Biomedical Engineering Reference
In-Depth Information
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Figure 1.13
Impedance devices where chemistry is conducted in an interdigital
electrode gap.
Primarily the system has been used to detect biomolecular recognition events
taking place at an electrode, much as the situation with the other elec-
trochemical arrangements outlined above.
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One interesting variation on this
theme was the detection of electrical changes in the gap between interdigitated
electrodes associated with immunochemical interactions.
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As can be seen in
Figure 1.13 changes in the gap conductivity and capacitance are connected with
alteration of the real and imaginary impedance components, respectively.
In a similar fashion, EIS has been employed to detect nucleic acid duplex
formation in the gap between interdigitated electrodes. The signal in this case
has its origins in the conductivity differences in the gap associated with single
and double strand DNA species.
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An attractive feature of this technology is its
capability to be exploited in tandem with nucleic acid microarray assays. In this
method a large array of single strand oligonucleotides or DNA are exposed for
hybridization with a sample containing possible target complementary strands.
Normally, the gold standard for the detection of the various levels of duplex
formation is confocal fluorescence microscopy, which requires the use of
luminescent labels.
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1.4.1.4 Electrochemical Detection and the 'Nano' World
Science has witnessed an explosion of relatively recent interest in nanotech-
nology and this has found its way into electrochemical research. Devices have
been constructed from nanowires,
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nanorods, nanoparticles and nanotubes
in addition to 'nanofilms' deposited on electrode surfaces. Much of this activity
has been concentrated on nanowires where the diameter of the structure is on
the nanometric scale, although the length dimension may well be much greater.
The main property of interest with respect to such devices is defined by the very
significant surface-to-volume ratio for nanowire atoms compared with more
macroscopic systems. Accordingly the Debye screening length is expected to
penetrate the width of the wire to a large extent.
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This leads to the consideration
that conduction in the wire will be controlled by quantum state effects.
Accordingly, any changes in surface charge, such as instigated by biomolecular
interactions, will result in highly sensitive changes in wire conduction. Among
several attempts to exploit this projected high sensitivity has been work on
nanowires used in tandem with FET technology.
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Such a configuration, in
principle, allows multi-analyte sensing as depicted in Figure 1.14.
