Biomedical Engineering Reference
In-Depth Information
composition.of.mixed.MPCs..This.shows.that.gel.electrophoresis.is.the.most.powerful.
tool. to. determine. the. degree. of. functionalization. on. monolayer-protected. clusters. or.
mixed.monolayer-protected.clusters.
The.scope.of.MPC.synthesis.is.not.limited.to.the.1-5.nm.region,.since.it.was.realized.that.
the. basic. two-phase. protocol. in. the. absence. of. thiols. leads. to. the. formation. of. somewhat.
larger,.stable.nanoparticles..The.stability.of.these.particles.is.provided.by.the.bromides.and.
tetraoctyl. ammonium. ions,. which. adsorb. to. the. surface. of. the. particles.. These. tetraoctyl.
ammonium.bromide.(TOABr)-stabilized.particles.are.in.the.5-8.nm.size.region..Increasing.
the.dielectric.constant.of.the.medium,.e.g.,.by.adding.alcohols.to.the.particles,.leads.to.the.
formation.of.irreversible.aggregates;.however,.they.have.been.used.as.readily.available.start-
ing.materials.for.the.construction.of.self-assembled.mono-.and.multilayers.on.different.sub-
strates.since.the.ionic.capping.can.be.easily.removed.by.thiols.and.amine.functionalities.
82
Apart. from. the. two-phase. solution. synthesis,. single-phase. methods. have. also. been.
developed,. to. obtain. MPCs. of. comparable. sizes,. including. the. use. of. NaBH
4
. to. reduce.
gold. in. an. alcoholic. solution-containing. thiols.
83.
However,. this. is. only. applicable. when.
the.thiols.to.be.used.are.soluble.in.alcohols..In.essence,.particles.with.both.hydrophobic.
and.hydrophilic.characteristics.are.obtainable,.as.was.demonstrated.using.mercaptound-
ecanoic.acid.(MUA).as.a.stabilizing/capping.agent.
84
Again,.the.5-8.nm.particles.are.not.limited.to.the.two-phase.approach;.MPCs.of.about.
6 nm.can.also.be.prepared.by.irst.preparing.borohydride-stabilized.particles,.which.can.
be.directly.converted.to.the.desired.MPCs..Even.larger.particles.(10-40.nm).can.be.pre-
pared. following. the. classical. Frens. method
60.
of. using. citrate. as. the. reducing. and. stabi-
lizing.agent..By.this.widely.used.approach,.gold.and.silver.MPCs.of.different.sizes.and.
shapes.can.be.prepared..The.adsorption.of.citrate.ions.to.the.cluster.surface.is.essential.to.
electrostatically.overcome.the.van.der.Waals.attraction,.and.hence.to.preserve.the.stabil-
ity.of.the.metal.nanoparticles..These.particles.can.be.easily.converted.to.MPCs.by.a.self-
assembly.process,.as.demonstrated.by.Weisbecker.et.al.
85
Since.the.ultimate.goal.in.preparing.Au.and.Ag.MPCs.is.to.explore.their.unique.prop-
erties
86-91
. in. biological. applications,. these. materials. require. more. than. water. solubility..
Key.issues,.such.as.functionality,.stability,.and.biocompatibility,.have.to.be.addressed.in.
particular.silver.known.for.its.propensity.to.oxidatively.corrode
92,93
.and.aggregate.in.elec-
trolytic.solutions.
24.3.3 Attachment of Biomolecular Functionality on Nanoparticles
The.rationale.to.attach.biomolecules.on.nanoparticle.surfaces.is.mainly.to.use.nanopar-
ticles.as.probes.to.investigate.biological.and.intracellular.processes..Many.applications.of.
nanoparticles.in.biology.and.medicine.have.been.reviewed.by.Salata,
94.
including.(1) bio-
detection. of. pathogens,. (2). detection. of. proteins,. (3). drug. and. gene. delivery,. (4). tumor.
destruction. via. heating. (hyperthermia),. (5). probing. DNA. structure,. and. (6). luorescent.
biological.labels..All.these.applications.require.the.attachment.of.a.biomolecule.of.interest.
to.the.nanoparticle..This.bioconjugation.step.must.be.performed.under.mild.conditions.to.
retain. the. biological. activity. of. the. resulting. nanohybrid. material.. Many. strategies. have.
been.attempted.and.utilized.to.date.to.attach.biomolecules.to.nanoparticles..Reviews.by.
Kumar
95.
and. Katz. and. Willner
96
. outlined. relevant. bioconjugation. strategies. that. can. be.
categorized.as.electrostatic.binding,
97
.covalent.coupling,
98-101
.and.speciic.recognition.
102-104.
Some.of.these.strategies.are.shown.in.Figure 24.3.
Adsorption.of.biomolecules.to.the.nanoparticles.by.electrostatic.interaction.is.illustrated.
in.Figure 24.3A.and.B..Gold.and.silver.hydrosols.prepared.by.the.citrate.reduction.method,.
