Biomedical Engineering Reference
In-Depth Information
Fig. 1.6 Electrical characterization of Al
2
O
3
nanopores [
89
], reprinted with permission. (a)
Current-Voltage characteristics of a 11 nm diameter pore measured in 10 mM KCl, 100 mM
KCl and 1 M KCl. Linear current-voltage characteristics suggest pore geometry is symmetric in
the
z-
direction (b) Pore conductance of 11 nanopores ranging in diameter from 4 to 16 nm. Black
and gray solid lines represent conductance models for geometries 1 (double cone) and 2 (cylindri-
cal), shown in the inset. Black dashed line is a least squares fit to the measured data, used to extract
pore parameters
h
eff
26.5 nm and
a
24
. (Right Inset) Predicted cylindrical, double cone
geometry of an Al
2
O
3
pore from conductance measurements and energy filtered TEM imaging
k
1
(where
K
2
2
e
2
n
KCL
=k
B
Tee
0
in
models as the Debye screening length given by
ΒΌ
1MKCl)
<< d
pore
. At these high salt concentrations, charge carriers in the bulk were
expected to dominate current flow. Electro osmotic flows resulting from counterion
condensation on the charged pore surface should be negligible. These results con-
firmed that a symmetric, double cone nanopore structure was formed through
decompositional sputtering. TEM tomograms taken by Wu et al. and Kim et al. on
SiO
2
/SiN/SiO
2
and Si
3
N
4
nanopores further confirmed the double cone, symmetric
structure of a TEM sputtered solid-state nanopore [
46
,
48
,
98
].
Asymmetric current-voltage characteristics have been reported in synthetic
nanopores formed in polyethylene terephthalate (PET) polymer membranes [
76
,
78
].
The track etch method used to form these nanopores produced an asymmetric,
conical geometry resulting in a structure that significantly rectifies the ionic
current. This conically shaped, highly charged nanopore is cation selective,
exhibiting diode like behavior in fluid with a preferential direction for the cation
flow from the narrow entrance toward the wide opening of the pore. Siwy et al.
further demonstrated some of the novel characteristics of this architecture by
pumping ions against a concentration gradient using a fluctuating electric force
applied across the membrane in the form of an AC voltage signal [
76
].
1.3.4.2 Surface Charge Effects
The effects of surface charge on pore conductance were investigated by Ho et al.
using nanopores formed in 10 nm thick Si
3
N
4
membranes [
33
]. At low electrolyte
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