Chemistry Reference
In-Depth Information
Fc
Glucose
H
2
O
2
GOx/(FAD)H
2
e
−
Gl
Gluconolactone
l
Fc
+
O
2
GOx/FAD
Electrode
SCHEME 8.7
E
red
S
red
M
red
e
−
M
ox
E
ox
P
ox
Electrode
SCHEME 8.8
An important limitation of the amperometric sensors based on the electrochemical
determination of H
2
O
2
is the high operating potential required for its measurement.
One of the methods aimed tominimize interferences and improve the sensitivity is the
modification of the electrode surface with catalysts that are able to decrease the
overvoltage (Scheme 8.7).
The restricted solubility of oxygen may limit the enzyme reaction and the high
potential necessary causes interfering reactions. One way to circumvent these effects
is to reach direct electron transfer reactions between enzymes and electrodes in
anaerobic conditions using artificial mediators whose role is to shuttle electrons
efficiently between the electrode and enzyme (Scheme 8.8).
Many amperometric enzyme electrodes with electroactive species acting as
mediators replacing the natural electron acceptor, dissolved oxygen, have been
widely investigated. Ferrocene derivatives represent one of the major groups of
mediators owing to their attractive redox properties and fast reaction with the reduced
oxidases and peroxidases [110,111].
8.3.1 Ferrocenyl Silicon-Based Dendrimers
The construction of carbon paste electrodes prepared doping carbon paste with
enzyme and the mediator is simple and in addition, the enzymatic mediator and the
sensing sites are located in close proximity. Monomeric mediators as ferrocene were
initially used as electron-shuttling redox couples but the solubility of the oxidized
form causes instability due to the loss of the mediator from surface elec-
trode [110,112]. Increase of the molecular weight of the mediator should significantly
prevent this loss and ferrocenyl silicon-based dendrimers with relatively high
molecular weights in which the ferrocene/ferrocenyl units are at the end of long
