Organic Nanoparticles in the Aqueous Phase-Theory, Experiment, and Use - 图文

Organic Nanoparticles in Aqueous Phaseactive substancegelatin stabilizer -carotene

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OrganicNanoparticlesintheAqueousPhase–Theory,Experiment,andUse

DieterHornandJensRieger*

Manyactiveorganiccompoundsandorganiceffectmaterialsarepoorlysolubleinwater,oreveninsoluble.Aqueousformsofapplicationthusrequirespecialformulationtechniquestoutilizeoroptimizethephysiological(pharmaceuticals,cosmetics,plantpro-tection,nutrition)ortechnical(var-nishes,printinginks,toners)action.Themostinterestingpropertiesofnanodispersionsofactiveorganiccom-poundsandeffectmaterialsincludetheimpressiveincreaseinsolubility,theimprovementinbiologicalresorption,andthemodificationofoptical,elec-trooptical,andotherphysicalproper-tieswhichareachievableonlywithparticlesizesinthemiddleorlowernanometerrange(50±500nm).Henceinadditiontoeconomicandecologicalconstraintstherearealsotechnicaldemandswhichappeartourgentlyrequirethedevelopmentofnewproc-essesfortheproductionoforganicnanoparticlesasalternativestotheestablishedmechanicalmillingproc-esses.Inthiscontextattentionisdrawntotherecentincreaseinresearchactivitieswhichhaveastheirobjectivethecontinuous,automaticpreparationofnanodispersedsystemsbyprecipita-tionfrommolecularsolution.Inthisreviewthecurrentstateofknowledgeofthefundamentalsofparticleforma-tionfromhomogeneoussolutionandtheeffectofsolventandpolymeradditivesonthemorphologyandsu-pramolecularstructureofthenano-particlewillbediscussed.Thepracticalimplementationofthisnewformula-tiontechnologywillbeexploredindetailforthecarotenoids,aclassofcompoundsofbothphysiologicalandtechnicalinterest.Keywords:carotenoids￥dispersesys-tems￥nanoparticles￥nanostructures￥phasetransformations1.Introduction

Theimportanceofnanoparticles,thatis,particleswithdimensionsintherangeofabout10nmtoafewhundrednanometersisobvious:theydetermineourlifeintheformofproteincomplexesandothercellcomponents,asviruses,colloidalparticlesindrinkingwater,surfacewaterandseawater,andasaerosols;theyfinduseasdispersioncolorsandasadhesives;inindustrytheyplayanimportantroleintheformulationofpigmentsandintheproductionofcatalysts;numerousattemptsarebeingmadetodelivernanoparticulateformsofpharmaceuticallyactivecompoundsspecificallytothedesiredsiteoftheactioninthebody;finallynanoparticlesfinduseasquantumdotswithspecialpropertiesforelectroniccomponents.Beyondthesepracticalaspectsthereisscientificinterestinnanoparticlesowingtotheirspecialpropertieswhichliebetweenthepropertiesofmoleculesandthoseofbulkmaterial.

[*]Dr.J.Rieger,Dr.D.Horn

BASFAG

PolymerResearch,DepartmentofPolymerPhysics67056Ludwigshafen(Germany)Fax:(??49)621-60-92281

E-mail:jens.rieger@basf-ag.de

Angew.Chem.Int.Ed.2001,40,4330±4361

Inathoroughstudyofthescientificliteratureonthetopicofcolloidalsystemsitbecameevidentthatmuchhasbeenwrittenoninorganicnanoparticles,polymerdispersions,andontheprinciplesofparticleformationingeneral.Incontrast,surprisinglylittleislearntofthemechanismsofparticleformationoforganicsystems.Moreover,thereisacleargapbetweenthatwhichisreportedinmanytextbooksonparticleformationandthecurrentstateofknowledge.Thisunsat-isfactorysituationwasthemotivationforthiscontributionwhichontheonehanddealswiththepreparationandpropertiesoforganicnanoparticles,andwithmodernaspectsofparticleformationontheother.

