Biomedical Engineering Reference
In-Depth Information
Fig. 6.2 (a) The Schematic of a solid-state nanopore detection system for protein translocation
experiments. (b) The nanopore cross-section profile (across the center) illustrated. (c) An ion beam
sculpted ~8 nm diameter pore imaged by TEM
only connection between two electrolyte filled chambers. The bandwidth of the
Axopatch 200B was set at 10 or 100 kHz depending on the time duration of the
protein signal and noise level of the nanopore.
6.3.2 Solid-State Nanopore Fabrication
Solid-state nanopores have been fabricated by low energy ion beams and by electron
beams [
25
,
26
] with several insulating materials such as silicon nitride [
27
,
28
],
silicon dioxide [
29
,
30
], and aluminum oxides [
31
,
32
]. The data discussed in this
chapter were measured with silicon-nitride nanopores made by low energy noble
gas ion beams. The typical size of nanopores used for protein measurements are
between 4 and 30 nm selected according to the size and conformational states of
proteins to be measured. Practically, nanopores with diameters 2-3 times larger than
the protein are better choices for sensing protein molecules. The 275 nm low stress
silicon nitride membrane was deposited by LPCVD at CNF (Cornel Nanofabrica-
tion Facility). The freestanding membrane window was created by photolithogra-
phy, reactive ion etching, and anisotropic wet KOH etching. A single ~100 nm hole
or a FIB hole (Fig.
6.2b
) is milled through the freestanding membrane by a 50 keV
Ga ion beam from a focused ion beam machine. The nanopores are made by
shrinking the ~100 nm FIB hole to a desired diameter using low energy noble gas
ion beam sculpting [
27
,
29
,
33
,
34
]. The nanopores made by this method have a
length or thickness between 10 and 20 nm depending on ion beam parameters
selected [
34
]. A TEM image of a nanopore made by this method is shown in
Fig.
6.2c
.
6.4 Measuring Native State Proteins
The volume (
) a protein molecule occupies in space or solution is approximately
proportional to the protein's molecular weight (
M
) or size,
L
L/M
. When a protein
molecule is at its native or folded state and if the protein length is smaller than the
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