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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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roughness of the film compared to the active area of platinum electrode
is 1.01, which again confirms the smooth surface of the film. The smooth
film of polymeric (l,10-phen)(TMHPP)2Ni is due to optimal spatial
arrangement of porphyrinic molecules in the film. This is reflected also
on its low resistance to charge transfer (63 ohms and diffusion
coefficient for charge transfer 6 x 10 - 7cm2 s1) which is significantly
lower than that observed for (TMHPP)Ni. The electrical characteristics
combined with high electrocatalytic activity make polymeric (l,10-
phen)(TMHPP)2)Ni very suitable for the development of amperometric
sensors.
236
Malinski
Figure 4. Scanning electron micrograph of a thin film of Ni 1,10-
phen(TMHPP)2 Ni, (a), p-NH2 (TMHPP)Ni (b) and copolymer above two (c)
deposited electrochemically on Pt electrode at constant potential of 0.6
V from 0.1 M NaOH.
The desired properties of polymeric film can also be obtained during
the copolymerization process of two different monomeric properties.17
Polymeric tris (3-methoxy-4-hydroxyphenyl) (p-aminophenyl) porphyrin
Ni(II)(p-NH2)(TMHPP)Ni film obtained by oxidative electropolymerization
shows a rough surface with significant craters and modest R,., and DC1
values (120 ohms, and 8xl0~8cm2s _l respectively). However, when (p-
NH2)(TMHPP)Ni is copolymerized with (1,10-phen)(TMHPP)2)Ni, morphology of
the copolymeric film is relatively smooth with no visible craters (Figure
4c). While the diffusion coefficient for charge transfer is 7 x 10~7cm2s
~', similar to that of polymeric (1,10-phen) (TMHPP)2Ni, the resistance
to charge transfer 55-ohms is significantly lower. Also, a significant
enhancement
in the catalysis was observed for copolymeric film over that for the film
based on its more active constituent (1,10-phen)(TMHPP)2Ni.
B. SENSORS FOR DETECTION OF METAL CATIONS
Application of chemically modified electrodes for trace-metal analysis
offers the advantage of selectivity coupled with the sensitivity
enhancement of preconcentration. Electrode surfaces are designed and
fabricated to preconcentrate a particular metal cation by reaction and
bonding with a specific functional group. The preconcentration, which has
a purely chemical character, can be affected by ion exchange, covalent
linkage, or complex formation reactions. Therefore, selectivity is
determined by the chemical reactivity of the electrode-modifying agents
rather than the redox potential of the cation. This allows construction
and use of sensors specifically optimized for a metal cation of interest.
These sensors do not depend on the electrochemical process performed in
or on the film and therefore, make possible selective preconcentration of
metal ions that are either difficult or impossible to reductively deposit
onto untreated electrodes.
Monomeric porphyrins are thought to metalate and demetalate according
to the overall chemical equilibrium
H2(P) + 2 M(P) + 2H' (I)
where (P) represents porphyrin and M + 2 represents metal. While equation
(1) does not consider mechanistic details of either process, a low pH
environment at the electrode surface would be expected to favor the
reverse reaction. Demetalation, or acid-catalyzed solvolysis, involves
the replacement of a coordinated metal by protons. Metalloporphyrin
stability has been defined as the stability order for the central cations
Pt(ll) > Pd(II) > Ni(II) > Co(II) > Ag(II) >Cu(II)> Fe(II) > Zn(II)>
Mg(II) > Cd(II)>Li(lI) > Na(II) > Ba(II) > K(I).
The best example of a porphyrinic sensor for detection of metal
cations is a sensor for Ni(ll) detection.2X (TMHPP)Ni can be used as an
agent to modify either glassy carbon or carbon-fiber electrodes. This
sensor has suitable properties for practical application for trace-level
determination of Ni(Il) by voltammetric methods and can be used as an
amperometric sensor as well. Initial oxidation of the monomeric metalated
porphyrin units leads to polymerization and the formation of a highly
conductive polymeric film on the electrode surface. The polymer undergoes
facile demetalation in acid solution leaving an intact, adherent,
conductive film on the electrode surface. This demetalated film is then
capable of selective chemical incorporation of Ni(II) cations from
analyte solutions. The analytical signal is the current from the
Ni(II)/Ni(IlI) oxidation (Figure 5a). Because both Ni(II) and Ni(IIl)
cations remain in the porphyrin-film electrode, stripping is not
involved. The nickel (II) sensor can operate in either the voltammetric
or amperometric mode.
The rate of polymerization and morphology of the film formed strongly
depends on the inclusion of the central metal within the porphyrin. Free-
base TMHPP polymerizes
44/Porphyrin-Based Electrochemical Sensors
Copper wire
Carbon fiber Polymeric porphyrin
t
-P-PNi+2-P-

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