Organicnanoparticlestakeonmanyforms(Figure1).Herewewilllimitourselvestotheconsiderationofpharmaceuti-callyactiveorganiccompoundsandorganiceffectmaterialsastheyoccur,forexampleinpharmaceuticalsapplicationsandintheformofvitaminsandpigments.Manyofthesematerialsarepoorlysolubleinwater,oreveninsoluble.Aqueousformsofapplicationthusrequirespecialformulationtechniquestooptimizethephysiological(pharmaceuticals,cosmetics,plantprotection,nutrition)ortechnical(varnishes,printinginks,toners)action.Animportanttargetinthiscontextistheconversionofthegenerallycoarsecrystallinesynthesisproductintothefinestparticulatedispersionpossible,with

1433-7851/01/4023-4331$17.50+.50/0

1WILEY-VCHVerlagGmbH,D-69451Weinheim,2001

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man-madenanoparticlesnatural

nanoparticles

active substances(pharmaceuticals)cell components

pigmentsviruses

polymer dispersions

micellar systems

supermoleculesand dendrimers

J.RiegerandD.Horn

environmental colloids(on surfaces, in water, in air)

protein aggregates(as nuclei for crystals)

Figure1.Classificationoforganicnanoparticles.

particlesizesintherangeof10to500nm.Inprincipletwostrategiesareconceivableforthispurpose:1)themechanicalmillingoftherawmaterialbywetordrymillingprocesses;2)theconversionoftheproductsoreductsdissolvedinsuitablesolventsintonanodispersedsystemsbyprecipitation,condensation,orbyspecificsynthesisprocedures(Figure2).Inthesecondcasetheundesirablesolventmustoftenthenberemoved.Thedifferentiationbetweenprecipitationandcondensationprocessesmakesitclearthatintheactualprecipitationprocessfurtheradditivessuchassurfactantsandpolymersassumetheroleofsurface-activecolloidalstabil-izers;inthecondensationprocesstheseadditivesthemselvesformthenanoparticulatephase(pseudolatexes)whichcon-tainstheactivecompoundoreffectmaterialboundbyadsorptionorabsorption.Thesyntheticpreparationofpolymerdispersions[1]asaspecialclassoforganicnano-particlesofconsiderableeconomicsignificancewillnotbediscussedhere.Otherprocedureswhichhavealsofound

Figure2.Methodsforthepreparationofnanoparticles.

DieterHorn,born1936inH?chst/Odenwald,studiedchemistryattheTechnicalUniversityDarmstadtandtheUniversityofHeidelberg,wherehegainedhisdoctorateinphysicalchemistryin1967workingwithProf.KlausSch‰fer.HethenmovedtotheUniversityofCalifornia,Berkeley,whereheworkedinthephysicalchemistrygroupofProf.GeorgeC.PimentelintheDepartmentofChemistry.AsamemberoftheNASAMariner6and7projectteamhewasresponsibleforthedevelopmentandapplicationofanalyticalmethodsforthequantitativeinterpre-tationofinfraredspectroscopicdataoftheatmosphereandsurfaceoftheplanetMarstransmittedbythespaceprobes.HejoinedthemainlaboratoryofBASFAGinautumn1970whereD.HornJ.Riegerhefirstworkedonthephysicsoforganicpigments.Acolloidalandbiophysicalworkgroupwasbuiltupunderhisleadership,withresearchprioritiesinthepreparationoforganicnanoparticlesandthedevelopmentoflaseropticmethodsforthecharacterizationofdispersedsystems.In1987hewasappointedheadofthepolymerphysics/solid-statephysicsresearchdepartmentinthePlasticsLaboratory,withresearchinterestsinthephysicsandthephysicalchemistryofpolymericstructuralandfunctionalmaterials,polymericeffectmaterials,anddispersedactivecompounds.ForhisfundamentalcontributionstoappliedcolloidsciencehehasbeenawardedtheSteinkopffPrizeoftheKolloid-GesellschaftandtheBonhoeffer-Eucken-ScheibelectureshipoftheDeutscheBunsenGesellschaftf?rPhysikalischeChemie.JensRieger,born1958,gainedhisdoctorateattheUniversityoftheSaarlandworkingwithProf.ArnoHolzintheoreticalphysicsonrandomwalks.In1989hejoinedthedepartmentforpolymerphysics/solid-statephysicsinBASFAGinwhich,nowasseniorscientistinchargeofthearea?StructureFormation∫,heisworkingonthefollowingtopics:deduc-tionofthestructure-propertyrelationshipsinpolymeric,colloidal,andhybridsystems,developmentofmethodsforthetime-resolvedtrackingofstructureformationprocessesincomplexsystemsonalllengthscales,controlofcrystallizationandparticleformationprocessesbypolymers.Inadditionheisexploringthepotentialofhigh-performanceradiationsourcesforpolymerandcolloidalphysics(small-angleneutronscattering,X-raymicroscopy,SAXS,andWAXSatthesynchroton).4332

