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Raman Micr oscopy for Biomedical Applications 259

in oligo- and polysaccharides. The observed dominance of vibrational
bands due to protein and sugar moieties is consistent with the known
chemical composition of the staphylococcus surface. As for all Gram-
positive bacteria the cytoplasmic membrane is surrounded by pepti-
doglycan, also known as murein, which is made up by linear chains
of the two alternating amino sugars N-acetyl glucosamine (NAG)
and N-acetyl muramic acid (NAM). The NAM chains are cross-
linked by short (4 to 5 residues) amino acid chains. The peptidogly-
can layer which is assumed to be around 50 nm thick is pervaded
by other polysaccharides like teichoic acid and a variety of surface
proteins. Furthermore, on the cell membrane there are also differ-
ent catalysis centers of enzymes and anchoring and binding sites
for adhesion on surfaces and cell recognition. Therefore, at the sur-
face the cell exposes a mixture of sugar derivatives from the pepti-
doglycan layer and the teichoic acids and polysaccharide intercellular
adhesin (PIA), and several different proteins. 67
Tip-enhanced Raman spectroscopy bears a high potential for the
noninvasive investigation of surfaces because it allows the recording of
the topography of the investigated surface with highest spatial resolu-
tion below the diffraction limit, while at the same time rich chemical
information from those surface structures can be obtained via vibra-
tional spectra. Measuring times per spectrum can be kept very short
(1 second or less), because the silver particle at the tip apex enhances
the Raman signals. From the signal-to-noise ratio and the probed sam-
4
ple area the enhancement factor can be estimated to be around 10 to
5
10 . If the reduced contact time between tip and sample due to the
intermittent contact mode oscillations is taken into account enhance-
68
6
8
ment factors of 10 to 10 can be calculated. Very recently, the applica-
tion of TERS was extended to the investigation of a single tobacco
69
mosaic virus. The development of fast identification techniques is
another important research topic which takes advantage of the unique
prospects of TERS.
8.5 Conclusions
This contribution described recent biomedical applications of Raman
spectroscopy that have been reported since 2005. Raman images were
collected from dried sections and nondried specimens of murine and
human brain tumors, colon tissue and lung malformations. Raman
microscopic imaging was also applied to study single cells at the sub-
cellular level. Classification models were developed to determine the
strain and species of bacteria based on Raman spectra. The topology
and chemical composition of bacterial surfaces were probed by tip-
enhanced Raman spectroscopy. Further progress of Raman spectros-
copy in life sciences is expected within the next years. Dedicated
miniaturized, biomedical, fiber-optic probes will enable to collect
Raman spectra and images of tissue in vivo under minimal invasive