Plasmonic Nanostructures in Biosensing - Bionanotechnology: Global Prospects - page 108

5.3 ControlledSynthesisandNanofabrication
ofPlasmonicNanostructures
The.localized.surface.plasmon.resonance.exhibited.by.plasmonic.nanostructures.is.sensi-
tive.to.the.surrounding.environment.as.well.as.composition,.structure,.size,.and.shape.of.
the.nanoparticle. 8b .The.ability,.then,.to.tune.the.morphology.of.these.“sensing”.elements.
is.essential.for.LSPR.biosensing.applications..Recent.advances.in.control.over.nanoparticle.
size.and.shape.allow.electromagnetic.energy.to.be.concentrated.and.transported,.as.well.
as.plasmon.energy.to.be.transferred. 29 .When.chemical.and.biological.molecules.are.com-
bined. with. these. nanostructured. surfaces,. both. in. vitro. and. in. vivo. static. and. dynamic.
molecular.interactions.can.be.detected.
The.current.design.toolbox.for.engineering.nanostructures.with.different.morphologies.
is.based.on.nanofabrication.routes.utilizing. bottom up ,. top down ,.or.both..With.bottom-up.
methods,.metal.nanocrystals.are.generally.synthesized.using.solution-based.chemical.pro-
cesses,.which.are.also.called.assay-based.or.“wet.chemistry”.methods..The.composition,.
size,. and. shape. of. the. nanoparticles. depend. heavily. upon. control. of. chemical. reactions.
and.conditions.during.synthesis. 8a .Top-down.lithographic.techniques.provide.repeatable.
and.precise.nanopattern.generation.of.over.large.areas,.taking.advantage.of.the.well-estab-
lished.micro-.and.nanofabrication.tools.available.
5.3.1 Bottom-up Chemical Synthesis
Bottom-up. chemical. synthesis. routes. include. sol-gel,. micelles,. chemical. precipitation.
from.supersaturated.conditions,.hydrothermal,.and.pyrolysis. 30 .Achieving.speciic.size.
and. shape. of. nanoparticles. is. facilitated. by. either. adding. organic. ligands. or. capping.
materials.to.inhibit.further.crystal.growth.or.by.replacing.the.growth.environment.with.
an.inert.one. 30 .For.metal.nanoparticle.synthesis,.chemical.reduction.of.metal.salts.with.a.
stabilizer.is.often.used,.where.the.stabilizer.functions.as.a.growth.inhibitor.in.particular.
directions,. thus. controlling. shape. as. well. as. providing. colloidal. stability. 8a . A. two-step.
process. of. metal. nanoparticle. seeded. growth. is. particularly. successful,. where. diverse.
shapes.such.as.rods,.plates,.and.pyramids.are.reproducible. 8a .Typically,.these.solution-
based. synthesis. routes. have. three. stages:. irst,. nucleation;. second,. transformation. of.
nuclei.to.seeds;.and.inally,.seed.evolution.into.nanocrystals. 31 .Additional.parameters.to.
control.nanoparticle.morphology.include.reaction.time.and.temperature.as.well.as.con-
centration.of.solvents,.reagents,.and.surfactants. 30 .For.seeded.growth,.the.inal.shape.of.
the.metal.nanocrystal.is.determined.by.the.structure.of.the.seed.and.the.binding.afinity.
of.capping.meterials
Due.to.recent.signiicant.advances.in.nanoparticle.synthesis, 5a,5c .“wet.chemistry”..meth-
ods.can.now.be.used.to.fabricate.plasmonic.NPs.with.controllable.sizes,.narrow.size.distri-
butions,.and.a.wide.variety.of.shapes..Nanospheres.are.the.simplest.form.of.nanoparticles.
and.are.formed.from.seed-mediated.growth,.as.described.earlier. 32 .Due.to.shape.symmetry,.
the.plasmon.resonance.mode.for.solid.gold.colloidal.nanospheres.typically.occurs.around.
520. nm. and. can. only. be. tuned. around. that. mode. at. approximately. 50. nm. 33 . Nanorods.
provide.additional.functionality.due.to.shape.anisotropy,.which.results.in.two.plasmonic.
resonance. modes,. dependent. on. the. radial. and. longitudinal. axes. of. the. nanorod, 29 . and.
can.be.tuned.by.modifying.the.aspect.ratio. 34 .Other.fabricated.shapes.include.cubes,.tetra-
hedrons,.octahedrons,.triangular.plates,.bipyramids, 35 .and.prisms. 36 .Mixed.metallic-alloy.