106
6 Applications to Biosensors
6.2 Selectivity of Electrochemical Biosensors
The substrate specificity of enzymes is usually high, while the selectivity of the
electrode reaction is quite poor, as evidenced by LFER-type Butler–Volmer equation
(LFER holds only for non-specific reactions). Therefore, once undesirable redox
active species in a sample solution contact with an electrode, the electrode would
show some response to these species. The body fluid contains various types of
electroactive species at unknown and non-constant concentrations. Therefore, for
example, oxygen reduction, dopamine oxidation, ascorbate oxidation, and urate
oxidation will interfere the response of electrochemical biosensors. Therefore,
the detection potential should be set to avoid the interference by these reactions. At typical carbon electrodes, detection potentials of around 0 V versus
Ag|AgCl|KCl(sat.) are suitable to avoid the interferences due to these reactions [3,
8]. On the other hand, in order to avoid the instability in the current detection due
to the changes of the electrode kinetics and the reference electrode potential, the
detection potential must be set in the limiting current region for the target species.
Therefore, the selection of the detection potential is one of very important factors in
the actual analysis.
When the selectivity of the electrode reaction is not guaranteed by the working
potential setting unfortunately, one of typical strategies is the utilization of permselective membranes on the electrode surface [9]. For example, electropolymerized
1,2-diaminobenzene plays as a barrier for the penetration of interfering substrates
and for the leaking of immobilized enzyme [10]. As a simpler method for the formation of permselective membrane, casting of Nafion
® dispersion on enzyme-modified
electrodes is frequently employed and effective to suppress the permeation of ascorbate and urate from test solutions to the electrode [11, 12]. Since Nafion
® is a cationic
exchanger resin, the suppression effect of Nafion
® toward such interfering substrates
seems to be caused by the electrostatic repulsion between the interfering species and
the membrane. It is also important that the membrane does not hinder the permeation
of the target substrate.
6.3 Electrodes for Amperometric Biosensing
Amperometric biosensors are most widely studied in bioelectrocatalysis-based
devices [2, 3, 13]. In the amperometric sensing, the steady-state response must be
required for accuracy reasons. In the case of MET-type amperometric biosensors,
the steady-state current response (i s ) is given as follows (see also Sect. 2.4);
1
i s
=
1
i s,elec
+
1
i s,enz
+
1
i s, perm
+
1
i s,mt
,
(6.1)
6 Applications to Biosensors
6.2 Selectivity of Electrochemical Biosensors
The substrate specificity of enzymes is usually high, while the selectivity of the
electrode reaction is quite poor, as evidenced by LFER-type Butler–Volmer equation
(LFER holds only for non-specific reactions). Therefore, once undesirable redox
active species in a sample solution contact with an electrode, the electrode would
show some response to these species. The body fluid contains various types of
electroactive species at unknown and non-constant concentrations. Therefore, for
example, oxygen reduction, dopamine oxidation, ascorbate oxidation, and urate
oxidation will interfere the response of electrochemical biosensors. Therefore,
the detection potential should be set to avoid the interference by these reactions. At typical carbon electrodes, detection potentials of around 0 V versus
Ag|AgCl|KCl(sat.) are suitable to avoid the interferences due to these reactions [3,
8]. On the other hand, in order to avoid the instability in the current detection due
to the changes of the electrode kinetics and the reference electrode potential, the
detection potential must be set in the limiting current region for the target species.
Therefore, the selection of the detection potential is one of very important factors in
the actual analysis.
When the selectivity of the electrode reaction is not guaranteed by the working
potential setting unfortunately, one of typical strategies is the utilization of permselective membranes on the electrode surface [9]. For example, electropolymerized
1,2-diaminobenzene plays as a barrier for the penetration of interfering substrates
and for the leaking of immobilized enzyme [10]. As a simpler method for the formation of permselective membrane, casting of Nafion
® dispersion on enzyme-modified
electrodes is frequently employed and effective to suppress the permeation of ascorbate and urate from test solutions to the electrode [11, 12]. Since Nafion
® is a cationic
exchanger resin, the suppression effect of Nafion
® toward such interfering substrates
seems to be caused by the electrostatic repulsion between the interfering species and
the membrane. It is also important that the membrane does not hinder the permeation
of the target substrate.
6.3 Electrodes for Amperometric Biosensing
Amperometric biosensors are most widely studied in bioelectrocatalysis-based
devices [2, 3, 13]. In the amperometric sensing, the steady-state response must be
required for accuracy reasons. In the case of MET-type amperometric biosensors,
the steady-state current response (i s ) is given as follows (see also Sect. 2.4);
1
i s
=
1
i s,elec
+
1
i s,enz
+
1
i s, perm
+
1
i s,mt
,
(6.1)