The Biosynthesis of Anisotropic Nanoparticles - Bionanotechnology: Global Prospects - page 187

A B

50 nm

200 nm

FIGURE 9.4
TEM.analyses. of.the.gold.nanoparticles. formed. by. R. capsulata .at.(A).pH.7.and. (B). pH.4..(From. He,.S.,.et.al.,.
Mater. Lett. ,.61,.3984,.2007..With.permission.).

tion. of. extracellular. nanoparticles,. whereas. incubation. under. shaking. conditions. led. to.
intracellular.gold.nanoparticle.formation.

9.4 MechanismofBiologicalNanoparticleSynthesis
While.the.indings.discussed.above.give.an.indication.of.the.potential.for.cellular.manipu-
lation. to. achieve. control. of. the. process. within. the. cells,. it. is. acknowledged. that. much.
research.still.remains.to.be.done,.especially.in.understanding.the.detailed.mechanism.of.
the.reaction.on.a.molecular.and.cellular.level,.including.isolation.and.identiication.of.the.
compounds.responsible.for.the.process.
So. far,. little. is. known. about. the. interaction. between. biomolecules. and. nanoparticles,.
though.various.models.for.the.enzymatic.production.of.nanoparticles.have.been.hypoth-
esized..NADH,.a.coenzyme.occurring.in.most.cells.could.serve.as.a.reductant.in.various.
metabolic.processes..There.are.indications.that.NADH-.and.NADH-dependent.enzymes.
are. important. factors. in. the. biosynthesis. of. metal. nanoparticles. 9,43-46 . The. models. pro-
posed.are.very.similar,.as.they.all.point.toward.the.possible.role.of.an.NADH-dependent.
reductase.or.hydrogenase.being.responsible.for.nanoparticle.production.
An.example.is.the.bacterium. R. capsulate ,.known.to.secrete.cofactor.NADH.and.NADH-
dependent. enzymes,. which. may. be. responsible. for. the. bioreduction. of. Au 3+ . to. Au 0 . and.
the.subsequent.formation.of.gold.nanoparticles.(Figure 9.5A)..The.reduction.seems.to.be.
initiated.by.electron.transfer.from.the.NADH.by.NADH-dependent.reductase.as.an.elec-
tron.carrier;.the.gold.ions.obtain.electrons.and.are.reduced.to.Au 0 . 11
The.same.observation.was.reported.with.a.strain.of. F. oxysporum ,.where it.was.demon-
strated.that.a.nitrate.reductase.and.anthraquinone.are.involved.in.the.in.vitro production.
of.silver.nanoparticles. 9 .The.authors.were.able.to.purify.the.nitrate.reductase.and.utilized.
the. enzyme. for. the. synthesis. of. nanoparticles.. It. is. thought. that. the. Ag 0 . reduction. was.
mainly.due.to.a.conjugation.between.the.electron.shuttle.systems,.with.the.reductase.par-
ticipating.as.shown.in.Figure 9.5B.