Biomedical Engineering Reference
In-Depth Information
high-quality.QDs,.is.not.affected.by.the.bioconjugation.process,.in.contrast.to.the.organic.
luorophores,.whose.yield.is.dramatically.reduced.during.functionalization.with.biomole-
cules. 59 .Bawendi.and.co-workers 60 .estimated.that.the.molar.extinction.coeficients.of.CdSe.
QD.are.about.10 5 -10 6. M -1 .cm -1 ,.depending.on.the.size.and.excitation.wavelength.of.the.par-
ticle..Since.this.value.is.about.10-100.times.larger.than.that.of.organic.dyes, 60,61 .QD.absorp-
tion.rates.will.be.~10-50.times.faster.at.the.same.excitation.photon.lux.(number.of.incident.
photons.per.unit.area)..This.superiority.makes.them.brighter.probes.under.photon-limited.
in. vivo. conditions,. where. light. is. extremely. diminished. by. scattering. and. absorption. 39 .
The. large. two-photon. absorption. cross. section. of. QDs,. which. is. two. to. three. orders. of.
magnitude.greater.than.traditional.organic.luorophores,.permits.more.eficient.probing.of.
thick.specimens.by.the.use.of.multiphoton.excitation.microscopy. 31,48 .With.this.technique,.
luorescence.signals.of.CdSe-ZnS.QDs.have.been.detected.hundreds.of.micrometers.deep.
through. the. intact. skin. of. living. mice. 31 . In. a. report. by. Stroh. et. al., 62 . tumor. vessels. have.
been. differentiated. from. both. the. perivascular. cells. and. the. extracellular. matrix. by. the.
use.of.micelle-encapsulated. QDs.and.multiphoton.microscopy.in.GFP-expressing.trans-
genic. mice.. In. comparison. with. the. traditional. high. molecular. weight. dextran,. a. much.
sharper. boundary. was. visualized. between. intra-. and. extravascular. spaces.. In. addition,.
the.relatively.broad.excitation.spectrum.of.QDs.permitted.the.simultaneous.monitoring.of.
demarcated.vessels.and.GFP-expressing.perivascular.cells.
Finally,. the. suficient. electron. density. of. the. QD's. core. enables. transmission. electron.
microscopy.imaging..This.ability,.coupled.with.luorescent.imaging.of.the.same.sample,.
can. provide. information. on. both. temporal. dynamics. and. high. spatial. resolution. of. the.
precise.localization.of.nanoparticles.within.cells.and.tissues. 63
2.5 Synthesis,Solubilization,andBioconjugation
There.has.been.progressive.research.based.on.quantum.dots.in.the.ield.of.nanobiotech-
nology,.which.accounts.for.the.considerable.improvements.in.their.synthesis.and.surface.
chemistry. since. 1982. 64,65 . Typical. synthesis. strategies. of. semiconductor. QDs. involve. the.
growth. of. nanocrystals. using. molecular. precursors. in. either. aqueous. or. organic. solu-
tions. 54 . Before. 1993,. QDs. were. mainly. prepared. in. aqueous. solutions. containing. stabi-
lizing.agents.(e.g.,.thioglycerol.or.polyphosphate)..This.procedure.generates.nanocrystals.
with. low. quantum. yields. and. large. size. variation,. resulting. in. broad. emission. spectra.
(~50.nm.FWHM). 41 .In.1993,.Bawendi's.group 60 .developed.a.novel.approach.for.synthesis.
of.highly.luminescent.monodispersed.QDs.by.using.a.high-temperature.organometallic.
process.. In. contrast. to. the. former. synthesis. methods,. the. nanocrystals. had. nearly. per-
fect.crystalline.structures.and.fewer.size.variations..However,.the.luorescence.quantum.
yields.were.still.relatively.low..The.deposition.of.a.surface-capping.layer.such.as.ZnS.was.
found. to. dramatically. improve. their. luorescence. quantum. yields. 61,66,67 . Alternative. pre-
cursor. materials. such. as. CdO. could. also. be. utilized. in. the. synthesis. of. CdS,. CdSe,. and.
CdTe.nanocrystals,.which.resulted.in.the.creation.of.QDs.with.a.relatively.narrow.FWHM.
(45-65.nm).and.acceptable.quantum.eficiency.without.the.need.for.an.inorganic.capping.
layer.surface.coating. 68
Generally,.the.QDs.synthesized.in.organic.solvents.have.hydrophobic.surface.ligands,.
which. should. be. rendered. water. dispersible. for. their. utilization. in. biological. applica-
tions. 69 . This. could. be. achieved. by. two. general. strategies:. ligand. exchange. and. coating.
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