Noble Metal Nanoparticles in Drug Delivery - Bionanotechnology: Global Prospects - page 484

O

O

O

O

Au

O

O

O

OH

O

O -

O -

O

O -

i

NaBH 4

Au S

HAuCl 4

HS

ii

iii

Au

FIGURE 24.1
Synthesis. of. gold. nanoparticles. by. the. reduction. of. the. gold(III). salt. by. (i). citrate. reduction/capping. or. col-
loids,.(ii).the.Brust.biphasic.method,.and.(iii).the.direct.reduction.route.in.the.presence.of.a.bi-/multifunctional.
capping. ligand.. Both. i. and. ii. are. regarded. as. ligand-stabilized. or. monolayer-protected. clusters. in. this. case..
However,.note.that.others.would.prefer.i.as.ligand-stabilized.particles..Silver.nanoparticles.are.prepared.in.the.
same.way.

back.to.1981.to.the.work.that.was.carried.out.at.the.University.of.Essen.in.Germany.by.
Schmid.et.al..for.the.preparation.and.characterization.of.Au 55 (PPh 3 ) 12 Cl 6 .nanoclusters 67 .
(Figure 24.1).. These. indings. have. led. to. the. development. and. exploitation. of. ligand-
stabilized.metal.nanoclusters.in.diverse.areas,.including.catalysis, 68 .electronics,.etc..This.
approach.had.some.drawbacks.and.limitations,.including.lack.of.stability..However,.it.
was.later.realized.that.organic.sulfur.compounds.form.stable.self-assembled.monolay-
ers.on.gold.surfaces, 69 .and.interestingly,.it.was.also.discovered.that.thiols.can.be.used.to.
stabilize.gold.colloids. 70
In.1994,.the.irst.facile.synthesis.of.very.stable,.isolable.gold.clusters.was.developed.in.
Liverpool.(Figure 24.1). 71 .The.evolution.of.this.work.has.led.to.the.preparation.of.metal.
nanoparticles.of.deined.size.and.shape,.and.has.been.exploited.in.diverse.applications..
This.approach.takes.advantage.of.using.tetraoctyl.ammonium.bromide.as.a.phase.trans-
fer. catalyst,. to. transfer. aqueous. chloroauric. acid. to. the. organic. phase. (toluene). to. form.
an.intensely.orange.Au(III).complex.(tetraoctylammonium.tetrabromoaurate)..Reducing.
this.complex.in.the.presence.of.thiols.ligands,.mostly.alkanethiols,.by.NaBH 4. allows.the.
formation. of. small. clusters. (1-5. nm),. the. diameter. of. which. is. inluenced. by. a. number.
of. factors,. such. as. the. thiol:gold. ratio.. It. was. found. that. the. use. of. a. larger. thiol:gold.
molar.ratio.produces.very.small.clusters, 72 .which.are.characterized.by.their.unique.dark.