Nitric oxide (NO) electrochemical sensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 6

Following this publication, Schuhmann showed that pyrrole functionalized porphyrins,
containing metals such as Ni, Pd and Mn, can be immobilized on carbon microelec-
trode surfaces via oxidative polymerization and be used for NO detection [29, 30].
Other researchers have shown that carbon fi bers coated with a variety of porphyrins
such as iron porphyrin [31] and cobalt porphyrin [23, 27, 32-34] are also effective
for NO detection. Although metal porphyrin coated electrodes were successfully used
to some extent for various applications [35-37], subsequent studies have shown that
carbon fi bers modifi ed with unmetallated porphyrins as well as bare carbon fi bers can
detect NO with similar sensitivity [38, 39]. The sensitivity and selectivity of porphy-
rinic NO sensors vary signifi cantly from electrode to electrode and depended not only
on the potential at which NO oxidizes, but also on the effects of axial ligation to the
central metal in the porphyrin, modifi cation/treatment procedure and other experimen-
tal variations. Furthermore, because the surface of the electrode remained in direct
contact with the measurement medium a variety of biological species were shown to
interfere (i.e. give false responses) with the measurement of NO. Adding a Nafi on
layer to the porphyrin coated fi bers could minimize these interferences. Other practical
problems such as easy porphyrin removal and degradation have limited their useful-
ness for most applications [40]. Phthalocyanines, with a similar structure to porphy-
rins, containing metals such as Fe, Ni and Co have also been used to modify electrode
surfaces for NO sensing [41, 42]. Phthalocyanine modifi ed electrodes have comparable
detection limits and selectivity to porphyrin modifi ed electrodes with the added benefi t
of being more stable to degradation.

1.3.3 Integrated NO microelectrodes
During the mid- to late 1990s a new range of combination NO sensors with tip diam-
eters between 7

m were developed by this lab [43]. These sensors com-
bine a carbon fi ber working electrode with a separate integrated Ag/AgCl reference
electrode. The resulting combination sensor was then coated with a proprietary gas
permeable/NO-selective membrane. A high performance Faraday-shielded layer was
then added to the sensor outer to minimize susceptibility to environmental noise. This
electrode is then operated exactly as outlined above for the Clark type NO sensors.
The use of these proprietary diffusion membranes and the novel design allows for NO
measurement in small volumes and confi ned spaces with great selectivity against a
wide range of interferences such as ascorbic acid, nitrite, and dopamine. This sensor
design was elaborated upon by creating L-shaped sensors designed specifi cally for tis-
sue bath studies [22]. The above design was further elaborated upon by our lab by
creating fl exible, virtually unbreakable NO sensors designed specifi cally for use in
measuring NO concentrations in arteries and microvessels. This electrode combines a
Pt/Ir wire with a separate integrated Ag/AgCl reference electrode [44]. The resulting
combination sensor was again coated with a proprietary gas permeable/NO-selective
membrane and a high performance Faraday-shielded layer and operated as described
above. Recently, we developed a novel combination NO nanosensor, which had a tip
diameter of just 100 nm [45]. The design of this sensor can be seen in Fig. 1.2. This

µ m to 200 µ