Biomedical Engineering Reference
In-Depth Information
is stepped from a fixed initial value of -0.200 V/SCE to a final
value of 0.500 V/SCE.
3
As the final value of this potential step
becomes progressively more negative, the charge of K
+
ions ac-
commodated in the TEO spacer increases rapidly, attaining a max-
imum limiting value of about 45 PC cm
-2
at -0.8 V/SCE.
185
This
corresponds to three potassium ions per DPTL molecule, denoting
an appreciable hydration of the spacer. Moreover, EIS measure-
ments of the surface dipole potential of the TEO spacer tethered to
mercury yield values that compare favorably with the dipole mo-
ment of TEO molecules measured in organic solvents;
186
this sug-
gests a substantial order of the TEO chains in Hg-supported
tBLMs. Incidentally, tBLMs supported by Au
165
or Ag
187
do not
allow ionic charge measurements carried out by stepping the ap-
plied potential to final values negative of ~ -0.650 V. In fact, the
resulting charge vs. time curves show a linear section with a rela-
tively high and constant slope that is maintained for an indefinitely
long time.
165,187
The constant current responsible for this linear
increase in charge is ascribed to a slight water electroreduction
with hydrogen formation. The high hydrogen overpotential of
mercury avoids this inconvenience. When comparing interfacial
phenomena on different metals, rational potentials should be used,
namely potentials referred to the potential of zero charge (pzc) of
the given metal in contact with a nonspecifically adsorbed
1,1-valent electrolyte. The pzc equals -0.435 V/SCE for Hg
188
and
-0.040 V/SCE for polycrystalline Au.
189
Therefore, an appreciable
hydration of the TEO moiety, possibly accompanied by its elonga-
tion
3
, takes place in the proximity of a rational potential of about
zero on Hg, but at a much more negative rational potential of about
-0.600 V on Au,
167,168
close to the DPTL desorption from this met-
al. A drawback in the use of mercury-supported tBLMs is repre-
sented by the notable difficulty in using surface sensitive tech-
niques for their structural characterization.
With respect to solid metal supports, mercury has the ad-
vantage of providing a defect free, fluid and readily renewable
surface to the self-assembling thiolipid/lipid bilayer. Moreover, it
imparts lateral mobility to the whole mixed bilayer. In addition,
the self-assembly of a lipid monolayer on top of a thiolipid mono-
layer is readily carried out by simply immersing a thiolipid-coated
mercury drop in an aqueous electrolyte on whose surface a lipid
film has been previously spread.
3
Thanks to the hydrophobic inter-
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