Page 166 - Vibrational Spectroscopic Imaging for Biomedical Applications

142 Cha pte r F i v e

the ground, and may grow over rocks, other plant growth and soils,
but does not grow within the soil. Environmental factors may affect
gene expression of the polyketide synthase genes of lichen fungi
resulting in production of different levels or types of these natural
products. Properties of usnic acid include an antibiotic function as
part of the lichen’s antiherbivore strategy, a strong absorption in
the ultraviolet range, allowing it to serve as a lichen “sunblock,” as
well as anti-inflammatory properties. The compounds are formed
from the repeated condensation of carboxylic acids, and are insolu-
ble in water. The material is gradually exuded from the lichen body
and may be found as a surface deposit on the exterior of the lichen.
Crystalline usnic acid can be detected in Raman spectra of thin sections
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of C. uncialis. Spectra from cryostat sections of the lichen (Fig. 5.9)
show a strong signature from the usnic acid exuded by the lichen; the
identity is confirmed from spectra of the pure compound, extracted
from bulk lichen samples. Spectroscopic tracking of its appearance will
assist in identifying the site of production of the polyketide within the
thallus tissue.

5.6 SERS Discovery and Development
While spectral signatures in Raman spectroscopy are as unique as those
in infrared spectroscopy, the principal drawback to the development of
the technique for many decades was the much lower sensitivity. There
are many ways of expressing this, for example, only about 1 in 10 7
photons will be Raman scattered; hence, it is far less sensitive technique
than IR. This situation was improved by use of lasers as the exciting
source nearly 50 years ago but, more importantly, it was changed by the
discovery of the potential for surface enhancement of the Raman signal
about 30 years ago. The history of that discovery is well-documented,
beginning with the experimental observation of immensely and
unexpectedly enhanced scattering from pyridine adsorbed onto a rough-
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ened silver electrode surface. This experiment was confirmed 70,71 and
the first theoretical explanation was presented the following year,
wherein the remarkable enhancement was correctly attributed to reso-
nant or preresonant excitation of electrons in the conduction band of the
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roughened metal surface. The roughened surface was likened to a
pseudocolloid, with the bumps on the surface approximating isolated
colloidal dots; the resonance condition for absorption of light by the
colloid was given by

⎡ + ⎤ 12
/
λ = λ ⎢ 1 + ε + 2 ( q) ε ⎥ (5.4)
⎣ 1 ( q) ⎦
R P b − 0
in which the resonance wavelength λ is defined in terms of the
R
plasmon wavelength, λ , ε is the interband contribution to the complex
p b