Gold Nanoparticles in Biomedicine - Bionanotechnology: Global Prospects - page 58

3.4.2 Biological Synthesis
Biosynthesis.of.nanomaterials.is.gaining.popularity.as.an.environmentally.friendly.alter-
native.to.the.conventional.chemical.methods,.which.may.involve.high.energy.consump-
tion,.as.well.as.production.and.use.of.toxic.or.hazardous.material..Biological.synthesis.is.
relatively.low.cost,.energy.eficient,.and.nontoxic. 29,49,50 .Successful.bioreduction.of.gold.ions.
has.been.done.using.bacteria,.fungi,.algae,.plants,.and.plant.extracts.
Bacteria.can.deposit.the.nanoparticles.intracellularly.or.extracellularly. 49 .Konishi.et.al. 51 .
used.the.Fe(III)-reducing.bacteria. Shewanella algae .to.reduce.Au(III).with.H 2 .as.the.electron.
donor.and.under.pH.conditions.ranging.from.2.0.to.7.0..The.bacteria.were.cultured.under.
anaerobic. conditions,. where. lactate. or. H 2 . acted. as. the. electron. donor. and. Fe(III)citrate.
acted.as.the.electron.acceptor..The.bacteria.were.incubated.for.3-5.days,.harvested.by.cen-
trifugation,.and.resuspended.in.a.bicarbonate.buffer.of.pH.7.0..This.process.was.repeated.
several.times.before.the.bacteria.were.ready.to.be.used.in.bioreduction.
At. pH. 7.0,. AuNPs. of. sizes. between. 10. and. 20. nm. were. deposited. in. the. periplasmic.
space.of.the.bacterial.cells..At.pH.2.8,.AuNPs.of.different.shapes.ranging.in.size.from.15.
to. 200. nm,. especially. 100-200. nm. triangular. and. hexagonal. AuNPs,. were. deposited. on.
the. bacterial. cells.. At. pH. 2.0,. intracellular. deposition. of. 20. nm. AuNPs. occurred,. along.
with.extracellular.deposition.of.350.nm.AuNPs..Complete.reduction.of.1.mM.AuCl 4 - .was.
achieved.within.30.min.at.25°C,.corresponding.to.the.chemical.reduction.of.20.mM.citric.
acid.at.50°C.
Fungi.are.generally.easy.to.handle,.and.most.have.the.advantage.of.extracellular.secre-
tion. of. reducing. enzymes,. which. makes. it. easier. to. retrieve. the. synthesized. AuNPs. 49 .
Their.drawbacks.are.that.they.are.slow.reducers.(48-120.h), 52-55 .and.being.eukaryotes,.are.
harder.to.manipulate.genetically. 50 .Perhaps.the.most.studied.fungus.in.the.biosynthesis.
of.nanoparticles.of.different.kinds.is. Fusarium oxysporum . 49 .Using.this.fungus,.Mukherjee.
et.al. 56 .observed.the.extracellular.deposition.of.20-40.nm.stable.AuNPs.of.spherical.and.
triangular.morphologies.
Singaravelu.et.al. 57 .were.the.irst.to.report.the.use.of.marine.alga.in.the.biosynthesis.of.
AuNPs.. Sargassum wightii .was.found.to.produce.extracellular.AuNPs.in.12.h.
Wang. et. al. 58 . used. Barbated. Skullcup,. a. Chinese. herbal. extract,. to. produce. AuNPs. of.
an. average. size. range. of. 10-100. nm.. Almost. complete. reduction. of. HAuCl 4 . occurred.
in. 3. h. at. room. temperature,. and. the. AuNPs. produced. were. highly. stable. and. of. fair.
monodispersity.
3.4.3 Characterization
Transmission. electron. microscopy. (TEM). is. the. most. common. method. used. to. deter-
mine.the.shape.and.size.of.the.AuNPs,.as.well.as.their.dispersity..Other.characterization.
methods.used.to.determine.the.dimensions.of.AuNPs.include.scanning.electron.micros-
copy,.atomic.force.microscopy,.small-angle.x-ray.scattering.(SAXS),.and.x-ray.diffraction..
Spectroscopic. methods. rely. on. the. absorption. and. scattering. properties. of. AuNPs.. The.
SPR.measured,.however,.is.based.on.the.collective.oscillations.of.a.group.of.AuNPs,.which.
may.be.of.various.shapes.and.sizes..It.is.therefore.used.as.a.measure.of.the.heterogeneity.
of.the.AuNPs.produced. 19 .Dark.ield.scattering 59 .and.absorptive.properties 60 .of.the.AuNPs.
can. be. measured. to. determine. the. spectrum. of. each. nanostructure. individually.. SAXS.
can.be.used.to.monitor.the.growth.of.AuNPs,.and.the.thermodynamics.of.growth.can.be.
measured.by.monitoring.changes.in.temperature.during.growth.by.isothermal.titration.
calorimetry. 61 .TEM.can.also.be.used.to.determine.the.mean.number.of.gold.atoms.(N Au ).in.