Biomedical Engineering Reference
In-Depth Information
the.core.determines.the.energy.of.the.emitted.photon, 28 .which.can.be.continuously.tuned.
from. the. ultraviolet 35 . to. near. infrared, 36 and. even. to. the. mid-infrared. 37 . The. smaller. the.
diameter.of.a.semiconductor.QD,.the.larger.the.band.gap.becomes,.and.thus.the.luores-
cence.of.the.shorter.wavelength.is.achieved. 28 .A.photon.with.greater.energy.than.that.of.
the.semiconductor.band.gap.is.required.to.generate.an.exciton.that.has.a.lifetime.of.about.
tens.of.nanoseconds.in.QDs. 38 .This.long.luminescence.duration.along.with.broad.Stoke's.
shift.can.be.as.large.as.300-400.nm,.depending.on.the.wavelength.of.the.excitation.light, 39 .
and.enables.time-gated.imaging,.which.improves.the.sensitivity.of.luorescence.detection.
in.live.cell.imaging.by.reducing.background.autoluorescence. 40 .QDs.can.be.excited.efi-
ciently.at.any.wavelength.shorter.than.their.emission.peak, 41 .which.results.in.a.broadband.
absorption. spectrum. extending. from. the. ultraviolet. to. a. cutoff. visible. spectrum. wave-
length. 42 .However,.it.has.an.increased.probability.toward.shorter.wavelengths..This.allows.
simultaneous.excitation.of.many.spectrally.distinct.QD.emitters.with.a.single.excitation.
source,.in.contrast.to.the.organic.luorophores.for.which.a.certain.wavelength.for.every.
different.luorophore.is.required. 28 .When.the.exited.electron.turns.back.to.its.lower.energy.
state,.a.photon.with.a.narrow.symmetric.energy.band.(typically.20-40.nm.full.width.at.
half.maximum.(FWHM)).is.emitted.upon.radiative.recombination. 43 .Therefore,.QDs.with.
different.emissions.can.be.used.for.simultaneous.visualization.of.multiple.cellular.targets.
or. different. cell. populations. without. any. spectra. overlap,. which. is. of. particular. interest.
for. both. in. vitro. and. in. vivo. multicolor. imaging,. besides. their. ability. for. polychromatic.
extracellular.membrane.labeling.with.up.to.eight.colors.in.low.cytometric.analysis. 26,44-46 .
By.using.different.QD.conjugates.and.a.single.excitation.source,.various.cellular.compo-
nents,.including.microtubules,. mitochondria,.and.nucleosomes,.have.been.detected.in.a.
HeLa.cell. 47 .Negatively.charged.dihydroxylipoic.acid.(DHLA)-capped.QDs.with.different.
emission.wavelengths.have.also.been.utilized.for.real-time.tracking.of.metastatic.cells.in.
a. living. mouse.. By. the. use. of. luorescence. emission. scanning. microscopy,. ive. different.
QD-tagged. populations. of. melanoma. cells. have. been. simultaneously. identiied. as. they.
extravasated.into.lung.tissue. 48
The.high.photobleaching.threshold.and.the.strong.resistance.to.chemical.and.metabolic.
degradation 49,50 . in. QDs. permit. continuous. monitoring. and. imaging. over. an. extended.
period. of. time. 51 . In. the. study. designed. by. Wu. et. al.,. nuclear. antigens.and. microtubules.
were.simultaneously.stained.with.Alexa.Fluor488.(green).and.QD630.(red).in.the.same.cell..
Regardless.of.which.target.was.labeled.with.Alexa.dye,.the.luorescent.signal.faded.quickly.
and. became. undetectable. within. 2. min,. in. contrast. to. QD630,. which. was. highly. photo-
stable.for.the.entire.3.min.illumination.period. 45 .It.is.believed.that.the.QD.photobleaching.
process.is.related.to.a.prolonged.photochemical.destruction.of.the.core.components..This.
hypothesis.is.supported.by.the.observation.of.a.blue.shift.in.the.QD.emission.spectra.upon.
continuous.UV.illumination..Henglein.and.co-workers.speculated.that.optical.excitation.
is. responsible. for. initiating. the. CdS. decomposition. process. by. the. formation. of. S. or. SH.
radicals..These.radicals.are.able.to.form.an.SO 2 .complex.via.an.air.oxidation.reaction.that.
ultimately.leads.to.slow.nanoparticle.degradation. 52 .An.appropriate.capping.with.a.thick.
semiconductor.layer.is.applied.to.improve.the.photostability.of.QDs.by.protecting.the.core.
surface.atoms.from.oxidation. 53,54 .Moreover,.in.contrast.to.organic.luorophores,.QDs.are.
less.susceptible.to.metabolic.degradation.due.to.their.inorganic.nature. 55,56 .It.was.shown.
that.QDs.remained.intact.in.live.cells.and.organisms.for.several.weeks.to.months,.with.no.
signs.of.toxicity.or.QD.breakdown. 32,50
The.superior.brightness.of.QDs.arises.from.a.combination.of.a.high.quantum.yield.(the.
ratio.of.emitted.to.absorbed.photons).and.a.large.extinction.coeficient,.so.that.they.are.able.
to.re-emit.much.of.the.absorbed.light. 57,58 .The.quantum.yield,.which.could.be.up.to.85%.in.
Search WWH ::




Custom Search