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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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should be noted that NO2 concentrations of > 10 mM are found in airway
lining fluid and saliva, representing a potential problem. Nafion
coatings may therefore need to be individualized for different biological
systems.
6. Dimensions of Porphyrinic Sensors for Nitric Oxide
The classical procedure used in the fabrication microelectrodes, of
inserting a carbon fiber into a glass capillary followed by polishing,
produces electrodes too large for insertion into single cells
(approximately 10 in diameter).74 Because the electroactive area is
limited to the area of the exposed electrode tip and the current
(analytical signal) is directly proportional to the surface area, it is
desirable to have this area as large as possible for the smallest
possible diameter. Instead of a flat disk, porphyrinic sensors have a
cone-shaped electroactive electrode surface, for which reason the surface
area is dependent on its slant height rather than the diameter. The
carbon fiber can be sharpened by gradual burning at a
244

Malinski
Figure 9. Microscopic photograph of a carbon fiber (a), Electron scanning
micrograph (ESM) of the part of the sensor covered with a coat of
isolating wax-rosin mixture (b), ESM of a thermally sharpened tip covered
with polymeric TMHPPNi (c). (Reprinted with permission from Malinski T,
Taha Z. Nature, 1992, 358: 676)
temperature of about 1000 C. Typical electrode dimensions (slant height
5 |am, fiber diameter 7 ) = 5 mm produce a cone shape-disk electrode
surface area ratio of 20. The resulting analytic signal will be higher by
the same factor.
Fibers of 6-8 in diameter are sharpened to 0.2-0.8 mm in order to
avoid cell-membrane rupture during implantation. The tip of a thermally
pointed carbon-fiber sensor covered with polymeric (TMHPP)Ni in the
sensing tip area is shown in Figure 9. For NO measurements on the cell
surface an L-shaped carbon fiber can be mounted in an L-shaped capillary.
Multiple electrodes (an electrode array) for studies of NO release in
tissues can be constructed in the disk form using glass microcapillaries.
A bundle of 2-6 fibers mounted in a glass capillary in a similar way to
that described from a single fiber can be effective in many large cell or
tissue experiments. However, the use of large electrodes (diameter > 100
) does not improve the performance of the sensor. This is due to an
increase of the background current (capacity current) as well as the
inability to place the sensor at the site where the highest concentration
of NO is expected (the cell membrane). This last limitation can be
overcome by using a large surface sensor on which cells can be grown
directly. In this case, the sensor consists of vitreous carbon support
covered with a polymeric film and Nation (Figure 10). The detection limit
of this sensor is similar to that obtained for the carbon-fiber sensor.
This sensor provides a convenient way to measure NO released in cell
culture under various conditions.
Carbon-fiber-based sensors are too fragile to be used as deep-tissue
measurements. However, these sensors can be protected by insertion into
an intravenous catheter that has been implanted in tissue or blood
vessels.4-65-75-80 To make the catheter-protected sensor, a bundle of
carbon fibers is mounted inside a truncated needle (Figure ll).5-7 After
curing, the shaft of the truncated needle is coated with nonconducting
epoxy. Then conductive porphyrinic film
and Nafion are deposited on the protruding carbon fiber. A puncture
needle is used to place a Teflon catheter with a perforated tip deep
inside the desired tissue or in a blood vessel. The Teflon catheter's
position is secured and the puncture needle is removed. Then a NO sensor
mounted at the end of a truncated needle shaft is inserted into the
hollow Teflon catheter. The NO sensor should be just short enough
Copper wire
Polymeric
porphyrin
Figure 10. Schematic diagram of porphyrinic sensor for detection of NO
released from cells grown directly on its surface.
needle
<
100 |im pores
calheler
Porphyrinic sensor
Tnincaled needle
<
Copper wire

I
\
Gold crimp pin
Figure 11. Schematic diagram of intravenous catheter-protected
porphyrinic sensor for in vivo measurement of NO.
44/Porphyrin-Based Electrochemical Sensors
245
to fit inside the perforated catheter and protect the NO sensor tip from
mechanical damage.
Single-fiber sensors and catheter-protected sensors can operate in an
amperometric or voltammetric mode. In both methods a current proportional
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