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REVIEWOpenAccessPlantsandmicrobesassistedseleniumnanoparticles:characterizationandapplicationAzamalHusen1*andKhwajaSalahuddinSiddiqi2AbstractSeleniumisanessentialtraceelementandisanessentialcomponentofmanyenzymeswithoutwhichtheybecomeinactive.
TheSenanoparticlesofvaryingshapeandsizemaybesynthesizedfromSesaltsespeciallyseleniteandselenatesinpresenceofreducingagentssuchasproteins,phenols,alcoholsandamines.
ThesebiomoleculescanbeusedtoreduceSesaltsinvitrobutthebyproductsreleasedintheenvironmentmaybehazardoustofloraandfauna.
Inthisreview,therefore,weanalysedindepth,thebiogenicsynthesisofSenanoparticles,theircharacterizationandtransformationintot-Se,m-Se,Se-nanoballs,Se-nanowiresandSe-hollowspheresinaninnocuouswaypreventingtheenvironmentfrompollution.
Theirshape,size,FTIR,UV–vis,Ramanspectra,SEM,TEMimagesandXRDpatternhavebeenanalysed.
Theweakforcesinvolvedinaggregationandtransformationofonenanostructureintotheotherhavebeencarefullyresolved.
Keywords:Nanotechnology,Selenium,Plantextracts,Microbes,BiofabricationIntroductionSeleniumwasknownasanotoriouselementuntilitwasrecognizedbySchwarzandFoltzin1957asanessentialtraceelementforbothplantsandmammals.
NormallySeisavailableasselenateandseleniteoxoanions.
ThereductionofsolubleSe4+andSe6+bymicrobestoinsol-ublenontoxicelementalSeisaneffectivewaytore-moveitfromcontaminatedsoil,wateranddrainage[1].
SeisoneofthechalcogensoccurringasselenateSeO42,seleniteSeO32andselenideSe2whichmaybereducedtoatomicstatebyaprecursorcontaininganappropriatereducingagent.
BiogenicsynthesisofSenanoparticlesisfrequentlyachievedbyreductionofselenate/seleniteinpresenceofbacterialproteinsorplantextractscontainingphenols,flavonoidsamines,alcohols,proteinsandalde-hydes.
ThedeficiencyofSeisknowntobeassociatedwithover40diseasesinman[2,3].
Atlowdosageitcanstimu-latethegrowthoftheplantwhereasathighdosagesitcancausedamagetoit[4-6].
Sehasalsobeenshowntobeef-fectiveagainstcancer[7,8].
Theircompoundsintheformofselenocysteineandselenomethioninearemetabolizedinbiologicalsystem[7,8].
Avarietyofmicroorganisms,enzymesandfungi,besidesplantextractshavebeenusedtosynthesiseSenanoparti-clesofdifferentsizeandmorphology.
Seitselfisusedinrectifier,solarcells,photocopierandsemiconductor.
Inaddition,theyexhibitbiologicalactivityowingtotheirinteractionwiththeproteinsandotherbiomoleculespresentinthebacterialcellsandplantextracts,contain-ingfunctionalgroupssuchasNH,C=O,COOandC-N[9].
Se-nanobeltshavebeensynthesisedonlargescalewithanapproximatediameterof80nmandlengthupto5μm[10].
Seexistsinmanycrystallineandamorph-ousformsbuttheshape,sizeandstructureofthenano-particlesdependontheconcentration,temperature,natureofbiomoleculesandpHofthereactionmixture.
ThepropertiesofSenanoparticlesvarieswithsizeandshapeforinstance,SenanosphereshavehighbiologicalactivityandlowtoxicitywhileSenanowiresoft-Sehavehighphotoconductivity[11].
VariousmethodshavebeenemployedtoproducelargescaleSe-nanowiresandtrigonalselenium(t-Se)[12,13].
Pulselaserablation,electro-kinetictechnique,hydrothermaltreatment,vapourdepositionmethods[10,14-16]generallyusedforproductionofSenanoparticlesonlargescalerequireeithersophisticatedin-strumentsorspecificchemicalswhicharetimeconsuminganduneconomical.
Suchmethodsoftenemploytoxic*Correspondence:adroot92@yahoo.
co.
in1DepartmentofBiology,CollegeofNaturalandComputationalSciences,UniversityofGondar,P.
O.
Box196,Gondar,EthiopiaFulllistofauthorinformationisavailableattheendofthearticle2014HusenandSiddiqi;licenseeBioMedCentralLtd.
ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttributionLicense(http://creativecommons.
org/licenses/by/4.
0),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycredited.
TheCreativeCommonsPublicDomainDedicationwaiver(http://creativecommons.
org/publicdomain/zero/1.
0/)appliestothedatamadeavailableinthisarticle,unlessotherwisestated.
HusenandSiddiqiJournalofNanobiotechnology2014,12:28http://www.
jnanobiotechnology.
com/content/12/1/28chemicalsorhightemperatureandhighpressurewhichfurtherpollutetheenvironment.
Inordertocircumventtheeffectoftoxicchemicalsinthefabricationofnanoparticles,biogenicprotocolisgenerallyfollowed[17,18].
Scientistshavedevelopedbe-nignandharmlessmethodsforthefabricationofnano-particlesusingplantextracts,bacteriaandfungi.
Forinstance,Capsicumannum,EscherichiacoliandBacillussubtilis[19-21]haverecentlybeenusedtoproducenanoparticles.
Over16differentspeciesofbacteriaandArechaeahavebeenfoundtoreducecolourlessselenateandselenitetoredelementalSeofdifferentshapeandsize[22,23].
Plantsandmicrobesactasproducersandprotectorsoftheenvironmentwhentheyareproperlyused.
Pureelementinits,atomicstatemaybeproducedbymanybacteria[24,25]mainlyduetothechemicalspresentinthemorproteinexudedbythem.
WehavelimitedknowledgeofthemechanismoftheformationofSenanoparticlesbymicrobesandplantextract,never-thelessforabetterunderstandingattemptsarebeingmadetoexplorethechemicalreactionsoccurringinthesemedia.
Manybacterialstrainshavebeenfoundtoreduceselenate/selenitetoSenanoparticlesindifferentenvironment[26]eveninsewageandsludgeunderbothaerobicandanaerobicconditions[27-29].
Ithasbeensuggestedthatsubstantialquantityofsolubletoxicselen-ate/seleniteisreducedbybacterialstraintoproducenontoxicinsolubleSenanoparticles,althoughindoingsomanysuchmicrobeswoulddie.
TheproductionofSe0/Te0bytwoanaerobicbacteriaSulfurospirillumbar-nesiiandBacillusselenireducenshasbeendemonstratedbyOremlandetal.
andBaesmanetal.
[25,24].
Themainobjectiveofthisreviewistoidentifytheplantextractsandbacterialstrainsinvolvedinthebio-synthesisofSenanoparticles.
AlsothecharacterizationandidentificationofSe-nanoballs,nanorods,nanowiresandhollowsphereshavebeenundertakenwithaviewtoupdatethenanobiotechnologyofSenanoparticlesandtheirapplicationindiverseareas.
Senanoparticlesfromplants,characterizationandapplicationThereisafinelinebetweenoptimumlimit/ordeficiencyandexcessofSeinlivingsystemwhichmaycausetox-icity.
ItisknownthattheSenanoparticlespreparedfrombiologicalmaterialarelesstoxicthanthebulkSenanoparticlespreparedfromchemicals.
Thebiomole-culespresentintheextractactbothasreducingagentandstabilizersofSenanoparticles.
Bacteria,algae,dryfruitsandplantextractsareusedtoproducenanoparti-cles.
Greensynthesisofseleniumnanoparticlesfromsel-eniousacidwasachievedbydriedextractofraisin(Vitisvinifera)[30].
Likeotherbiologicalmaterials,raisinalsocontainssugar,flavonoidsandphenolsinadditiontomineralssuchasiron,potassiumandcalcium[19,31].
Achangefromcolourlesstodeeplybrick-redcolourindi-catedtheformationofnanoparticles.
TheformationofSe-nanoballswasexaminedatdifferentintervaloftime.
Ittooknearly6mintostartconversionofSeiontoSenanoparticleswhichwasindicatedbyadecreaseinSeionconcentrationinthesolution.
Thenatureofnano-particleswasanalysedbyTEMimages.
Itshowedthatthediameterofnanoballsrangesbetween3and18nm.
Theywerefoundtobeencapsulatedwithathinpoly-morphiclayer.
TheformationofSenanoparticleswasconfirmedfromtheenergydispersivex-rayspectros-copy.
TheSenanoballswereidentifiedfromtheirchar-acteristicabsorptionpeaksat1.
37KeV,11.
22KeVand12.
49KeV[32].
ThemorphologyofSenanoparticlescanbeanalysedbyx-raydiffraction(XRD)analysis.
