Biomedical Engineering Reference
In-Depth Information
of AOI through liquid/solid interface can be an interesting approach for use
in such in situ study. If measurements have to be performed in an opaque
solution or medium, an internal reflection approach can be applied as atten-
uated total reflection (ATR) (Tiwald et al. 1998) or total internal reflection
ellipsometry (TIRE) (Rekveld 1997; Poksinski et al. 2000). In these measure-
ments the optical path is critical and the probe beam needs to be reflected at
its second interface to achieve sensitivity for layers on the surface. It is also of
paramount importance to maintain a “clean” surface without contamination.
It is known that when the sample surface passes through a liquid-air interface,
a contamination layer can be easily formed by the Langmuir-Blodgett (LB)
effect. One should also note that several surface layer formations and phenom-
ena such as the double layer formation and the presence of a stagnant diffu-
sion layer (even under mechanical stirring) could vary the optical function in
these interface layers before a steady quasiequilibrium condition is attained in
dynamic or transient measurements. Although metallic surfaces such as those
with Au, Ag, Ti, Fe, Cu, Cr, Hg, and their oxides when available, are used in
the ellipsometric studies, most of the studies are often carried out on Au or
oxidized silicon surfaces. Oxidized silicon is of particular interest for macro-
molecules and porous media, because of its reasonable cost, availability with
flat surface, high purity, well-known optical properties, high refractive index,
thus high contrast, sensitivity, and resolution. The oxidized silicon surface can
be modified chemically to become more hydrophobic or hydrophilic (Arkles
1977; Pluddeman 1980; Welin-Klintstrom et al. 1993a; Welin-Klintstrom et al.
1993b) to facilitate the study of molecular interaction on such surfaces. Some
interesting studies of protein adsorption in porous oxidized silicon have been
demonstrated (Zangooie et al. 1998; Arwin et al. 2000). Another example by
using infrared spectroscopic ellipsometry (IRSE) to detect residual water in
annealed sol-gel thin films is illustrated in Figure 13.8 (Bruynooghe et al.
1998). By FTIR spectrophotometry, ATR configuration with spectroscopic
guided wave absorption, SIMS, and IRSE measurements, residual amounts of
water remains in the thermally annealed sol-gel silica samples were shown,
and IRSE is the most sensitive technique among all.
The application of ellipsometry to life science has been reviewed by Arwin
(2005) recently. The approach used in the study of macromolecular adsorption
could be applicable to the situations with porous media. For future application
in nanotechnology, ellipsometry with nanoscale sensitivity to follow kinetic
measurements at the solid/liquid interface is particularly interesting. Due to
the size of microbial species (in the vicinity of micron range), ellipsometric
techniques might have limited value for in situ studies. However, applications
to biofilms remain interesting.
Although the high sensitivity in the nanoscale and in situ capability with
temporal and spatial resolutions make ellipsometry very attractive as a char-
acterization tool, there are pitfalls that need to be mentioned. One of the
drawbacks in the ellipsometry is the prerequisite of very inert, smooth, and
flat surfaces that needs to be used as the model surface to permit quantitative
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