Biomedical Engineering Reference
In-Depth Information
Localized.surface.plasmon.polaritons.are.nonpropagating.collective.oscillations.of.the.
conduction. electrons. of. metallic. nanostructures. against. the. background. of. ionic. metal.
cores
16
. coupled. to. the. exciting. electromagnetic. ield. (Figure. 5.1b).. For. a. particle. much.
smaller.than.the.wavelength.of.the.exciting.light,.the.dipolar.plasmon.is.dominant.in.the.
oscillation,.which.contains.an.effective.restoring.force.on.the.driven.electrons..When.the.
exciting.laser.light.is.in.resonance.with.the.dipolar.plasmon,.the.metal.particle.will.radiate.
light.characterized.by.dipolar.radiation,
17
.leading.to.electrical.ield.(E).ampliication.both.
inside.and.in.the.near-ield.zone.outside.the.particle..This.resonance.is.called.the.localized.
surface.plasmon.resonance.(LSPR)..Typical.materials.for.plasmonic.applications.are.noble.
metals,.particularly.silver.or.gold..Silver.displays.sharper.and.more.intense.LSPR.bands.
than.gold,.while.the.gold.nanostructure.is.chemically.more.stable.than.silver.
The.response.of.both.SPR.and.LSPR.sensors.to.change.of.refractive.index.can.be.described.
by.using.the.following.equation,
18
.which.was.initially.developed.for.propagating.SPR.
19
.
Δλ
max
.≈.
m
(
n
adsorbate
.−.
n
medium
).[1.-.exp.(−2
d
/l
d
)].
(5.1)
where.Δλ
max.
is.the.wavelength.shift.when.the.extinction.reaches.maximum.due.to.resonant.
absorption,.
m
.is.the.sensitivity.factor,.
n
adsorbate
.and.
n
medium
.are.the.refractive.indices.of.the.
adsorbate.and.medium.surrounding.the.nanoparticle,.respectively,.
d
.is.the.effective.thick-
ness. of. the. adsorbate. layer,. and. l
d
. is. the. electromagnetic. ield. decay. length.. Therefore,.
both.sensors.are.highly.sensitive.to.the.dielectric.properties.of.the.medium.adjacent.to.the.
dielectric-metal.interface..Figure.5.1a.shows.a.typical.experimental.setup.and.a.resulting.
relectivity.spectrum.obtained.in.an.angle-solved.mode.(measured.against.angle.of.inci-
dence.at.a.ixed.wavelength).that.demonstrates.a.resonance.angle.shift.upon.binding.of.
analyte.in.a.SPP.biosensor,.while.Figure.5.1b.illustrates.a.typical.measurement.scheme.for.
LSPR. biosensors. and. a. resulting. scattering. spectrum. that. shows. resonance. wavelength.
shift.upon.binding.of.DNA.(Au-Ph-DNA,.red.line).to.a.phosphine.(Ph).surfactant-coated.
gold.nanoparticle.(Au-Ph,.green.line).
20
.To.selectively.bind.and.detect.analytes.of.interest.
in. biosensing,. the. metallic. surface. of. both. sensors. is. generally. modiied. using. various.
strategies,.such.as.bioreceptors.(Figure.5.1).or.hydrophilic.coating.
7b,7c
One.of.the.important.driving.forces.in.biosensor.research.is.to.develop.large-scale.bio-
sensor. arrays. composed. of. highly. miniaturized. signal. transducer. elements. that. enable.
the.real-time.and.parallel.monitoring.of.multiple.species,.especially.for.high-throughput.
screening. applications. such. as. drug. discovery. and. proteomics. research. where. many.
thousands. of. ligand-receptor. or. protein-protein. interactions. must. be. rapidly. measured..
To.meet.this.need,.LSPR.sensors.stand.out.as.better.candidates.due.to.the.“nano”.advan-
tages. over. SPR.
18,21
. For. example,. SPR. sensors. require. a. minimum. of. 10. ×. 10. µm. area. for.
sensing.experiments..In.contrast,.to.deliver.the.same.information.as.do.SPR.sensors,.the.
spot.size.for.LSPR.sensors.can.be.minimized.down.to.the.sub-100.nm.regime.(around.the.
size. of. a. single. nanoparticle). using. single-nanoparticle. LSPR.
22
. What's. more,. because. of.
the.large.refractive.index.sensitivity.(around.2.×.10
6
.nm.RIU
-1
).SPR.sensors.require.accu-
rate.control.of.temperature.and.complex.optical.instrumentation,.while.LSPR.nanosensors.
do. not. due. to. a. lower. refractive. index. sensitivity. (around. 2. ×. 10
2
. nm. RIU
-1
).. Today,. the.
LSPR.not.only.has.been.recognized.as.an.ultrasensitive.method.for.detecting.molecules.
of. both. biological. and. chemical. interest,. but. also. plays. a. major. role. in. all. other. surface-
enhanced. biosensing. techniques,. such. as. surface-enhanced. Raman. scattering. (SERS),
9b
.
