Biomedical Engineering Reference
In-Depth Information
The field of solid-state nanopores has to thank mother nature for the inspiration.
In biology, on cell membranes, there are plenty of tiny nanometer-sized holes,
called pores. Cells need them to regulate ion contents, DNA and RNA molecules
move through them, all as parts of the intricate machinery of life. It is now possible
to make pores of similar dimension in solid-state materials. There have been intense
interests in recent years in using these solid-state nanopores to explore biology, to
read out DNA sequences, and to perhaps build artificial cells mimicking life.
Perhaps the single most important applications scientists had in mind for nano-
pores is for the development of a new method for DNA sequencing. The concept of
direct DNA sequencing using nanopores was first introduced by Kasianowicz et al.
[
1
] in a seminal paper on DNA translocation through a biological pore
a
-hemolysin,
a toxin secreted by
Staphylococcus aureus
. Their basic idea, patented by Church
et al. [
2
], is simple and appealing: since the ionic current through an ion channel is a
measure of the opening of the pore, it seems possible that if a macromolecule like
DNA enters the pore, the ionic current will be suppressed by an amount which is a
measure of the size of the molecule. Since the four nucleotides comprising a single-
stranded DNA (ssDNA) or RNA have slightly different physical sizes, they may
give rise to measurable fluctuations in the ionic current through the pore if the
molecule “translocates” in a linear fashion under the drive force of an applied
electric field across the ion channel. In this method, the DNA sequence is read out
by the temporal variation of ionic current through the pore.
The subsequent experiments by a number of groups [
3
,
4
] using
-hemolysin
failed to detect any sign of individual nucleotides. There are a number of reasons
that the proposed direct sequencing approach may be difficult. Among these is the
fact that the spacing between nucleotides is about 0.4 nm, but the
a
-hemolysin
channel length is about an order of magnitude longer, hence the current blockade
effect is an averaged signal from several nucleotides.
Motivated by the desire for a more robust and more tunable system for studying
the DNA translocation phenomena, the first solid-state nanopore device was devel-
oped by Li et al. [
5
] using ion-beam techniques. Since then, several alternative
methods have been reported (Storm et al. [
6
]; Chang et al. [
7
]; Wu et al. [
8
];
Park et al. [
9
]).
a
8.2 Fabrication Techniques for Single Solid-State Nanopores
With the standard electron-beam lithography techniques [
10
], one can routinely
produce features at 20-100 nm. To produce pores at 1-20 nm range, one has to
resort to non-conventional methods. The ion-beam technique developed by Li et al.
[
5
] was the first such example. Here, the latter methods are discussed including the
TEM-drilling technique [
6
] which is more popular at present with researchers.
It should be emphasized that the solid-state nanopore technology is an evolving
field, there is no “best” method of fabricating nanopores. It is hoped that the readers
will be inspired to develop even “better” techniques.
Search WWH ::
Custom Search