Electrochemical Analysis of Ion Channels and Transporters in Pore-Suspending Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 304

vious advantage is that pore-spanning membranes overcome the
problem of an underlying substrate, as it is required for SSMs 22,79
or tBLMs, 9,13 which serves at the same time as the electrode. For
example, Gritsch et al. 77 performed an impedance analysis on
gramicidin-doped SSMs obtained from fusing gramicidin-doped
liposomes (1 mol%) on an indium-tin-oxide surface. In the pres-
ence of 4 mM NaCl, a relative conductance change of 0.4 mS cm -2
was observed. On pore-suspending membranes we obtained a 20-
fold change in conductance of about 8 mS cm -2 at a similar ionic
strength (5 mM NaCl). Krishna et al. 55,80 considered the influence
of the bilayer supporting material in detail. Whereas in SSMs the
bilayer is in direct contact with the underlying substrate, tBLMs
are separated by 10-40 nm from the substrate, thus providing a se-
cond buffer-containing reservoir, which facilitates the detection of
an ion transport through transmembrane proteins. However, it
turned out that the aqueous reservoir in tBLMs is very small so
that the ions cannot move freely; at higher concentrations they
even form ion pairs, which reduce the effective number of con-
ducting species and, thus, the expected conductivity of the buffer
system. 55 With dielectric constants of İ r = 27-45, the aqueous res-
ervoir is more of a gel-like texture compared to the bulk aqueous
phase with İ r = 80. 80 Under these conditions, the apparent conduct-
ance of gramicidin was 7-fold smaller than expected. 55 Similar re-
sults were obtained from experiments on Į -hemolysin reconstitut-
ed in tBLMs. 81 tBLMs were established on gold substrates using
tethered lipids of various lengths. The change in conductance was
a function of the linker length and, thus, the volume of the aqueous
reservoir. In contrast to experiments with tBLMs, the activity of
ion channels in pore-spanning membranes or nano-BLMs reflects
the expected conductivity. 23,26,69 This led us conclude that the buff-
er-filled pores of the alumina substrates can be considered as simi-
lar to the bulk aqueous phase. However, there are still some limita-
tions, which will be discussed in the following chapter.
( ii ) Mass Transport Phenomena
Besides the conductance changes calculated from R m,o , a se-
cond parameter is obtained from impedance analysis, namely the
concentration resistance ı W . In previous studies, the Warburg ele-
ment has been shown to account for the mass transport of ions in