Biomedical Engineering Reference
In-Depth Information
A
B
s
s
s
s
C
D
E
Streptavidin
Linker
Biotin Protein Mediator Protein
Nanoparticle
DNA
FIGURE 24.3
Strategies.commonly.used.for.biomolecular.functionalization:.(A).direct.physisorption.of.biomolecules.to.nano-
particles,.(B).assisted.physisorption.using.prebound.biomolecules,.(C).bioconjugation.via.cross-linkers.on.the.
chemisorbed.functional.ligand.on.nanoparticles,.(D).direct.conjugation.of.thiolated.biomolecules.to.nanopar-
ticles,.and.(E).attaching.biotinylated.molecules.to.avidin.physisorbed.on.nanoparticles..(From.Kumar.C.S.S.R.,.
Biofunctionalisation of nanomaterials
(Nanotechnologies.for.the.Life.Sciences.1),.Wiley-VCH,.Weinheim,.2005).
for.instance,.are.negatively.charged,.allowing.simple.adsorption.of.positively.charged.pro-
teins
97
.(Figure 24.3A)..This.has.been.commonly.used.to.prepare.bioconjugates.for.immu-
nolabeling.since.the.1960s.
105
.Multilayer.assemblies.of.proteins,.as.shown.in.Figure 24.3B,.
can.be.constructed.by.attaching.a.polyelectrolyte.polymer.to.the.protein-coated.nanopar-
ticles,.which.then.allows.the.deposition.of.a.second.protein.layer.
98,99
.The.major.drawbacks.
of.protein.adsorption.by.electrostatic.interaction.are.that.(1).it.is.very.dificult.to.control.
the.number.of.biomolecules.attached,.(2).the.particles.lack.stability,.and.(3).the.activity.of.
biomolecules.generally.decreases.gradually.with.time.
Chemisorption.of.biomolecules.to.nanoparticles.is.shown.in.Figure 24.3C.and.D..In.the.
simplest. case,. proteins. can. be. chemisorbed. to. the. nanoparticles. through. thiol. or. amine.
groups.that.can.be.engineered
106,107
.or.are.already.present.in.the.protein.
108,109
.Either.way,.
the. biological. activity. of. the. resulting. bioconjugates. usually. decreases. dramatically. due.
to.conformational. change. upon. adsorption.
96,109
.In.addition,. it.has.been.shown.that.pro-
teins.are.often.denatured,.leading.to.the.loss.of.their.biological.activity..To.avoid.some.of.
these. problems,. bifunctional. linkers. containing. a. thiol. group. to. link. to. the. particle. and.
functional. groups. such. as. carboxylic. acid. to. link. to. the. protein. have. been. used
101,110,111
.
(Figure 24.3C).. Particles. engineered. in. this. way. are. stable;. however,. site-speciic. attach-
ment.of.biomolecules.by.covalent.means.is.generally.a.substantial.synthetic.challenge.
113
.
Nucleic.acids.have.been.widely.used.following.the.approach.illustrated.in.Figure 24.3D..
Since.they.can.be.synthesized.terminated.with.alkane.thiol.groups,.they.have.been.used.
to.directly.stabilize.gold.nanoparticles.
113-115
Figure 24.3E. shows. physisorption. of. biomolecules. to. nanoparticles. and. the. immobili-
zation. of. other. biomolecules. by. the. biotin-avidin. system.. Avidin. and. antibodies. can. be.
adsorbed.to.nanoparticles,.which.are.then.used.to.bind.to.biotinylated.functionalities.and.
antigens,.respectively..This.is.an.attractive.and.frequently.used.strategy;.however,.the.par-
ticles.often.lack.stability.
116,117
