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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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metal can change dramatically. Porphyrins are insulators when absorbed on
the surface of a bare electrode. But metalloporphyrins are conductive
when they are electropolymerized to form thin films on the surface of a
bare electrode. The intrinsic redox and ionic conductivity of polymeric
metalloporphyrins combined with their extensive conjugated 7t-system
electronic conductivity provide significant observed conductivity,
approaching that of metals; hence, they belong to a family of conductors
called molecular metals.1112
Currently available amperometric and voltammetric porphyrinic sensors
for detection of electroactive analytes are based on their
electrochemical oxidation or reduction on polymeric conductive films of
metalloporphyrins. If the current generated during the process is
linearly proportional to the concentration of an analyte, the current can
be used as an analytic signal. This current can be measured in either the
amperometric or the voltammetric mode. In the amperometric mode, the
current is measured while the potential applied to the sensor is fixed at
a level where the current from the redox reaction is at its peak. In the
voltammetric mode, the current is measured while the potential is
linearly scanned across a region that includes the redox reaction. Both
methods of measurement provide a quantitative signal while the
amperometric method is faster.
Recently, promising anion-selective potentiometric sensors based on
the use of nonconductive, monomeric metalloporphyrins as membrane active
components have been developed.13-16 When these reagents are incorporated
into plasticized nonconductive polymeric membranes, the resulting sensors
display significantly different selectivity then those observed for the
membranes doped with classical anion exchangers like tetraalkylammonium
or phosphonium salts. The potentiometric response of these metalloporphy-
rins-doped membranes involve direct interaction of the anion as an axial
ligand of the central metal. With potentiometric porphyrinic sensors,
substitution on the porphyrin ring and coordination of different central
metals to the porphyrin ring can influence the ability of the analyte to
bind as an axial ligand, thereby altering the sensor selectivity. Again,
the broad possibilities for changing a porphyrin structure, usually
followed by a change of its coordination chemistry, permit the design of
highly specific potentiometric sensors.
II. Amperometric/Voltammetric Porphyrin-Based Sensors
The feasibility of metalloporphyrins for use as electrocatalysts and
their use in amperometric sensors and biosensors has been firmly
established.5-7'17-23 The discovery that several transition-metal
porphyrins can reduce oxygen directly, by a four-electron pathway,
suggested that other heterogenous electrocatalytic reductions or
oxidations of small molecules are also possible. In order for
heterogenous electrocatalysis to be possible, successful attachment of
the metalloporphyrins to solid electrodes is achieved by a variety of
techniques including chemisorp-tion, chemical reactions with previously
functionalized electrodes, chemical reactions with functionalized polymer
incorporation of a monomeric porphyrin within a conductive polymer film
and electrochemical polymerization. Electrochemical polymerization offers
unique advantages in porphyrinic polymer film synthesis because it
produces the direct immobilization of stable, electrically conductive
films on solid electrodes. In addition, the morphology of the layers as
well as adherent film thickness can be precisely controlled. Film
thickness of conductive polymeric films can vary from one to several
hundred monolayers. The simplicity in the preparation of these polymeric
films is a salient feature, especially when considering the challenging
task of designing new polymeric porphyrinic films.
With the knowledge that aniline, pyrrole and phenol groups may be
involved in the formation of electroactive
44/Porphyrin-Based Electrochemical Sensors
polymeric films, research in the amino-, pyrrole-, and hydroxy-
substituted porphyrins became important, especially for those forming
films via oxidative or reductive electropolymerization.7'9-u'25 In
addition, it has been shown that electropolymerization of conductive
polymeric films can readily occur from porphyrins substituted with (2.2)-
paracyclophane, as well as from several diporphyrin species in which
1,10-phenanthroline acts as a link between the various porphyrins and is
attached by carboxamide-p-phenylene bridges at the 4- and 7- positions of
phenanthro-line.6-17-22-26 Many of the polymeric films cited above
demonstrated catalytic properties in the four-electron reduction of
oxygen directly to water, as well as in the oxidation of several other
small molecules like nitric oxide, hydrazine and methanol.11-17'20'25
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