Thebroaddiffractionpeaksuggeststhepresenceofamorph-ousnatureofSenanoparticles[33].
Theirparticlesize[34]hasbeenfoundtobeoftheorderof12nm.
Sharmaetal.
[30]havecharacterizedSenanoballsfab-ricatedfromV.
viniferabyFTIRspectralstudies.
Thespectrumexhibitedtwosharpabsorptionpeaksat3420cm1attributedtoOHand,thesecondpeakat1620cm1toC-Hvibrationoftheorganicmolecules.
Adistinctpeakat1375cm1hasbeenassignedtophenolicOH.
Theotherpeaksofmediumintensityaredueto–CH3andOCH3groupsassociatedwiththebiopolymers,presentintheV.
viniferaextractactingasreducingagentandstabilizerfortheSenanoballs.
Sinceligninisacomponentofallvegetables,fruitsandcellwall,itcanbeextractedfromthemandthecompoundspresentinthemmaybeidentified.
Inthepresentwork,phenolicgrouphasbeenidentifiedwhichgenerallyactsasredu-cingagentand,itisoxidisedtoketoneduringtheredoxprocess.
However,theextractalsocontainsfairlysub-stantialamountofreducingsugarsandtherefore,theyalsohelpinthereductionandformationofSenanoballs.
TheseauthorshavegivenaflowdiagramforSenanopar-ticlessynthesisbutitdoesnotrevealthechemicalchangeswhichoccurasaconsequenceofredoxreac-tions.
WenowproposethefollowingschemeFigure1basedonthegeneralsyntheticroute.
Although,biochemicalsmayoftenbeusedforthesynthe-sisofnanomaterials,thebiogenicsyntheticrouteisfre-quentlyappliedduetoitseaseandsimplicityand,alsobecausenohazardousandtoxicresiduesarereleasedintheenvironment[35,36].
Ingeneral,avarietyofSenanoparti-clesareproducedwhenH2SeO3istreatedwithplantex-tractsforinstance,α-SenanoparticleshavebeenfabricatedfromCapsicumannumextractinaqueousmediumatlowpHandatambienttemperature[19].
ThelightgreenextractofC.
annumturnspaleafter5hoftheadditionofH2SeO3,andthengraduallyturnedredafter12h(Figure2a).
ThisredcolouristhecharacteristicsignatureHusenandSiddiqiJournalofNanobiotechnology2014,12:28Page2of10http://www.
jnanobiotechnology.
com/content/12/1/28ofα-Seinthex-rayphotoemissionspectroscopy(XPS)whichisduetoexcitationoftheirsurfaceplasmonvibra-tion[37].
ItsXPSspectrum(Figure2b)showsasharppeakat54.
4eVwhichcorrespondstotheelementalselenium[38].
TheXRDpatternoftheSenanoparticlesshowsabroadpeakat2θanglesof15-350(Figure2c)whichsug-geststhatthenanoparticlesarenotcrystalline.
TheirRa-manspectrumdisplayedaresonancepeakat263.
7cm1which(Figure2d)furtherconfirmstheformationofα-Senanoparticles[39].
Anadditionalpeakat474cm1hasbeenattributedtotheproteinvibrationwhichismixedwithamorphousSe.
SEMandTEMimagesoftheα-SenanoparticlesshowedthattheyconsistofnanorodsandnanoballslacedwithC.
annumproteinwhichmakesthemslightlyirregularinshape.
Thelengthanddiameterofrodsandnanoballsrangebetween1–3μmand0.
4μm,respect-ively.
Acloserlookatthehighlymagnifiedfieldemissionscanningelectronmicroscopy(FESEM)imagesuggeststhatrodlikenanoparticlesareactuallyaggregatesofsphericalparticleswithproteincoating,makingthesur-faceroughanduneven.
ItisquitelikelythatproteinsareheldtogetherbyhydrogenbondingandSenanoparticlesareheldbyvanderWaalsforces.
WhenthepHofthereactionmixtureisloweredto2thetimetoproduceα-Senanoparticlesincreases.
IthasbeenobservedfromtheirFESEMimagesthatavarietyofpolygonalSenanoparticlesareproducedwithsizevaryingfrom200–500nm.
Itisofinteresttonotethatsomehollowsphericalparticleswerealsoproducedwithaporediameterof160nm.
Lietal.
[19]havehypothesizedthathollowspheresareformedasaconsequenceofriseintemperaturewhenthereactionproductisplacedinanelectricfield.