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OrganicNanoparticles

extensiveuseinthepreparationofinorganicnanoparticles,[2]suchassol±gelprocesses,[2,3]synthesisinmicroemulsiontemplates,[2,4]andaerosolprocedures[5]havehithertoplayednopartinthepreparationoforganicnanoparticlesfromthestandpointdiscussedhere.Anydiscussionofnanoparticlesbyaggregationofblockpolymers[6]orbythetargetedsynthesisofextendedmoleculessuchasdendrimers[7]andotherbranchedsystems[8]willalsobeomitted.

Todiscussthemechanismsofparticleformationitisusefultodescribethecurrentstateofknowledge–initiallyinde-pendentlyofthenatureofthesystemformed.Figure3givesaninitialviewofthecomplexityofparticleformationandhighlightsanumberoftheoutstandingquestions.Aswillbecomeclear,fiveessentialpointsneedtobeemphasizedwithinthiscontext.

1.Thereisafardeeperunderstandingoftheformationofinorganicparticlesthanoforganicparticles.

2.Interestingadvancesinproteincrystallizationhavebeenachievedrecentlywhichalsoaffecttheclassicalareaofnucleationtheory.

3.Thereisstillaconsiderableneedforresearchintheareaofnucleationtheory,forexample,wheretheinteractionof

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severalcomponentsduringparticleformationiscon-cerned,whetheritbethesimultaneousprecipitationoftwomaterialsorthecontrolofparticlemorphologybytheuseofadditives.

4.Certainparticlesizesarenecessarytoachievecertaineffects,thesecanbeobtainedeitherthroughthe?physicalchemistry∫oftheparticle-formingsystemorbytheuseofadditives(protectivecolloids).Foranefficientprocedureinbothcasesthestructure-formationprocessesmustbeunderstoodonalllengthscales.

5.Littleisknownaboutthemolecularprocesseswhichtakeplaceduringthemixingoftwostartingsolutionsforproducingthestateofsupersaturationwhichinitiatesparticleformation.

Thepreparationofcolloidalsystemshasoccupiedscientistsforalongtime.[9,10]Numerousmonographsandreviewarticleshavebeendevotedtothistopic,afewofwhicharementionedherebywayofexample.[2,3,11±15]Severalpointsarisefromaperusaloftheliteratureonthistopic:astronglyphenomenologicalapproachisfrequentlyencountered,thatis,attemptsaremadetodeduceamechanismofformationfromthestructureoftheproduct,oftenapurelydescriptive

treatmentisconsideredsatisfactory.Ifonelooksbeyondtextbookknowledgeon?supersaturation,nucleation,growth∫a?zoo∫oftheoriesandinter-homogeneousHow to achievepretationsisencounteredwhichmustbetakenintohomogeneous supersaturation?solutionHow to exploitaccountifthereisadesiretounderstandatabasic,inhomogeneous supersaturation?thatis,molecular,levelhowananoparticulatesystemisformed.Thisknowledgeisofprimeprecursorimportanceasonlywithaknowledgeoftheexistence?mechanisticaspectsofparticleformationcantheprocessbemanipulatedspecifically,thatis,con-trolled–whetheritbebyvariationoftheprocesscriticalcrystallineamorphousparametersorbytheuseofsuitableadditivenucleistructure?molecules.Thisareaofresearchistrulyinterdisci-plinarysincechemists,physicists,andengineers,eachwiththeirspecialistknowledgeareindemand.intermediatestabilizationaggregationgrowthHowever,withinacademiathiscooperationisstagesnowherenearaswide-spreadaswouldappearappropriateforanoptimaltreatmentofthisprob-structure?lem.

