Electrochemical Analysis of Ion Channels and Transporters in Pore-Suspending Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 276

suspending membranes, it turned out to be advantageous to func-
tionalize the gold-covered porous substrate with cholesterylpoly-
ethylenoxy thiol (CPEO3), a cholesterol derivative with a hydro-
philic linker terminated with a thiol group, which renders the upper
surface hydrophobic. 35 CPEO3 was already successfully used by
others to establish solid supported membranes on gold elec-
trodes. 36,37 To generate pore-suspending membranes, LUVs com-
posed of 1,2-diphytanoyl- sn -glycero-3-phosphocholine (DPhPC)/
1,2-dioleoyl- sn -glycero-3-phosphocholine (DOPC) (6:4) were
spread and fused on CPEO3-functionalized porous alumina, lead-
ing to a pore-suspending membrane, whose electrical characteris-
tics could be monitored by EIS. The setup was a three-piece Tef-
lon chamber with the porous substrate horizontally fixed between
the two upper parts ( Fig. 1B ). Sealing rings allow insulating the
substrate area from its environment. However, to prevent leak cur-
rents, a small amount of n -decane was applied in the sealing ring
region prior to the addition of liposomes. The organic solvent did
not alter the electrical properties of the sample. A characteristic
course of the absolute value of the impedance | Z |( f ) after 30 hours
of incubation with LUVs is depicted in Fig. 4 A (open squares). To
extract the electrical parameters of the membrane system, again
the equivalent circuit depicted in Fig. 2 A (inset) was used.
Good accordance between model and experimental data is
found with a membrane capacitance of C m = 4 nF and a membrane
resistance R m = 82 Mȍ. Taking the porous area into account, the
absolute capacitance value translates into a specific capacitance of
0.4 μF cm -2 . From independent experiments, a mean specific
membrane capacitance of (0.5 r 0.1) μF cm -2 ( n = 5) was deter-
mined. This is in agreement with values found for solvent-free
BLMs, which are 0.5-1.0 μF cm -2 . 27 The membrane resistance R m ,
which varied between 10 6 -10 8 ȍ, is still lower than those obtained
for nano-BLMs. However, it is already sufficiently high to monitor
channel activities in an integral manner, 38 as will be shown in de-
tail in Section IV.
Pore-spanning membranes feature a high degree of mechani-
cal and long-term stability. The membrane resistance R m was ex-
tracted by impedance data analysis and given as a function of time
( Fig. 4 B ). Up to 50 hours after preparation, R m remains almost
constant followed by a decrease, which is attributed to the rupture
of single pore-suspending membranes. It turned out that adding