Biomedical Engineering Reference
In-Depth Information
removal of the solvent, leaving an about 5 nm thin bilayer behind.
This procedure allows to quickly assess whether a nano-BLM has
been successfully formed.
26
In order to elucidate if a lipid bilayer is fully covering the po-
rous substrate, characteristic membrane parameters are extracted
from the impedance spectra. The data analysis is based on an elec-
trical model (
Fig. 2 A
, inset). The very simple equivalent circuit
comprises three elements, which represent the different compo-
nents of the electrochemical system. It is composed of a parallel
RC
-element, namely an Ohmic resistor
R
m
and a capacitor
C
m
,
which represent the electrical behaviour of a lipid bilayer, in series
to another Ohmic resistor
R
el
that represents the electrolyte solu-
tion and the wire connections. The obtained impedance spectra are
characterized by the electrolyte resistance
R
el
in the high frequency
regime (5·10
4
-10
6
Hz), and the capacitance
C
m
at frequencies be-
low 5·10
4
Hz. At frequencies below 3·10
-1
Hz a second Ohmic re-
sistance is discernable, which is attributed to the membrane re-
sistance
R
m
. Fitting the parameters of the equivalent circuit to the
data results in good agreement between data and fit with a mem-
brane capacitance of
C
m
= 11.7 nF and a membrane resistance of
R
m
= 4.7 Gȍ. Taking the porosity of the alumina substrate of 33 %
into account, which was determined by scanning electron micros-
copy,
24
an active area of
A
= 2.3 mm
2
is calculated. Thus, the mean
capacitance of
C
m
= (14.9 ± 3.3) nF (
n
> 50) translates into a spe-
cific capacitance of
C
m,sp
= (0.65 ± 0.2) μF cm
-2
.
C
m,sp
is defined as
C
m
A
-1
. This value agrees well with those obtained for classical
BLMs
27
and supports the idea that single lipid bilayers have been
formed across the pores.
(
ii
)
Long-Term Stability of Nano-BLMs
The achieved membrane resistances of the nano-BLMs are
similar to those of traditional BLMs and are obviously sufficient to
perform single channel recordings. However, the suitability of
classical BLMs in biosensor applications is limited, as the BLM
ruptures at a certain point, i.e., owing to mechanical distortion,
leading to the loss of the insulating membrane in one single event.
In contrast, in nano-BLMs each bilayer, suspending a single pore,
is decoupled from the others and can therefore rupture separately,
as proven by visualizing the process by fluorescence microscopy
28
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