Although,themeltingpointofα-Senanopar-ticlesisnotverylow(~490K)eventhenthistemperatureisseldomachievedinsuchsystem,sothatitmaymeltandproducehollowspheres.
Itistobenotedthatevenifmicrowaveenergyissuppliedwithoutriseintemperatureonlytheoutersurfaceofα-Senanoballs,madeofproteinlayerwouldmelt,becauseorganicmaterialshaveinher-entlylowermeltingpointthanmetalloidSe.
However,iftheseα-Separticlesalsomeltwithelectronicimpacteventhenthehollowspherewouldnotbeproducedbecausethelatticewouldruptureresultingintheformationofirregularsheetsanddotlikestructures.
Itismorelikely,thathollowspheresofα-Searealsoformedalongwithsolidnanoballsandpolygonalstructuresduringthesynthesisofnanoparticles.
AcomparisonofFTIRspectrumofpureC.
annumextractwiththereactionmixture(C.
annumextract+SeO32)showedmanypeaksat1652,1542and1241cm1correspondingtoamideI,IIandIIIbandsowingto(C=O),(N-H)and(C-N)respectively[36].
Thesebandsslightlyshiftaftertheformationofnano-particles.
TheUV–visspectrumoftheC.
annumpro-tein(washedwithSDS-PAGEgel)withmolecularweightof30kDa,showedpeak(210nm)correspondingtopeptidebondsandaminoresidues(280nm).
Asthesearereducingagentstheyhelpintheformationofnano-particles.
Ithasalsobeenconfirmedfromcyclicvoltam-mogramthattheredoxreactionoccursbetween-0.
7and0.
9V[19].
+SeleniousacidVitisviniferaaq.
extractH2SeO34Lignin–OH(phenolicgroup)Se+3H2O+4Lignin=OCappedSenanoparticles+SeleniousacidVitisviniferaaq.
extractH2SeO34Lignin–OH(phenolicgroup)Se+3H2O+4Lignin=OCappedSenanoparticlesFigure1SenanoparticlesynthesisusingVitisviniferaextract.
HusenandSiddiqiJournalofNanobiotechnology2014,12:28Page3of10http://www.
jnanobiotechnology.
com/content/12/1/28InorganicSe(seleniteorselenate)alsooccurasseleno-methionine,selenocysteine,selenocystathione,methylselenol,dimethylselenideandseleniummethylseleno-cysteine.
AbsorptionofSedependsonitsmorphologyandsolubilityinaqueousmedium.
SodiumsaltsofSearegenerallysolubleinwater.
Oneformmaychangeintoanothertosuitthebasicrequirementsofbindingtocertainfunctionalgroupssuchasproteins.
DifferentplantsabsorbSeindifferentquantitiesforinstance;wheataccumulatesSeproportionaltoitsavailabilityinthesoilwhileAstragalusgrowninthesamesoilhadmanifoldexcessoftheelementinit.
BroccoliisknownasSeaccumulator.
Finley[40],showedfromanexperi-ment,onbroccoligrowninsodiumselenateladensoil,thatitaccumulated~103μgSe/gmdryweightoftheplanttissue.
Broccoliisknowntocontainfairly[41]largequantityofseleniumasmethylselenocysteine,perhapsduetoitsgreatersolubilityinaqueousmedium.
However,itisstrangethatSefrombroccolidoesnotaccumulateefficientlyinmanorrats[42,43]becauseitsmajorpartasseleniummethylselenocysteineisperhapsmetabolisedtomethylselenol[44].
Ithasbeendemon-stratedexperimentallythat,methylsubstitutedformsofSeisaneffectiveanticanceragentthantheotherderiva-tivesoforgano-Secompounds[45].
GarlicwithasmuchSeas1000μg/gmofdryweighthasbeengrown[46]andfoundtocontainSeasseleniummethylselenocysteinebutwhentheSeconcentrationfallsbelow200μg/gm,itisfoundasq-glutamylseleniummethylselenocysteine.
Al-thoughSefromhighSegarlicisachemoprotectiveagent[47]itisparticularlyselectiveagainstbreastcancerinrats[48,49]inducedby7,12-dimethylbenzene(a)anthracene.
Eventheaqueoussolutionofgarlicischemoprotective[50].
GlucosinolateassecondaryplantcompoundknownFigure2SenanoparticlesynthesisusingCapsicumannumextract(a)Thetime-dependentcolorchangesofthereactionsolution;(i),(ii),(iii)represents0,5,15h,respectively.
