Gold Nanoparticles in Biomedicine - Bionanotechnology: Global Prospects - page 57

3.4.1.2 Thiolated AuNPs
The. Brust-Schiffrin. method. produces. thiolated. AuNPs. with. a. size. range. from. 1. to. 6.
nm. (depending. on. the. gold/thiol. ratio). by. reacting. the. AuCl 4 - . salt. with. sodium. boro-
hydride.in.the.presence.of.a.thiol.ligand..Brust.et.al. 35 .used.a.two-phase.system,.where.
tetraoctylammonium.bromide.is.used.to.transfer.AuCl 4 - .from.aqueous.solution.to.tolu-
ene,.followed.by.reduction.using.sodium.borohydride.in.the.presence.of.dodecanethiol..
This.yielded.1-3.nm.thiol-stabilized.AuNPs..In.a.later.experiment,.Brust.et.al. 36 .further.
developed.the.method.to.a.single-phase.reduction.of.AuCl 4 - .by.sodium.borohydride.to.
form.2.nm.gold.nanoclusters.stabilized.by.mercaptophenol..The.Brust-Schiffrin.method.
has.thus.paved.the.way.for.the.simultaneous.synthesis.and.functionalization.of.AuNPs.
with.thiol.groups..The.thiol.groups.impart.higher.stability.to.AuNPs.and.can.be.used.to.
conjugate.AuNPs.to.other.functional.groups.
3.4.1.3 Gold Nanoshells
For.gold.nanoshells,.a.dielectric.core,.such.as.silica,.is.used.in.a.four-step.process.devel-
oped.by.Oldenburg.et.al. 37 .to.produce.a.silica.core.enclosed.within.a.thin.gold.shell..The.
silica. nanoparticles. are. synthesized. using. the. Stober. method, 38. where. alkyl. silicates. are.
hydrolyzed. followed. by. the. condensation. of. silicic. acid. in. alcohol. in. the. presence. of. an.
ammonia.catalyst..The.silica.nanoparticles.then.undergo.amination.by.means.of.surface.
adsorption.of.organosilane.(3-aminopropyltriethoxysilane),.with.amine.terminals.extend-
ing. from. the. surface. of. the. silica. nanoparticles.. A. gold. colloid. solution. is. prepared. by.
reduction.of.HAuCl 4 .by.tetrakis(hydroxymethyl)-phosphonium.chloride.(THPC),.produc-
ing.1-2.nm.sized.spherical.AuNPs. 39 .Addition.of.the.colloidal.gold.to.the.silica.nanopar-
ticle. solution. leads. to. their. attachment. via. covalent. bonding.. The. Au-silica. particles. are.
then.added.as.seeds.to.a.solution.of.aged.HAuCl 4 .and.potassium.carbonate,.and.reduced.
by. sodium. borohydride. or. citrate,. to. give. the. inal. uniform. thin. gold. shell. with. a. silica.
core..The.aspect.ratio.(core/shell.ratio).is.controlled.by.varying.the.size.of.the.silica.core.
or.the.initial.colloidal.AuNPs..Nanoshells.are.typically.synthesized.with.total.diameters.
between.80.and.150.nm, 4,5 .though.smaller.nanoshells.with.diameters.between.10.and.60.
nm.have.also.been.produced. 40 .Alternatively,.the.silica.core.can.be.replaced.with.an.iron.
oxide.core.to.give.sizes.between.45.and.60.nm. 41
3.4.1.4 Gold Nanorods
The.seed-mediated.two-step.method.is.most.popular.for.the.synthesis.of.gold.nanorods. 42 .
In.the.irst.step,.sodium.borohydride.is.added.to.a.solution.of.cetyl.trimethylammonium.
bromide. (CTAB). and. tetrachloroauric. acid. (HAuCl 4 ). to. form. the. initial. gold. seeds.. In.
the.second.step,.the.gold.seeds.are.then.added.to.a.solution.of.CTAB.with.silver.nitrate,.
HAuCl 4 , . and.ascorbic.acid.as.the.reducing.agent..
There.are.numerous.modiications.that.have.been.made.to.this.method..For.example,.to.
obtain.longer.rods,.and.thereby.a.higher.aspect.ratio,.the.rods.produced.in.the.second.step.
are.used.as.seeds.in.a.third.step. 43 .Alternatively,.longer.nanorods.may.also.be.produced.
by.omitting.silver.nitrate.from.the.growth.solution,.giving.an.aspect.ratio.of.up.to.25. 44,45.
Gold.nanorods.can.also.be.synthesized.by.using.a.cosurfactant.of.benzyldimethyl-hexa-
decyl.ammonium.chloride.(BDAC).and.CTAB. 42,44,45 .Other.controlling.parameters.include.
the. size,. shape,. and. concentration. of. the. seeds;. the. pH. and. temperature. of. the. growth.
solution 46,47 ;.and.the.growth.time, 42 .concentrations.of.ascorbic.acid,.surfactant,.and.initial.
gold.precursor. 48