Thearticleisarrangedasfollows:inSection2thecurrentstateofknowledgeofthefundamentalsofparticleformationisdiscussed,however,theclas-sicaltheoryofnucleationistreatedonlybriefly.EmphasisisplaceduponmorerecentthoughtsonHow to controlfinalstructure formationmechanismsofparticleformationincludingcom-particlesby processputersimulations,aswellasonthequestionoftoand/or additives?whatextenttheprocessesoccurringduringinitia-growth /tionofparticleformation–mainlybythemixingofstabilizationaggregationgrowthrecrystallizationtwoeducts–areunderstood?Finally,howtheparticleformationprocesscanbecontrolledbytheuseof(mainlypolymeric)additiveswillbediscussed.InSection3processesfortheprepara-tionoforganicnanoparticlesareconsidered.InSection4thepropertiesandareasofapplicationoftheseparticleswherethenanoparticulatestateisarequirementwillbeintroduced.Finallyopenques-Figure3.Stagesofparticleformationandopenquestions(seetextforfurther

tionsonthistopicwillagainbeaddressed.explanation).

process of particle formationAngew.Chem.Int.Ed.2001,40,4330±4361

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2.TheoreticalApproaches

2.1.ClassicalNucleationTheory

Textbookknowledgemaybesummarizedinthefollowingway:[11,14,16,17]Amulticomponentsystemexistsinitiallyasasinglephase.Bymodificationofboundaryconditionssuchastemperatureandpressureorbyhomogenousmixingwithafurthercomponentthefreeenergychangesinsuchawaythatphaseseparationisenergeticallymorefavorable.Theap-proachtakenassumesthatparticles(atoms,ions,molecules)oftheonecomponentcoalesceandformnucleioftheseparatingphase,initiallyitisimmaterialwhetheritis,forexample,theseparationofasolidphaseintheliquidmedium(thesubjectmatterofthisarticle),condensationfromthegaseousphase,orbubbleformationinaliquid(foaming).Thefreeenergyofasphericalnucleuswithradiusrcanbedescribedinrespectofthesingle-phasestatetoafirstapproximationasinEquation(1)(thesubscriptsSandV

DG??DGs??DGv??4pr2g??4/3pr3Dgv

(1)

refertosurfaceandbulkvolume,respectively)wheregstandsforthesurfacetensionbetweenthetwophasesandDgvforthedifferenceinfreeenergyperunitvolumebetweenthetwophases.ThetwotermsontherightoftheequationhaveoppositesignssothatDGasafunctionofrpassesthroughamaximum(Figure4).Thecriticalnucleusradiusr*isdefined

?GSgrowth ofstable particlesEr*?GV?Gsize of nucleus, rFigure4.Energydiagramtoexplanethenucleationprocess(DG:freeenergyofaparticlewithradiusr,DGs:surfaceenergy,DGv:bulkenergy,r*:radiusofthecriticalnucleus).Theparticlesizesfluctuatebecauseofstatisticalprocesses.Particleswitharadiusrr*growfurther.

bythepositionofthemaximumofthefreeenergyandgivenbyEquation(2).Particleswitharadiussmallerthanr*redissolve,whilstparticleswhichbyreasonofstatisticalfluctuationsexceedthissizearestableandcangrowfurther.

r*??à2g/Dgv

(2)

Withinthescopeofaquasi-equilibriumapproach(Arrhe-nius)therateofnucleationthatis,thenumberofnucleiwhich4334

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formperunittimeandvolumeisdescribedbyEquation(3),inwhichAisdeterminedbythefrequencyofthemolecular

J??Aexp(àDG*/kT)

(3)

processesandkandThavetheusualmeanings.TherateofnucleationisthusEquation(4),whereinaccordancewithEquation(5)thesupersaturationS??c(r)/c*iscoupledwiththeparticleradius;visthemolecularvolume,c(r)denotesthesolubilityofaparticlewithradiusr,andc*theequilibriumsolubility.

J??Aexp(à(16pg3v2)/(3k3T3[lnS]2))(4)kTln(S)??2sv/r

(5)

Inclassicalcolloidchemistryafurthermodelconceptisusedespeciallytoexplainmonodispersityincertainsystems(Figure5):[18]theconcentrationofadissolvedsubstancecontinuestorise,forexample,byreleaseinareactionuntilthecriticalnucleationconcentrationisreached.Atthispointashowerofnucleiareformedwhichbegintogrow.Inthiswaytheconcentrationfallsmomentarilybelowthecriticalthresh-oldsothatnonewnucleicanform.Thenucleialreadyformedgrowuntiltheconcentrationofthestill-dissolvedmaterialhasfallentotheequilibriumconcentration.

precipitationcritical nucleationconcentrationsoluteconcentrationsaturation limitt

Figure5.SchematicrepresentationoftheconcentrationrelationshipsincontrolledparticleformationaccordingtothemodelrepresentationofLaMer,ref.[18](forfullexplanationseetext).

