Chemically Modified Carbon Paste Electrodes - Electroanalysis with Carbon Paste Electrodes - page 86

This.concept.was.then.extended.to.the.analysis.of.alkali.metal.and.alkaline.earth.metal.cations.
after.separation.by.ion.chromatography.[1111,1116]..Zeolites.can.also.be.doped.with.cationic.redox.
mediators.such.as.Methylene.blue.[1129,1130],.Methylene.green.[1128,1141],.Fe III .[1012,1124,1144],.
Cu II . or. Cu II . porphyrin. [1085,1112,1113],. methylviologen. [1106],. or. trinuclear. ruthenium. amine.
complexes.[1126],.and.the.resulting.zeolite-modiied.CPEs.have.been.applied.to.electrocatalytic.
detection. of. various. analytes,. for. example,. H 2 O 2 ,. O 2 ,. NADH,. amino. acids,. and. other. biologi-
cally.active.species;.such.systems.are.promising.for.biosensors.[876,1118]..Other.applications.are.
potentiometric. determinations. (i.e.,. HAsO 4 2− . at. Fe 2+ -doped. clinoptilolite. [1137]. or. PO 4 3− . at. sur-
factant-modiied.zeolites.[1149]).or.electrogenerated.chemiluminescence.(i.e.,.heroin.detection.at.
Ru(bpy) 3 2+ -doped.zeolite.[1119]).

4.3.1.3 Metal Oxides and Sol-Gel-Derived Inorganic Materials
4.3.1.3.1 Silica-Based Materials
Silica. gels. (or. fumed. silicas). are. rather. old. materials. exhibiting. adsorption. and. catalytic.
properties.. Such. attractive. features,. associated. with. usually. large. speciic. surface. areas.
(>100.m 2 .g −1 ),.are.at.the.origin.of.their.wide.use.as.CPE.modiier.[89,831]..At.irst,.silica.particles.
were.simply.introduced.into.the.carbon.paste.matrix.to.enhance.its.electroanalytical.performance.
[419,682,1000,1150-1163].. Examples. are. available. for. preconcentration. and. detection. of. organic.
pollutants.[682,1152,1154-1156],.for.the.accumulation.of.heavy.metal.cations.before.their.voltam-
metric.detection.[1157,1158,1160-1162],.as.well.as.to.improve.the.response.of.amperometric.bio-
sensors.[1000,1153],.or.for.potentiometric.determinations.[419]..Rapidly.after,.the.versatility.of.the.
sol-gel. process. (i.e.,. room-temperature. synthesis. of. silica. gels. by. hydrolysis. and. condensation. of.
alkoxysilane. reagents. [1164]). was. exploited. to. encapsulate. biomolecules. (enzymes. or. redox. pro-
teins),.and.the.resulting.biocomposite.was.introduced.into.CPE.as.a.way.of.durable.immobilization.
with.promising.applications.in.the.ield.of.biosensors.[1165-1170]..Electrocatalysts.(e.g.,.metal.por-
phyrins.[1171,1172]).have.also.been.entrapped.in.sol-gels.and.then.applied.as.CPE.modiiers..Silica.
was.then.exploited.to.adsorb.electrocatalysts.acting.as.electron.transfer.cofactors,.with.applications.
mainly.in.electrochemical.biosensors.[1173-1176].
A.signiicant.advance.in.the.ield.was.made.by.the.groups.of.Kubota.and.Gushikem.(among.some.
others)..They.proposed.to.extend.the.properties.of.silica.gels.by.modifying.their.surface.by.very.
thin.reactive.layers.of.metal.oxides.(i.e.,.M x O y .monolayers,.which.could.eventually.be.further.trans-
formed.into.corresponding.metal.phosphates).[1177]..The.synthetic.pathway.involves.the.reaction.of.
surface.silanol.groups,.≡SiOH,.or.siloxy.bonds,.(≡Si) 2 O,.with.a.metal.halide,.MX n .(Equations.4.1a.
and.b),.and.their.subsequent.hydrolysis.(Equations.4.2a.and.b),.giving.rise.to.monolayers.of.metal.
oxide.after.drying:

m ≡ SiOH + MX → ≡ ( SiO) MX + m HX .

(4.1a)

.

n m n m −

( Si) O + MX ≡ →≡ SiX + SiOMX ≡

. (4.1b)

.

2 n n − 1

( SiO) MX ≡ + ( n m − )H O → ≡ ( SiO) M(OH) + ( n m − ) HX

. (4.2a)

.

m n m − 2 m n m −

≡ SiOMX + ( n − 1)H O →≡ SiOM(OH) + ( n − 1) HX

. (4.2b)

.

n − 1 2 n − 1

These.metal.oxide.layers.can.be.further.transformed.into.metal.phosphates.by.treatment.with.phos-
phoric.acid,.as.illustrated.in.Equation.4.3.for.titania.on.silica.((≡SiO) 2 Ti(OH) 2 ):

( SiO) Ti(OH) + H PO ≡ → ≡ ( SiO) Ti(OPO H) + 2H O

. (4.3)

.

2 2 3 4 2 3 2 2