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 71

of probing very small volumes, owing to the fact that the SERS signal comes
from the immediate vicinity of a metal nanostructure whose size and shape
dictate the range of the electromagnetic field enhancement. Individual metal
nanostructures or their aggregates can act as tiny probes that can be brought
to the analyte of interest or be arranged in space. The concept of a SERS
label, where 'reporter' molecules with a specific SERS signature are attached
to metal nanostructures [25, 32, 33], is generally accepted and already being
commercialized. SERS labels provide significant advantages over fluorescence
labels in particular because of their higher stability and their multiplexing ca-
pability. The latter results from the fingerprint nature of a Raman spectrum
compared to broad and relatively unspecific fluorescence bands of conventional
optical labels. If resonant enhancement of the reporter is used in addition to
SERS, SERRS labels can provide a higher sensitivity than fluorescence and
enable quantitative detection of analytes; for a summary see e.g. [34].
SERS can provide new vibrational spectroscopic perspectives on biolog-
ical and physiological processes. This is true literally in a topological sense
when SERS spectra are obtained from a specific location within a cell or
complex biomatrix, but also since SERS provides selectivity regarding which
compounds in a heterogeneous mixture have access to the SERS substrate
and can be detected. The spectra of the biological samples themselves, rather
than those of label molecules, are of great interest in many bioanalytical ap-
plications of SERS. Not only the characterization and identification of bac-
teria [27, 35-38], but also the sensitive detection of their spores [39] can be
achieved by SERS. With tip-enhanced Raman spectroscopy (TERS), where
a silver-coated AFM tip serves as substrate for SERS [40], membranes of
individual bacterial cells can be studied [41]. SERS-active nanoparticles as-
sembled on surfaces form ecient sensors for important metabolites, such as
glucose and lactate, also through additional selectivity implemented through
self-assembled monolayers (SAMs) [42, 43].
In this chapter, we will discuss the SERS signals from two different types
of eukaryotic cells. SERS in cells from cell cultures will be discussed regarding
the ability to detect intrinsic cellular molecules of physiological importance.
Emphasis of this chapter will be put on a second type of eukaryotic system,
the water-soluble cellular fraction of pollen grains. As will be explained, SERS
on the cells from pollen grains gives us additional information to the normal
Raman information we use to study pollen biochemistry and will enable sen-
sitive detection for fast allergy warning.
4.2 Raman Spectroscopy of Pollen
4.2.1 Questions and Motivation
It has become clear that pollen grains, owing to their biological function, con-
sist of a broad variety of molecules. These range from an extremely stable