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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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interactions. In more recent studies, Sessler et al. have found that
sapphyrins appended to silica gel show high affinities for phosphate and
arsenate, as well as an ability to separate a wide range of anionic
species when used as HPLC solid phases.242 These types of materials may
be applicable to more specific anion-sensing applications.
b. Cation Detection
While detection of cations via porphyrin-based materials has been
explored less than anion sensing, the ability of a porphyrin to
coordinate different metals and the unique spectral signatures that
result form the basis for metal ion detection. Use of free-base
porphyrins in polymer matrices has allowed for the detection of heavy
metal ions by Ache et al.243 Immobilization of 5,10,15,20-tetrakis(4-N-
methylpyridyl)porphyrin on Nation membranes permitted detection of
cadmium and mercury in solution with detection limits of 5 x 10~8Mand2 x
10 " 7 M, respectively over a 20-minute measuring period. The method is
subject to interferences from other metal ions, but the researchers were
able to detect several ions simultaneously using pattem-recognition
techniques such as principal component analysis. Sol-gel films doped with
5,10, l5,20-tetra(p-suIfonatophenyDporphyrin have also been used by Ache
and coworkers for the fluorimetric determination of mercury in solution,
with a detection limit of approximately 7 x lO^M.244
Porphyrin-doped electrodes have been employed for the electrochemical
and potentiometric detection of nickel(II) in aqueous solutions. Malinski
and coworkers have utilized polymeric tetrakis(3-methoxy-4-
hydroxyphenyl)porphyrin, H2(TMHPP), film electrodes for nickel
determination.24^ The films are grown by electrochemical deposition from
a solution of 0.1 M NaOH containing 5 x 10 4 M Ni(TMHPP). The film is
then demetalated in acid solution, yielding a polymer that the authors
postulate to have enhanced selectivity for nickel cations due to its
formation from the nickel monomer. Using differential pulse voltam-metry
to monitor the Ni(II) / Ni(III) oxidation, a detection limit of 8 x 10 "8
M was obtained for a 60-second exposure to a nickel containing solution.
The calibration curve shown in Figure 136 shows linear response over a
range of 10 " 7 to
2 x 10 " 6 M. In the presence of interferences, for example, zinc(II),
cadmium(II) or copper(II), some reduction in signal is seen; however, no
ion showed redox activity in the region of detection. Gupta and coworkers
have utilized H2(TPP)and H2(TTP) in PVC-based membranes for the
potentiometric detection of nickel(II).246 The demonstrated linear
response in the range of 5.6 x 10
1.0 x 10 1 M, with a response time of 20 seconds. The primary observed
interferences were Co- and Na 1 ions. Quantitative determination of Ni ~
1 in chocolate samples using this detector gave good agreement with
values obtained from atomic absorption spectroscopy.
c. Sensors for Organic Molecules
Many current organic molecular sensors employing metalloporphyrins are
based upon the ability of the

sensor

- e
124
Chou et al.
Mn(TPP)L

Mn(TMP)L
Mn(TXNP)CI
Mn(TDDPP)CI
Mn(TDDIP)CI
Figure 133. Structures of quaternary ammonium and manganese
metalloporphyrin species used for membrane preparation.
porphyrin to act as a catalyst for the reduction or oxidation of organic
substrates. This concept has been used by Saini and coworkers, who have
recently used porphyrin-modified electrodes for the determination of
organohalide species in aqueous solution.247 By precipitating (TPP)Co on
a graphite-foil working electrode, the researchers were able to induce
the reduction of various organohalides, such as carbon tetrachloride,
perchloroethylene and iodobenzene. The sensor is believed to function via
a porphyrin-mediated, two-electron-reduction pathway shown below:
Co11 (TPP) + e" -* Co1 (TPP) -
Co1 (TPP) " + RX -> Co1 (TPP)(R) + X"
Co '(TPP)(R) + e -> Co "(TPP) + R~ (5)
Amperometric studies, with a working potential of - 500 mV, gave a
detection limit of 0.75 ppm for carbon tetrachloride. The amperometric
calibration curves for carbon tetrachloride and perchloroethylene are
shown in Figure 137. Greatest sensitivities were seen for haloalkanes,
with less sensitivity for haloalkenes or haloarenes. Interference from
nonhalogenated organics (e.g, ethanol, allyl alcohol or phenol) was not
seen, although the addition of phosphate ion gave slight decreases in
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