2.2.MoreRecentKnowledge

Inthefollowing,newideasonparticleformationfromsupersaturatedsystemswillbepresentedinchronologicalorderfromthestageofthehomogeneousstartingstatetothatofthecolloidalparticle.Anassessmenthasbeenintentionallyomittedasthereisrapiddevelopmentinthisarea.Forreasonsofspacecompletecitationhastobeomitted;acomprehensivereviewofthistopicisinpreparation.2.2.1.Precursors

Afundamentalassumptionofclassicalnucleationtheoryisthatpriortoproductionofthesupersaturationthatinitiates

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BaldygaandBourne.[76]Finallyitisnotedthatinrecentyearscomputer-supportedsolutionstofluid-dynamicsproblemsaregainingincreasinglyinimportance.[77,86]

2.7.ExperimentalAspects

Tounderstandthestagesthroughwhichtheformationofcolloidalparticlespassitisnecessarytorecordexperimentallytheparticleformationprocessbytimeresolutionfromthetimepointwhensupersaturationisrealized.Thatthetime-resolveddescriptionofprocessesisinnowaytrivialisrecognizedfromthelimitednumberofpublicationsinthisarea.Inmostcasesadescriptionofthemechanismofparticleformationisstilllimitedtoaretrospectivederivationfromthestructureoftheparticleformed.Thattheamountofworkpublishedinthisproblemareaisquitelimitedisbecause,unlikecrystallization,particleformationinprecipitationreactionsoccursonaclearlyshortertimescale,downtotheregionofmilliseconds.

Wheretimeresolutionofthechosenmeasurementmethodissufficientthestopped-flowmethodhasprovedeffective:twoeductvolumesareledthroughamixingcellintoameasurementcell.Atapredeterminedtimepointtheflowisinterruptedandtheparticleformationprocessismeasuredbyasuitablemethod,forexamplelightscatteringorthescatteringofintenseX-rays.Ifthemeasurementmethodrequireslongermeasurementtimestheflowreactororprecipitationjetmaybeused.[31,87±89]Here,botheductflowsarepassedthroughamixingcellandthenthroughareactortubeor,afteradefineddistance,directedasafreebeamintoadefinedatmosphere.Assumingthatthesystemisquasista-tionary,thatis,thatoveranextendedperiodoftimetheprecipitatingsystemisinthesamestateatdefinitepositionsoftheprecipitationtube,ataknownflowratevinthetubethedetectionpointx(measuredfromthemixingcell)atwhich,forexample,spectroscopicordiffractivecharacterizationmethodsareattached,maybeconvertedintoreactiontimet??x/v.Alternativelythetubelengthmaybevariedandsamplesforoff-linemethods(especiallymicroscopicmeth-ods)obtained,forexample,byquenchingliquidsamplesatthebeamoutlet.[31]Itisassumedwhenusingthesesystemsthatthetimescaleofthereactionprocessislongerthanthetimetohomogeneousmixingoftheeductsinandafterthemixingcell;typicalmixingtimeslieintherangeofmilli-secondsforthestopped-flowtechniqueandintherangeofmicrosecondswithspecialflowcells.[88]Hithertotherehavebeenfewinvestigationsofprocessesduringmixing.Morerecentexperimentsshow,however,thatprecursorstructureswhichbreakdowntonanoparticlesarealreadyformingattheboundaryinterfaceofthetwoturbulentlymixingeductflows.[31]

Inthefollowinganumberofexperimentalmethodswillbedescribedbrieflywithwhichparticleformationprocessesmaybeinvestigatedonlengthscalesofnanometers.Staticanddynamiclightscatteringareestablishedtechniquesfortheinvestigationofcolloidalsystems.[90]Morerecentlycorrela-tionspectroscopicmethodssuchasfluorescencecorrelationspectroscopy(FCS)andRamancorrelationspectroscopy4340

J.RiegerandD.Horn

(RCS)havebeendevelopedforthecharacterizationofnanodispersedsystems.[91]Concentrateddispersionscanalsobecharacterizedreliablybyfiberopticquasielasticlightscattering(FOQELS).[92]Small-angleX-rayandneutronscattering(SAXSandSANS)arealsoclassicalmethodsforthecharacterizationofcolloidalsystems[93]andhavebeenusedinmanyinvestigationsforthedeterminationofprimaryparticlesizeandaggregatestructures.Investigationsonthefollowingsystemsarecitedasexamples:lysozymeclustersasprecursorforcriticalnuclei(SANS),[94]precursorformationinzeolitecrystallization(SAXS),[95]aggregationandcompactionintheformationofSiO2andTiO2particles(SAXS),[96]hydrolysisandcondensationofmetalalkoxides,[89]andparticleformationinquinacridoneandboehmiteprecipita-tion,[31]inthelattertwocasestheabove-describedprecip-itationtubetechniquewasused.

