Biomedical Engineering Reference
In-Depth Information
behind Al-rich nanocrystals resulting in a partially metalized nanopore. Similar
phenomenon has been observed in hole drilling experiments conducted in Na-
b
Al
2
O
3
[
24
]. Coupled with studies by Berger et al. demonstrating the formation of
continuous Al regions and “plugs” in e-beam irradiated metal
-aluminas, [
3
] this
work provides a unique method to potentially form nano-scale metallic contacts
within a nanopore for bio-sensing applications. Simulation work by Lagerqvist
et al. demonstrated the ability to achieve single nucleotide resolution by employing
a nanopore sensor with embedded transverse sensing electrodes, with potential
application to nanopore-based DNA sequencing [
50
]. These results could help
enable the possible realization of such a structure. Local nanopore stoichiometry
is also very important when chemically modifying or functionalizing a nanopore
with various biomolecules or organosilanes. The packing density of these
molecules in the lumen and barrel of the nanopore are dependent on the density
of Aluminol surface groups in the nanopore. Thus a thorough oxidation in an O
2
plasma is required before any surface functionalization steps.
b
1.3.4
Ionic Conduction Through Solid-State Nanopores
1.3.4.1 Nanopore Conductance
The conductance of the nanopore can be measured in monovalent electrolyte, typi-
cally KCl, by placing the nanopore between two electrically isolated, fluidic reser-
voirs. Typically, high salt solutions well in excess of physiological conditions are used
(~1 M KCl, 10 mM Tris-HCl, pH 7.5) to obtain sufficiently high baseline current
levels that can be monitored using a Pico ammeter. Faradaic Ag/AgCl electrodes are
placed in each reservoir allowing for a localized redox based exchange reaction to
occur at each electrode. After a 30 s O
2
plasma treatment, immediate wetting and ionic
conduction through the nanopore is observed. Linear current-voltage characteristics
are typically observed for nanopores in SiO
2
,Si
3
N
4
and Al
2
O
3
membranes formed
using TEM based decompositional sputtering processes [
30
,
81
,
89
]. The linear
current-voltage characteristics of a ~11 nm pore in 1 M, 100 and 10 mM KCl
electrolyte are shown in Fig.
1.6a
. To further probe the performance of Al
2
O
3
nanopores in electrolyte, the conductance,
G
, of 11 different nanopores of varying
diameter (4-16 nm) were measured in 1M KCl, as shown in Fig.
1.6b
. Two geometric
models were proposed to fit
G
[
46
,
48
,
81
]. The first model assumed a symmetric
double cone structure with cone angle,
30
,[
46
,
48
]anupper
conductance bound can be derived (solid black curve of Fig.
1.6b
). The second model
assumed a purely cylindrical channel of length,
L
pore
¼
a
[
81
]. Assuming
a ¼
60 nm with a cross sectional
diameter equal to the pore diameter,
d
pore
(solid gray curve of Fig.
1.6b
). This model
provided a lower bound for the measured pore conductance. Applying a least squares
fit to the measured data (black dashed curve of Fig.
1.6b
), an effective length of
h
eff
24
were extracted for Al
2
O
3
pores formed via
decompositional sputtering. The effects of surface charge were neglected in these
26.5 nm and cone angle of
a
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