Biomedical Engineering Reference
In-Depth Information
barriers.include.polymeric.ligands.such.as.poly(ethylene.glycol).(PEG)
85
.and.small.molecu-
lar. ligands. like. bis(p-sulfonatophenyl). phenylphosphine. surfactants.
20
. These. ligands. for.
water.solubilization.are.then.replaced.by.phase.transfer.or.ligand.exchange.or.modiied.
by.ligand.addition.with.desired.biofunctional.ligands.
7a
.Water.solubilization.may.be.per-
formed.either.as.the.inal.stage.of.the.biofunctionalization.process.of.NPs.or.as.an.inter-
mediate.stage.
Four.strategies.are.generally.used.to.functionalize.the.surface.of.plasmonic.nanomateri-
als.(mostly.Au,.Ag,.and.Cu.surfaces).with.biomolecules.such.as.bioreceptors..The.irst.is.
electrostatic. adsorption. of. positively. charged. biomolecules. to. negatively. charged. nano-
particles,.or.vice.versa..For.example,.gold.and.silver.NPs.synthesized.by.citrate.reduction.
are.stabilized.by.citrate.ligands.at.a.pH.slightly.above.their.isoelectric.point..This.results.
in.the.anionic.citrate.coated.NPs.that.can.be.bound.to.the.positively.charged.amino.acid.
side. chains. of. immunoglobulin. G. (IgG). molecules.
86
. The. second. strategy. is. ligand-like.
binding.to.the.metallic.surface.of.plasmonic.nanomaterials.by.chemisorption.of,.e.g.,.thiol.
groups.. In. the. former. case,. metal. nanostructures. can. be. functionalized. with. L-cysteine.
through.the.Au-S.bonds.and.then.bound.to.target.proteins.through.peptide.bonds.with.
the. cysteine. moieties.
87
. They. can. be. directly. bound. to. thiol. derivative. analytes. such. as.
protein-containing.cysteine.residues.(e.g.,.serum.albumin)
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.or.thiolated.DNA.
89
.The.third.
strategy. is. covalent. binding. through. biofunctional. linkers,. exploiting. functional. groups.
on.both.particles.and.biomolecules..The.bifunctional.linkers.have.anchor.groups.that.can.
be.attached.to.NP.surfaces.and.functional.groups.that.can.be.further.covalently.coupled.
to.the.target.biomolecules..They.are.extensively.used.to.covalently.conjugate.biomolecules.
with. various. NPs,
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. especially. when. no. linking. moieties. like. thiol. groups. are. available.
in. biomolecules.. The. common. anchor. groups. include. thiols,. disulides,. and. phosphine.
ligands.that.are.used.to.bind.the.bifunctional.linkers.to.Au,.Ag,.CdS,.and.CdSe.NPs..The.
fourth.strategy.is.based.on.noncovalent,.afinity-based.receptor-ligand.systems..More.spe-
ciically,.nanoparticles.are.functionalized.with.bioreceptors.(e.g.,.antibodies).that.provide.
afinity. sites. for. binding. of. the. corresponding. ligand. (e.g.,. antigens). or. ligand. modiied.
proteins.and.oligonucleotides..The.most.well-known.example.in.the.last.several.decades.is.
the.avidin-biotin.system.
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.For.example,.biotinylated.proteins.(e.g.,.immunoglobulins.and.
serum.albumins).or.biotinylated.oligonucleotides.(e.g.,.single-stranded.DNA.(ssDNA))
89b
.
have. been. widely. used. to. modify. streptavidin-functionalized. Au. NPs. by. afinity. bind-
ing.
90
.Regarding.molecular.recognition,.the.system.consists.of.a.ligand,.the.small.molecule.
biotin.(vitamin.H),.and.a.receptor,.the.globular.protein.avidin.that.is.present.in,.e.g.,.egg.
whites..Avidin.consists.of.four.identical.subunits,.yielding.four.binding.pockets.that.spe-
ciically.whites.recognize.and.bind.to.biotin..The.dissociation.constant.is.of.the.order.of.
10
15.
M,.and.the.afinity.bond,.though.not.covalent,.is.found.to.be.extremely.stable,.resisting.
harsh.chemical.and.physical.(e.g.,.elevated.temperature).conditions.
5.5 Application
5.5.1 Case 1: Molecular Plasmonic Rulers
Förster. resonance. energy. transfer. (FRET). has. served. as. a. molecular. ruler. to. monitor.
conformational. changes. and. measure. intramolecular. distances. of. single. biomolecules.
92
.
However,. such. a. ruler. sometimes. suffers. from. dificulty. in. distinguishing. changes. in.
