Biomedical Engineering Reference
In-Depth Information
Scheme 4 Type of
complexes proposed
K
16
·Cd(NO
3
)
2
+
K
17
·M(NO
3
)
2
, M=Cd, Zn
(A1) and
0.61 V (A3). The latter peak disappears in CVs if the potential is
reversed at potentials close to
1.25 V, suggesting that peak A3 corresponds to
the oxidation of any species generated during electrode process C2 (Fig.
17b
).
The observed response is described on the basis of the well-known electrochem-
istry of the aromatic and nitroaromatic compounds in aprotic solvents, consisting of
two successive one-electron-transfer processes yielding an anion radical and a
dianion. The voltammetric profile, however, depends largely on the intermediate
radical anion's stability. If this is unstable if compared with the disproportio-
nateness into dianion and the parent aromatic compound, the voltammogram
resembles a single two-electron wave. This is the case of the studied receptors;
here the obtained half-peak width value of process C1, measured in square wave
voltammetries (SQWVs) as 115 mV, is clearly lower than that expected for a
reversible one-electron process (126 mV). The overall electrochemical process
can be represented as:
2
e
L
2
:
L
þ
!
As the SQWVs show in Fig.
18
, noncoordinated 17 displays isolated peaks C
1
and C
2
. On addition of Cd
2+
, additional peaks appear at
0.49 (C
5
) and
1.64 V
(C
6
), whereas peak C
1
is resolved in two peaks overlapping at
1.03 V.
Electrode process C
5
is described in terms of the biphenyl-centered reduction of the
complex. This is represented as:
0.92 and
2
e
xM
2
þ
þ
M
2
þ
Þ
x
ð
L
2
Þ
:
L
þ
!ð
Electrode process C
6
is ascribed to metal-centered reduction:
M
2
þ
Þ
x
ð
L
2
Þþ
2x
e
xM
0
L
2
:
ð
!
þ
This process takes place at considerably more negative potentials than reduction
of uncomplexed Cd
2+
, which is as expected for the electrochemical reduction of
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