Electroanalysis with Carbon Paste–Based Electrodes, Sensors, and Detectors - Electroanalysis with Carbon Paste Electrodes - page 204

Neurotransmitters.on.catechol.base.can.be.enzymatically.oxidized.with.HRP.and.hydrogen.per-
oxide.[2697]..Noradrenaline.( syn ..norepinephrine).is.detectable.with.unmodiied.CPEs.by.differen-
tial.pulse.voltammetry.[3020].or.by.amperometric.detection.in.HPLC.[3021]..Hydroquinones.(after.
oxidation).and.chloranil.catalytically.oxidize.the.analyte.[442,991,3022].
Serotonin.was.determined.either.with.unmodiied.carbon.paste.electrodes.[659,2737,2738,3023].
or. with. modiied. dodecyl. sulfate. [1657],. mesoporous. silicagel. [1852],. or. a. clay. mineral. [1011]. to.
facilitate.accumulation.and.oxidation.

8.4.9 n uCleiC a Cid , n uCleiC b ases , and r elaTed C oMpounds
Table.8.23.surveys.electrochemical.studies.with.nucleic.acids.or.their.constituents..These.bioess-
ential.substances.have.attracted.some.attention.also.by.electrochemists.[3024-3028]..Apart.from.
mercury,.they.adsorb.strongly.to.carbon;.thus,.carbon.paste.electrodes.have.been.widely.applied.in.
studies.with.nucleic.acids,.because.they.allow.simple.addition.of.additional.modiiers.to.character-
ize.the.target.molecules.and.interactions.with.them.
Apart. from. studies. on. the. constituents. of. nucleic. acids. (bases,. nucleosides,. and. nucleotides),.
investigations.may.be.categorized.mainly.into.three.major.topics:

. 1.. Adsorption.studies.including.stripping.analysis

. 2.. Interaction.studies.with.adsorbed.nucleic.acids.(as.modiiers).to

. a.. Accumulate.other.analytes

. b.. Study.interactions.of.substances.to.evaluate.protective.or.damaging.effects

. c.. Investigate.possibilities.for.substances.as.reporter.(indicator).molecules.by.intercalation

. 3.. Hybridization.studies.with.single-stranded.DNA.or.oligomers

Adenine,.guanine,.cytosine,.and.thymine.can.be.oxidized.directly,.but.the.corresponding.potentials.
are.rather.in.the.positive.range.starting.at.around.+0.7.V.vs..SCE.with.guanine.and.increasing.in.
the.order.guanine.<.adenine.<.thymine.<.cytosine.([328,723,1901,3029-3031,3268];.see.also.Figure.
5.8)..Binding.to.ribose.or.deoxyribose.and.polymerization.to.nucleic.acids.does.not.change.the.elec-
trochemical.behavior.signiicantly..Therefore,.nucleotides.and.oligonucleotides.can.be.accumulated.
and.directly.oxidized.[3033]..Direct.oxidation.of.nucleic.acid.relies.on.the.oxidation.of.the.bases,.but.
mainly.the.guanine.signal.is.monitored.because.it.appears.lowest.in.potential.
Often. background. current. correction. is. performed. to. improve. the. signal. evaluation;. carbon.
pastes.with.a.wide.positive.potential.window.are.preferable;.this.is.particularly.necessary.if.also.
other.bases.are.monitored.by.direct.oxidation.
Preanodization.in.an.oxidizing.medium.creates.strongly.oxidizing.groups.at.the.carbon.paste.
surface. which. facilitate. the. accumulation. of. oxidized. adenine. under. open. circuit. conditions.
[3030]..Ionic.liquids.usually.improve.the.resolution.of.signals.[723].and.even.facilitate.the.distinc-
tion.between.guanine.and.guanosine.[412,484]..Additives,.such.as.cyclodextrin.[1421].or.mont-
morillonite.[1066],.improve.the.signal..It.is.also.possible.to.use.electrocatalysts,.such.as.cobalt.
phthalocyanine.[907,1043],.Mo(VI).complexes.[997],.and.hexacyanoferrates.[1033,1038]..Inosine,.
a.nucleoside.with.hypoxanthine.bound.to.ribose.and.able.to.pair.with.adenine,.cytosine,.and.ura-
cil,.is.not.electroactive.but.can.be.detected.as.a.copper.complex.with.La-hydroxide.nanowires;.
the.same.is.possible.with.uracil.[3032]..A.biosensor.for.nucleosides.with.nucleoside.oxidase.has.
been.developed.[788].
DNA,.single.or.double.stranded,.and.RNA.easily.adsorbs.on.the.surface.of.carbon.paste.elec-
trodes.[361,474,491,629,661,3034-3036]..Usually.the.electrodes.are.preanodized,.and.adsorption.is.
performed.at.a.positive.potential.around.0.5.V.vs..SCE..Ionic.liquids.or.carbon.nanotubes.improve.
the.analytical.conditions.by.decreasing.the.oxidation.potentials.[444,661,918,394]..Dissolved.DNA.
has.been.used.to.detect.the.voltage-dependent.release.of.ethidium.from.ethidium-tetracyanoquinodi-
methane. by. complex. formation. and. laser-induced. luorescence. [3037,3038].. Sorption. of. DNA. on.