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In-Depth Information
Reagent
Substrate
Substrate
Mediator
M
Ox
S
Red
R
Ox
Electro-
chemical
oxidation
(signal)
Electro-
chemical
re-oxidation
(signal)
S
Red
Catalytic
cycle
R
Ox
S
Ox
S
Ox
M
Red
(A)
Solution
CPE
(B)
Solution
CPE
FIGURE 5.2
(A). Electrochemical. mediation. and. (B). electrochemical. catalysis. (“electrocatalysis”). in. a.
closer.sense.
unchanged,. but. the. signal. is. magniied,. and. apparent. effective. cycle. numbers. may. be. evaluated..
Both.cases.may.improve.the.analytical.characteristics.of.the.method.signiicantly.with.respect.to.
interferences.or.detection.limit.
Nevertheless,.direct.mediation.of.the.analyte.will.still.show.quite.a.high.degree.of.nonspeciic-
ity. because. any. substances. reacting. with. the. meadiator. will. inluence. the. response. of. the. target.
compound..Electrocatalysis.may.increase.the.current,.but.will.still.be.the.subject.of.interferences.
if.the.corresponding.potentials.for.the.direct.measurement.are.to.be.set.rather.positive.or.negative.
5.2.2 e
nzyMes
The.unsatisfying.analytical.situation.where.the.direct.or.even.electrocatalytic.electrochemical.con-
version.of.the.analyte.lacks.speciicity.can.be.very.much.improved.by.the.use.of.an.enzyme.that.
speciically.attacks.one.single.or.a.group.of.similar.target.compounds..The.resulting.sensors.are.
termed.as.enzyme.electrodes.and.may.show.quite.high.speciicity.for.certain.analytes.depending.on.
the.nature.of.the.enzyme.used.
As.most.of.the.biosensors.described.in.the.literature.are.amperometric.or.voltammetric.devices,.
it.is.evident.that.the.preferred.types.of.enzymes.used.are.the.ones.that.change.the.oxidation.state.of.
the.substrate,.that.is,.oxidases.and.dehydrogenases..Less.frequently,.peroxidases.are.used.and.only.
occasionally.catalases..In.fact,.all.these.enzymes.belong.to.the.group.of.oxidoreductases..Their.way.
of.action.is.depicted.in.Figure.5.3.
Substrate
Enzyme
Substrate
Electro-
chemically
inactive
H
2
O
S
Red
Ox
Red
Inhibition
Co-
substrate
Coenzyme
cofactor
Enzyme
Electro-
chemical
signal
Ox
Red
S
Ox
Products
P
1
P
2
(A)
(B)
FIGURE 5.3
Way.of.action.of.oxidoreductases.(A).and.hydrolases.(B).
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