Biologically Modified Carbon Paste Electrodes - Electroanalysis with Carbon Paste Electrodes - page 121

oxidases,.the.latter.for.phenol.oxidase.(see.Section.8.4,.Table.8.26)..Yeasts.may.serve.as.a.source.of.
alcohol.dehydrogenase..Algae,.mosses,.and.lichens.were.dispersed.in.carbon.pastes.and.exploited.
for.the.preconcentration.of.metal.species.. Mitochondria .have.been.immobilized.on.carbon.paste.as.
well.[1640].
5.2.6 o Ther b ioMoleCules
Other.biological.materials.used.to.modify.CPEs.include.cytochrome.C,.hemoglobin,.myoglobin.(see.
Section.8.4,.Table.8.20),.melanins,.and.heparin.[1777]..Polysaccharides.(cyclodextrins).and.chitosan.
are.used.for.accumulations.and.immobilization.
5.3 MODIFICATION STRATEGIES (FOR BIOSENSORS)
Biomolecules. have. often. a. high. afinity. toward. carbon. paste. so. that. they. can. be. either. admixed.
directly.to.the.bulk.of.the.paste.or.adsorbed.to.the.surface..Enzymes.often.maintain.their.activity.in.
the.carbon.paste.matrix,.and.can.even.show.increased.temperature.and.acid.resistance.[58,90-92]..
In.order.to.prevent.leaching,.different.strategies.can.be.applied.to.immobilize.biomolecules.
. (i). Cross-linking . with. glutaraldehyde. is. a. common. procedure. to. connect. biomolecules. via.
aminogroups.to.larger.molecular.networks.with.lower.solubility..Addition.of.bovine.serum.
albumin. (BSA). provides. even. a. large. pool. of. -NH 2 . groups. to. which. the. molecules. can.
be.attached..Other.possibilities.are.the.entrapment.in.porous.membranes.(e.g.,. Naion ® ),.
electropolymerized.layers.(e.g.,.polypyrrole),.or.behind.dialysis.membranes..Usually.mem-
branes. deteriorate. the. kinetics. of. the. accompanying. chemical. reactions. and. diffusion..
Membranes.can.be.applied.from.the.corresponding.solutions.via.drop.or.spin.coating,.by.
electrolysis.from.the.monomer,.or.by.preformed.layers.(e.g.,.dialysis.membrane).
. (ii). Electrostatic immobilization .via.surfactants,.functionalized.nanoparticles,.and.ion.exchang-
ers.is.convenient.if.the.modiier.has.an.electric.net.charge..For.instance,.popular. ionic liq-
uids .as.binders.or.modiiers.may.exert.a.very.positive.effect.(see.also.Section.2.2.3.3).
.(iii). Magnetic nanoparticles . enable. special. interactions. that. can. often. be. accomplished. by.
using. Fe 3 O 4 . for. instance.. Outside. the. core,. they. may. be. covered. with. a. functionalized.
shell,.to.which.biomolecules.can.be.bound.(for.other.details,.go.to.Sections.2.2.1.and.4.2).
. (iv). Avidin-biotin complexation . represents. another. important. way. of. immobilization..
Streptavidin. modiied. surfaces. (e.g.,. nanoparticles). and. many. biotinylated. biomolecules.
and.biocomponents.are.commercially.available..As.shown.in.Section.8.4,.the.avidin-biotin.
pair.plays.a.particular.role.with.immunoassays.
Other possibilities of modiication .. (v). Surfactants. change. the. character. of. the. carbon. paste. sur-
face.from.lipophilic.to.more.hydrophilic.and.can.be.used.as. in situ .ion.exchangers.in.case.they.are.
charged;.(vi).nucleic.acids.can.be.immobilized.at.positive.potentials.(around.+0.5.V.vs..reference.
electrode).after.activation.of.the.electrode.surface.at.very.positive.potentials.(preanodization)..Such.
(vii).preanodization.of.carbon.pastes.or.also.(viii).thermal.activation.of.the.graphite.moiety.prior.to.
preparation.of.the.paste.may.signiicantly.improve.the.analytical.signals.of.interest.
Finally,.(ix).carbon.nanotubes.and.(x).other.nanosized.particles.(both.as.plain.or.functionalized,.
see.also.Section.4.2).possess.catalytic.properties.for.many.chemical.and.electrochemical.reactions.
(e.g.,.reduction.of.H 2 O 2 ).and.can.be.therefore.useful.modiiers.or.even.main.constituents.of.the.past..
Here,.it.should.be.repeated.that.modiiers.in.the.form.of.nanoparticles.usually.manifest.consider-
ably.higher.activity.than.macro-.or.microsized.particles.due.to.their.substantially.larger.surface.in.
relation.with.the.volume.