Bionanotechnology Strategies for Cell Detection - Bionanotechnology: Global Prospects - page 17

2.4 QuantumDots
After.the.irst.publications.in.1998.by.Bruchez.et.al. 22 .and.Chan.and.Nie 23 .on.the.possibil-
ity.of.using.QDs.as.competent.lourescent.probes.in.biological.imaging.and.related.appli-
cations,.QD-based.research.has.progressively.improved..In.less.than.a.decade,.QDs.have.
prevailed. over. many. of. the. intrinsic. limitations. encountered. by. conventional. organic.
dyes. and. genetically. encoded. luorescent. proteins. and. have. become. powerful. tools. in.
many. ields,. such. as. molecular. biology,. cell. biology,. molecular. imaging,. and. medical.
diagnostics. 24
Quantum.dots.are.luminescent.colloidal.semiconductor.nanocrystals.typically.in.the.size.
range.of.2-10.nm.comprising.200-10,000.atoms, 25 .in.which.the.electron.motion.is.conined.
by.potential.barriers.in.all.three.dimensions. 26 .The.general.structure.of.QDs.is.composed.
of. an. inorganic. core. that. frequently. consists. of. group. II-VI. (e.g.,. CdSc,. CdTe,. CdS,. and.
ZnSe).or.III-V.(e.g.,.InP.and.InAs).atoms.of.the.periodic.table. 27 .The.core.is.overcoated.by.
a.shell.of.another.semiconductor.with.a.larger.spectral.band.gap,.mostly.ZnS, 28 .and.a.sub-
sequent.aqueous.organic.coating.is.utilized,.to.which.the.biomolecules.are.conjugated, 29 .
as.shown.in.Figure 2.1.
QDs.exhibit.several.unique.optical.and.electrical.properties,.including.broad.excitation.
and.size-tunable.narrow.emission.spectrum,.long.luminescence.lifetime,.exceptional.pho-
tochemical. stability,. high. brightness,. and. large. two-photon. action. cross. sections. 22,23,30,31 .
These.remarkable.properties.make.QDs.luorophores.of.choice.in.applications.requiring.
long-term,.multitarget,.and.high-sensitivity.detection,.such.as.dynamic.live.cell.imaging.
and.in.vivo.whole.organism.imaging. 28,32
QDs. gained. their. name. from. the. quantum. coninement. effect,. which. only. occurs. in.
structures.with. physical. dimensions. smaller. than.the.Bohr. exciton. radius. (electron-hole.
spacing.in.excited.state). 33,34 .A.direct.consequence.of.this.phenomenon.is.that.the.energy.
levels.of.the.exciton.(electron-hole.pair).will.become.quantized,.and.therefore.the.size.of.
FIGURE 2.1
The. general. structure. of. a. QD.. The. nanocrystal. core. is. passivated. by. a. semiconductor. shell,. mostly. ZnS.. An.
organic.coating.is.used.that.enables.conjugation.of.different.functional.groups.(antibody,.peptide,.oligonucle-
otide).for.desired.bioactivity.