0 – Δυ, (5.13)
anti-Stokes: υ = υ0 + Δυ. (5.14)
where ΔEυ is the change in energy and Δυ is the change in frequency. Although
there is a change in wavelength, no energy is lost during the interaction.
This is because the electron cloud in a bond becomes deformed
during interaction with the impinging light. The amount of deformation is
the polarizability, and this determines the shift Raman of intensity and frequency.
74
Raman scattering comprises a very small fraction of the scattered light,
only one Raman scattered photon from 10 6 to 10 8 excitation photons.
Therefore, the main limitation of Raman scattering is discerning the weak
inelastically scattered light from the intense Rayleigh scattered light. Furthermore,
depending on the incident light energy, photoluminescence may
occur which can obscure the Raman spectrum. The Raman scattering intensity
is inversely proportional to the fourth power of the emission wavelength.
75 So decreasing the wavelength of the light source should result in
an increase in the Raman signal. However, decreasing the wavelength increases
the likelihood of observing photoluminescence. 76 Stokes shifts are
susceptible to photoluminescence interference, but not anti-Stokes. If the
sample is highly fluorescent then a different excitation, one that will not
cause electronic excitation, can be employed to circumvent this problem.
Unlike IR spectroscopy, where water contamination can block out entire
regions of the spectrum, a Raman spectrum is less susceptible to the presence
of water.
The Raman signal is inherently weak, which prevents achieving low detection
limits with normal Raman spectroscopy. 77 The Raman signal can
be enhanced if molecules are adsorbed on roughened metal surfaces,
typically gold or silver. This is called Surface Enhanced Raman Spectroscopy
78 (SERS), 79 and exploits changes in analyte polarizability perpendicular
to the surface, which enhances scattering by factors of more than a
million-fold. 77 The metal surface must have a plasmon in the frequency region
close to that of the excitation laser. Surface roughness or curvature is
required for the scattering of light by surface plasmons, however, it is not
the defining factor, as an intrinsic surface enhancement effect plays a fundamental
role. 80