Electroanalysis with Carbon Paste–Based Electrodes, Sensors, and Detectors - Electroanalysis with Carbon Paste Electrodes - page 196

with.glutamate.decarboxylase.[842,2729-2731]..The.detection.proceeds.via.hydrogen.peroxide.with.
manganese.dioxide.as.mediator.
Biosensors.for.phenylalanine.contain.phenylalanine.dehydrogenase.which.oxidizes.it.to.phenyl.
pyruvate. and. releases. NADH;. the. latter. can. be. reoxidized. with. dihydroxy. benzaldehyde. [2732]..
Oxygenation. removes. interferences. from. ascorbic. acid. (AA). and. uricase. from. UA.. A. trienzyme.
biosensor. was.suggested. by. Scheller's. and. Kuwana's. groups.[2733].. Besides. phenylalanine. dehy-
drogenase,. salicylate. hydroxylase. and. tyrosinase. were. co-immobilized;. the. hydroxylase. converts.
salicylate. to. catechol. in. the. presence. of. oxygen. and. NADH. from. the. dehydrogenase.. The. other.
enzyme,.tyrosinase,.then.oxidizes.the.catechol.to. o -quinone,.which.is.electrochemically.detected.by.
the.reduction.to.catechol.at.an.operating.potential.of.around.−50.mV.vs..Ag/AgCl..The.analyte.can.
be.detected.in.blood,.serum,.and.urine,.which.is.important.to.uncover.phenylketonuria,.the.condi-
tion.of.not.being.able.to.metabolize.phenylalanine.that.leads.to.mental.retardation.
l-Proline.interacts.with.cyclodextrins.which.is.exploited.for.enantioselective.determination.with.
potentiometry.[2734]..Complexation.studies.of.selenoamino.acids.with.Ag + .were.performed.with.
cyclic.voltammetry.and.chronocoulometry.[1309].
Tryptophan. shows. a. high. afinity. for. carbon. paste. and. is. electroactive;. it. can. be. oxidized. at.
relatively.high.positive.potentials..Therefore,.it.can.be.detected.with.adsorptive.stripping.voltam-
metry. [223,2735-2738].. Preanodization. or. over-oxidized. polypyrrole. ilms. improve. the. sorption.
[181,1797,2739]..Mediated.oxidation.can.be.achieved.with.quinizarine.on.titanium.dioxide.nanopar-
ticles. (with. the. possibility. of. simultaneous. determination. of. penicillamine). [1880],. ferrocenes.
[920,1545],.or.Co-salophen.with.carbon.nanotubes.[384]..Potato.juice.as.a.carbon.paste.modiier.
increases. the. selectivity. [2741];. polyamide. [2740]. and. clay. minerals. [1012,1122,1140,2742]. assist.
accumulation..Doping.with.Fe III .increases.the.anodic.peak.current.
Tyrosine. can. be. oxidized. at. unmodiied. CPEs. [2743,2744];. an. effect. similar. to. tryptophan.
was. observed. with. clay. minerals. and. with. Fe 3+ . [1124,2745].. Polypyrrole. molecularly. imprinted.
with.l-phenylacetic.acid.increases.the.selectivity.[2746];.polyvinylpyrrolidone.with.cetyltrimethyl.
ammonium.bromide.(CTAB).supports.accumulation.[2747].
8.4.4 a nTioxidanTs and p henoliC C oMpounds
Table. 8.18. gives. an. overview. of. the. determination. of. antioxidants. with. carbon. paste. electrodes..
AA will.be.reviewed.with.vitamins.in.Table.8.25;.phenolic.amino.acids.can.be.found.in.Table.8.17..
An.overview.of.the.antioxidant.activity.of.natural.products.is.given.in.[2748].
Ni-porphyrin. on. phosphate. silica-niobia. was. used. as. a. catalyst. for. the. oxidation. of. pheno-
lic. compounds,. in. particular,. p -aminophenol. and. hydroquinone. [1201].. The. electrochemical.
behavior. of. antioxidants. was. critically. compared. with. different. electrodes. in. [2749].. Mn(II)-
hexacyanoferrate. catalytically. oxidizes. butylated. hydroxyanisole. via. hexacyanoferrate(III).
[226,2750];.Ni-phthalocyanine.is.probably.reduced.to.Ni(I).by.the.analyte.and.can.be.electrochemi-
cally. re-oxidized. [651,895].. Butylated. phenols. are. used. as. antioxidants. in. food. industry.. Caffeic.
acid. (3-(3,4-dihydroxyphenyl)-acrylic. acid). in. the. presence. of. hydrogen. peroxide. is. catalytically.
oxidized. by. peroxidase. (in. green. bean. tissue). to. the. quinone. which. can. be. electrochemically.
reduced.[2751]..β-Cyclodextrin.supports.the.extraction.of.(+)-catechin.from.tea;.the.analyte.can.be.
oxidized.to.an. o -quinone.[1419].
Catechol. is. oxidized. by. tyrosinase. and. polyphenol. oxidase. and. also. by. laccase. (monophenol.
oxidase.and.blue.copper.enzyme).[222,279,364,554,987,2752-2758]..The.enzymes.can.be.applied.
in.puriied.form,.and.also.with.plant.tissues.[279,2756-2758]..DNA.improves.the.stability.of.tyros-
inase-modiied.carbon.pastes.and.prolongs.the.shelf.lifetime.[2755]..The.detection.proceeds.via.the.
reduction.of.the.enzymatically.produced.quinone.
DA. pyrocatechol,. and. lavonols. can. be. determined. with. plant. tissue. electrodes. (banana. and.
green.apple);.the.tissue.acts.as.a.source.for.polyphenol.oxidase.[278,2759,2760]..Dihydroxybenzenes.
[41].and.lavonoids.can.be.oxidized.with.unmodiied.CPEs.[2761-2763];.lavonoids.have.been.also.