Biomedical Engineering Reference
In-Depth Information
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Nanopillars and Nanoelectrode Ensembles
The terms nanopillars and nanoelectrode ensembles (NEE) are
often used interchangeably. In general, these nanostructures are
created by depositing metal within a porous material that is re-
moved afterwards, leaving behind freestanding structures. They
are another way to introduce nanostructure on electrodes. A few
examples are presented here, in which these pillar/mesh structures
have been employed for detecting biologically relevant molecules.
Nickel nanopillars have been specifically cited as exhibiting
innate selectivity. For example, Lu et al. employed nickel nanopil-
lars as a nonenzymatic glucose sensor.
194
They postulated that glu-
cose oxidation is catalyzed by nickel species on the electrode sur-
face. Hubalek et al. employed a similar approach to detect and
distinguish between native and denatured urease, which is a nick-
el-binding protein.
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In 2003, Gooding and co-workers
196
, and Rusling and co-
workers
197
independently presented the first two examples of elec-
trodes modified with a
forest
of end-on oriented carbon nanotubes.
Gooding et al. achieved this by covalent linkage of the carboxylic
acid functionalities of the nanotube ends with a cysteamine SAM
on gold, while the Rusling group assembled a nanotube forest non-
covalently on a mixed Nafion/Fe(OH)
3
layer on graphite. Both
groups applied these new nanowire array electrodes to covalently
attach redox enzymes (microperoxidase by Gooding et al.; myo-
globin and horseradish peroxidase by the Rusling group) via amide
bonds to the water-exposed carboxylic acid functionalities of the
nanotube ends, and observed the electrochemical response of these
wired
enzyme molecules. This was shortly followed by a report
from the Willner group
198
on the wiring of glucose oxidase by a
carbon nanotube forest electrode similar to that of the Gooding
group, via a direct amide bond to the amine-functionalized FAD
cofactor.
Ugo et al. demonstrated that a gold nanoelectrode ensemble
(NEE) is much more sensitive to
cytochrome c
than standard elec-
trodes. This added sensitivity has an added benefit because
cyto-
chrome c
undergoes concentration dependent adsorption, therefore
a significantly lower amount of the protein is needed for electro-
chemistry experiments involving the protein when these structures
are employed.
199
A similar system was used to detect phenothia-
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