Biomedical Engineering Reference
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sent in the bilayer, which shuttles electrons to or from the
electrode surface;
(2) an electron tunneling across the bilayer, possibly assisted by
some lipophilic molecule possessing conjugate S-bonds,
and therefore capable of providing an electron-conductive
pathway across the lipid bilayer.
The latter view, uphold by Tien and some other authors
146
, is in-
spired by the existence in natural membranes of sites, small in ex-
tent, where electron transfer between hemes occurs as a result of
the overlapping of the orbitals of neighboring molecules. Howev-
er, the probability of electron tunneling through the entire thick-
ness of a lipid bilayer, even in the presence of molecules with con-
jugated double bonds such as ferrocene, TCNQ or TFF
142
, seems
doubtful. In fact, it would require overpotentials higher than those
observed experimentally.
A different procedure for the realization of a lipid bilayer non-
covalently attached to a silver surface was adopted by Salamon,
Tollin et al.
147
It consists of spreading a small amount of a solution
of a lipid in a suitable solvent across a small orifice in a Teflon
sheet sandwiched between a thin silver film, deposited on the ex-
ternal surface of a prism, and an aqueous solution. The hydrophilic
surface of silver attracts the polar heads of the lipid molecules,
thus forming a lipid monolayer with the hydrocarbon tails directed
toward the bulk lipid phase. This phase becomes progressively
thinner through the accumulation of the excess lipid and its solvent
at the edge of the Teflon orifice, with formation of an annulus (the
Plateau-Gibbs border), until the orifice is covered by a lipid bilayer
enclosed between the silver surface and the aqueous solution. The
authors claim that a water layer remains in contact with the silver
electrode during this process, thus favoring the incorporation of
integral proteins into the bilayer. The above sBLMs, noncovalently
attached to the metal support, are reported to be stable for 25-35 h.
However, the capacitance
C
of these bilayers, measured by EIS, is
about one order of magnitude greater than that of a conventional
BLM, denoting an appreciable amount of defects.
148
By a detailed
analysis of the minimum, depth and half-width of the SPR reflec-
tivity curves of these silver-supported lipid bilayers, Salamon and
Tollin studied the structural changes following the incorporation of
a number of integral proteins into the bilayer and the interaction of
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