Carbon Paste as an Electrode Material - Electroanalysis with Carbon Paste Electrodes - page 58

Note: . For.instance,.a.research.group.dealing.with.modeling.the.processes.at.CP.biosensors.intro-
duced.[574,643,644].and.later.eagerly.advocated.[645].a.hypothesis.based.on.the.so-called. pinhole
mechanism ..This.theory.assumes.that.the.binder.forms.a.uniform.thin.layer.or.ilm.coating.inti-
mately.the.individual.graphite.particles.and.consisting.of.many.miniature.holes.(with.diameter.in.
the. micrometer. range).. Reportedly,. such. a. perforated. structure. then. enables. effective. charge. and.
mass.transfer.when.the.species.involved.in.electrode.reactions.may.pass.freely.through.the.holes.
to.reach.the.active.sites.at.the.graphite.particles.and.beneath.the.liquid.layer..The.model.also.tries.
to.explain.the.effect.of.surface.treatments.and.the.proper.alteration.to.more.hydrophilic.status.by.
means.of.enlargement.of.the.individual.holes.after.activation..It.is.speculated.that.the.entire.process.
would. depend. primarily. on. the. activation. conditions. selected,. whereas,. for. example,. the. carbon-
to-pasting. liquid. is. of. lesser. importance.. In. addition,. it. is. elucidated. that. the. more. intensive. the.
activation,.the.larger.the.holes.formed,.which.may.expose.the.hydrophilic.graphite.surface.to.action.
in. more. extent.. Despite. the. logical. deductions. behind. the. whole. model,. the. pinhole. mechanism.
based.mainly.on.mathematic.modeling.has.not.been.conirmed.experimentally,.and.only.a.few.sub-
stances.were.found.to.match.the.theoretical.predictions.(e.g.,.highly.hydrophilic.ferricyanide.ion,.
[Fe(CN) 6 ] 3− )..Such.insuficient.veriication,.as.well.as.the.fact.that,.for.example,.none.of.the.power-
ful.microscopic.techniques.was.able.to.reveal.and.conirm.the.existence.of.perforation.in.the.binder.
layer,.seems.to.be.the.reason.why.the.pinhole.theory.has.not.gained.further.support.and.remained.
abandoned..On.the.other.hand,.there.have.been.some.interpretations.based.on.spontaneous.diffusion.
or.penetration.through.the.binder.layer.that.may.remind.the.pin-hole.mechanism.used.to.deine.the.
role.of.the.binder.in.the.behavior.of.CP-based.amperometric.and.potentiometric.sensors.assuming.
either.an.ensemble.of.miniature.membranes.[276].or—more.simply—a.liquid.membrane.[159],.both.
ensuring.eficient.and.relatively.rapid.mass.transport.
Speciic. kinetics. at. carbon. pastes. was. experimentally. veriied. for. a. number. of. organic. com-
pounds.(see.[2,3].and.Section.7.1),.which.resulted.in.some.practical.advice.on.how.to.accomplish.
the.measurements.in.electrochemical.practice..As.a.consequence.of.moderated.reaction.kinetics.
and.the.corresponding. overpotential .( overvoltage ,.Δ E ),.oxidation.peaks.of.various.organic.sub-
stances.appear.at.highly.positive.potentials,.often.close.to.the.anodic.limit.(with.the.starting.elec-
trolyte.decomposition)..In.such.cases,.the.respective.signals.suffer.from.a.very.high.background,.
deteriorating. the. quality. of. measurements. with. respect. to. unfavorable. signal-to-noise. ratio. and.
resulting,. naturally,. in. lower. selectivity. and. sensitivity.. Moreover,. as. shown. earlier,. such. unfa-
vorable.behavior.can.be.principally.changed—and,.in.certain.limits,.also.controlled—by.means.
of. activation procedures . that. enable. to. substantially. lower. the. overpotential. at. the. carbon. paste.
surfaces.
Then,.due.to.altered.reaction.kinetics.at.hydrophilized.CPEs,.the.signals.of.interest.are.shifted.
toward. less. positive. potentials. and. out. of. rather. noisy. potential. regions. nearby. the. anodic. limit,.
which.is.suficient.to.bring.the.desired.effect—a.marked.improvement.in.the.overall.signal-to-noise.
characteristics.
In.electroanalysis.with.CPEs,.beneicial.effect.of.activation—after.Adams's.classical.study.[68].
investigated.in.detail,.for.example,.by.Ravichandran.and.Baldwin.[63])—is.well.known.and.exam-
ined.in.numerous.studies.(e.g.,.[182,254,600,628-630,642,646]).
The.most.frequent.electrolytic.treatment.is. anodization, .when.highly.positive.potentials.(typ-
ically. from. +1.25. up. to. +1.75.V. vs.. SCE). are. applied. for. a. period. of. 30-120.s.. After. anodic.
oxidation,. the. activated. carbon. paste. surface. can. be. “equilibrated”. by. a. short. cathodization .
(e.g.,. at. −1.0.V. for. 15.s).. Occasionally,. the. CP. surface. is. treated. by. cathodization. only,. which.
can. be. advantageous. in. measurements. requiring. a. negatively. charged. active. surface. [182]. or. a.
lower.background.during.the.reduction.by.constant.current.in.the.stripping.potentiometric.mode.
[254]..In.contract.to.anodic.activation,.however,.only.little.is.known.about.the.chemistry.behind.
cathodic.treatments..Perhaps,.in.analogy.and.by.following.some.interpretations.for.solid.carbon.
surfaces. [3,167],. it. can. be. deemed. that. the. reductive. activation. would. release. fragments. such.