Chemically Modified Carbon Paste Electrodes - Electroanalysis with Carbon Paste Electrodes - page 76

to.immobilization.of.proteins,.where.smaller.units.are.linked.with.each.other.by.an.agent.with.two.
reactive.groups,.which.bind.covalently.to.the.target.molecules,.yielding.a.huge.polymeric.network.
with.very.low.solubility.in.the.solvents.to.which.the.electrode.surface.will.be.exposed..Often.this.
form.of.immobilization.is.combined.with.an.additional.polymeric.backbone.structure.(membrane).
such. as. Naion ® .. Typically,. enzyme. molecules. are. cross-linked. either. to. each. other. or. to. bovine.
serum.albumin.with.glutaraldehyde..In.fact,.cross-linking.is.possible.not.only.with.surface.but.also.
with.bulk.modiication.

4.1.2.2 Bulk Modiication
Although.surface.modiication.can.be.effected.also.with.homogeneous.solid.electrodes,.hetero-
geneous.materials.offer.the.advantage.of.simple.modiication.of.the.whole.electrode.material.
under. mild. conditions. due. to. its. composite. nature.. In. the. simplest. case,. the. modiier—either.
liquid.or.solid—is.added.directly.to.the.carbon.paste.(the.term.“direct.mixing”.for.this.proce-
dure.being.coined.by.Baldwin.et.al..[53,54,683]).and.is.indeed.the.mildest.and.simplest.way.to.
modify. an. electrode.. Alternatively,. the. modiier. may. be. mixed. to. or. adsorbed. on. the. carbon.
powder.during.the.preparation.of.the.carbon.paste.[799];.another.way.is.its.dissolution.in.the.
pasting.liquid.[800].
The. quality of modiication .( homogeneity ).with.particulate.matter.depends.on.the.particle.size.
of.the.modiier.and.the.care.with.which.it.is.mixed.into.the.electrode.material..In.these.cases,.for.
example,. sonication. may. improve. the. results.. Due. to. better. homogeneity,. liquid. modiiers. yield.
more.reproducible.results.than.solid.ones.

4.2 REASONS AND STRATEGIES FOR MODIFICATION
The.general.reason.for.modiication.of.an.electrode.surface.is.to.improve.either.the.mass.transport.
or.the.electron.transport.limitations.at.an.electrode.surface.
This.can.be.achieved.either.via.generation.or.immobilization.of.electroactive.species.or.reactants.
on.or.close.to.the.electrode-solution.interface.or.electron-transfer.improving.compounds.or.func-
tional.groups..Immobilization.of.electroactive. species.directly.at.the.electrode.surface.decouples.
mass. transport. from. the. electrochemical. transformation. process,. that. is,. oxidation. or. reduction,.
during.the.measurement.and.is.commonly.accompanied.by.a.considerable.decrease.in.the.detection.
capabilities.of.the.system.given.
Thus,.the.main.reasons.for.modiication.of.CPEs.involving.modiier-substrate.interactions.are.
the.following.[801]:

•. Immobilization.and.preconcentration.(accumulation).of.the.analyte.at.the.electrode.surface

•. Exploitation. of. catalytic. (mediating). electrochemical. reactions,. or. placement. of. electron.

transfer.accelerating.or.improving.groups.at.the.electrode.surface

•. Immobilization.of.reactants.generating.electrochemically.active.products

•. Protection.and.stabilization.of.the.electrode.surface

•. Change.of.physical.or.physicochemical.properties.of.the.CP-surface.in.a.favorable.way

4.2.1 C riTeria r equired For M odiFiers
Modiiers.should.meet.some.criteria.to.be.useful.for.the.intended.purposes:.they.should.be.insoluble.
in. solutions. to. which. they. will. be. exposed. or. at. least. they. should. adsorb. strongly. on. the. carbon.
particles;.otherwise.a.“bleeding”.of.the.electrode.can.occur,.yielding.poorly.reproducible.results..
Improvements.can.be.made.by.making.modiier.molecules.more.lipophilic.[801].
Except.if.electrocatalytic.effects.are.to.be.exploited.the.modiier.should.not.be.electroactive.at.
the.potential.of.interest.because.otherwise.high.background.currents.deteriorate.or.even.impair.elec-
trochemical.responses.[802,803]..With.bulk.modiication,.the.usual.concentrations.of.the.modiier.