Biomedical Engineering Reference
In-Depth Information
section.
3
.Nanoshells.largely.in.use.range.in.size.from.80.to.150.nm.
4,5
.Smaller.nanoshells.
ranging.from.20.to.60.nm.with.a.core.of.iron.oxide.nanocrystals.have.also.been.explored.
6.
An.advantage.of.using.iron.oxide.is.its.magnetic.properties,.which.can.be.harnessed.for.
example.in.directing.the.magnetic.AuNPs.to.a.target.site.using.an.external.magnet.
Though. the. spherical. gold. nanostructures. have. enjoyed. wide. popularity. and. applica-
tions,.they.do.have.their.limitations.with.regard.to.their.physical.properties,.such.as.their.
limited.tunability.into.the.near-infrared.(NIR).region.
7
.This.urged.scientists.to.develop.new.
and.innovative.methods.by.which.anisotropic.nanoparticles.can.be.obtained,
8,9
.paving.the.
way.for.a.multitude.of.different.shapes.and.sizes.of.AuNPs,.as.well.as.applications..Now.
AuNPs.can.be.synthesized.with.geometries.such.as.nanorods,
7,10
.nanocages,
11,12
.triangular.
nanoprisms,
7,13-15
.nanocubes,
7,15-17
.and.ellipsoidal,
18
.bipyramidal,
17
.and.branched
7,17
.AuNPs.
Gold. nanorods,. as. opposed. to. the. spherical. shapes,. have. two. dimensions:. length.
and. diameter,. also. referred. to. as. short. and. long. axes,. with. transverse. and. longitudinal.
bands.
19
Gold.nanocages,.on.the.other.hand,.are.hollow.nanocubes,.like.boxes,.with.holes.
in.their.walls.
20
3.3 Properties
3.3.1 Optical Properties (Radiative)
3.3.1.1 Surface Plasmon Resonance
Metals. have. conductive. electrons,. which,. upon. undergoing. collective. excitation,. are.
referred. to. as. plasmons.
21
. The. free. conduction. band. electrons. on. the. surface. of. AuNPs.
interact.with.the.electromagnetic.component.of.light.and.oscillate.in.a.collective.dipolar.
manner,. known. as. surface. localized. plasmon.
21
. When. the. surface. plasmon. frequency. is.
close.to.that.of.the.excitation.of.light,.it.is.referred.to.as.surface.plasmon.resonance.(SPR),.
which.lies.within.visible.frequencies.
22
Controlling.the.size.and.shape.of.AuNPs.alters.their.properties.by.controlling.the.motion.
and.degree.of.coninement.of.their.electrons.
19
.This.is.why.AuNPs.exhibit.properties.that.
are.very.different.from.gold.in.its.bulk.form,.and.surface.properties.are.more.pronounced.
and.dominant.
19,22
.As.a.result,.the.SPR.enhances.the.radiative.properties.of.AuNPs,.such.as.
light.scattering.and.absorption.
19
The.size,.shape,.and.composition.of.the.gold.nanostructures.affect.their.SPR.properties..
The.SPR.of.gold.nanospheres.lies.within.the.visible.region,.and.increasing.their.size.causes.
a. limited. shift. into. the. NIR.
19
. In. nanoshells,. changing. the. shell. thickness-to-core. ratio.
causes.a.greater.shift.in.SPR.from.the.visible.region.to.as.far.as.the.mid-infrared.region.
23,24
.
Gold.nanorods,.on.the.other.hand,.have.two.axes,.upon.which.oscillations.occur.in.two.
directions..The.short.axis.has.a.transverse.SPR.band.similar.to.that.of.a.nanosphere,.and.
therefore.exhibits.similar.absorption.and.scattering.properties.in.the.visible.region,.while.
the. longitudinal. axis. exhibits. a. much. stronger. absorption. band.
19
. The. nanorods. can. be.
tuned.to.absorb.650-1,500.nm,
21
.not.necessarily.by.changing.the.diameter.or.short.axis,
18
.
but.rather.by.changing.the.aspect.ratio:.length/diameter.
21
.The.higher.the.aspect.ratio,.the.
more.red.shifted.the.longitudinal.axis.becomes.
19
.In.nanospheres,.nanoshells,.and.nano-
rods.at.a.ixed.aspect.ratio,.absorption.increases.in.smaller.sizes.and.scattering.increases.
in.larger.sizes..Thus,.a.nanoparticle.of.20.nm.mainly.absorbs.incident.light.with.little.or.
no. scattering,. whereas. for. a. nanoparticle. of. 40. nm,. scattering. begins. to. emerge,. and. its.
