Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 168

surface plasmon excitation due to changes in the interfacial refrac-
tive index. For a simple dielectric material there exists, as a first
approximation, a nearly linear relationship between the change in
refractive index caused by biomolecule adsorption and the number
of biomolecules at the interface; on the other hand, SPR is practi-
cally insensitive to the presence of water molecules. Hence, the
mass sensed by a QCM exceeds that estimated by SPR by the
amount of water molecules bound or dynamically coupled to the
adsorbed biomolecules. However, the dissipation monitoring of the
QCM-D allows a distinction between lipid layers and loosely
bound water molecules in adsorbed vesicles. In fact, under shear
stress, lipid layers behave more elastically (or, differently stated,
less viscously) than adsorbed vesicles, coupling strongly to the
motion of the crystal surface. Conversely, adsorbed vesicles are
substantially larger and less compact structures filled with water;
moreover, water is also trapped between the vesicles and the sur-
face. Consequently, vesicles are subject to larger deformations
under shear stress. These viscoelastic features are invisible by
simple resonance frequency determination. Viscoelasticity can,
however, be visualized by measuring the energy loss, or dissipa-
tion ( D ) of the shear movement of the crystal in water. The dissi-
pation D is measured by driving the crystal with a.c. current at the
resonant frequency, followed by disconnection and analysis of the
resulting damped sinusoidal oscillations. This dissipation parame-
ter allows a distinction between intact, adsorbed vesicles (high
dissipation) and lipid bilayer patches (low dissipation). Figure 9
shows the changes in resonance frequency (' f ) and in dissipation
(' D ) under different situations. If vesicles do not adsorb, no
change in f or D takes place ( Fig. 9A ). If vesicles adsorb and re-
main intact, forming a supported vesicular layer, their high mass
causes an appreciable frequency decrease, and also a high dissipa-
tion ( Fig. 9B ). If vesicles are initially adsorbed, but subsequently
fuse forming a water-free lipid bilayer of lower mass, ' f shows a
minimum before attaining a constant limiting value, while ' D
shows a maximum ( Fig. 9C ). Figure 9D exemplifies the case in
which the vesicles adsorb and rupture instantaneously. Combined
measurements by QCM-D and SPR have shown that isolated vesi-
cles of egg-phosphatidylcholine (egg-PC) remain intact when
bound to a silica support; a minimum critical vesicle coverage is
required to initiate the conversion of surface-bound vesicles into