Biomedical Engineering Reference
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layers on Au(111) single crystal faces have been prepared by Lip-
kowski and coworkers either by vesicle fusion
134
or by Langmuir-
Blodgett and Langmuir-Schaefer transfers.
33
The latter procedure
yields bilayers with a higher packing density and with smaller tilt
angles of the alkyl chains with respect to the surface normal. These
bilayers have been characterized by charge density measurements,
photon polarization modulation infrared reflection absorption
spectroscopy (PM IRRAS)
134,33
and neutron reflectivity.
135
The
minimum differential capacitance of these bilayers is about equal
to 2 PF cm
-2
, and is attained at charge densities V
M
on the metal
higher than -8 PC cm
-2
. The differential capacitance being greater
than that of a solvent-free BLM denotes the presence of a number
of defects, while neutron reflectivity reveals the presence of water
molecules within the lipid bilayer. As V
M
becomes negative of
-8 PC cm
-2
, the lipid bilayer starts to detach from the electrode,
but remains in close proximity to the electrode surface. In fact,
neutron reflectivity indicates that the lipid bilayer is suspended on
a thin cushion of the aqueous electrolyte, which screens the metal
charge and contains a large fraction of the overall interfacial po-
tential difference; consequently, the electric field across the lipid
bilayer becomes weak. This suspended lipid bilayer is essentially
defect-free and its structure resembles that of lipid bilayers sup-
ported by a quartz surface. The IR data demonstrate that the car-
bonyl and phosphate groups are more hydrated when the bilayer is
adsorbed on the electrode surface at V
M
> -8 PC cm
-2
than when it
is detached from the electrode at more negative charge densities.
134
Neutron reflectivity shows that a significant amount of water en-
ters the polar head region of the lipid bilayer when it is in contact
with the metal surface. The detachment of the lipid bilayer from
the electrode surface is accompanied by a decrease in the tilt angle
of the alkyl chains with respect to the surface normal from about
55° to 35°, with a resulting increase in bilayer thickness. The PM
IRRAS measurements indicate that, as long as the lipid bilayer
remains attached to the metal surface, changes in the local electric
field by several orders of magnitude have only a small effect on
the orientation of the phospholipid molecules.
136
Electrochemical
scanning tunneling microscopy (EC-STM) images of adsorption of
dimyristoylphosphatidylcholine (DMPC) on Au(111) show that
the lipid molecules are initially adsorbed flat, with the alkyl chains
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