Biomedical Engineering Reference
In-Depth Information
eral different preparation techniques, by exploiting the attractive
hydrophobic interactions between the hydrocarbon tails of the al-
kanethiol and those of the phospholipid. The most common proce-
dure for lipid deposition onto alkanethiol monolayers is vesicle
fusion. The architecture of the mixed alkanethiol/lipid bilayer, in
which the lipid forms a well-ordered, non-interdigitated monolayer
on the alkanethiol, with its polar heads directed toward the aque-
ous solution, was confirmed by surface enhanced Raman spectros-
copy (SERS), reflection absorption infrared spectroscopy
(RAIRS), neutron reflectivity and EIS. A dipalmitoylphosphati-
dylcholine (DPPC) monolayer on top of a gold-supported al-
kanethiol monolayer in air was found by RAIRS to be highly or-
dered, with 95% of all-trans conformers. 76 SERS indicates that
short chain alkanethiols, which are relatively disordered, are more
affected by addition of a phospholipid monolayer than longer
chain alkanethiols. Neutron reflectivity in D 2 O of a
DPPC/octadecanethiol bilayer on Au yields a scattering length
density (SLD) profile that was fitted to a model of three layers: the
sulfur layer, a hydrocarbon layer consisting of the alkyl chains of
both the alkanethiol and DPPC, and the polar head region of
DPPC. 77 At 60°C, where DPPC is in the liquid crystalline state, the
thickness of the hydrocarbon layer region equals 3.3 nm, and that
of the polar head region equals 1.0 nm. However, the flat mini-
mum of the SLD profile, corresponding to a totally unhydrated
region, is only about 2.2 nm wide, because the polar head region,
which is estimated to contain 10 water molecules per DPPC mole-
cule, is also significantly hydrated. AFM images of the gold-
supported hybrid bilayer provide clear evidence that the phospho-
lipid monolayer on top of the alkanethiol monolayer is substantial-
ly complete and homogeneous. Electrochemical impedance spectra
of alkanethiol/phospholipid bilayers on gold have been analyzed
with an equivalent circuit consisting of a capacitance C hb , simulat-
ing the hybrid bilayer, with in series the resistance R : of the aque-
ous solution. 25,78 By considering the capacitance C t of the thiol
monolayer in series with that, C m , of the phospholipid monolayer,
C hb is given by C t C m /( C t + C m ). Upon measuring independently the
differential capacitance C t of the sole thiol monolayer on gold, the
above equation allows an estimate of C m . For a palmito-
yloleoylphosphatidylcholine (POPC) monolayer on top of an octa-
decanethiol monolayer tethered to gold, C m amounts to about
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