Biomedical Engineering Reference
In-Depth Information
Yet,.small.amounts.of.this.protein,.normally.present.in.animals,.are.not.suficient.when.
cells.are.exposed.to.high.levels.of.Cd
2+
.ions.
171
Protons.in.low-pH.environments,.such.as.lysosomes.and.peroxisomes,.on.the.other.hand,.
can. lead. to. detachment. of. coordination. groups. from. the. QD. surface,. releasing. cationic.
ions,. resulting. in. subsequent. cellular. toxicity.
176,177
. In. addition,. it. is. known. that. cellular.
toxicity.of.cadmium.is,.in.part,.related.to.alteration.of.intracellular.calcium.homeostasis,.
which.can.competitively.reduce.extracellular.calcium.inlux.or.increase.intracellular.cal-
cium.levels.by.inhibiting.calcium-dependent.ATPase.
178,179
Free. radical. formation. is. considered. an. alternative. mechanism. involved. in. QD.
cytotoxicity.
162,173
.During.the.excited.state,.QDs.are.able.to.transfer.electrons.or.energy.to.
nearby.oxygen.molecules,.thus.generating.a.reactive.oxygen.species.(ROS),.such.as.hydroxyl.
radical,.super.oxide,.or.singlet.oxygen,.depending.on.their.composition.
180
.ROS.elevation.
could. induce. oxidative. stress. by. releasing. the. endogenous. antioxidants,. and. thus. their.
subsequent.depletion.
181
.Oxidative.stress.in.turn.results.in.irreversible.cellular.damage.or.
death.by.peroxidizing.lipids,.altering.proteins,.disrupting.DNA,.interfering.with.signal-
ing.functions,.and.modulating.gene.transcription.
182,183
.However,.sensitivity.of.cells.to.ROS.
may. depend. on. the. cell. type,. the. level. and. duration. of. oxidant. production,. the. species.
of.ROS.generated,.and.the.speciic.site.of.ROS.production..Furthermore,.oxidative.stress.
in. living. organisms. may. have. a. role. in. the. induction. or. enhancement. of. inlammation.
through.upregulation.of.redox-sensitive.transcription.factors.and.the.related.kinases.
184,185
Considering.the.nanoscale.size.of.QDs,.which.is.smaller.than.cellular.organelles,.they.
are.able.to.penetrate.into.the.cells.and.interact.with.intracellular.components,.resulting.in.
their.structural.alterations.and.dysfunction..QD.aggregates.were.found.localized.around.
the.mitochondria,.and.after.72.h.morphological.effects.included.swollen.mitochondria.and.
enlarged.Golgi.cisterns.
157
.Kirchner.et.al..have.proposed.that.in.addition.to.the.release.of.
Cd
2+
.ions.from.semiconductor.QDs,.their.precipitation.behavior.also.contributes.to.cyto-
toxicity.after.studying.the.effect.of.QDs.on.MDA-MB-435S.breast.cancer.cells.
160
2.15 MagneticResonanceImaging
Magnetic. resonance. imaging. uses. hydrogen. nuclei. signals. for. image. generation.. Recall.
from. the. periodic. table. that. a. hydrogen. atom. consists. of. a. nucleus. containing. a. single.
proton.with.positive.charge.and.a.single.electron.with.negative.charge..The.proton.rotates.
about.its.axis.like.a.spinning.top,.and.so.has.an.angular.momentum.that.strives.to.retain.
the.spatial.orientation.of.its.rotation.axis,.and.magnetic.moment.as.a.rotating.mass.with.an.
electrical.charge..The.orientation.of.the.proton.rotation.axis.can.be.identiied.by.magneti-
zation.vector.B.and.can.be.measured.by.signal.generation.in.a.receiver.coil.
Electromagnetic. waves. and. an. external. magnetic. ield. can. alter. the. orientation. of. the.
hydrogen.nuclei.rotational.axis.to.align.the.magnetic.moments.with.the.direction.of.the.
external.magnetic.ield,.B0,.a.process.that.is.called.precession..However,.in.the.quantum.
ield.proton.alignment.changes.are.due.to.an.external.force.and.have.some.limitation..Like.
electrons,.the.protons.can.only.spin.in.speciic.orbits,.so.they.cannot.align.exactly.in.exter-
nal.ield.direction,.and.there.is.an.angle.width.between.the.nuclei.magnetic.momentum.
and. the. external. magnetic. ield.. Therefore,. after. this. enforcement,. protons. align. in. dif-
ferent.phases.or.energy.levels:.one.low.energy.and.one.high.energy,.which.are.separated.
