Chemistry Reference
In-Depth Information
10.3.2
Null Ellipsometry
In soft-matter chemistry, null ellipsometry is commonly used to observe the
adsorption of lipids, polymers, or surfactants from aqueous solution. Ellipsom-
etry is an optical technique to evaluate the thickness and the dielectric proper-
ties of materials on surfaces by measuring the change of the elliptically
polarized light upon refl ection (Azzam and Bashara, 1977; Landgren and
Jonsson, 1993). By monitoring the change of amplitude (
), the
optical thickness (
d
) and refractive index (
n
) of the surface layer can be
modeled. The absorbed amount,
ψ
) and phase (
Δ
, then can be calculated using, for instance,
the de Feijter approximation (Defeijter et al., 1978):
Γ
(
nnd
dn
−
)
0
Γ=
/
dc
where
n
0
is the refractive index of the solvent and
dn/dc
is the concentration
dependence of the refractive index of the adsorbed layer. A more detailed
description can be found in the literature (Azzam and Bashara, 1977; Land-
gren and Jonsson, 1993; Tompkins and Irene, 2005).
10.3.3
Quartz Crystal Microbalance with Dissipation
Quartz crystal microbalance with dissipation (QCM-D) is an acoustic tech-
nique that has been used to evaluate the adsorption of the lipid nanoparticle
on surfaces and monitor the structural and conformational changes of the
adsorbed layers (Hook et al., 2001; Rodahl et al., 1997; Vandoolaeghe et al.,
2009b). In QCM-D, an alternating current (AC) voltage is applied across a
quartz crystal so that the quartz oscillates at its resonance frequencies. When
the voltage is turned off, the oscillation decays exponentially. The mass of an
adsorbed layer is sensed by the decrease in resonance frequency, and the
structural and viscoelastic properties of the layer are deduced by the change
in dissipation obtained in the decay of the oscillations. For a rigid fi lm, the
adsorbed mass is directly proportional to the change in resonance frequency
following the Sauerbrey equation (Sauerbrey, 1959). For a soft fi lm, viscoelas-
tic models, such as the Voigt and Maxwell models, must be used to correct for
the energy loss from dissipation. Ellipsometry measures the adsorbed mass
from the changes in refractive index and thickness. The measured mass in
QCM-D is the adsorbed mass plus the mass of the solvent coupled to the layer.
This and the ability to measure the viscoelastic properties of the layer makes
it a useful tool to monitor the structural change due to, for instance, vesicle
fusion with a surface as well as LCNP adsorption.
10.3.4
Attenuated Total Refl ectance and Fourier Transform Infrared
Fourier transform infrared (FTIR) spectroscopy in attenuated total refl ec-
tance (ATR) mode has been used to study the interfacial interactions of LCNP
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