(b)XPSspectrumoftheproductobtainedfromreactionsolution(I).
(c)TypicalXRDpatternofthesameproductofreactionsolution(I).
(d)Ramanspectrumofthesamereactionproductasin(c)[19].
HusenandSiddiqiJournalofNanobiotechnology2014,12:28Page4of10http://www.
jnanobiotechnology.
com/content/12/1/28toinducephaseIIenzymes[42]ischemoprotectiveagainstbladdercancer.
Sefedexperimentalratsintheformsofselenite,selenate,selenomethioninedidnotacumulateinmostratswhichmeansitiseithernotabsorbedorexcretedthroughurine.
TrigonalSenanowiresandnanotubeshavebeensyn-thesizedinabsolutelyecofriendlyway.
TheSenanowiresof70–100nmwidthandlengthinseveralμmwerepre-paredinabsoluteethanolatroomtemperaturewhiletri-gonalSenanotubesofdiameter180–350nmwereobtainedinaqueousmediumat85°C(358K).
Itwasob-servedthatamorphousSenanoparticleswereformedinthebeginningandsubsequentlytransformedintonano-wiresandtubes[51].
StableSenanoparticlesincolloidalformwerepreparedfromTerminaliaarjunaextractinaqueousmedium.
TheywerecharacterizedbyUV–vis,energydispersiveX-rayanalysis(EDAX),transmissionelectronmicroscopy(TEM),FTIRandXRDanalysis.
Thecolloidalsolutionhadanabsorbancemaximumat390nm.
ItsIRspectrumshowedpeakscorrespondingtoO-H,NH,C=OandC-Ostretchessuggestingthepresenceofhydroxyl,amino,ketonicandcarbonylfunctionalgroupsintheextractwhichactbothasstabilizerandcappingagentfortheSenanoparticles[52].
TheSenanoparticlessynthesizedfromfenugreekseedextractinaquousmediumatroomtemperaturearebetween50–150nm.
Theyhavebeenfoundtobeactiveagainsthumanbreastcancercells[53].
Senanoparticlesofapproximately35nmhavebeensynthesizedfromgumarabicwhichremainstableinso-lutionforabout30days.
ThegumarabicwasfoundtobeabetterstabilizerforSenanoparticlesthanthehy-drolysedgumarabic[54].
TheSenanoparticlessynthe-sizedfromlemonleafextractexhibitedanabsorptionmaximumat395nmintheUV–visregion.
Initially,themixtureofleafextractandSeO32remainscolourlessbutafterstirringandincubatingitfor24hrat30°C,itturnsred[55].
Thephotoluminescencespectraexhibitedexcitationpeakat395andemissionpeakat525nm(Figure3).
IthasbeenfoundfromTEMimagethatthesizeofparticlesrangesbetween60–80nm.
Theyarepolydispersedincolloidalsolutionbutcrystallineinna-ture[55].
TheFTIRspectraofthesampleswithandwithoutSenanoparticleswerecomparedtoexaminethechangesinthefunctionalgroupsofthebiomolecules.
Thebroadpeakat3415dueto(NH)shiftsto3418cm1butnewpeaksappearat2930and3456cm1inthecolloidalsolutioncontainingSenanoparticles.
Theregion1500–1800cm1isduetovariousamidebandswhichsplitintosomenewbandsincolloidalsolution.
However,afterreductionoftheNa2SeO3toSenanopar-ticlesbythebiomoleculesintheextractcontainingfunc-tionalgroupssuchasalcohol,aldehyde,phenoletc.
,theyareoxidizedtothefollowingspecies:Na2SeO3+H2O→H2SeO3+Na2OH2SeO3SeO32+2H+Alcohol+SeO32→Se+CarboxylicacidAldehyde+SeO32→Se+KetonePhenol+SeO32→Se+PhenoneIthasalsobeendetectedfromgelelectrophoresisthatSenanoparticlespreventedDNAdamagewhencellswereexposedtoUVB[55].
PolyphenolgallicacidnanoparticlesfromplanthavebeenusedtofabricateSenanoparticlessincethegallicacidnanoparticlesmaybehavedifferentlythanthebulkgallicaicd.
SincegallicacidcanquantitativelycoordinatewiththeSeions,anotherreducingagentdithioerthreitolwasaddedtogallicacid-coatedwithSeions.
AchangeincolourwastakenasanindicationoftheformationofnanoparticleswhichwasconfirmedbyUV–visandemis-sionspectroscopy[56].
AslightlydifferentmethodhasalsobeenemployedbyIngoleetal.
