Biomedical Engineering Reference
In-Depth Information
zine, an organic compound whose structure occurs in various anti-
psychotic and antihistaminic drugs that also exhibits concentration
dependent adsorption. 200
So far, empirical evidence has been demonstrated for these
new and improved sensor properties employing nanoscale struc-
tures. Much work remains to be done to gain a better understand-
ing of how nanostructure affects chemical properties. A very nice
overview of nanowire ensemble devices is provided by Walcari-
us. 201 At the moment, pillars and electrodes in these arrays cannot
be individually addressed, but in the future, these structures may
have individual sensors poised at different potentials, coated with
different modifications or even located within different regions of
a sample matrix.
( iii ) Other Techniques
Tyagi et al. employed a nice nanofabrication trick to create
gold nanowires with a thickness of 30 nm, that they then used to
detect dopamine. 202 They exploit undercut, a typical problem of
microfabrication processing, to create nanostructures. By inten-
tionally overetching nickel that is covered by photoresist, they
create a recessed structure. Gold is then electrodeposited in the
positions where the nickel is exposed, but since it is covered by
resist, the gold structure is of the same thickness as the nickel film.
The resist and nickel are removed, leaving behind gold nanowires.
While optical lithography hails as a champion of mass produc-
tion, the equipment and processes necessary to achieve sub-micron
resolution are still very expensive, and in the case of electron beam
lithography, quite slow. Another way to achieve nanometer scale
separation between electrodes is to partially remove an insulating
material that is sandwiched between two electrodes. 203 This creates
a series of nanopores, inside which proteins can be selectively pat-
terned. Some amplification is achieved due to redox cycling in the
confined space that is created.
Wolfrum et al. fabricated thin-layer cells with electrodes sepa-
rated by a few tens of nanometers by employing a thin film as a
sacrificial material and demonstrated their sensor capabilities by
detecting catechol in the presence of excess ascorbic acid. 204
Another alternative fabrication scheme utilizes nanoimprint li-
thography. The process employs a stamp that transfers a design
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