Biologically Modified Carbon Paste Electrodes - Electroanalysis with Carbon Paste Electrodes - page 120

On.the.other.hand,.a.signal.increase.with.hybridization.will.be.observed,.if.the.reporter.molecule.
intercalates.strongly.into.the.double.helical.structure,.as.is.the.case.with.Co(II)-bipyridyl.complex.
Bovine.serum.albumin.(BSA).may.be.used.to.prevent.nonspeciic.adsorption..Peptide.nucleic.acids.
(PNA),.in.which.the.deoxyribose-phosphate-backbone.is.substituted.by.a.proteinic.structure,.may.
replace.ssDNA.with.hybridization.studies..Substitution.of.guanine.by.electroinactive.inosine.in.sin-
gle.strands.allows.monitoring.hybridization.via.the.guanine.moiety.of.the.complementary.strand.
Occasionally,.aptamers—DNA.or.RNA.oligomers.with.less.than.100.mers.(i.e.,.replicates).that.
can.bind.speciically.to.certain.biomolecules—are.used.to.speciically.bind.proteins.or.antibodies.
to.the.electrode.surface.[58,90-92].

5.2.4 i MMunosensors
Immunoreactions.involve.the.combination.of.an.antibody.with.an.antigen,.which.can.be.realized.
with. carbon. paste. electrodes.. Antibodies. are. usually. immobilized. on. the. electrode. surface;. two.
assays.are.very.common.to.determine.antigens.from.the.sample:.sandwich.and.competitive.assay.
(Figure.5.10).
For.the.sandwich.assay,.the.antibodies.are.reacted.with.the.antigens.from.the.sample;.in.an.ensuing.
step.the.antibody-antigen.complex.is.incubated.with.a.labeled.antibody;.the.label.can.be.an.enzyme.
producing.an.electroactive.product.from.an.inactive.substrate.(e.g.,.alkaline.phosphatase.with.a.pheno-
lic.phosphate.as.a.substrate).or.metal.nanoparticles.that.can.be.determined.by.dissolution.and.stripping.
voltammetry..With.sandwich.complexes.the.signal.increases.with.an.increasing.amount.of.antigen.
In.competitive.assays.a.labeled.antigen.is.added.to.the.sample,.which.competes.with.the.unla-
beled.sample.constituent.for.the.antibodies..As.a.consequence,.the.higher.the.signal.of.the.label,.the.
lower.the.concentration.of.the.antigen.in.the.test.solution..Carbon.paste.immunosensors.will.also.
be.discussed.in.Section.8.4.

5.2.5 T issues and C ells
Tissues. and. whole. cells. were. sometimes. used. as. such. to. modify. CPEs. for. two. goals:. (i). to. cir-
cumvent.the.need.to.isolate.the.corresponding.enzyme.in.its.pure.form.while.even.preserving.its.
stabilizing.environment,.and.(ii).to.bioaccumulate.analytes.(e.g.,.metal.ions).prior.to.their.voltam-
metric.detection..Mostly.plant. and .mushroom.tissues.or.their.crude.extracts.were.used.as.sources.of.

Carbon paste Carbon paste

Immobilized antibodies Immobilized antibodies

e -

e - e - e - e -

AG

P

P P P

P

E

E

S

E E

S

E

S S

S

Ab Labeled Ab Ag Labeled Ag

(A) (B)

FIGURE 5.10 Immunosensors:.sandwich.(A).and.competitive.assay.(B).