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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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several methods (biochemical assays, UV-Visible spectroscopy, chemilumi-
nescence, EPR) electrochemical methods are considered to be the most
suitable for in situ detection of NO in biological milieu.4-20-37-56"70
Generally, the electrochemical oxidation of nitric oxide on solid
electrodes proceeds via a two-step (EC mechanism) with an electrochemical
reaction as the initial step (heterogenous electron transfer) followed by
a chemical reaction. The first electrochemical step is a one-electron
transfer from a NO molecule to the electrode resulting in the formation
of a nitrosonium ion:
NO 5 NO+ + e" (2)
NO + is a relatively strong Lewis acid and in the presence of OH- is
converted to nitrite (N02):
NO+ + OH HN02 (3)
The rate of the chemical reaction (3) increases with increasing pH.
Because the oxidation potential of nitrite in aqueous solution is only
60-80 mV more positive than that of NO, oxidation of NO on solid
electrodes with scanned potential results in the transfer of two
additional electrons. Thus nitrite is ultimately oxidized to nitrate,
NOJ, the final product of electrochemical oxidation of NO. Therefore, by
242
Malinski
0.1 nA
0.40
0.65
0.90
Potential, V
Figure 7. Differential pulse voltammograms obtained for oxidation of 5
NO on a carbon fiber electrode (a) and carbon fiber electrode covered
with polymeric porphyrin (TMHPP)Ni and Nafion (b).
using a bare metal or carbon electrode and voltammetry, it is impossible
to differentiate nitrite produced electrochemi-cally from that derived
from NO chemical oxidation. In addition, the oxidation of NO on metals
(platinum, gold, etc.) or carbon produces a relatively low current at
neutral pH. This is due to the strong adsorption of NO on the
electrode surface as well as slow electron exchange between NO and the
electrode.
A concentration of 5 NO produces a very small current at about 0.9
V on the carbon-fiber electrode operating in the differential pulse
voltammetry mode (DPV) (Figure 7). However, this same carbon fiber
covered with a layer of polymeric porphyrin produces a much larger
current at peak potential shifted to 0.63 V (about 270 mV in comparison
to bare carbon fiber). The significant shift of potentials indicates that
oxidation of NO on the porphyrin-coated electrode is much easier than in
the bare carbon electrode. In order to prevent oxidation of NO j to NO y,
a barrier has to be placed between the electrode and the analyte. This
has been achieved in the porphyrin sensor by deposing a layer of cation
exchanger (Nafion), which repels negatively charged species such as NO 2
, and also several other anions (like ascorbic anion) which can be
oxidized at similar potential as NO (Figure 8). Nafion also serves to
attract NO+ from the underlying catalytic surface and prevent its further
oxidation to NO 2
The sensitivity and selectivity of the porphyrinic sensor depends not
only on the potential at which NO oxidizes, but also on the catalytic
process of NO oxidation which generates the high current. In addition,
surface effects, the nature of the central metal, NO permeability through
Nafion layer(s) and fast removal of NO + (the oxidation product of NO) by
Nafion are all important in promoting fast and selective oxidation of NO.
The wide difference observed between the sensitivity of porphyrinic
sensor for NO and other readily oxidizable molecules like dopamine is not
related to their relative affinity for axial ligation to the metallated
porphyrin in its Nafion-shrouded environment. The affinity of NO to
Ni(II)/Ni(III) is much lower than its affinity to Fe(II)/Fe(III).
However, a replacement of nickel with iron does not improve the
sensitivity of the sensor. This is an indicator that axial ligation of NO
to the central metal is not a crucial factor affecting the sensor's
sensitivity. Generally, the sensitivity of the porphyrinic sensor is the
highest with nickel as the central metal and decreases
Copper wire
Carbon fiber-------->
Polymeric porphyrin " Nafion ---------------->
Aqueous phase------>

N02-, ascorbic anion, etc.
Figure 8. Schematic diagram of electrochemical nitric oxide sensor based
on conductive polymeric porphyrin.
44/Porphyrin-Based Electrochemical Sensors
243
significantly for other metals in the following order Ni > Fe " Co > Cu
> Zn. A signal generated during NO oxidation on poorly defined (high
resisitivity) polymeric porphyrin film is low and does not depend on
central metal.71-72 Typical current density for NO oxidation on a
porphyrinic sensor is
0.3-1.8 mA cm-2 -1 and at least four to seven times higher than that
which can be obtained on an activated carbon fiber covered with Nafion.
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