Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 172

also be monitored by fluorescence microscopy. In fact, the energy
of a photoexcited fluorophore is transferred nonradiatively to the
metal support (quenching), if the fluorophore is at a distance from
the metal surface shorter that a critical transfer distance (the Foer-
ster radius). This critical distance amounts to about 20-30 nm for
gold. Therefore, unquenched bright spots in the fluorescence mi-
croscopy images of gold-supported lipid films obtained by fusion
of fluorophore-labeled vesicles mark the presence of heightened
areas ascribable to adsorbed or hemifused vesicles. 30

3. Langmuir-Blodgett and Langmuir Schaefer Transfer

Another procedure for forming a lipid monolayer on a hydropho-
bic substrate or a lipid bilayer on a hydrophilic substrate makes use
of a Langmuir trough equipped with a movable barrier. By spread-
ing a lipid dissolved in an organic solvent on the surface of the
aqueous electrolyte contained in the trough and by allowing the
solvent to evaporate, a lipid film is formed at the air/water inter-
face. This film is compressed with the movable barrier until it is
brought in proximity of the liquid crystalline state by adjusting the
surface pressure. To form a lipid monolayer on a hydrophobic
slab, this is immersed vertically through the lipid monolayer. This
brings the hydrocarbon tails of the lipid monolayer, turned toward
the air, in direct contact with the hydrophobic surface of the slab,
which remains coated by a lipid monolayer (see Fig. 10A ). This
technique, called Langmuir-Blodgett (L-B) transfer, is also used to
form a lipid bilayer on a hydrophilic slab. 31,32 In this case, the slab
is initially immersed vertically through the lipid monolayer into
the trough. No significant change in surface pressure is observed at
this stage. The slab is then withdrawn at a speed slow enough to
permit water to drain from the surface. During the withdrawal, the
polar heads of the lipid monolayer are turned toward the surface of
the hydrophilic slab, giving rise to noncovalent self-assembly (see
Fig. 10B ). The resulting decrease in the surface pressure of the
lipid monolayer at the air/water interface is prevented by reducing
its area with the moving barrier. A further lipid monolayer is then
self-assembled on top of the first one by the so-called Langmuir-
Schaefer (L-S) transfer. In practice, the lipid-coated slab is brought
into horizontal contact with the lipid monolayer at the air/water
interface, after compressing it at a preset surface pressure. Finally,