Chemistry Reference
In-Depth Information
e
-
e
-
e
-
M
Red
M
Ox
e
-
e
-
M
Red
M
Ox
S
Ox
S
Ox
e
-
S
Red
e
-
S
Red
e
-
(A)
(B)
FIGURE 4.4
Mediation.of.the.oxidation.(A).and.reduction.(B).of.a.substrate.
S
.by.a.modiier.
M
.
The.reason.for.designing.an.electrocatalytically.active.electrode.surface.is.mainly.the.improve-
ment.of.the.electron.transfer.toward.the.analyte,.which.can.be.accomplished.with.some.envisaged.
follow-up.effects,.such.as
•.
Generation.of.electroactivity.of.the.substrate.at.the.applied.potential
•.
Improvement.of.the.electrochemical.signal,.either.in.shape,.height,.or.potential
•.
Separation.of.otherwise.overlapping.signals.and.elimination.of.interferences
•.
Improvement.of.linear.dynamic.ranges.of.the.response.or.of.the.detection.limit
•.
Improvement.of.the.general.analytical.situation.by.decreasing.matrix.effects
Modiiers.that.are.able.to.facilitate.the.aforementioned.tasks.may.be.quite.different.in.their.chemi-
cal.nature..Most.frequently.used.mediators.are.metals,.metal.complexes.and.salts,.polymers.with.
redox. properties,. and. recently. nanosized. materials,. such. as. nanoparticles. (NPs),. nanorods,. and.
nanotubes.. In. fact,. the. electrochemical. generation. of. functional. groups. at. the. surface. of. carbon.
particles.by.application.of.a.highly.positive.potential.may.also.be.classiied.as.electrocatalytically.
active.modiication.
Metals
..Compact.metallic.elements.used.to.enhance.the.catalytic.activity.of.carbon.paste.electrodes.
belong.often.to.the.platinum.metal.group.and.comprise.Pt,.Pd,.Ru,.or.Rh.[838];.they.can.be.used.
for.the.catalytic.oxidation.of.hydrogen.peroxide,.hydrazines,.or.related.compounds.[839]..The.met-
als.may.be.applied.in.dispersed.form.but.can.also.be.plated.on.the.carbon.particles..Nevertheless,.
metals.of.less.noble.electrochemical.character.can.also.be.used.for.speciic.applications.[840],.such.
as.nickel.for.the.oxidation.of.methanol.[841]..More.recently.applications.with.NPs.are.becoming.
dominating.
Metal oxides
..These.binary.compounds.are.useful.catalytic.mediators.if.they.exist.in.various.oxida-
tion.states;.thus,.MnO
2
,.FeO,.Fe
2
O
3
,.SnO
2
,.RuO
2
,.ReO
2
.[842],.and.similar.compounds.are.useful.
for.the.determination.of.corresponding.substrates..In.many.cases,.the.target.is.hydrogen.peroxide,.
which.shows.high.overpotentials.to.both.oxidation.and.reduction..Hydrogen.peroxide.is.of.particular.
interest.because.it.may.occur.as.intermediate.in.enzymatic.reactions.of.many.oxidases..However,.
other.substrates.such.as.amino.acids.can.also.be.detected.[556]..Oxides.as.modiiers.attract.increas-
ing.attention.in.the.form.of.NPs.with.usually.improved.action.over.the.macro-.or.microsized.powder.
Metal complexes and salts
..The.biggest.group.of.electrocatalytically.active.modiiers.is.made.by.
metal.complexes.or.salts..Hexacyanoferrates.and.analogues.with.platinum.metals.or.nickel.(for.the.
detection.of.peroxides).[196,843-852],.also.coupled.synergistically.with.carbon.nanotubes.[360,853],.
ferrocenes.(as.electron.transfer.mediator.in.biosensors.mainly).[785,854-881],.and.phthalocyanines.
(Co
II
,. Fe
II
,. Ni
II
,. Mo
VI
. salts. for. the. detection. of. peroxides,. thio-compounds,. oxalic. acid,. phenols,.
ephedrine,.vitamin.B1,.pesticides,.and.sugars.[557,651,652,655,765,882-919]).are.most.frequently.
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