Biomedical Engineering Reference
In-Depth Information
surface-enhanced. hyper-Raman. scattering,
23
. surface-enhanced. infrared. spectroscopy,
24
.
and.surface-enhanced.luorescence.
25
Raman.scattering.is.inelastic.scattering.of.a.photon.from.a.molecule.in.which.the.fre-
quency.change.precisely.matches.the.difference.in.vibrational.energy.levels.
9b
.Raman.spec-
troscopy.is.a.highly.speciic.Raman.scattering-based.technique.that.detects.and.identiies.
molecules.by.their.unique.vibrational.ingerprints..However,.as.a.scattering.process,.Raman.
scattering.is.exceedingly.weak:.typical.Raman.scattering.cross.sections.per.molecule.are.
in.the.range.of.10
-30
.and.10
-25
.cm
2
.
23
.Thus,.Raman.spectroscopy.usually.needs.a.relatively.
large.volume.of.molecules.to.produce.detectable.scattering.signal.intensities..This.disad-
vantage.has.prevented.Raman.spectroscopy.from.many.important.sensing.applications,.
such.as.surface.and.in.vivo.sensing.where.the.number.of.molecules.that.produce.Raman.
scattering.is.small..SERS.is.about.Raman.scattering.of.a.single.molecule.or.an.ensemble.of.
molecules.of.interest.appearing.(e.g.,.by.binding.and.absorbing).in.close.proximity.(within.
a.few.nanometers.of.the.nanoparticle.surface).of.a.plasmonic.nanoparticle.that.will.produce.
an.enhancement.of.the.Raman.signal..It.was.discovered.in.1974
26
.and.correctly.explained.
in.1977.
27
.Two.primary.mechanisms.are.generally.thought.to.be.responsible.for.large.SERS.
signals.
9b,14,23,28
. First,. chemical. enhancement. (CE). corresponds. to. any. modiication. of. the.
Raman.polarizability.tensors.(α
R
).upon.adsorption.of.the.molecule.onto.the.metal.surface..
It.can.result.from.changes.in.the.molecular.electronic.state.or.resonant.enhancements.from.
either.existing.molecular.excitations.or.newly.formed.charge.transfer.states..Second,.both.
optical. excitation. and. Raman. scattering. resonate. with. the. LSP. modes. in. the. metal. NPs.
simultaneously,.resulting.in.local.ield.electromagnetic.(EM).enhancement.and.radiation.
EM,.respectively.
14,28b
.The.EM.contribution.is,.in.any.case,.believed.to.be.much.larger.(>10
2
).
than.the.CE.effect.
8b
.More.speciically,
28b
.if.we.assume.that.
g
.is.the.local.ield.enhancement.
averaged.over.the.surface.of.the.nanoparticle,.the.average.magnitude.of.the.local.ield.(
Es
).
around.the.surface.will.be.
Es
.=.
g
.×.
E
0
,.where.
E
0
.is.the.magnitude.of.the.incident.ield..The.
average.molecule.on.the.surface.will.then.be.excited.by.the.enhanced.local.ield.
E
s
,.result-
ing.in.the.Raman-scattered.light.near.the.surface.with.a.ield.strength.
ER
.∝.α
R
.×.
E
s
.∝.α
R
.×.
g
.×.
E
0
..The.Raman-scattered.(or.radiated).ields.(
E
R
).at.the.Raman-shifted.wavelength.will.
be.further.enhanced.by.the.metal.nanoparticle.with.the.radiation.enhancement.factor.
g
'.
in.the.same.way.as.the.optical.excitation.was,..giving.the.magnitude.of.the.SERS-scattered.
iled:.
E
SERS
.=.α
R
.×.
g
'.×.
E
s
.∝.α
R
.×.
g
'.×.
g
.×.
E
0
..As.the.average.light.intensity.is.proportional.to.
the.square.of.its.electromagnetic.ield,.we.will.theoretically.have.the.SERS.enhancement.
factor.(assuming.that.the.difference.in.α
R
.between.SERS.and.non-SERS.(
E
non-SERS
.=.α
R
.×.
E
0
)
.
ields.can.be.ignored.compared.to.EMs):
2
I
E
SERS
SERS
EF
=
=
(
g
)
.
g
.
(5.2)
2
2
I
E
.
non SERS
−
non SERS
−
Assuming. that.
g
. ≈.
g
',. we. will. have.
EF
. fourth-power. dependence. on.
g
,. the. key. to. the.
extraordinate. enhancements. SERS. provides.. This. approximation. takes. advantage. of. the.
fact. that. the. plasmon. width. is. generally. large. compared. to. the. Stokes. shift,. especially.
the.low-frequency.one..However,.for.SERS.on.isolated.homogeneous.particles.where.the.
plasmon.width.is.small,.this.assumption.leads.to.an.overestimate.of.the.EF.by.factors.of.3.
or.more.