UV/Visspectroscopicmethodscanbeusedprincipallyforonlineanalysisofparticlesizedevelopmentbymeansofparticlesizedependencyofturbidityspectraand,wherepossibleabsorptionspectra(seeSection4.3).Aquantitativeevaluationismadedifficult,however,becausemodelconceptsrelatingtoparticlesizedistribution,particleshape,andrefractiveindexdifferencesmustbeavailable.

Inconclusionanumberoftechniquesforthetime-resolvedinvestigationofparticleformationprocessesarelistedaboutwhichonlyafewreportsarecurrentlyavailable.Theearlieststageofparticleformationinthehydrolysisofmetalalkoxideswasstudiedwiththelaser-inducedliquidbeamionization/desorption(LILBID)technique.[97]Anequallyexotictechni-queisX-raymicroscopywithwhichaqueoussystemscanbeinvestigatedundernormalpressurewithtimeresolutionintheminuterangeandaspaceresolutionofabout30nm.[83]Finallyaninterestinguseoftheanalyticalultracentrifuge[98]hasbeenreported:thismethodallowstheparticlesizedistributionofprecursorsinthecrystallizationoflysozymeandCdStobedetermined.[99]Forcompletenessthepulse-radiotechniqueismentionedwithwhichtheformationandgrowthofcolloidalmetalclustersinanaqueousenvironmentmaybestudiedwithatimeresolutioninthesubmillisecondrange.[100]

3.PreparativeMethodsfortheProductionofOrganicNanoparticles

Asdiscussedattheoutset,nanodispersedsystemscanbeobtainedintwoways(Figure2):1)bymechanicalmillingoftherawmaterialbywetordrymillingprocesses,or2)byprecipitationorcondensationoftheproductsoreductsdissolvedinsolventswithsubsequentseparationoftheunwantedsolvent.Inbothvariantsadditivessuchassurfac-tantsandpolymerstakeoverthefunctionofboundarylayeractive,colloidalstabilizersor–inthecondensationprocedureofmethod(2)–alsoformthenanoparticulatephaseitselfwhichcontainstheactivecompoundoreffectsubstance,boundbyadsorptionorabsorption.

Millingprocesses[101]areinprincipleunsuitablefortheproductionofnanodispersedsystemswithnarrowsizedis-tributionsincewithdecreasingparticlesizeitbecomes

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increasinglymoredifficulttousetheappliedmechanicalenergyintheformofshearingandcavitationforces[102]forparticlemillingwithoutsimultaneouslyinducingparticleagglomeration.[101,103]Moreover,theunavoidablemillingelementabrasionwhichcontaminatestheendproductandisdifficulttoseparate,especiallyinactivecompoundfor-mulations,isoftenundesirableornottolerated.[104]Theconsiderablepracticalsignificanceofmillingprocessesliesmainlyinthatinpigmentanddyestuffsformulationstheachievableparticlesizedistributionsinthelowermicrometerregion,inspecialcasesalsobelow,satisfytechnicalde-mands.[101,105]

Inspiteofthesedisadvantagesmillingprocessesdofindwidespreaduseintheformulationofpoorlysolubleactivecompounds[106±108]sincealternativetechnologieswhichcoulddelivernanoparticulateproductsareessentiallystillinthedevelopmentstage.Theseincludeinparticularthedescribedprecipitationprocessesfromhomogeneoussolutionwhichwithsuitableprocesscontrolnotonlyallowthepreparationofextremelyfineparticulatedispersions,butalsoallowacontinuousand,inrespectofprocessparameters,easilycontrollablemethodofproduction.Thesetechnicaladvan-tagesalsomakeprecipitationprocessesparticularlyattractivefromaneconomicviewpoint.

WiththetheoreticalprinciplesdiscussedinSection2inmindtheprecipitationprocessesareillustratedbelow.Variousmethodologicalvariantsfortheproductionofnanodispersedpolymerdispersionsbyphysicalcondensationprocesseswillalsobedescribedbriefly.[109±111]Figure10givesastructuredmethodicaloverviewofprecipitationandcondensationproc-essesfortheproductionoforganicnanoparticlesinaqueousmedia.Startingfromamolecularlydispersedsolutionofthe?activecompound∫threegroupsorprocesseswhichallowa

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restrictionofparticlegrowthtothenanometerregioncanbedifferentiated.

