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O
O
O
O
O
O
p
-Benzoquinone
Naphthoquinone
9,10-Anthraquinone
O
OH
O
HO
O
HN
O
CH
3
H
3
C
H
3
C
O
H
HO
N
O
O
CH
3
6-10
O
O
Ubiquinone
Pyrroloquinoline quinone
FIGURE 4.11
Some. quinone. derivatives. used. as. carbon. paste. modiiers. and. redox. mediators.. (Chemical.
structural.formulas.)
to.direct.catalytic.determinations. or.enzyme-based. biosensing.. More.sophisticated.structures. are.
ubiquinone.(coenzyme.Q
10
).[1456,1468],.pyrrolo-quinoline.quinone.[1458],.menaquinone.(vitamin.K
2
).
[1457],.quinizarin.[1459],.and.a.series.of.[alcanediylbis(nitriloethylidyne)]-bis-hydroquinone.deriva-
tives.[422,993,1460-1462].
4.3.2.2.2 Phenothiazine, Phenoxazine, and Phenazine Derivatives
These. redox-active. dyes. constitute. another. extremely. popular. class. of. electrocatalysts;. the. most.
widely.used.in.CPEs.are.depicted.in.Figure.4.12..In.the.phenothiazine.family,.CPE.was.often.modi-
ied. with. Methylene. blue. [289,1463-1467],. Methylene. green. [244,656,873,1468-1472],. Toluidine.
blue. [321,1473-1477],. and. to. a. lesser. extent.
Promazine
. (an. antipsychotic. agent. [323,1478,1479]);.
the.behavior.of.these.mediators.was.sometimes.mutually.compared.to.ind.one.leading.to.the.best.
performance.[314,873,1478-1480].
While.Methylene.blue.was.often.applied.as.an.electrochemical.hybridization.indicator.in.DNA.
sensors,.Methylene.green.and.Toluidine.blue.acted.in.oxidase-.and.dehydrogenase-based.biosens-
ing. devices.. These. mediators. were. often. immobilized. onto. inorganic. particles. (silica-supported.
metal.oxides.or.metal.phosphates),.again,.with.examples.of.Methylene.blue.[1087,1178,1184,1190],.
Methylene.green.[1196],.or.Toluidine.blue.[1204,1205,1249].
In. the. phenoxazine. family,. Meldola. Blue. holds. a. prominent. position. to. modify. CPEs,. either.
dispersed.as.powder.in.the.matrix.[783,863,1481-1483].or.incorporated.after.adsorption.onto.inor-
ganic.particles.[87,1106,1173-1175,1185,1205],.while.some.examples.also.report.the.use.of.Nile.blue.
[783,870,1193,1289,1492-1494].. These. compounds. have. been. widely. used. for. the. determination.
of. the. coenzyme. NADH. generated. during. analytical. methods. employing. certain. dehydrogenase.
enzymes.. A. mini-review. compares. the. performance. of. phenoxazine-. and. phenothiazine-modi-
ied. CPEs. [784].. Finally,. CPEs. modiied. with. the. title. derivatives. (i.e.,. phenazine. methosulfate.
[873,1484,1485]).have.also.been.reported.
4.3.2.2.3 Other Organic Mediators
Within.these.substances,.one.can.select.widely.used.molecular.electrocatalysts,.such.as.tetrathiaful-
valene.[1486-1491].and.tetracyanoquinodimethane.[280,1492-1494],.alone.or.together.[1495,1496],.
ribolavin.[1198,1294],.as.well.as.the.cationic.methylviologen.and.some.other.4,4′-bipyridyl.derivatives.
(mostly.immobilized.on.inorganic.particles.before.incorporation.in.CPE.[830,1099,1106,1497,1499];.
see. Figure. 4.13).. Finally,. the. catalytic. properties. of. some. less. common. compounds. were. also.
exploited,.including.(nitro).luorenone.derivatives.[439,1295,1296],.aminobenzoic.acid.[1021,1500],.
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