Electroanalysis with Carbon Paste–Based Electrodes, Sensors, and Detectors - Electroanalysis with Carbon Paste Electrodes - page 206

Caffeine. is. preconcentratable. on. carbon. pastes,. and. can. be. directly. oxidized,. but. at. rather.
positive.potentials.[39,1691,3121-3122],.whereas.1,4-benzoquinone.forms.an.electroinactive.com-
plex. with. caffeine,. which. allows. its. indirect. determination. via. the. quinine. signal. [1444].. DNA.
enhances.the.accumulation.[430]..Mesoporous.TiO 2 .improves.the.oxidation.current.of.hypoxan-
thine.[1875];.this.analyte.is.frequently.detected.with.xanthine.oxidase.[572,858,945,1497],.where.
the. electrochemical. signals. can. be. improved. by. Au. nanoparticles. or. mediated. by. ferrocene. or.
methyl.viologen.
Nicotine.can.be.quantiied.indirectly.by.inhibition.of.choline.oxidase.with.choline.as.a.substrate.
and.benzoquinone.as.an.electron.acceptor;.the.enzyme.converts.choline.into.betaine.[2452]..Some.
pyrimidine.derivatives.can.be.directly.oxidized.on.carbon.paste.electrodes,.which.can.be.exploited.
for.amperometric.detection.in.chromatography.[3123].
Theophylline. also. undergoes. direct. oxidation. at. around. +1.0.V. vs.. Ag/AgCl. [3125];. cetyl.
trimethylammonium. bromide. supports. accumulation. [741];. and. cobalt. phthalocyanine. and.
K-hexachloroplatinate. serve. as. catalysts. [908,932,2626].. Thiouracil. like. many. other. thio-com-
pounds.is.catalytically.oxidized.by.cobalt.phthalocyanine.[908].
UA. is. another. most. frequently. studied. compound. when. using. various. carbon. paste-based. elec-
trodes. and. sensors. [119,135,203,226,247,422,427,442,453,456,590,659,660,676,916,969,995,1122,.
1420,1446,1509,1513,1590,1632,1722,1847,1900,2738,2826,2876,3009,3016,3022,3126-3133];.mainly,.
because.of.the.fact.that.UA.in.biological.matrices.AA.is.quite.a.common.interferent..UA.is.a.inal.
product.of.the.purine.metabolism.in.many.animals..It.shows.a.poorly.developed.direct.oxidation.sig-
nal.which.can.be.improved.with.glassy.carbon.powder.[659,2738,3126],.carbon.nanoibers.[203,660],.
carbon.nanotubes.[3128],.or.ionic.liquids.[453,456]..Preanodization.improves.the.electron.transfer.and.
allows.simultaneous.determination.with.xanthine,.hypoxanthine,.or.AA.[1900].
Naion. eliminates. AA. interferences. by. electrostatic. repulsion. [1722].. Cationic. surfactants.
[1632,2971,2972,3129],.cyclodextrin.[1420],.and.Pd-nanoparticles.[969].improve.the.resolution.with.
other. compounds. yielding. overlapping. peaks. (particularly. AA).. Mesoporous. SiO 2 . increases. the.
oxidation.signal.by.adsorption.[1847].
Quite.a.few.mediators.may.catalyze.the.oxidation.of.UA:.Fe(III).on.zeolites.[1122],.chloromer-
curi.ferrocene.[3130],.Fe(II)-phthalocyanine.[916],.Co(II)-5-nitrosalophen.[1590],.4-nitrophthalo-
nitrile. [3131],. quinones. and. hydroquinones. [422,397,442,995,1446,3016],. thionine. [1509],. Congo.
red.[1513],.poly(calconcarboxylic.acid).[135],.and.chloranil.[3022]..Uricase,.an.enzyme.occurring.
in.many.animals.which.metabolize.(oxidize).UA.to.allantoin,.can.be.exploited.for.corresponding.
biosensors.[247,590,3132];.the.enzyme.can.be.also.placed.on.a.precolumn.reactor.in.low.injection.
analysis.[2826].
Xanthine.is.also.a.metabolite.of.purine.degradation;.it.occurs.in.caffeine-containing.plants.and.many.
animal.tissues,.in.plasma,.and.in.canned.ish..It.can.be.oxidized.directly.on.carbon.paste.with.poorly.
established.signal.[1900];.the.electron.transfer.is.signiicantly.improved.with.carbon.nanotubes.and.ionic.
liquids.[3128]..The.most.common.way.of.determination.is.with.xanthine.oxidase.[722,943,3134-3139].
8.4.11 V iTaMins
A. myriad. of. analytical. methods. for. vitamins. [2748]. that. have. been. employing. various. types. of.
carbon.paste.electrodes.are.summarized.in.Table.8.25.
One. of. the. most. frequently. studied. substances. is. AA. [53,63,86,123,135,144,416,422,438,453
,456,458,653,655,660,783,874,916,930,933,969,1003,1029,1043,1044,1087,1094,1110,1122,1133
,1186,1202,1209,1229,1285,1288,1316,1318,1430,1446,1460,1461,1472,1491,1509,1516,1528
,1542,1548,1549,1570,1583,1586,1590,1609,1610,1612,1617,1661,1704,1729,1742,1763,1764,.
1799,1802,1806,1820,2002,2681,2748,2957,2964,2969-2972,2985,2989,3009,3131,3140-3177])..
It.is.the.main.water-soluble.antioxidant.and.is.widely.distributed.in.the.animal.and.plant.kingdom;.
it.is.also.widely.employed.in.food.industry.as.a.preservative.