Biomedical Engineering Reference
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orded in an alkali cation-free buffer (10 mM TRIS, 50 mM
N(CH 3 ) 4 Cl, pH 7.4, further referred to as tetramethylammonium
chloride (TMA) buffer). The membrane resistance of the nano-
BLM should remain constant even after insertion of the carrier, as
TMA cannot be complexed by valinomycin and, thus, no transport
activity should be monitored. If, however, different potassium ion
concentrations were added to both compartments, the impedance
spectra changed considerably ( Fig. 9 A ).
The equivalent circuit shown in Fig. 2 A can be applied to ex-
tract the membrane resistance. The membrane resistance can also
readily be approximated from the magnitude of the impedance at
low frequencies, where | Z | becomes independent of the frequency
and equals the sum of R el and R m ( Fig. 9 A ). In pure TMA buffer, a
specific membrane resistance of R m,sp = 4.4 Mȍ cm 2 was moni-
tored. After exchanging the solution with TMA buffer containing
500 mM K + , the resistance decreased considerably to R m,sp = 0.3
Mȍ cm 2 . By varying the potassium ion concentration in a range of
50-500 mM, a linear relation of the membrane conductance as a
function of c K was obtained ( Fig. 9 B ). The same holds for the so-
dium ion dependency. A linear dependence of the membrane con-
ductance as a function of c Na was found in a concentration range of
50-500 mM ( Fig. 9 B ). Obviously, the conductance increase in the
presence of potassium ions is significantly larger than in the pres-
ence of sodium ions, if valinomycin is embedded in the nano-
BLM. The slope, which is a measure of the transport of the cation
mediated by valinomycin, is approximately 20 times larger for K +
than for Na + . This result is in good accordance with the known se-
lectivity of valinomycin for potassium over sodium ions found in
different membrane systems. 12,43-46 A parameter that cannot be
controlled is, however, the valinomycin concentration in the mem-
brane, which also determines the macroscopic transport efficiency.
Thus, a direct comparison between different cation transport effi-
ciencies can only be performed on one nano-BLM after insertion
of valinomycin. As a nano-BLM lasts various buffer exchanges,
even in a continuous flow system, 47 experiments on one nano-
BLM can be easily performed. To verify that a nano-BLM without
valinomycin is not conducting for potassium cations, it was im-
mersed in a TMA buffer containing either 500 mM KCl or 500
mM NaCl. Impedance analysis revealed no significant change in
membrane resistance in the presence of high salt.
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