[34]toprepareSenanoparticlesfromglucose.
Na2SeSO3preparedfromSepowderwastreatedwithglucosesolutionaccordingtothefollowingchemicalreactions:Sepowder+Na2SO3→Na2SeSO3Na2SeSO3+H2O→H2SeO4+Na2SH2SeO42H++SeO42SeO42+Glucose→Se+GluconicacidThecolourlesssolutioninthebeginningbecomesyellowthenorangeandfinallyturnsredwhichdoesnotchangeevenafterheatingforover1h.
ThesechangeshavebeenascribedtothechangesinsizeofSenanoparticles.
Senanoparticlesfrommicrobes,characterizationandapplicationMicroorganismsreducethetoxic,selenateandseleniteoxoanionsintonontoxicelementalseleniumwhichisinsolubleinwater.
ContinuoususeofwateroredibleplantsfromSerichsoilcancauseskinlesionandearlyhairfall.
Effortistherefore,madetoreduceSecom-poundstoelementalSewiththehelpofbacteria.
Itisasimpleprocessofdetoxificationofselenites/selenatestoSenanoparticlesasthereversereactionistooslowtoproduceSecompounds[57].
Fast(forwardreaction)Se(IV)/Se(VI)Se(o)Slow(backwardreaction)DuetotheiruniquepropertySenanoparticlesarephotovoltaicandsemiconductor,antioxidantandchemo-protectiveagents[58].
SinceSenanoparticlesinhibitthegrowthofStaphylococcusaureusitcanbeusedasaHusenandSiddiqiJournalofNanobiotechnology2014,12:28Page5of10http://www.
jnanobiotechnology.
com/content/12/1/28medicineagainstS.
aureusinfection.
Differentconcentra-tionofSestartingfrom65to230mg/LofSe(IV)wereallowedtointeractwithdifferenttypesofmicrobes.
Ap-pearanceofredcolourwastakenassignofreductionofSe(IV)toSe(0)asshowninforwardreactionabove.
How-ever,therewasnodecolourationlater,indicatingtheab-senceofanyspeciescausingoxidationofSe(IV)→Se(VI)(Figure4).
Theredoxprocessistimeandconcentrationdependent.
Whenbacterialculturewasgrowninpresenceof40–100mg/Lselanate,nochangeincolourwasob-servedevenafterlongtime.
ItappearsasifthebacteriaareresistanttoSe(VI)reduction.
However,suchbacterialculturemaybeusedtoreducesolubleandtoxicSe(IV)tonontoxicandinsolubleSenanoparticles.
Itisalsoindica-tiveofbioremediationofSefromselenites.
AmporphousSenanoparticleshavebeensynthesizedfromsodiumseleniteinpresenceofShewanellasp.
HN-41inaqueousmediumunderanaerobicconditionstakingcareofreactiontime,seleniteconcentrationandbiomassofShewanella[59].
DifferenttypesoftheSenanopar-ticlesaresynthesizedusingprotein,peptidesandseveralotherreducingagents[1,60,61].
NanowiresandnanorodshavebeenfabricatedfromRhizobiumselenireducenssp.
,Dechlorosomasp.
,Pseudomonassp.
,Paracoccussp.
,Enter-obactorsp.
,Thaureasp.
,Sulfurospirilliumsp.
,Desulfovibriosp.
,andShewanellasp.
,[61-63].
IthasbeenreportedthattheparticlesizeisdecreasedinpresenceofO2.
Itisobvi-ousthatoxygenwillpromoteoxidationofSe(forwardre-action)asaconsequenceofwhichtheredoxstepbecomesslowerproducingsmallerSenanoparticles.
Selenitereduc-taseisalsohelpfulinthesynthesisofSenanoparticles.
Awiderangeofseleniteconcentrationstartingfrom0.
01,0.
05,0.
15,0.
25,0.
050,0.
75and1mMwereusedtostudytheeffectofconcentration,sizeandmorphology.
Averageparticlesizeforalltheaboveconcentrationswasnearly103±5.
1nm.
Forlargequantityofnanoparticlestheselen-iteconcentrationnotexceeding0.
1mMisneeded.
IthasbeenobservedfromTEMandSAEDimagethatSenano-particlesareamorphous[59].
Extracellularsynthesis[64]offairlysmallerparticlesoftheorderof47nmfromthefun-gus,Aspergillusterreuswasdonein60min.
Figure4Selenitereductionbythemixedmicrobialcultureisolatedfromagriculturalsoil.
