Nanoelectrochemistry: Fundamentals and Applications in Biology and Medicine - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 46

tions techniques, such as chromatography or electrophoresis 174, 175 ,
or in combination with other, mainly optical detection schemes,
such as fluorescence 139 or SERS. 143 Two possible strategies that
may alleviate these problems include the functionalization of
nanoelectrodes with enzymes and the deployment of selective
membranes. The inherent selectivity and high catalytic activity of
enzymatic reactions can be effectively utilized provided the long-
standing problems of robust functionalization and effective wiring
of the enzymes are overcome. The coupling between redox en-
zymes and nanoscopic electrode structures has been the subject of
several reviews. 172, 176-179
These above limitations notwithstanding, electrochemical sen-
sor applications have still found their way into the marketplace.
The most common and commercially significant application of
electrochemical sensors is in the detection of glucose. 180 Hundreds
of millions of people suffer from diabetes and require daily moni-
toring of blood glucose levels. Further improvements are still
needed, most notably in the development of continuous and in-vivo
sensing strategies. The use of nanostructured materials, especially
as electrical interconnects with the enzymes, together with nanoe-
lectrodes holds considerable potential for improvement of sensor
response of these devices. 181 An interesting device made by Tao's
group, comprised of a nanojunction formed by bridging of two
nanoelectrodes separated by 20-60 nm, with polyaniline/glucose
oxidase. The signal transduction mechanism relied on the change
in the conductance of the nanojunction as a consequence of glu-
cose-oxidation induced changes in the polymer redox state. Due to
the small size of the nanojunction sensor, the enzyme was regener-
ated naturally without the need of redox mediators and gave a very
fast response (<200 ms). 182
Another direction in continuous monitoring is in the develop-
ment of implantable devices. These types of devices will have to
be very small, robust, resistant to fouling, and bio-compatible. 183
Carbon nanotube based devices are actively investigated for these
applications, as they may exhibit many of these desired quali-
ties. 184 In one of the first examples of carbon nanotube sensors, Lin
et al. created a Clark type glucose sensor by covalently immobiliz-
ing glucose oxidase on carbon nanotube nanoelectrode ensem-
bles. 185