1.Withlipophilicsolvents(processesIandII)particledimensioningoccursthroughanemulsionstepasanintermediatestage.Theparticlesizedistributionofthisoilinwater(o/w)emulsionisadjustedmechanicallybyhomogenization.[101,103,106]Theconversionoftheemulsionintoananodispersionisthencarriedoutbyseparationofthesolventbyevaporationordiffusionprocedures.Thesizedistributionofthenanodispersionisdeterminedbythatoftheo/wemulsion,themeanparticlesizebytheconcentrationofthesubstrateintheemulsionphase.

2.Ifhydrophilic,fullywater-misciblesolventsareused(processesIVandV)particleformationoccursbyprecip-itation[103]eitheraccordingtotheprinciplesofnucleationandgrowthoutlinedinSection2or,atextremelyhighsupersaturation,byspinodalphaseseparation.Ineachcaseanagglomerationstepcanfollowtheseelementaryproc-esses(Section2.4.2).[112]

3.Withuseofamphiphilicsolventsorsolventmixtures(processIII)nanoparticleformationtakesplacethroughatransientemulsionphasewhichformsspontaneouslyandwhichthentransformsintoananodispersivestate.

Onlywithprocessgroups(2)and(3)issizedistributioncontrolledbytheleveloftheadjustablesupersaturationaswellasbysurfactantadditives,whichpossiblyintercedespecificallyintheelementarystepsofnucleation,growth,phasebreakdown,andagglomeration.Theindividualprocessvariantsdifferinrespectoftheadjustmentofthetemporalsupersaturationprofileaswellasinthechoiceandfunctionoftheadditivesintroduced.

InthestructuringindicatedinFigure10onlyinthelimitingcasesIandVdoesparticleformationtakesplacebypure

hydrophobic organic compound (HOC)lipophilic solventamphiphilic solventhydrophilic solventadditivesadditivespolymersadditivesadditivesHOC solutionHOC-polymersolutionHOC-polymersolution + waterHOC-polymersolutionHOC solutionwater + stabilizerwater +stabilizer +solventspontaneous emulsificationwater + stabilizermechanical homogenizationspontaneous particle formationwatero/w emulsionHOC-hydrosolHOC/pseudolatexHOC/polymer coprecipitateHOC/pseudolatexseparation of the solventHOC/polymer coprecipitateHOC/pseudolatexHOC precipitateHOC-hydrosolIIIIIIemulsification diffusion methodsIVVemulsification evaporation methodssolvent-displacement methodsFigure10.Precipitationandcondensationprocessesforthepreparationoforganicnanoparticles.MoredetailedexplanationsinFigures11±15andthetext.

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precipitationprocesses(inrespectofactivecompound).Incontrast,withvariantsII,III,andIVprocessesaredescribedwhichleadtoso-calledpseudolatexsystemsthroughphysicalcondensationofdissolvedmacromolecules,thatis,the?sup-port∫isprimarilyprecipitated.[111]Thephysico-chemicalelementaryprocessesofparticleformationandthebondingofdissolvedactivecompoundsontoorintotheparticulatephasearestillessentiallyunexplained.Thetermsnucleationfromahomogeneoussupersaturatedphaseandparticlegrowthnormallyusedinthedescriptionoftheelementalstepsinprecipitationreactionsarehereoflimitedsuitabilityindescribingthecomplexrelationshipsinparticleformation.ItshouldbenotedthatprocessesI±IIIcontainanemulsifi-cationsteppriortotheactualparticleformation.

ProcessvariantsI±Vareillustratedbelowwithselectedexamples.Inallprocessespolymers,surfactants,andsurface-activeprotectivecolloidsplayasignificantroleinparticleformation,evenifinquitedifferentfunctions.Detailedknowledgeofmolecular-specificpossibilitiesforinfluencingparticleformation,especiallyforthetargetedadjustmentofparticlesizeandparticlemorphology,isonlyavailableinindividualcases.

