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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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to NO concentration is measured. Of the several voltammetric methods
available, differential pulse voltammetry (DPV) is most suitable for the
measurement of NO. In DPV, a potential modulated with rectangular pulses
is linearly scanned from 0.4 to 0.8 V. The resulting voltammogram
(alternating current versus voltage plot) contains a peak due to NO
oxidation. The peak current should be observed at a potential of 0.63-
0.67 V which depends on the pulse amplitude. This potential is the
characteristic potential for NO oxidation on Nafion coated porphyrinic
sensor.
Single-step amperometry is the best method to monitor rapid release
and decay of NO concentration. In amperometry, a current is measured at
constant potential of
0.63-0.67 V with time resolution better than one millisecond. Amperometry
provides rapid quantitative response to minute changes in NO
concentration. Amperogram can be recorded in several millisecond time;
however, the amperogram provides no information on the basal
concentration of NO.
DPV provides quantitative information concerning basal concentration
of NO but requires approximately 5-40 seconds for the voltammogram to be
recorded. The relatively long recording time required in DPV limits its
application for in situ measurement of NO (half-life 2-5 seconds). Many
other electroanalytic techniques, including normal pulse voltammetry,
square wave voltammetry, coulometry and double-potential step amperometry
can also be used to measure NO with a porphyrinic sensor. Amperograms and
voltammograms are recorded using a three-electrode system which consists
of an NO-sensor working electrode, a platinum counter electrode and
silver/ silver chloride or calomel electrode as the reference electrode.
The current measured with both techniques is (at constant NO
concentration) controlled by mass transport (laminar flow of diffusion)
of NO to the sensor. Therefore, for accurate determination of the NO
concentration, the sensor has to be calibrated under mass transport
conditions similar to those used for in vivo and in vitro measurement.48
The flow rate of blood in which the NO is monitored has to be measured or
calculated based on the diameter of the blood vessel and the blood
pressure. A stopped-flow system or controlled stirring of the mimetic
solution of the blood (e.g., an aqueous solution of dextran, MW
70,000 with a viscosity of 2.8 cP at 37 C) can be used to calibrate the
NO-sensor response under dynamic flow conditions.
D. PREPARATION AND MEASUREMENT OF
NITRIC OXIDE WITH AMPEROMETRIC SENSORS
Molecules which undergo fast random polymerization on the electrode
surface, like tetraphenylporphyrins with amino or pyrrole substituents on
the phenyl ring, will form films with a well-developed surface but with
poor catalytic properties for NO oxidation and low electrical
conductivity.
Out of more than forty different porphyrinic and non-porphyrinic organic
and inorganic electrocatalysts tested for these properties, only three
show the desired characteristics: tetrakis (3-methoxy-4-
hydroxyphenyl)porphyrin,
m<?.TO-tri(7V-methyl-4-pyridinium)-/}-phenylene-5 '-0-2',3 '-0-
isopropylideneuridine (PUP), and M/V'-bis[5-p-phenylene-10,15,20-tris(3-
methoxy-4-hydroxyphenyl)-porphyrin]-l,10-phenanthroline-4,7-diamide(
1,10-phen)(TMHPP)2.4 Detection limit for NO depends on film quality and
varies between
10 8 and 10" 9 M. The best sensitivity is usually obtained using a PUP
and (l,10-phen)(TMHPP)2 films with Ni(II) as the central metal. If a
reproducibility of preparation procedure is considered (l,10-
phen)(TMHPP)2 Ni(II) is much superior (failure rate 1:10) in comparison
to two other porphyrins.
1. Single-Cell Sensors
Sensors for measurement of NO release from a single isolated cell are
produced by threading a carbon fiber (diameter 7 ) through the pulled
end of a capillary tube with about 1 cm left protruding. Nonconductive
epoxy is put at the glass-fiber interface. After the epoxy cement drawn
into the tip of the capillary has cured, the carbon fiber is sealed in
place. The carbon fiber is then sharpened by gradual burning (propane-air
microbumer, 1000-1100 C). The sharpened fiber is immersed in molten bee
wax-rosin (5:1 w/w) at a controlled temperature for 5-15 s and, after
cooling, is sharpened again. The flame temperature and the distance of
the fiber from the flame need to be carefully controlled. The resulting
electrode is a slim cylinder with a small diameter (0.5-2 ) rather than
a short taper, a geometry that aids in implantation and increases the
active surface area. The tip (length 2-6 ) is the only part of the
carbon fiber where electrochemical processes can occur. For the sensor to
be implanted into a cell, this length must be less than the cell
thickness. The unsharpened end of the fiber is attached to a copper wire
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