Chemically Modified Carbon Paste Electrodes - Electroanalysis with Carbon Paste Electrodes - page 84

Nevertheless,. many. clay-modiied. electrodes. have. been. prepared. by. the. bulk. dispersion. of. clay.
particles.within.the.carbon.composite.matrix.
Several. types. of. clays. have. been. used. as. modiiers. for. carbon. pastes,. including. mainly. mont-
morillonites. [716,1011,1058-1070]. and. sepiolite. [823-827,1071-1073]. but. also. bentonites. [1074-
1079],.vermiculites.[542,1080-1082],.kaolinites.[1069,1083],.kaolin.[1084,1085],.(luoro)hectorites.
[826,1086],. muscovite. [1087],. and. some. other. natural. smectites. [110,1033,1088-1090].. Another.
family. of. layered. materials. are. anion-exchanging. clays. exhibiting. a. hydrotalcite-like. structure,.
called.“layered.double.hydroxides”.(LDHs),.have.been.exploited.in.electroanalysis,.mostly.as.thin.
ilms. [1091,1092]. but. also. in. CPEs. [1092-1095].. Friedel's. salt. is. a. special. case. of. this. category.
[1096]..Crude.soil.samples.were.also.incorporated.directly.into.CPEs.[1097]..Although.Wang.with.
Martinez.acted.as.pioneers.[1058],.the.groups.of.Kula.and.Navratilova,.L..and.H..Hernandez,.and.
Kalcher.were.the.most.active.in.the.ield.of.clay-modiied.CPEs.and.that.of.Mousty.in.using.LDH.
anion.exchanging.clays.
Heavy. metal. cations. such. as. Hg II . [1061,1064,1082],. Cu II . [1072,1080],. Ag I . [1080,1082],.
Fe III [1058],.or.Bi III .[1065],.alone.or.in.mixture.[1060],.have.been.preconcentrated.at.clay-modiied.
CPEs.to.enhance.the.sensitivity.of.their.subsequent.detection..Interestingly,.cation.exchanging.clays.
were.also.likely.to.accumulate.anionic.species.(as.exempliied.for.AuCl 4 − .[1062,1063]).as.a.result.of.
positive.charges.on.the.edges.of.clay.sheets.inducing.both.cation-.and.anion.exchange.properties.[716]..
On.the.other.hand,.organic.molecules.(especially.pesticides.[110,825-827,1071,1072,.1084-1086].
but. also. other. pollutants. [823,824,1086],. biologically-relevant. molecules. [1011,1066,1073],. phar-
maceuticals.[1098],.or.dyes.[1068]).can.be.determined.after.accumulation.at.clay-modiied.CPEs..
Improvement.of.their.analytical.characteristics.can.be.achieved.by.modifying.the.clay.before.use..
For. example,. doping. the. clay. by. cationic. electrocatalysts. such. as. methylviologen,. porphyrins,. or.
Methylene.blue.was.found.to.enable.detection.of.molecular.oxygen.[1099],.manganese.[1079],.or.
ascorbic. acid. [1087],. respectively.. Another. approach. consists. in. doping. the. clay. with. long-chain.
alkylammonium.moieties.(that.are.easily.incorporated.in.the.interlamellar.space.by.ion.exchange),.
resulting. in. highly. hydrophobic. accumulation. centers. likely. to. preconcentrate. organic. analytes.
(see,. e.g.,. phenol. derivatives. accumulated. at. cetyltrimethylammonium. exchanged. montmorillon-
ites. [1100,1101]).. Using. organically. functionalized. alkyl-ammonium-based. surfactants. further.
improved. the. sensor. selectivity,. as. exempliied. for. clays. intercalated. with. 2-mercapto-5-amino-
1,3,4-thiadiazole-. or. 2-thiazoline-2-thiol-functionalized. hexadecylammonium,. which. were. then.
applied.to.selective.detection.of.Hg II .[1102,1103].
4.3.1.2.2 Zeolite-Based Molecular Sieves
These. materials. are. microporous. crystalline. aluminosilicates. characterized. by. a. regular. spatial.
arrangement.of.TO 4 .tetrahedra.(T.=.Si,.Al),.leading.to.well-deined.structures.made.of.uniform.cages,.
cavities,.or.channels.of.deined.dimensions.(typically.in.the.4-15.Å.range).with.minimum.and.maxi-
mum. diameters. that. can. readily. discriminate. against. molecules. with. dimensional. differences. less.
than.1.Å.(size.selectivity)..Substitution.of.Si IV .by.Al III .in.the.framework.makes.it.negatively.charged.
and.requires.the.presence.of.charge-compensating.“extra-framework”.cations.to.maintain.electroneu-
trality..These.cations.are.mobile.enough.to.be.exchanged.by.other.ones,.contributing.thereby.to.the.
rich.ion-exchange.chemistry.of.these.materials..Ion-exchange.capacity.is.directly.related.to.the.Al/Si.
ratio.in.the.framework..Zeolites.are.unique.materials.exhibiting.both.size.selectivity.and.ion-exchange.
capacity.in.a.single.solid..This.attractive.feature.has.attracted.the.huge.development.of.zeolite-modi-
ied.electrodes.[1104],.and.most.advances.made.in.electroanalysis.are.based.on.zeolite-modiied.CPEs.
[717,718,1105]..The.reason.is.probably.related.to.the.particulate.nature.of.zeolites,.making.dificult.
their.immobilization.as.mechanically.stable.ilms.onto.electrode.surfaces.(other.than.in.the.presence.
of.a.polymeric.binder.or.overcoating),.contrary.to.their.simple.incorporation.into.CPE.matrices.
By.far,.the.most.widely.used.molecular.sieve.to.modify.CPEs.was.the.large-pore.(8.Å.in.diam-
eter).faujasite-type.zeolite.Y.[533,753,755,830,850,876,1012,1106-1126]..Its.isostructural.zeolite.X.
[700,1115,1127-1130].and.the.small-pore.(4.Å.in.diameter).zeolite.A.[700,830,1106,1115,1116,1131].