SelenitereductionatdifferentSe(IV)concentrations(a)anddevelopmentofredcolorationinculturesafter5.
5h(b),23h(c)and48h(d)ofincubation[58].
Figure3Photoluminescencespectraofseleniumnanoparticlessynthesizedusingleafextract[55].
HusenandSiddiqiJournalofNanobiotechnology2014,12:28Page6of10http://www.
jnanobiotechnology.
com/content/12/1/28MicrobeslikeKlebsiellapneumoniae[31]andPseudo-monasalcaliphila[65]havealsobeenusedtosynthesizeSenanoparticlesingoodyield.
WhenNa2SeO3wasaddedtotheactivatedcultureofP.
alcaliphilathereactionstartedimmediatelybutcompletedafter48h[65].
Agradualcolourchangewithtimewasobservedinthefollowingorder:Grey→Red→IntenseRed0h6h48hThecharacteristicredcolourofSenanoparticles[37]wasdetectedspectrophotometricallyandhasbeenas-cribedtotheexcitationofthesurfaceplasmonvibrationofthemonoclinicSe.
Ithasbeennoticedthatparticlesizeisdirectlyproportionaltoreactiontimeanditrangesbetween50–500nm[21].
Fieldemissionscan-ningelectronmicroscopic(FESEM)imageshowsnano-particlesofvaryingsizeandshape.
TheFTIRspectraofthesampleswithandwithoutSenanoparticlesshowedthattheintensityofthespectralpeakscontainingSenanoparticlesisdrasticallydimin-ished[65]whichsuggestsstronginteractionbetweenSeatomsandtheproteinmoleculespresentintheP.
alcali-phila.
ThisistobenotedthattheinteractionbetweenSenanoparticlesandproteinissimplyelectrostaticbe-causetheintensityofsamplecontainingSeatomswasdecreasedfollowedbyanincreasein(NH)from3421to3435cm1.
TheRamanspectraalsosupportthefor-mationoftrigonalselenium(t-Se)andmonoclinicselen-ium(m-Se)bytheappearanceofpeaksat234and254cm1,respectively(Figure5).
Apeakat235cm1ismainlyduetochainlikestructureoft-Se.
Asthepeakat234cm1appearsafter48hofinoculation,itisconsid-eredasthetransformationofoneformofSeintoother.
TheFESEMimageswhichshowtheaccumulationofnanorodsonthenanoballs.
ThesizecanbecontrolledbyPVPatdifferenttimeofincubationofnanospheresrangingfrom20–200nm.
OnthebasisofabovestudiesapossiblemechanismfortheformationandtransformationofSenanoballstoSenanorodshasbeenproposed.
Zhangetal.
[65]pre-sumethatSeO32isreducedtoelementalseleniumbytheproteinexcretedbytheP.
alcaliphilaand,theirag-gregationgivesSenanoparticlesofvaryingsize[13].
ItistruethatproteinactsasreducingagentforSeO32butitisavailableasexcretionfromP.
alcaliphilaisnotcon-vincing.
Theexcretioncontainstoxins,pyrogensandtracesofproteinbuttheymaynotbesufficientforre-ductionofselenite.
Authorsalsosuggestthatlargem-SenanoballsarenotstableinsolutionandtheydissolvetoformSeatoms.
AfractionofdissolvedSeatomscrystallizeast-Seformingnanorods[66].
ItisnotruebecauseSeinatomicstateisnotsolubleinasolventbutstaysincolloidalform.
Itisamisconception.
However,PVPcontrolsthesizeofSenanoparticles.
IftheSenano-particleswithoutPVPareleftfor2–3weekstheyformaggregatesofdifferentshapesandsize.
SinceSeO32inionicformistoxictobacterialculture,thebacteriainselenitesolutionmaytherefore,dieandthedisintegratedproteinmaythenactasreducingagentforselenite.
Thelargem-SenanoparticlecannotdissolveinsolutiontogiveSeatomformingSe-nanorods.
Itisquitelikelythattheymaybesegregatedandrearrangedintonanorods.
SenanoparticlessynthesizedfromsodiumseleniteandglutathioneinaqueousmediumhadbeentestedforitsgrowthinhibitionefficacyagainstStaphylococcusaureus[67].
ItwasfoundthatgrowthofS.
aureuswasinhibitedinpresenceofSenanoparticleswithin3–4hwithaslowconcentrationas7–15μg/mlwhichsuggeststhatSenanoparticlesmaybeusedagainstbacterialinfections.
