Biomedical Engineering Reference
In-Depth Information
the presence of characteristic peaks at particular energies indicates the presence
of a specific element on the sample. Hence, unlike ATR-FTIR, XPS provides a
quantitative analysis of the surface chemical composition making it also called
as electron spectroscopy for chemical analysis (ESCA). 215 The intensity of the
peaks is directly related to the concentration of the element within the sampled
region. Furthermore, the shape of each peak and the binding energy can be
slightly altered by the chemical state of the emitting surface component atoms,
thereby providing chemical bonding information as well. 216 Although XPS can
be used for all other elements, it is not sensitive to hydrogen or helium. An addi-
tional advantage of XPS is its capability to provide information about the cover-
age and thickness of molecules adsorbed on a surface. 217-222 This has been used
in comparing initial protein adsorption on NMs compared with conventional
surfaces for tissue regeneration. 215
2.6.3 Time-of-Flight SIMS
A technique with sampling depth between 1 and 2 nm that is more sensitive
than XPS is time-of-flight SIMS. SIMS is used for analyzing elemental and
chemical compositions of the outermost molecular or atomic layer of a solid
surface. 215 With this technique, the NM surface is bombarded with a beam of
energetic ions of argon or gallium, usually. As the ions hit the surface, atoms
and molecules are removed from the material surface such as in sputtering
process. 215 A portion of the sputtered particles is ionized to produce second-
ary ions that are accelerated to a constant kinetic energy. These are allowed to
travel a certain distance in a field free environment and collected by a detec-
tor that measures their time-of-flight which is correlated with their individual
mass. The positively charged secondary ions are characteristic fragments of
chemical structures on the material surface that provide significant information
about surface chemistry.
Two kinds of SIMS have been developed: static and dynamic SIMS. Static
SIMS uses low-energy primary ion beam (10 −9 A/cm 2 ) to scan the sample sur-
face to obtain a “static” surface analysis. 215 Dynamic SIMS uses a high-energy
primary ion beam (1 A/cm 2 ) at a short length of time to erode the material
surface continuously while the real-time signal is recorded. A plot of the signal
against the depth of the sample provides a profile of the chemical composition
and/or structures from the surface into the bulk of the sample with high-resolu-
tion depth of several nanometers.
SIMS is used to study the composition, conformation, orientation, and
denaturation of proteins on NM surfaces. 223 Very little is known about protein
orientation and conformation after adsorption on NM surfaces because of the
complexities of protein structures. Compared with XPS, SIMS provides more
chemical structure information based on the fragmentation pattern. In addi-
tion, SIMS offers small sampling depths of 10-15 Å providing an amino acid
level detail about the adsorbed protein on the exterior 10-15 Å portion of the
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