Biomedical Engineering Reference
In-Depth Information
that.are.dependent.on.size,.shape,.etc.,.have.long.been.exploited,.for.instance,.in.decorative.
pigments. during. the. Roman. period.. They. exhibit. a. strong. UV-vis. absorption. band. not.
present.in.the.spectrum.of.the.bulk.material,.near.IR..The.origin.of.the.light.absorption.by.
these.noble.metal.nanoparticles.is.the.collective.oscillation.of.the.conduction.band.elec-
trons.induced.by.the.interacting.electromagnetic.ield..The.absorption.band.arises.when.
the.incident.photon.frequency.is.resonant.with.the.collective.oscillation.of.the.conduction.
electrons.and.is.commonly.known.as.surface.plasmon.resonance.(SPR)..In.addition.to.the.
transverse. plasmon. band. for. Au. nanoparticles. observed. at. regions. around. 520. nm,. an.
additional,.stronger.band.at.longer.wavelengths.arising.from.longitudinal.plasmon.oscil-
lation.is.observed.for.gold.nanorods.
34
.The.longitudinal.plasmon.resonance.can.be.shifted.
into.the.NIR.region.by.increasing.the.nanorod.aspect.ratio.
35
These.optical.properties,.in.particular.those.of.gold.colloids,.have.long.been.used.as.
biomolecular. labels. in. electron. microscopy
36-38
. because. they. are. much. more. electron.
dense.than.organic.molecules,.and.hence.yield.excellent.contrast.in.comparison.to.cells.
and.tissues..Exploitation.of.gold.nanoparticles.as.optical.markers.in.dark.ield.micros-
copy.of.biological.samples.can.be.regarded.as.a.more.recent.development,.as.this.area.
has. always. been. dominated. by. luorescence. dyes.. These,. however,. have. limitations,.
such. as. photobleaching. and. often. a. relatively. high. detection. threshold,. while. light.
scattering.by.gold.nanoparticles.even.has.the.potential.for.single-particle.detection.
39
.
This.opens.the.ability.to.track.single.molecules.within.a.cell,.and.hence.to.monitor.sin-
gle.molecular.binding.events.in.biological.systems.
39
.Owing.to.the.limitations.accom-
panying. the. use. of. luorescent. dyes,. gold. and. silver. nanoparticles,. and. in. particular.
semiconductor.quantum.dots,.have.now.gained.popularity.as.alternative.biolabeling.
probes.
40-44
. Apart. from. their. unique. optical. properties,. gold. and. silver. nanoparticles.
are. also. known. to. act. as. strong. Raman. enhancers,
45,46
. and. may. also. amplify. luores-
cence. under. certain. circumstances.
47.
An. important. requirement. for. their. biological.
applications,.apart.from.making.them.stable.in.aqueous.environments,.is.the.ability.to.
attach.biomolecules.of.interest.
24.3.2 Nanomaterials Preparation
The.preparation.of.gold.and.silver.nanoparticles.has.indeed.become.an.area.of.consider-
able. interest. in. both. applied. and. fundamental. research,. largely. due. to. their. fascinating.
properties.and.attractive.potential.applications.in.electronics,
48
.optics,
49-53
.biomedical.and.
bioanalytical.areas,.etc.
54-57
.It.is.documented.that.ever.since.Faraday
58
.published.a.report.
on.the.synthesis.and.properties.of.colloidal.gold,.a.reasonable.insight.into.the.nature.of.
gold.sols.(nucleation,.growth,.and.kinetics.of.coagulation).has.developed.
59.
Considerable.
attention.has.been.drawn.during.the.last.few.decades.to.develop.and.optimize.methods.
for.the.preparation.of.gold
60-63
.and.silver
64,65
.nanoparticles..Ongoing.research.in.this.ield.
has. primarily. been. focused. on. developing. stable. monodisperse. particles. of. controllable.
size.and.shape,.as.it.is.generally.believed.that.current.and.novel.potential.applications.of.
nanostructured.materials.are.dependent.on.these.properties.
The.terms.
colloids
.and.
clusters
.are.often.used.interchangeably.in.the.literature..The.cri-
teria.to.distinguish.clusters.(particles.described.herein.as.ligand-stabilized.or.monolayer-
protected. clusters. (MPCs)). from. colloids. (electrostatically. stabilized;. note. that. this.
can. be. confusing. since. others. prefer. “charge. and. ligand. stabilized”). are. based. on. the.
notion. that. clusters. are. isolable,. reproducible. molecules. with. a. precise. composition.
and. structure,. while. colloids. are. much. less. deined. and. typically. have. a. distribution.
of.size.(Figure 24.1).
66
.Basically,.the.preparation.of.ligand-stabilized.nanoclusters.dates.
