Chemically Modified Carbon Paste Electrodes - Electroanalysis with Carbon Paste Electrodes - page 82

The.primary.role.of.a.modiier.is.to.bring.new.or.additional.features.to.the.CPE.(preconcentra-
tion.of.analytes,.rejection.of.interferences,.electrocatalytic.properties).to.improve.both.the.sensitiv-
ity.and.the.selectivity.of.the.electrode..Notwithstanding.the.very.high.number.of.modiiers.used.to.
date,.most.of.them.have.been.selected.for.their.accumulation.capability.and.electrocatalytic.proper-
ties..Due.to.the.wide.variability.of.these.modiiers,.it.is.quite.dificult.to.provide.a.universal.clas-
siication..They.are.presented.hereafter.according.to.three.main.categories:.(i).inorganic.materials,.
(ii).organic.compounds,.and.(iii).organometallic.compounds.and.other.“novel”.materials.(such.as.
nanostructured.solids,.NPs,.hybrids,.or.other.substances.used.in.the.recent.approaches.for.modiica-
tion)..For.each.family,.subcategories.have.also.been.considered..Note.that.biomolecules.constitute.a.
special.category.that.is.described.in.Chapter.5.and.largely.also.in.Section.8.4.

4.3.1 i norganiC M aTerials
Inorganic.materials.are.usually.manufactured.in.the.form.of.powders.or.particulate.solids,.making.
their.immobilization.onto.a.solid.electrode.surface.rather.dificult.(especially.if.long-term.mechani-
cal. stability. is. required).. Therefore,. their. physical. entrapment. or. embedding. into. a. CPE. matrix.
constitutes.an.ideal.way.of.immobilization.

4.3.1.1 Prussian-Blue Derivatives and Polyoxometallates
•. Prussian Blue and Related Compounds ..The.complex.structure.of.Fe II 4 . [Fe II (CN) 6 ] 3 ).and.
related. metal. hexacyanoferrates. (where. Fe III . is. replaced. by. another. transition. metal. ion).
offer. attractive. electrocatalytic. and. ion-exchange. properties. that. can. be. advantageously.
exploited.in.electrochemistry.[1031]..After.the.pioneering.work.by.Boyer.et.al..[845],.the.
incorporation. of. metal. hexacyanoferrates. in. CPEs. has. been. largely. applied,. especially.
for. electrocatalytically. oriented. determinations.. They. were. mainly. based. on. Prussian.
Blue. itself. [196,847,848,850,1032,1033]. and. also. on. copper. hexacyanoferrate. [436,610,.
1034-1037],. cobalt. hexacyanoferrate. [1038],. or. hybrids. based. on. copper. and. cobalt.
hexacyanoferrates.[1039-1041]..Ru III -.[1042],.Rh III -.[849],.Sn IV -.[1043],.and.Th IV -.[1044].
hexacyanoferrates.have.also.been.used,.and.a.comparison.between.hexacyanoferrates.con-
taining.Fe III ,.Cu II ,.Cu I ,.Co II ,.or.Ni II .with.respect.to.hydrogen.peroxide.detection.is.available.
[843]..CPEs.modiied.with.such.compounds.were.applied.to.electrocatalytic.detection.of.
various. species,. including,. for. example,. amino. acids. [1036,1040-1042]. or. other. biomol-
ecules.[1034,1038,1043,1044],.nitrite.[847,1032,1037],.sulite.[436,1041],.persulfate.[945],.
and.hydrogen.peroxide.[196,843,849,].as.well.as.in.biosensing.applications.[610,848,1035]..
Ion-exchange.properties.of.Prussian.Blue.derivatives.were.also.exploited.for.potentiomet-
ric.detection.of.cations.[584,1045].
•. Polyoxometallates .. Polyoxometallates. (POMs). are. heteropolyanions. that. are. likely. to.
form.Keggin-type.(XM 12 O 40 n − ),.Dawson-type.(X 2 M 18 O 62 n − ),.mixed-addenda,.or.transition.
metal-substituted. structures.. They. can. exhibit. well-deined. electrochemical. responses.
that. have. been. largely. exploited. for. electrocatalytic. purposes. [1046].. Most. POM. struc-
tures. used. in. connection. with. electrochemistry. are. Keggin-type. and. transition. metal-
substituted. heteropolyanions.. Except. the. irst. report. on. a. phosphomolybdate. ilm. on.
CPE. [696],. pioneering. works. on. POM. bulk-modiied. CPEs. have. been. reported. by. Xiu.
Li.Wang.and.coworkers.[140,1047,1048]..The.most.widely.used.Keggin-type.POM-based.
solids.to.modify.CPEs.involved.phosphomolybdate.(PMo 12 .[126,142,1049,1050]).and.sili-
comolybdate. (SiMo 12 . [1049,1051]). but. less. commonly. phosphotungstate. (PW 12 . [1052]),.
silicotungstate. (SiW 12 . [1053]),. or. vanadomolybdate. (VMo 12 . [487]).. The. use. of. 1:8-type.
heteropolymolybdate. with. tetravacant. Keggin. structure. has. also. been. reported. [1054]..
Sometimes,.organic.cations.(pyridine-.or.bipyridine-based.derivatives).acted.as.counter-
ions.of.the.heteropolyanions.[1049-1051],.leading.to.more.sophisticated.crystal.structures.
(see.two.examples.in.Figure.4.6).