Chemistry Reference
In-Depth Information
0.0
no H
2
PO
4
−
−
0.2
7 x 10
−
6
M
120
−
0.4
100
−
5
80
5
x
10
M
60
−
0.6
40
20
2 x 10
−
4
M
−
0.8
0
0
25
50
75
100
125
150
175
200
225
−
1.0
[H
2
PO
−
](µM)
1.10
0.90
0.70
0.50
0.30
0.10
E
(V) vs. SCE
FIGURE 8.11
OSWV curves for a GC electrode modified with poly-
25
measured in H
2
O/Li
[B(C
6
F
5
)
4
] in the absence and presence of different concentrations of H
2
PO
4
. Shift of the
oxidation peak potential. Adapted from Mart´nez et al. [45].
investigations of the ferrocenylamidoalkyl dendrimers show their ability to recognize
and sense anionic species. Robust modified electrodes can be successfully prepared
with the dendrimers by different procedures and their voltammetric response is also
sensitive to thepresenceandconcentrationof anions inbothorganicandaqueousmedia.
The electrochemical behavior of this dendritic family has been investigated by cyclic
voltammetry, normal pulse voltammetry, andOsteryoung square-wave voltammetry of
thematerials in homogeneous solution, aswell as confined to electrode surfaces. For all
generations a single redox process is observed in CH
2
Cl
2
indicating that the multiple
ferrocenyl units are seemingly identical and sufficiently remote from one another to
render the electrostatic factor negligible [53].
The electrochemical behavior of the dendrimers is strongly affected by the
addition of increasing amounts of H
2
PO
4
anion. A “two-wave” behavior is observed
both in solution as well as in the modified electrodes (Figure 8.12A) [54]. This is
attributed to a large increase in the association constants on oxidation, due to the effect
of the synergy between the H-bonding and strong electrostatic interactions between
H
2
PO
4
and the oxidized ferricinium, enhanced by the dendritic structure that
improve the electrochemical sensing properties toward the dihydrogenphosphate
anion. A progressive decrease of the initial Fc/Fc
þ
wave is observed along with the
growth at less positive potentials of a new peaks system (Figure 8.12A curves b-d).
The binding of anions effectively stabilizes the positive charge of the oxidized film,
shifting the redox potential (E
p
) of the Fc/Fc
þ
system to less positive values until the
potential peak reaches a constant value. The intensity of the peaks is depressed in the
presence of H
2
PO
4
ions, even at low concentrations. The loss of electroactivity has
been attributed to the doping of the films that is responsible for a restricted transport of
counter ions due to the formation of strong ion-pairs. This leads to a decrease in the
rate of charge propagation that is controlled by counter ion diffusivity in the films and
ion-trapping effects. Calibration graphs have been established upon the analysis of the
OSWV curves for electrodes modified with the films of dendrimers as function of
the [n-Bu
4
N][H
2
PO
4
] concentration and it has been concluded that the strength of the
hydrogen bonding and electrostatic interactions between the amidoferrocene groups
