Characterization Techniques of Living Cells Encapsulated with Nanomaterials - Cell Surface Engineering: Fabrication of Functional Nanoshells - page 82

and a detector to record the fluorescence emission but confocal micro-
scopy has a confocality property and a laser is used as a monochromatic
light source. It is also possible to collect Raman scattering in the

confocal mode.
Plasmonics is a term used to define the light-metal interactions. 4,5 Upon
interaction of light with metallic NPs, two fundamental processes take place:
absorption and scattering of a portion of impinging light simultaneously.
While the absorbed light causes the formation of surface plasmons, a
magnetic field, on the surface of the metal structure, the scattered light can
be used for imaging. 5 Since noble metals, such as gold, and silver absorb the
light in the visible region of the spectrum, they are widely used in sensing,
imaging and therapy benefiting from their extraordinary plasmonic prop-
erties. 6,7 When NPs with plasmonic properties such as gold and silver are
used, they and their aggregates can be visualized using an ordinary light
microscope or confocal microscope.
In order to overcome the diffraction-limited resolution problem in light-
based microscopic techniques, electron- and probe-based microscopic
techniques were developed. For example, with SEM, down to subnanometer
resolution is possible and with TEM atomic-level resolution can be easily
achieved.
In probe-based imaging techniques, a probe such as a tip is used to scan a
defined surface area. Atomic force microscopy (AFM) and scanning tunnel-
ing microscopy (STM) are the most commonly employed modes of this group
of microscopic techniques. AFM provides topographic information down to
a few nanometer resolution and the obtained image offers similar infor-
mation to the images obtained with SEM. However, SEM is much faster to
acquire the image compared to AFM but it may not be suitable to image
biological samples due to the destructive nature of the electron beam.
The STM is based on quantum tunneling, and a conductive tip is scanned
across the surface by applying a bias voltage. Although the technique is
mentioned here, it is a dicult technique for the applications addressed
here since it requires an extremely clean environment and it is performed in
a vacuum.
Another mode of scanning-probe microscopy called near-field scanning
optical microscopy (NSOM) combining optical microscopy and scanning
probe mode is also available. 8 In NSOM, a fiber optic tip tapered down to
about 30 nm coated with aluminum is used as a scanning probe. Since light
cannot escape from the tapered end because the diameter of a fiber optic is
much smaller than the wavelength of the light, an electromagnetic field is
formed at the end of the fiber optic tip. This magnetic field is used to excite
the molecules either for fluorescence Raman scattering measurements.
However, NSOM is a dicult technique for probing the surfaces with high
roughness due to the fact that the fiber optic tip should be located as close as
a few nm to the sample surface but a spatial resolution down to 20 nm can
be achieved. 9 Tip-enhanced Raman scattering (TERS) is also becoming
popular in recent years for characterization of not only polymeric surfaces

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