Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 167

is controlled by vesicle adsorption at free sites of the surface, ac-
cording to a Langmuirian type behavior. A second, lower stage is
ascribed to vesicle unrolling and spreading processes. It has been
proposed that the balance between the gain in adhesion energy (as
given by the adhesion area) and the cost in the vesicle curvature
energy (as given by the bilayer bending rigidity) is determinant for
the adsorption, deformation and rupture of vesicles. Rupture oc-
curs when strong adhesive forces cause the tension in the mem-
brane of a partially fused vesicle to exceed the threshold for dis-
ruption of the membrane. 17
Among hydrophilic substrates, those allowing the formation
of lipid bilayers by vesicle fusion more easily are freshly oxidized
surfaces of silica, glass, quartz and mica. 18 However, hydrophilici-
ty is a necessary but not a sufficient condition to promote vesicle
fusion. Surfaces of oxidized metals and metal oxides (e.g., TiO 2 , Pt
and Au) allow adsorption of intact vesicles but resist the formation
of bilayers, presumably due to weak surface interactions. 19 Elec-
trostatic, van der Waals, hydration and steric forces cause the non-
covalently supported lipid bilayer to be separated from the solid
surface by a nanometer layer of water. 20 This water layer prevents
the support from interfering with the lipid bilayer structure, thus
preserving its physical attributes, such as lateral mobility of the
lipid molecules.
The quartz crystal microbalance with dissipation monitoring
(QCM-D) has proved quite valuable to monitor the macroscopic
features of vesicle deposition. A typical QCM sensor consists of a
MHz piezoelectric quartz crystal sandwiched between two gold
electrodes. The crystal can be brought to resonant oscillation by
means of an a.c. current between the electrodes. Since the resonant
frequency can be determined with very high precision (usually by
less than 1 Hz), a mass adsorbed at the QCM surface can be de-
tected down to a few ng cm -2 . At an ideal air/solid interface, there
is a linear relationship between an increase in adsorbed rigid mass
and a decrease in resonant frequency. The mass obtained from
QCM-D measurements corresponds to the total mass coupled to
the motion of the sensor crystal, including the mass of the ad-
sorbed biomolecules and of the solvent bound or hydrodynamical-
ly coupled to the molecular film. This feature distinguishes mass
measurements by QCM-D from those by SPR. In fact, the meas-
ured SPR signal originates from altered conditions for resonant