Applications of Raman and Surface-Enhanced Raman Scattering to the Analysis of Eukaryotic Samples - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 70

provide superimposed vibrational information on structure, composition and
interactions from all classes of molecules in the samples. Thereby, information
obtained in Raman spectra of biological samples such as cells or pollen or other
tissues naturally integrates the genomic, proteomic and metabolic status of
the sample. Other kinds of complex biological materials that have been studied
by Raman spectroscopy include prokaryotes, fungi, animal cells and foodstuffs
[9-16].
Raman scattering is a very weak effect, with typical Raman cross-sections
being 10 − 30 -10 − 26 cm 2 per molecule. As described in a large body of litera-
ture, in surface-enhanced Raman scattering (SERS), we observe an increase
in Raman cross-section from molecules that are in close proximity to a noble
metal nanostructure [17-20]. Cross-sections in SERS can be on the order of
10 − 16 cm 2 per molecule and higher, and hence become comparable to typical
cross-sections of fluorophores [21]. The favourable optical properties of noble
metal nanostructures, based on their surface plasmon polaritons, provide the
key effect for the observation of SERS. The so-called electromagnetic field
enhancement, the major contributor to the enhancement observed in SERS,
is brought about by resonances of the optical fields (excitation and Raman
scattering field) with surface plasmons of gold or silver nanostructures. The
plasmon resonance condition for silver and gold is fulfilled in the visible and
near infrared wavelength range, and therefore nanostructures of these met-
als are particularly suited for SERS experiments excited with light of such
frequencies. Since the plasmon resonance covers a relatively wide frequency
range compared to the frequency shift between excitation and Raman scat-
tered light, both excitation and scattering fields can be in resonance with
the surface plasmons of the metal nanostructures. In such a case, the SERS
intensity enhancement scales approximately to the fourth power of the en-
hancement of the local optical fields. In addition to the electromagnetic field
enhancement, the so-called chemical or electronic enhancement takes place,

which yields an increase in Raman cross-section σ R ads due to the electronic
interaction of the analyte molecule with the metal surface. Several mechanisms
have been proposed for this chemical enhancement [17, 20, 22]. Expression 1
summarizes the two different enhancement contributions:

2 2

σ ads ,
with N being the number of molecules involved in the Raman process, I
the laser intensity, A ( ν L )and A ( ν S ) enhancements of the excitation and the
scattering field and σ R ads the Raman cross-section of the adsorbed molecule.
Since its observation more than three decades ago by VanDuyne and
Jeanmaire as well as Albrecht and Creighton [23, 24], SERS has been gain-
ing popularity in analytical and physical chemistry and very recently also
in the biomedical field [25-28]. The potential of SERS in bioanalytics lie in
the combination of sensitivity that can be achieved [29-31] with the struc-
tural information that is generated in Raman spectroscopy as a vibrational
method. In addition to the increased sensitivity, SERS offers the opportunity

P SERS ( ν S ) ∝ N · I ( ν L ) ·| A ( ν L ) | ·| A ( ν S ) | ·