3 Surface-enhanced Raman Scattering ...
47
of the cells, SERS offers considerably shorter collection times, which is especially
important in Raman mapping. Application of SERS for probing living cells was
reviewed in several papers [142–145]. First SERS report, which demonstrated the
capability of SERS for cell studies was published in 1991 [146], but broader interest of scientific society to this subject actually started in 2005, as shown in an
interesting review by vitol et al. [145]. In SERS experiments with cells, nanoprobes
that are responsible for enhancement of the Raman signal are inserted into the
cells. therefore, the key problem in intracellular SERS is development of efficient
and biocompatible metal nanoprobes that do not affect cell integrity and viability.
Nanostructures used in these experiments should be small enough to be transferred
through the cell membranes without damaging them. various SERS probes have
been used for intracellular experiments, starting from the most popular and easily
fabricated colloidal silver and gold nanoparticles [147, 148], through nanoshells
[49], nanorods [149], nanostars [150], nanoflowers [151], to more sophisticated
probes such as nanopipettes [152].
generally, there are two ways of performing intracellular SERS experiments. In
the majority of experiments, metallic nanoprobes are labeled with molecules that
exhibit strong SERS (or SERRS) spectrum. they are called reporter molecules.
then, the SERS spectrum of the reporter molecule (e.g. a dye), linked to the nanoprobe is recorded. In the second, label-free approach, metallic nanoprobes enhance
Raman spectrum of native constituents of the cell such as dNA, RNA, lipids or
aminoacids, delivering chemical structural information at the molecular level. Coupling of the modern Raman instruments with confocal microscope opened a new
possibility of obtaining Raman maps that show spatial distribution of investigated
species with high lateral resolution.
In case of using label-free SERS nanoprobes the signatures of biological molecules and structures constituting the cell in the immediate nanoparticle vicinity can
be delivered. In SERS experiments, nanoparticles are either introduced inside the
cell or located on the cell membrane surface. First important report demonstrating
power of SERS in such experiments was published in 2002 by Kneipp et al. [153].
It was possible to observe vibrational bands that were ascribed to amino acids, dNA
and RNA and to find their distribution inside the intestinal epithelial cells. In 2006,
Kneipp et al. reported SERS spectra from endosomes in living individual epithelial and macrophage cells at different times after the uptake of gold nanoparticles.
[147]. metal particles were found to be collected in endosomes inside the cells.
Spectra indicated that the molecular composition in the vicinity of the nanoprobes
changes over time. Spectra also varied between different cell lines. moreover, total
SERS intensity increased due to formation of nanoaggregates of which dimers and
trimers were found to exhibit the highest enhancement factors. on the other hand,
it is known that the increasing distance between aggregated nanoparticles results in
lowering of the electromagnetic enhancement factor, that in turn result in decreasing intensity of the spectra. uncontrollable aggregation of nanoparticles with time
is a major obstacle in using metal colloids for SERS inside the cells, because of
undesirable processes that result in changes of both the spectral pattern and SERS
intensity [147].
47
of the cells, SERS offers considerably shorter collection times, which is especially
important in Raman mapping. Application of SERS for probing living cells was
reviewed in several papers [142–145]. First SERS report, which demonstrated the
capability of SERS for cell studies was published in 1991 [146], but broader interest of scientific society to this subject actually started in 2005, as shown in an
interesting review by vitol et al. [145]. In SERS experiments with cells, nanoprobes
that are responsible for enhancement of the Raman signal are inserted into the
cells. therefore, the key problem in intracellular SERS is development of efficient
and biocompatible metal nanoprobes that do not affect cell integrity and viability.
Nanostructures used in these experiments should be small enough to be transferred
through the cell membranes without damaging them. various SERS probes have
been used for intracellular experiments, starting from the most popular and easily
fabricated colloidal silver and gold nanoparticles [147, 148], through nanoshells
[49], nanorods [149], nanostars [150], nanoflowers [151], to more sophisticated
probes such as nanopipettes [152].
generally, there are two ways of performing intracellular SERS experiments. In
the majority of experiments, metallic nanoprobes are labeled with molecules that
exhibit strong SERS (or SERRS) spectrum. they are called reporter molecules.
then, the SERS spectrum of the reporter molecule (e.g. a dye), linked to the nanoprobe is recorded. In the second, label-free approach, metallic nanoprobes enhance
Raman spectrum of native constituents of the cell such as dNA, RNA, lipids or
aminoacids, delivering chemical structural information at the molecular level. Coupling of the modern Raman instruments with confocal microscope opened a new
possibility of obtaining Raman maps that show spatial distribution of investigated
species with high lateral resolution.
In case of using label-free SERS nanoprobes the signatures of biological molecules and structures constituting the cell in the immediate nanoparticle vicinity can
be delivered. In SERS experiments, nanoparticles are either introduced inside the
cell or located on the cell membrane surface. First important report demonstrating
power of SERS in such experiments was published in 2002 by Kneipp et al. [153].
It was possible to observe vibrational bands that were ascribed to amino acids, dNA
and RNA and to find their distribution inside the intestinal epithelial cells. In 2006,
Kneipp et al. reported SERS spectra from endosomes in living individual epithelial and macrophage cells at different times after the uptake of gold nanoparticles.
[147]. metal particles were found to be collected in endosomes inside the cells.
Spectra indicated that the molecular composition in the vicinity of the nanoprobes
changes over time. Spectra also varied between different cell lines. moreover, total
SERS intensity increased due to formation of nanoaggregates of which dimers and
trimers were found to exhibit the highest enhancement factors. on the other hand,
it is known that the increasing distance between aggregated nanoparticles results in
lowering of the electromagnetic enhancement factor, that in turn result in decreasing intensity of the spectra. uncontrollable aggregation of nanoparticles with time
is a major obstacle in using metal colloids for SERS inside the cells, because of
undesirable processes that result in changes of both the spectral pattern and SERS
intensity [147].