Biomedical Engineering Reference
In-Depth Information
spectroscopy, XPS, time-of-flight secondary ion mass spectroscopy (SIMS),
colorimetric methods, and atomic force microscopy (AFM).
2.6.1 ATR-FTIR Spectroscopy
Chemical bonding interactions between structures can be studied using ATR-
FTIR spectroscopy when samples are placed in contact with internal reflec-
tion elements (IREs) such as zinc selenide (ZnSe) or germanium (Ge).
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In
this process, the IR radiation is first focused in order to penetrate the end
of the IRE and reflects down the length of the IRE. The IR radiation pen-
etrates a short distance (∼1 µm) from the surface of the IRE into the speci-
mens. The absorption of radiation is related to the fundamental vibrations of
chemical bonds, while internal reflection spectrometry provides information
with respect to the presence or absence of specific functional groups and the
chemical structures of polymer membranes. Absorption bands are assigned
to functional groups (such as the O
C- stretch and the C-H bend) such as in
ordinary IR spectra, which can be used to identify changes in chemical struc-
ture when shifts in the frequency of absorption bands and changes in relative
band intensities are observed. This can also be used to indicate changes in
the chemical structure or alterations in the environment around the polymer
membranes.
The ATR-FTIR analysis of vibrational spectra can be applied to characterize
the organic fouling layer that may be present on a nanopolymer surface.
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It
can also be used to determine changes in surface chemistry after application of
specific nanotechnology-based chemical or physical treatments or to identify
unique chemical features on the polymer surface after NaOH etching of poly-
mers to obtain nanosurface features. Information gathered with this technique
has been correlated with polymer performance in biological systems as well as
in understanding surface interactions with cells and proteins.
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But, the pen-
etration depth of ATR-FTIR has the same order of magnitude as the wavelength
of IR light that ranges from several hundred nanometers to more than 1 µm.
Thus, ATR-FTIR is not a very sensitive surface analysis method for NMs that
are in the tens of nanometer dimensions.
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2.6.2 XPS
XPS is a technique for surface characterization that is used for polymeric nano-
biomaterials.
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It has a <10 nm sampling depth that makes it a more sensitive
surface analysis technique than ATR-FTIR. This technique uses photoionization
and energy-dispersive analysis of the emitted photoelectrons to establish the
composition and electronic state of a sample. Every element has a characteristic
binding energy associated with a specific atomic orbital giving a characteris-
tic set of peaks in the photoelectron spectrum at specific kinetic energies con-
trolled by the photon energy and their respective binding energies. Therefore,
