Chemistry Reference
In-Depth Information
4
Chemically Modiied
Carbon Paste Electrodes
The.functionality.of.all.electrode.materials.is.limited.with.respect.to.their.potential.window.avail-
able..On.the.cathodic.side,.a.reduction.in.the.cation.of.the.supporting.electrolyte.results.in.large.
currents,.whereas.at.the.anodic.side,.the.oxidation.processes.with.the.respective.responses—either.
of.the.electrode.material.itself.or.of.the.anion.of.the.electrolyte—would.overlap.the.signal.of.inter-
est..Thus,.in.voltammetric.or.amperometric.measurements,.the.analytes.can.basically.be.monitored.
if.they.are.electroactive.and.their.responses.obtainable.within.the.operational.potential.range..Apart.
from.sorption.effects,.the.reactivity.of.the.analyte.is.then.limited.mainly.to.electrochemical.trans-
formations.at.the.electrode.surface,.that.is,.reactions.connected.with.the.electron.transfer.
The. concept. of. “chemically. modiied”. electrodes. (CMEs). appeared. in. the. 1970s,. propagated.
by.Murray.et.al..[756-758].and.later.also.by.Wang.[759]..In.fact,.chemical.modiication.of.carbon.
paste.had.already.been.performed.in.the.1960s.by.Kuwana.and.French.[14],.and.the.term.
chemi-
cally modiied electrode
.was.coined.around.1975.with.different.types.of.electrode.materials;.irst.
publications. on. explicitly. “modiied”. graphite. electrodes. dealt. with. the. electrosynthesis. of. opti-
cally.active.compounds.[760,761]..One.of.the.irst.publications.dealing.with.such.
modiied carbon
paste electrodes
.concerned.with.experimentally.rather.complicated.chemical.immobilization.of.the.
amino.functional.groups.into.the.carbon.paste.bulk;.nevertheless,.the.resultant.electrode.was.found.
particularly.powerful.for.the.determination.of.silver.at.subnanomolar.level.[51].
Initially,.the.term.was.quite.liberally.applied.also.to.the.electrodes.that.could.somehow.be.treated.
chemically..In.today's.understanding,.CMEs.are.electrodes.also.possessing.some.chemical.reactiv-
ity.besides.their.ability.to.transfer.the.electrons.to.or.from.the.analyte,.which.becomes.possible.by.
the. presence. of. chemically. reactive. groups. or. substances. at. the. electrode. surface.. Consequently,.
besides.electrochemical.transformations,. the.analytes.may.undergo.some.chemical.reactions.that.
principally.widen.the.applicability.of.each.electrode.material.
Although.the.modiication.of.solid.homogeneous.electrode.materials.is.usually.a.labor-intensive.
task,.heterogeneous.materials—in.particular,.carbon.paste.mixtures—can.be.easily.modiied.not.
only.at.the.electrode.surface.but.also.in.the.whole.bulk.with.comparable.effectiveness..This.is.one.
of.the.reasons.why.heterogeneous.electrodes.have.become.so.popular.over.the.past.three.decades.
4.1 CLASSIFICATIONS OF CHEMICAL MODIFICATION
Chemically. modiied. carbon. paste. electrodes. may. be. classiied. by. different. criteria. such. as. (a-i).
type.and.method.of.modiication.and.(a-ii).location.of.the.modiier..Basically,.one.can.differentiate.
between.
intrinsic
.and.
extrinsic modiication
,.depending.on.whether.the.components.of.the.paste.
(conductive.particles.or.pasting.liquid).bear.modifying.functional.groups.or.the.modiier.has.been.
added.to.the.material.as.a.further.component..Another.way.is.to.discriminate.between.(b-i).surface-.
and.(b-ii).bulk.modiication.based.on.the.placement.of.the.modifying.agent.at.or.in.the.electrode..
In.the.irst.case,.the.presence.of.chemically.active.components.is.restricted.to.the.electrode.surface,.
whereas.the.latter.employs.the.whole.bulk.(interior).of.electrode.material.modiied.
A.third.aspect.is.(c).time.of.modiication,.whether.it.occurs.
during the measurement
.or.such.a.
process.
precedes
.it;.the.latter.is.the.case.of.the.electrolytic.formation.of.a.metal.ilm.on.the.support.
or.substrate.electrode.prior.to.its.use.in.the.MeFE.coniguration..In.this.respect,.we.may.distinguish.
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