Biomedical Engineering Reference
In-Depth Information
brown.color.and.damped.plasmon.absorption.band.in.the.UV-vis.spectrum..This.plas-
mon.absorption.band.even.disappears.completely.when.very.small.clusters.are.formed..
Fast. addition. of. the. reducing. agent. and. cooling. the. reaction. mixture. again. lead. to. the.
formation.of.small.clusters,.with.improved.monodispersity.
72,73
.In.order.to.isolate.a.high.
yield.of.very.small.clusters.(∼2.nm),.the.reaction.must.be.carefully.terminated.immedi-
ately.after.reduction.
74,75
Templeton.et.al..demonstrated.the.usefulness.of.this.method.by.carrying.out.extensive.
investigations,. particularly. to. show. that. this. method. tolerates. various. modiications,. to.
include.straight-chain.alkanethiols,.glutathione,.tiopronin,.thiolated.PEG,.
p
-mercaptophe-
nol,.aromatic.alkane.thiol,.phenyl.alkane.thiol,.and.mercaptopropyl.trimethoxysilane.as.
stabilizing.agents.
76
.Murray's.group.also.characterized.these.materials.to.gain.insight.into.
their.physical.and.chemical.properties.
Upon.understanding.the.three-dimensional.(3D).self-assembly.process.of.these.mate-
rials,. in. contrast. to. the. two-dimensional. (2D). self. assembled. monolayers. (SAMs). of.
ligand. molecules. adsorbed. on. the. metal. surface,. Murray. coined. the. name.
monolayer-
protected clusters
.(MPCs).to.distinguish.them.from.2D.SAMs..Many.laboratories.today.
employ.this.original.protocol.for.the.size-selective.formation.of.MPCs.by.varying.the.
ratio. between. Au. and. HS-R,. including. the. preparation. of. very. small. Au. MPCs.
77
. Of.
particular.importance.again.has.been.the.ability.to.control.the.size.distribution.of.these.
MPCs,. since. it. is. known. that. their. optical. properties. depend. on. size. and. shape.. This.
has. been. achieved. by. using. a. number. of. techniques,. such. as. solvent. fractionation,
73
.
continuous. free-low. electrophoresis,
73,78
. size. exclusion. chromatography,
79
. and. high-
performance. liquid. chromatography. (HPLC).
80
. Successive. fractionation. appears. to. be.
a.reliable.means.of.obtaining.narrow.size.distributions.by.taking.advantage.of.allow-
ing. bigger. particles. to. aggregate. irst. due. to. the. stronger. van. der. Waals. interaction.
between.the.larger.spheres.
81
.The.use.of.chromatographic.techniques.for.the.separation.
and.determination.of.the.composition.of.MPCs.appears.more.attractive,.since.it.is.prac-
tically.not.possible,.using.TEM,.to.distinguish.the.size.and.composition.of.the.ligand.
shell,. but. size. exclusion. chromatography. does.
79
. As. shown. in. Figure 24.2,. the. assess-
ment.of.ligand.attachment.in.different.proportions.of.mixed.neutral.and.carboxylated.
(charged).ligands.on.gold.nanoparticles.was.investigated.using.horizontal.agarose.gel.
electrophoresis.. Increasing. the. carboxylated. ligand. from. 1%. (lane. 1). to. almost. 100%.
(lane.4).carboxy.demonstrates.the.different.degrees.of.mobility.on.gel.in.line.with.the.
1
2
3
4
-
+
FIGURE 24.2
A.typical.example.of.agarose.gel.electrophoresis.of.14.nm.gold.nanoparticles.stabilized.by.a.mixture.of.neutral.
and.carboxylated.ligands.at.different.ratios.(lanes:.1.=.1%,.2.=.10%,.3.=.50%,.4.=.100%.carboxylated.ligand)..The.
migration.of.gold.MMPCs.on.the.gel.increases.relative.to.the.amount.of.charge.on.their.surface.
