Nanofluidics - The Nanobiotechnology - page 424

of. biomolecules. toward. various. biomedical. applications,. which. have. already. been.
extensively.reported.in.microluidics.(Smith.et.al..1992;.Lyubchenko.et.al..1993;.Perkins.
et.al..1994,.1995).
18.2.2 electrokinetics in Nanofluidics
Electrokinetics.is.the.most.important.driving.force.in.nanoluidics..Major.electrokinetic.
phenomena. include. EP,. the. motion. of. charged. objects. relative. to. the. stationary. liquid.
by. an. applied. electric. ield;. electroosmosis. (EO),. the. motion. of. ionic. liquid. relative. to.
the. stationary. charged. surface. under. the. external. electric. ield;. and. dielectrophoresis.
(DEP),. the. phenomenon. caused. by. the. gradient. of. nonuniform. AC. or. DC. electric. ield.
and.the.difference.in.electrical.permittivities.of.medium.and.polarized.particles.(Shaw.
1992)..DEP.is.very.important.and.eficient.in.manipulating.particles.and.biomolecules.of.
small.charge.or.no.charge.at.all..For.more.information,.please.refer.to.Chapters.17.and.
19..Here,.we.only.discuss.properties.of.EP.and.EO.and.their.interaction.on.the.movement.
of.a.charged.object.
EP. of. a. rigid. particle. depends. on. both. its. size. and. the. thickness. of. the. surrounding.
counterion. cloud,. which. forms. an. electric. double. layer. (EDL). structure. (Probstein. 1994;.
Viovy. 2000).. The. counterion. cloud. deforms. in. the. electric. ield. due. to. the. force. balance.
between.the.external.electric.ield.and.the.static.electric.potential.of.the.charged.particle..
Essentially,.the.EP.of.a.charged.rigid.particle.is.its.movement.with.its.surrounding.deformed.
counterion.cloud..Nanostructures.affect.the.electrophoretic.mobility.of.a.rigid.particle.in.
that. they. conine. or. limit. the. distribution. of. the. counterion. cloud.. Also,. van. der. Waals.
forces.between.nanostructures,.rigid.particle,.and.ions.might.have.a.nonnegligible.effect..
EP.of.a.lexible.polyelectrolyte.in.nanoluidics.is.usually.hard.to.predict,.since.we.might.
need.to.consider.intramolecular.forces.between.different.segments.of.the.polyelectrolyte.
and.interactions.between.the.polyelectrolyte.and.nanostructures.such.as.walls,.pillars,.or.
posts.. This. is. the. situation. we. often. face. in. DNA. separation. (see. Section. 18.4.1).. If. walls.
of.nanostructures.are.charged,.the.effect.of.EO.needs.to.be.incorporated.with.EP.and.the.
situation.becomes.very.complicated.
EO.is.usually.shown.in.the.form.of.the.EO.low.(EOF),.which.is.caused.by.the.viscous.
drag.of.moving.ions.in.diffusion.layer.of.the.EDL.(another.ionic.layer.is.called.the.stern.
layer,. which. is. formed. by. immobile. ions. on. the. charged. surface).. EO. in. nanoluidics. is.
hard. to. explain. without. considering. the. nanoconinement. effect. on. the. distribution. of.
co-ions. and. counterions,. especially. when. the. thickness. of. the. EDL. is. comparable. to. or.
larger.than.the.size.of.nanochannel.(i.e.,.the.overlapping.of.EDL)..For.example,.both.ion.
depletion.and.ion.enriching.are.observed.in.nanochannels.when.the.EDL.overlaps.(Pu.et.
al..2004)..However,.the.quantitative.description.of.such.phenomena.needs.the.numerical.
simulation.to.solve.distributions.of.cations.and.anions.
As. we. mentioned,. EOF. also. strongly. affects. the. movement. of. a. charged. particle. or.
biomolecule. if. the. surface. charge. of. nanostructures. is. large. enough.. The. effect. of. the.
EO-EP.interaction.on.dynamics.of.charged.particles.and.biomolecules,.however,.has.not.
received. enough. attention,. although. such. interaction. in. microluidics. can. generate. very.
complicated.movements.of.charged.objects.(Juang.et.al..2005;.Hu.et.al..2006;.Wang.et.al..
2007).. EO-EP. interaction. in. nanoluidics. would. be. more. complicated.. For. example,. it.
can.produce.asymmetric.transport.phenomenon.in.polymeric.nanonozzle.arrays.(Wang.
et. al.. 2008a).. It. is. found. that. electrokinetic. migration. of. rigid. nanoparticles. is. easier.
in. the. diverging. direction,. while. lexible. DNA. molecules. transport. with. less. resistance.
in. the. converging. direction.. The. EO-EP. interaction. can. become. extremely. complicated.