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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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Scan rate 5 mV s "1
Voltage range 0.20-0.60 V
Amplitude 40 mV
tions (10_5-10-3 M) and thick films (5-20 equivalent monolayers) have to
be considered. A 10-monolayer film became saturated after immersion in 10
"3 M Ni(II) for 1 hour. The conditions for preparation of sensor and
measurements of Ni(II) are listed in Table 1.
1. Sensitivity and Interferences
The sensitivity of the sensor is relatively high with a detection limit
of 8x10"8 M. The use of polymeric porphyrin sensors for the detection of
cations might be hampered by types of interference not commonly
encountered in anodic and cathodic stripping voltammetry. In particular,
metal cations that can form stronger complexes than Ni(II) with
porphyrins might block the coordination centers in the polymeric
porphyrin film. Furthermore, these interfering metal ions may be
incorporated into, and diffuse through, the polymeric film. The latter
can be hindered by steric effects as well as hydration of the cation.
Almost every metal can be forced to form some kind of complex with
porphyrins. Synthetic mechanisms of metal incorporation involve ring
deformation, effective collisions, initial monoanion formation and
involvement of a receptor for the displaced protons.31 The chemical
conditions in the polymeric TMHPP film favor the formation of the Ni(II)
complex over all other metal ions. This is not just another example of
chemical serendipity. Re-incorporation of Ni(II) into polymeric TMHPP
film is an example of a sterically controlled substitution process. In
this case, any deformation or steric control is there prior to
demetalation, due to the rigidity of the film. High selectivity for
Ni(II) would be expected because the site was produced by Ni(II) leaving.
Similar concentrations of cations Zn, Cd, Co, Pb, Cu and Fe did not
appreciably influence the Ni(II) response. A 10-fold excess of Co results
in partial suppression of the Ni(II) signal. However, it changes the
detection limit of Ni(II) to
about 10" 6 M. Although no voltammetric peak due to the oxidation of
Co(II) to Co(III) incorporated in the polymeric TMHPP is observed.
2. Microsensor for Nickel Measurement in a Single Cell
Chemical preconcentration of metal cations by the polymeric porphyrin is
achieved by an ion-exchange mechanism in which two hydrogens of the
porphyrin free base are selectively replaced with the metal cation,
resulting in the incorporation of metal in the polymer. The ion-exchange
process depends not only on the time of exposure, film thickness and
concentration of metal ions in solution, but also on the area of the
sensor exposed to the analyte medium. The diameter and thickness of the
biological cell being investigated will limit the size and area of ion-
exchange sensor.32 Therefore, electrodes should always be fabricated with
dimensions determined by the particular biological cell being
investigated.
Carbon fibers with diameters of 6-10 צע have to be sharpened in order
to avoid cell-membrane rupture during the implantation process.33 The
traditional procedure for sharpening carbon fibers is by gradually
burning the fiber in the thermal gradient obtained with a microbumer or
by heating with an electrically generated plasma. However, for the
polymeric sensor, the carbon fiber is only a conductive support for the
ion-exchange polymeric film, that is, the ion-sensitive part of the
microsensor. In order to achieve a well-defined area of ion-selective
polymeric porphyrin, the remainder of the carbon fiber on which the
material is deposited has to be isolated from the solution of monomeric
porphyrin during its deposition. This isolation can be achieved by waxing
the electrode.
A three-step procedure is used in sensor fabrication. First, the
carbon fiber is sharpened using a microbumer. In
44/Porphyrin-Based Electrochemical Sensors
239
the second step the sharpened fiber is immersed in molten bee wax (90%
bee wax and 10% rosin) and kept at a controlled temperature for 5-15
seconds. After cooling to room temperature, the electrode is sharpened
again. During the burning, the flame temperature and the distance of the
electrode from the center of the flame need to be carefully controlled by
using a micromanipulator. Carbon fibers prepared according to this
procedure are cone shaped with a small tip diameter to aid implantation.
The classical procedure for fabricating microelectrodes, by inserting
a carbon fiber into a glass microcapillary followed by polishing, cannot
be applied. The diameter of the electrode produced includes the glass and
fiber, making it too large for insertion into single cells with a
diameter smaller than 10 צע. Because the electroactive area is limited to
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