Electroanalysis with Carbon Paste–Based Electrodes, Sensors, and Detectors - Electroanalysis with Carbon Paste Electrodes - page 192

biosensors..In.general,.electrochemical.sensors.have.a.much.longer.history.than.optical.sensors,.
whose.development.started.eventually.in.the.second.half.of.the.twentieth.century..A.brief.over-
view.has.been.given.in.[1911].
The.following.sections.with.Tables.8.15.through.8.28.summarize.electrochemical.methods.for.
determination.of.BIC.at.carbon.paste-based.electrodes,.except.for.a.few.classes.of.compounds.that.
have.already.been.reviewed—in.the.Sections.8.2.and.8.3.
8.4.1 a lCohols
Analytical.methods.of.alcoholic.compounds.involving.carbon.paste.electrodes.are.summarized.in.
Table.8.15.
Many. alcohols. can. be. oxidized. with. the. aid. of. Cu(I)-oxide;. the. actual. oxidation. mechanism.
proceeds.via.higher.oxidation.states.[1225]..Also.Ru,.as.RuO 2 .or.its.complexes,.can.be.exploited.for.
this.purpose.[1220,2642-2644]..Here,.again,.the.reaction.proceeds.via.higher.oxidation.states.of.the.
metal.with.assumed.tetra-.to.hexavalencies.
Choline. can. be. detected. with. biosensors. containing. choline. oxidase. and. ferrocene. or. pheno-
thiazine. as. electron. acceptors. and. membranes. to. immobilize. the. bioconstituent. [1531,2645]..
Phosphatidyl.choline.in.rat.saliva.can.be.determined.with.this.type.of.sensors.
Ethanol.is.the.most.frequently.described.alcoholic.analyte.in.the.literature..Its.non-enzymatic.
oxidation.may.be.catalyzed.by.Ni.nanoparticles.[139].or.Ni(II).or.Ru(II).complexes,.where.again.an.
involvement.of.higher.oxidation.states.of.the.metal.ion.can.be.assumed.[1299,2646].
The. majority. of. analytical. ethanol. assays. employ. oxidizing. enzymes,. most. frequently. alco-
hol. dehydrogenase. (ADH). [52,214,319,364,372,783,784,1000,1197,1471,1474,1599,1601,1750,.
1752,1772,1786,2647-2651].and.to.a.lesser.extent.alcohol.oxidase.[1755,2652-2659]..The.enzymes.
are.not.speciic.to.ethanol.but.oxidize.also.methanol.and.other.aliphatic.alcohols.
The.enzyme.ADH.can.be.present.in.the.solution.[52,319].but.is.usually.immobilized.as.a.modi-
ier.at.the.electrode.surface..ADH.needs.NAD + .as.a.cofactor.which.is.usually.added.to.the.solu-
tion.or.to.the.carbon.paste.itself;.immobilization.of.NAD + .with.an.aldehyde.is.also.possible.[52]..
The.metabolite.NADH.can.be.oxidized.directly.or.with.a.mediator;.the.latter.can.be.polymerized.
o -aminophenol. or. o -phenylenediamine. [1772,1786],. Ru. complexes. [1601],. phendione. complexes.
of. Fe. or. Re. [1599],. or. polymerized. vinyl-ferrocene. [2649].. Organic. mediators. can. be. toluidine.
blue. [784,1474],. Meldola. blue. [372,783,1197],. phenothiazine. derivatives. [1750,1752,2650,2651]. or.
Methyl.green.[1471].
Alcohol. oxidase. is. usually. combined. with. horseradish. peroxidase. (HRP). to. detect. hydrogen.
peroxide.[1755,2652-2659];.HRP.can.be.reconstituted.directly.via.wiring.with.an.Os-polymer.or.
mediated.with.ferrocene.[2652]..Yeast.can.be.used.as.a.source.of.ADH.in.corresponding.biosensors.
[2660-2662],. and. plant. tissue. (tomato. seeds). [2663]. or. whole. cells. of. Acetobacter . spp.. may. also.
serve.as.a.pool.of.the.enzyme.[780,2664]..As.a.matter.of.fact,.alcohol.sensors.are.mostly.applied.for.
the.determination.of.ethanol.in.beverages,.such.as.wine,.whiskey,.etc.
For.glycerol,.glycerol.dehydrogenase.was.used.to.construct.a.biosensor.with.NAD + .as.a.co-factor.
[654]..In.this.assay.NAD + .was.partly.overoxidized,.and.the.oxidation.products.strongly.adsorbed.to.
the.carbon.paste.and.showed.catalytic.effect.on.the.re-oxidation.of.NADH.formed.by.the.enzymatic.
reaction.
Methanol.can.be.catalytically.oxidized.by.various.metals.mainly.from.the.iron-platinum.group.
[134,841,2666-2668]..Ni(II)-complexes.can.be.used.as.well;.probably.they.act.via.NiO(OH).which.
is. formed. in. the. strongly. alkaline. medium. [131,424,2669-2671];. multi-wall. carbon. nanotubes.
(MWCNT).may.support.the.oxidation.[424]..Sulfur.analogues.of.alcohols,.thiols.(organic.mercap-
tans),.can.be.oxidized.easily.with.catalysts.such.as.cobalt.phthalocyanine.(see.Section.8.4.5);.thiols.
can.also.be.determined.by.the.use.of.sulfhydryl.oxidase.as.amperometric.detector.for.chromato-
graphic.separations.[2672].