BiogenicSenanoparticlesfabricatedfromproteinpro-ducedbyE.
colihavebeencomparedwiththosesynthe-sizedfromchemicalreactionviaredoxmechanism[68].
TherearespecifictypesofproteinproducedbyE.
coli(AdhP,Idh,OmpC,AceA)whichareassociatedwithSenanoparticles.
TheyarealsoresponsiblefortheirFigure5RamanscatteringspectraofSeNPstrappedatdifferentincubationtimes:(a)24hand(b)48h[65].
HusenandSiddiqiJournalofNanobiotechnology2014,12:28Page7of10http://www.
jnanobiotechnology.
com/content/12/1/28uniformsizeanddistribution.
Oneoftheproteins(AdhP)wasfoundtobindstronglytoSenanoparticles.
E.
coliwasfoundtoproduceSenanoparticlesfrom2mMofSeO32inabout48hwhichwasdistinguishedbythechangeincolourfromcolourlesstodarkred.
TheamorphousSenanoparticlesthusproducedwerebe-tween10–90nm.
Sincethebacterialgrowthcontinuedeveninpresenceofselenite,itisconfirmedthatseleniteisnottoxictoE.
coliatthisconcentration.
Theenzymesalcoholdehydrogenase,propanol-preferring(ADHP),ACEP(Isocitratelyase),ENO,KPYKI,IDHandGLPKrequiremetalionsascofactorfortheiractivitywhiletheenzymesDCEP,ASTCandTNAArequirepyridoxalphosphateascoenzymewithoutwhichtheiractivityislost.
Theauthorshavenotdistinguishedbetweencofac-torandco-enzyme,theyhavetermedbothasmetalliccofactorandnon-metalliccofactorwhichismisleading.
Thecofactorsintheenzymearemetalionsbondedthroughacoordinatecovalentbondandcanacceptlonepairofelectronsfromthedonoratomsintheenzymeintotheirvacantorbitaltoformthebond.
TheSenano-particleisintheelementalstateandnometalinatomicstatecanbindtoproteinoranyelectrondonatingspe-cies.
Itistherefore;propertousethewordassociationofSenanoparticlestoproteinratherthanbonding.
ThesizeofSenanoparticlesproducedinpresenceofthepro-tein,alcoholdehydrogenasepropanol-preferring(AdhP)weremuchsmallerthanthoseproducedintheirab-sence.
However,sinceE.
colicontainsmanyotherpro-teinsthanonlypAdhP(purifiedprotein),thedecreaseinSenanoparticlesizemaybethecumulativeeffectofallproteinsavailableinE.
coli.
ConclusionBioreductionofselenateorselenitefrommicroorganismsuchasbacteria,fungiandplantextracthavebecomethefavouritepursuitofbiologist,chemistandengineers.
Itisexpectedthatinfuturethemetalwouldbeex-tractedbybiomineralizationbecausetheyproducethepurestformoftheelement.
Manyrawmaterialslikewastevegetables,fruitpeelsandleathercuttingsmaybeutilizedtoproduceelementalmetal/metalloidfromtheiroxide,halide,nitrate,sulphideandcarbonates.
Generally,protein,phenol,alcohol,flavonoidorsugararerequiredforthereductionofSeO32,SeO42andatleastoneoftheaboveorganicmoleculesispresentinmicrobesandplantextracts.
Theymaytherefore,beexploitedforthebiotransformationofselenateandselenidetoelementalSeofvariousshapeandsize.
Sincethereducedmetalsormetalloidsareinsolubleinaqueousmediumtheycanbeeasilysequestered.
Growthinhibitionofsomeofthebacterialstainsoccursduringtheredoxprocesswhichsuggeststhatselenite/selenatemaybeusedagainstinfec-tioncausedbysuchmicrobes.
CompetinginterestsTheauthorsdeclarethattheyhavenocompetinginterests.
Authors'contributionsAHgatheredtheresearchdata.
AHandKSSanalysedthesedatafindingsandwrotethisreviewpaper.
Bothauthorsreadandapprovedthefinalmanuscript.
AcknowledgementsTheauthorsarethankfultothepublishersforthepermissiontoadopttheirfiguresforthisreview.
Authordetails1DepartmentofBiology,CollegeofNaturalandComputationalSciences,UniversityofGondar,P.
O.
Box196,Gondar,Ethiopia.
2DepartmentofChemistry,CollegeofNaturalandComputationalSciences,UniversityofGondar,P.
O.
Box196,Gondar,Ethiopia.
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