Chemically Modified Carbon Paste Electrodes - Electroanalysis with Carbon Paste Electrodes - page 104

[1895]. deposits,. when. almost. all. of. them. have. been. successfully. applied. in. ESA. of. metal. ions. and.
related.species.[83,255,262-264,268]..If.the.preplated.metallic.ilm.conigurations.are.combined.with.
.electrochemical.stripping.analysis,.the.electrode.process.involves.the.deposition.and.subsequent.dissolu-
tion.of.intermetallic.compounds.such.as.Me(Hg) X .amalgams.[255,1890],.Hg-Au.adducts.[255,260],.and.
Me A Bi B .alloys.[262,263],.expected.to.be.formed.in.the.respective.metallic.layer.
As.shown.in.experiments.with.metal.ilms.generated. in situ ,.these.processes.can.also.be.accom-
panied.by.additional.effects.as.a.consequence.of.mutual.competition.between.the.corresponding.ilm.
and.the.bare.carbon.paste.surface.[189,260,812],.which.would.also.occur.at.CPEs.modiied.with.metal.
powders. (e.g.,. inely. pulverized. bismuth. [266]. and. antimony. [1896]).. Finally,. the. identical. metallic.
layer.can.also.be.accomplished.in.the.conigurations.of.CPEs.modiied.with.solid.oxides,.namely,.HgO.
(as.yellow.allotrope.[617]),.Bi 2 O 3 .[268],.Sb 2 O 3 .[1897],.and.a.related.hydrolytic.product.SbOL.[1898]),.
or.some.inorganic.salts—NH 4 BiF 4 .[269].and.BiF 3 .or.SbF 3 .[1899]—being.“pseudo-covalent”.in.nature.
(forming.polymeric.structures),.insoluble.in.aqueous.media,.and.applicable.as.“normal”.bulk.modiier.
4.3.3.3.2 Electrolytic Modiications and Other Surface Treatments
By.referring.back.to.Section.3.2.and.highlighting.again.some.fundaments.here,.the.pretreatments.by.
electrolytic.activation.(mainly. anodization ,.which.may.sometimes.be.followed.by. cathodization ).
and.chemical.oxidation.are.applicable.to.CPEs.before.use.[63,181,254,629,646,675,868,1448,1900-
1904]..CPEs.treated.by.such.procedures.can.also.be.classiied.as.CMEs,.as.both.treatments.lead.
to.an.alteration.of.surface.states.[68]..(As.mentioned.earlier,.this.happens.through.the.formation.
of. various. functional. groups,. such. as. −C-OH,. −C = O,. −C-OH + ,. and. −C = OH + ,. after. intensive.
oxidation.of.carbon.particles.at.or.near.the.CP-surface.layer,.leading.to.its.partial.or.even.total.
hydrophilization .[3,68,83,157].).Beneicial.effects.of.anodization.(e.g.,.at.−1.5.V.for.180.s.[176]).
include. enhancements. in. both. heterogeneous. charge. transfer. rates. [868]. and. preconcentration.
eficiencies. [1902],. both. improving. the. overall. analytical. performance. of. CP-based. electrodes.
and. sensors.. After. anodic. oxidation,. the. activated. carbon. paste. surface. can. (or. even. must). be.
“equilibrated”.by.a.short.cathodization.(e.g.,.at.−1.0.V.for.15.s.[176]),.or.alternatively,.by.succes-
sive. potential cycling .between.the.properly.set.positive.and.negative.potential.limits.[63,646,868],.
where.the.“shock.from.aggressive.anodization”.is.compensated.yet.more.effectively.
Finally,.there.are.some.occasional.treatments.of.the.CP-surface,.e.g.,.ultrasonication.(to.facilitate.
mediator. immobilization. onto. CNT-modiied. CPE. [1511,1590]). or. magnetic. ield. (for. increasing.
the.electrochemical.lux.[1905].or.improving.the.DNA.detection.[299]);.for.other.examples.of.these.
rather.unusual.pretreatments,.refer.to.Section.2.2.1).
4.4 MODELING AND TESTING OF CHEMICALLY MODIFIED CPEs
Carbon.paste.electrodes.are.heterogeneous.systems.in.which. electrically conductive sites or areas .
and. insulating zones .are.both.present..And.if.there.is.a.modiier.on.the.surface.or.in.the.bulk,.the.
constellation.becomes.yet.more.complicated..Hence,.modeling.the.behavior.of.such.electrodes.is.
not.easy,.and.some.basic.procedures.that.are.available.have.been.developed.from.empirical.experi-
ence.with.modeling.and.testing.of.binary.or.unmodiied.CPEs.(see.Section.3.3).or.from.assays.for.
distinguishing.between.unmodiied.and.modiied.variants..Mostly,.the.respective.models.assumed.
that.such.sensors.would.behave.like.arrays.of.microelectrodes,.nevertheless,.considering.that—due.
to.the.overlay.of.the.diffusion.layers.of.hypothetical.microelectrodes—electrochemical.responses.
are.like.those.of.common.“macroelectrodes.”
4.4.1 b asiC p roCedures and s peCiFiCs
Due.to.the.presence.of.pasting.liquid,.the.voltammetric.signals.at.CPEs.are.not.ideal.even.for.elec-
trochemically.reversible.systems,.when.showing.deviations.as.(i).lattening.of.sigmoidal.curves.in.