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Electrochemical Surface Switches and Electronic Ion Pumps Based on Conjugated Polymers 403

160 5

10 s 20 s 30 s 40 s 150 0 s
+ – 140 10 s 4
20 s
V AB V CD Green intensity (arb. units) 130 30 s Approximate pH
40 s
–
+
50 s 60 s 90 s 120 s 50 s
60 s
180 s
+ – 120 120 s 3
240 s
180 s 240 s 300 s 360 s 110
V BC
1 2 3 4 5
Distance (mm) from release line
FIGURE 11.8 Left: The electronic ion pump with a pH paper on the C electrode as
an indicator of the proton gradient. Middle: The proton gradient formed over time.
Right: The associated measured pH gradients vs. time. (Reprinted from Ref. 20.
Copyright 2008, with permission from Elsevier.) (See also color insert.)
the proton current was recorded at the same time as the pH gradient
formed inside the target electrolyte was imaged using an ordinary pH
paper (Fig. 11.8). As soon as 10 to 20 s after biasing the B-C electrode,
the first delivered protons were observed. Then a dynamic gradient is
established by diffusion, which evolves and spreads over time. We also
demonstrated that proton oscillations can be induced by applying a
20
pulsed voltage difference to the B and C electrodes. Induction of pro-
ton gradients as well as proton oscillations, with the associated pH
ranging from 3 to 5, is of great interest since pH is an important regula-
tor for a vast array of cellular functions.

11.2.2 Electronic Ion Pumps to Regulate Intracellular
Ca Signaling
2+
The Ca ion is of major importance in a number of cell signaling pro-
2+
cesses in eukaryotic cells, e.g., to regulate metabolism, exocytosis, and
21
gene transcription. The specificity of a particular Ca flux is repre-
2+
sented by the spatial and temporal resolution of the Ca signal, which
2+
is often oscillatory in nature. It is well established that excessively high
extracellular concentrations of K (≥ 50 mM) depolarize the cell mem-
+
brane, whereby voltage-operated Ca pumps located in the plasma
2+
membrane are activated. This leads to a controlled influx of Ca , result-
2+
2+
ing in an increase of the intracellular [Ca ]. Using 0.1 M KCl as the
aqueous electrolyte in the reservoir on top of the A-B electrodes, and
2+
0.1 M Ca acetate aqueous electrolyte in the receiving compartment on
+
top of the C-D electrodes, we showed that K ions could be pumped
from the source to the target electrolyte in the ion pump at V = 10 V,
BC
V = V = 1 V (Fig. 11.9). An almost constant delivery rate was observed
AB CD
during the first 600 s followed by a pinchoff caused by consumption of
any of the A or D electrodes. Interestingly, we found that the ion pump
operates at a very high efficiency, close to 100%. Knowing that every
charge that is transported from the AB source to the CD compartment
+
is a K ion, experiments were performed in an attempt to mimic