Chemistry Reference
In-Depth Information
H
2
PO
4
anion causes a cathodic shift and a decrease in the intensity of the initial redox
wave (I) along with the appearance of a new redox system at more negative potentials
(II). The presence of the sulfate anion in the absence of H
2
PO
4
causes a small
potential shift, however it perturbs significantly the dihydrogenphosphate anion
determination. On the other hand, H
2
PO
4
can be unambiguously detected in the
presence of a 10-fold excess of Cl
and/or Br
salts.
Astruc and coworkers have reported the specificity of dendritic cationic aminoar-
ene-iron sandwich complexes for halides [106]. Whereas the amidoferrocenyl
dendrimers recognize the oxoanions but not the halides (see above), dendrimers
terminated with [Fe(
6
-C
6
H
5
NH-dendr)]
þ
recognize chloride and bro-
mide but not the oxoanions. The polycationic metallodendrimers show large dendritic
effects for recognition of Cl
and Br
according to
1
H NMR spectroscopy.
Dendrimers functionalized with [CpFe(
5
-C
5
Me
5
)(
Z
Z
m
3
-CO)]
4
clusters at the periphery have
also been applied as selective redox sensors for oxoanions including ATP
2
,by
monitoring the shifts of their single reversiblewave for the redox change Fe
4
!
Fe
4
þ
.
In contrast to ferrocenyl dendrimers, the largest cathodic shift is observed in the
presence of ATP
2
. This result is explained on the basis of a more facile interaction
between the Fe
4
clusters and ATP
2
, which are more similar in size, than between
ferrocene and ATP
2
[107].
8.3 ORGANOMETALLIC DENDRIMERS IN THE CONSTRUCTION
OF BIOSENSORS
In a biosensor a bioactive molecule is used in close contact with a transducing
element. The bioactive molecule recognizes the presence of a substrate and the
biochemical signal obtained from the sensing device, which reflects the concentration
of substrate, is converted into an electronic signal by the transducer. The specificity of
the enzymes enables sensors incorporating enzymes to achieve much greater selec-
tivity. Enzyme electrodes result from the combination of any type of electrochemical
sensor with a layer of immobilized enzyme in close proximity to the active surface of
the transducer [108,109]. The analyte can be either the substrate or a product of the
enzyme reaction. The redox enzymes have great interest especially in amperometric
biosensors in which an externally applied potential is employed to drive the electrode
reaction. When an oxidoreductase is used, the reaction implies oxygen consumption
according to Scheme 8.6. Since oxygen reoxidizes the enzyme and produces
hydrogen peroxide, the reaction may be monitorized either through the oxygen
uptake or through the formation of hydrogen peroxide.
S
E
red
H
2
O
2
O
2
P
E
ox
SCHEME 8.6
