Carbon Paste as an Electrode Material - Electroanalysis with Carbon Paste Electrodes - page 67

Even.the.electron.transfer.reaction.itself.can.lead.to. incorporation/ejection .of.matter.in.or.from.
the.paste..This.is.notably.the.case.when.a.hydrophobic.redox.probe.(i.e.,.ferrocene.derivative).is.intro-
duced.into.the.CPE.bulk.(via.solubilization.into.a.binder. iso -nitrophenyl-octylether),.the.electrochem-
ical.oxidation.of.which.into.positively.charged.ferricinium.derivative.gives.rise.to.anion.transfer.into.
the.organic.phase.(or.ejection.of.the.electrogenerated.ferricinium.derivative).to.maintain.the.charge.
balance.[455,725]..A.similar.example.is.the.ion.transfer.forced.by.generation.of.tert-butylferrocenium.
cation.from.the.neutral.substrate.present.in.a.hydrophobic.room-temperature.ionic.liquid.(1-decyl-3-
methylimidazolium.bis(triluoromethylsulfonylimide)).used.as.the.CPE.binder.[454].
The.combination.of. adsorption/extraction .was.applied.to.accumulate.organic.substances.[82,69],.
environmental.pollutants.[59,726],.BIC.[70,156].or.some.pharmaceuticals.[727,728],.in.view.of.their.
subsequent.electrochemical.detection..For.typical.examples.in.the.latter.category,.for.example,.some.
analgesics.[729],.antimycotics.[730],.antipsychotics.[731],.including.a.group.of.phenothiazines.[219,732],.
extraction.may.dominate.over.surface.adsorption,.which.can.be.exploited.for.their.detection.in.complex.
matrices.with.strongly.adsorbing.constituents..This.is.the.case.of.determining.chlorpromazine.(CLP).
in.the.presence.of.uric.acid.(UA).[732].based.on.simultaneous.accumulation.of.both.compounds.with.
the.subsequent.mechanical.removal.of.the.adsorbed.UA.(for.details.on.this.procedure,.see.Section.8.3).
A.combination.of. extraction with ion pairing .is.typical.in.procedures.employing.some.commercially.
available. ion. exchangers. (e.g.,. Aliquat 336 . [251],. Dowex 50W . [522],. Amberlite LA-2 . [733,734]). and.
surfactants.from.the.family.of.quaternary.ammonium.salts.(CTAB.[735,736],. Septonex ® .[695,736,737]).
or.alkyl.sulfonates.[182,703]..Of.related.interest.is.the.interaction.of.organic.molecules.with.surfactants.
to.form.hydrophobic.assembles.that.are.likely.to.be.accumulated.at.CPEs.and.subsequently.detected.
in.the.same.way.with.great.sensitivity.(examples.are.available.for.l-dopa.[738],.β-nitrostyrene.[739],.
Ciproloxacin . [740],. or. theophylline. [741].. Some. rather. sophisticated. schemes. comprising. several.
physicochemical. and. electrochemical. interactions. were. occasionally. reported.. For. example,. electro-
lytic.deposition.can.be.performed.at.the.electrocatalytically.active.carbon.paste.surface,.both.addition-
ally.combined.with.effective.ion.pairing.[182]..Alternatively,.accumulation.through.ion.pairing.can.be.
accompanied.by. intercalation .[742].or. precipitation .[743]..The.methods.that.use.such.sophisticated.pro-
cesses.usually.offer.high.analytical.performance.with.respect.to.both.selectivity.and.sensitivity.[157],.
thanks.to.the.multiplicity.of.the.individual.processes.combinable.in.a.single.sensing.device.
This.has.led.to.the.determination.of.trace.amounts.of.target.analytes.in.the.presence.of.a.large.
excess.of.potentially.interfering.species.(e.g.,.I − .vs..Cl − .in.a.ratio.1:10 6 .[639,737]).or.to.the.achieve-
ment.of.extremely.low.detection.limits.[182]..The.latter.is.the.case.of.a.method.that.employs.the.
whole.sequence.of.mutually.combined.and.supporting.processes;.see.Figure.3.5.and.its.upper.part..
Namely,.it.is.(i).specially.chosen.carbon.paste.with.TCP.as.chemically.active.binder;.(ii).cathodic.
activation.of.this.C/TCP.giving.rise.to.negatively.charged.functional.groups.at.its.surface;.(iii).elec-
trolytic.accumulation.of.the.analyte.(the.reduction.of.single.ions),.when.the.overall.eficiency.is.fur-
ther.ampliied.by.(iv).additional.transfer.of.the.target.ions.toward.the.electrode.(via.ion.pairing.with.
modiier. in situ )..Moreover,.these.processes.were.likely.accompanied.by.(v).extraction/reextraction.
of.the.ion.associate.formed.before.reduction.(not.shown),.making.the.entire.mechanism.selective.
prior.to.inal.(vi).reoxidation.at.a.favorable.potential.around.0.0.V.vs..ref..with.minimal.background..
As.a.result.[182],.the.procedure.based.on.a.series.of.these.consecutive.and.simultaneous.steps.(ii).
through.(vi).offered.an.extraordinary.sensitivity,.being.capable.of.operating.at.the.picomolar.level.
or.even.below—see.Figure.3.5,.lower.part.
Some.synergistic.mechanisms.can.also.be.found.in.electroanalysis.of.biologically.important.com-
pounds..For.example,.within.the.series.of.momentarily.popular.modiications.by.electrogenerated.
polymeric.ilms.(see.the.following,.in.Section.7.4),.there.is.a.report.on.a.monomer-modiied.bulk.CPE.
[744].that,.after. electropolymerization .by.potential.cycling,.gave.rise.to.a.poly(1-naphthylamine)-
coated.electrode.surface.with.electrocatalytic.activity.toward.glucose.oxidation..Another.interesting.
example.is.the.combination.of.the.so-called. enantioselectivity .of.some.special.modiiers.with.elec-
trochemical.detection.[745],.when.the.corresponding.method.may.include.yet.another.process.(e.g.,.
adsorption)..Enantioselective.sensors.act.as.chiral.differentiators,.and.the.respective.CP.biosensors.