Gold Nanoparticles in Biomedicine - Bionanotechnology: Global Prospects - page 56

contribution. to. total. light. extinction. (sum. of. absorption. and. scattering). increases. as. the.
particle. size. increases. 25 . This. allows. the. utilization. of. AuNPs. with. different. scattering/
absorption. properties. in. different. applications.. For. example,. large. nanoparticles. could.
be.used.in.relectance-based.imaging.due.to.their.higher.scattering.abilities,.while.small.
nanoparticles.could.be.used.in.absorption-based.imaging.due.to.their.higher.absorption.
abilities,.or.in.photothermal.therapy,.where.light.energy.is.converted.into.heat. 25
The. absorption. and. scattering. wavelengths. are. determined. by. the. size. and. shape. of.
AuNPs,.which.account.for.their.different.colors..Nanospheres.of.20.nm.have.SPR.wave-
lengths.in.the.520.nm.visible.region,.appearing.as.red, 26 .while.nanospheres.larger.than.100.
nm.have.a.grimy.yellowish.color. 27 .Nanospheres.generally.exhibit.a.yellow,.orange,.red,.
pink,. or. purple. color;. nanorods:. green,. blue,. magenta 28 ;. triangular. nanoprisms:. yellow,.
orange,. pink,. or. green. 29 . These. color. ranges. depend. on. particle. size. and. aspect. ratio,. as.
well.as.the.refractive.index.of.the.solvent.they.are.in. 26
3.3.1.2 Plasmon-Plasmon Interactions
Another.property.of.AuNPs.is.the.plasmon-plasmon.interaction..Spherical.AuNPs.are.syn-
thesized.in.the.form.of.a.colloidal.solution.that.has.an.intense.red.color..If.the.AuNPs.are.
brought.closer.to.one.another,.electromagnetic.coupling.occurs.and.the.AuNPs.aggregate. 30 .
These.interactions.between.adjacent.gold.nanoparticles.lead.to.a.decrease.in.the.SPR.energy.
absorption.band..This.increases.the.absorbance.of.the.red.wavelength.by.the.aggregated.
AuNPs 30 .and.changes.the.colloid.color.from.red.to.blue:.a.phenomenon.known.as.the.red.
shift. 24,31,32 .Some.changes.in.conditions.may.induce.the.aggregation.of.AuNPs,.such.as.the.
addition.of.NaCl.to.citrate.capped.or.negatively.charged.AuNPs..Citrate-capped.colloidal.
AuNPs.are.negatively.charged.and.remain.in.suspension.due.to.electrostatic.repulsions..
The.addition.of.NaCl.neutralizes.these.negative.charges,.and.the.AuNPs.become.destabi-
lized.and.aggregate,.the.color.of.their.solution.turning.from.red.to.blue..This.property.is.
especially.useful.in.colorimetric.detection.methods.in.diagnostic.applications.
3.3.2 Physical Properties (Nonradiative)
When. AuNPs. absorb. light,. the. electron. oscillations. lead. to. electron-electron. collisions,.
which.convert.light.energy.into.heat. 33 .This.is.followed.by.a.transfer.of.the.energy.through.
electron-phonon. interactions,. causing. a. further. rise. in. temperature.. The. fate. of. the. heat.
generated.is.either.to.be.transferred.to.the.surrounding.environment,.or.melt.or.ablate.the.
nanoparticles.
3.4 SynthesisofAuNPs
3.4.1 Chemical Synthesis
3.4.1.1 Colloidal AuNPs
Colloidal.AuNPs.can.be.synthesized.chemically.by.the.reduction.of.tetrachloroauric.acid.
(HAuCl 4 ).using.a.reducing.agent. 21 .Citrate.is.commonly.used.as.both.a.reducing.and.cap-
ping.agent.when.added.to.HAuCl 4 .in.water, 23 .producing.AuNPs.with.an.average.size.of.
10-60.nm. 34 .The.size.can.be.controlled.by.changing.the.gold-to-citrate.concentration.ratio.