Electrochemical Analysis of Ion Channels and Transporters in Pore-Suspending Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 321

From this result we suggest that the protons flow from the
purple membrane across the nano-BLM facilitated by the proton
ionophore CCCP into the pores of the underlying porous alumina
substrate. Since the stationary current does not decrease within a
prolonged time period of continuous illumination (10 min), we
conclude that the pores do not form a significant diffusion barrier
for protons. It is well conceivable that protons are well directed
along the pores down their concentration gradient, as the substrate
and, thus, the walls of the pores are composed of a negatively
charged oxide.
In Fig. 20 B , the stationary current is shown as a function of
the ionophore concentration. Theoretically, if the conductance G m
of the nano-BLMs is linearly dependent on the ionophore concen-
tration, 101 the stationary current changes with the CCCP concentra-
tion c CCCP in solution according to Eqs. (27) and (28):

GGs c (27)

m 0 CCCP

Gsc

0 CCCP

J J . (28)

sta t p,0

J

p,0

Gsc G

0 CCCP p

*

V

By keeping values for G 0 , G p and V * constant, Eq. (28) was fit
to the data, which results in the solid line shown in Fig. 20 B and
which is in good accordance with the data points. For the parame-
ter s , which indicates the change of the stationary current per con-
centration unit, a value of approximately 1 μS cm -2 μM -1 is ob-
tained. This value is in the same order of magnitude as that ob-
tained from experiments on BLMs with 6.6 μS cm -2 μM -1 . 101 How-
ever, no stationary photocurrent was detected when PM-fragments
adsorbed on SSMs were illuminated. 97 The SSMs in this study
were composed of an octadecanethiol monolayer on gold and a
second phospholipid monolayer. With no aqueous reservoir be-
tween the gold electrode and the bilayer, a stationary current can-
not be observed upon addition of ionophore as the underlying sub-
strate limits the ion flux. In this case, the membrane merely acts as
a capacitor, so that only the charging of this capacitor is observed.
In contrast, nano-BLMs are sensitive to changes in membrane re-