3.1.NanoparticleFormationbytheEmulsification±EvaporationProcessfromLipophilicSolution

3.1.1.HydrosolsofActiveCompounds(ProcessI)

Processesofthistypebelongtotheclassicalmethodsforthepreparationofwaterdispersiblenanoparticulatehydro-solsofwater-insolubleactivecompounds.Theyweredevel-opedespeciallyfortheformulationofcarotenoids.[113±115]Preparationofthenanoparticlesiscarriedoutbydissolvingtheactivecompoundtogetherwithanemulsifier,forexample,ascorbylpalmitate,inasuitablesolvent,forexample,chloro-formormethylenechloride,thenemulsifyingthissolutionwithanaqueoussolutionofaprotectivecolloid,forexamplegelatin,andremovingthesolventbydistillation(Figure11).Theactualprecipitation/crystallizationtakesplaceintheemulsiondropletduringdistillationwhenthesolubilitylimitiscrossed.Thesizeoftheactivecompoundparticleisthusproscribedbytheconcentrationoftheactivecompoundsolutionandthesizeoftheemulsiondroplet.Theparticlesizedistributioncanbeadjustedwithinwidelimitsbythedropletsizedistributionoftheo/wemulsionthroughthechoiceofhomogenizer(colloidmill,highpressurehomogenizer,ultra-sounddisperser).[101,103,106]Asolidnanoparticlewhichisprotectedagainstagglomerationbythesuitablechoiceoftheprotectivecolloidisobtainedfromeachemulsiondropletofawell-stabilizedemulsionuponremovalofthesolvent.[116]Theparticlemorphologyisusuallypolycrystallineinthethermodynamicallystablecrystalstructuresincethesolidformationtakesplacebyevaporationcrystallizationatlowsupersaturation.[117]Numerousrecipevariantscanberealizedbytheadditionoffurtherlipophilicadditivestotheemulsi-ficationformulation.Afundamentaldifficultyinthisprocessliesintheremovalofasmuchsolventaspossiblefromthefinalproduct.4342

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organic compoundwaterlipophilic solvent+

stabilizer

+emulsificationseparation of solventhydrosol of organic compound (I)

Figure11.Principleofthepreparationofactive-compoundhydrosolnanoparticlesbytheemulsification±evaporationprocess.

3.1.2.PseudolatexSystems(ProcessII)

Iflipophilicpolymers,suchasbiodegradablepolylactides(PLA),poly-b-hydroxybutyrates(PHB),polylactide-co-gly-colides(PLGA),polycaprolactones(PCL),orpolyalkylcyano-acrylates,areusedwiththeactivecompound[110,118±122]nano-particulatepolymerdispersionsareobtainedwhichcontainthelipophilicactivecompoundeitheradsorbedorem-beddedasmoleculardispersionsormicrocrystals(Fig-ure12).[110,119,123]Theformulationsthuspreparedareofincreasinginterestasparenteraldosageforms(seeSec-tion4.2).Aspecialversionofthismethodforthepreparationofprotein-loadedpolylactidenanoparticleswasrecentlypublished.[120]Inadoubleemulsionprocessanaqueoussolutionoftheactiveprotein(proteinCplasmainhibitor)wasemulsifiedinamethylenechloride/acetonepolylactidesolutionandthiso/wemulsionwasthenemulsifiedinanaqueoussolutionofpolyvinylalcohol(PVA)asprotectivecolloid.AfterremovalofthesolventPLAnanoparticles(200±250nm)wereobtained.Theproteinactivityinthenanodispersedformulationcouldbecontrolledbytheprep-arationconditions.

Onaccountoftheexcellentsolubilizingpropertiesofchlorohydrocarbonsforlipophilicactivecompoundsandgalenicallyinterestingadditives.[101,106,124]Theprocedurede-scribedisinprinciplewidelyapplicabletotheformulationoflipophilicactivecompounds.However,thereisoneintrinsicdisadvantageinthatthesetoxicologicallydubioussolvents

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organic compoundpolymer+waterlipophilic solvent+

stabilizer

+emulsificationseparation of solventHOC-loaded pseudolatex (II)

Figure12.Principleofthepreparationofactive-compoundpseudolatexnanoparticlesbytheemulsification±evaporationprocess.HOC??hydro-phobicorganiccompound.

mustbefullyremovedduringtheworkupoftheformula-tion.[106]Therefore,morerecentlyattemptsarebeingmadetochangetheemulsification±evaporationprocesstotheuseofmoreacceptablesolventssuchascyclohexane.WiththeexampleofcholesterylacetateasamodelcompoundSj?s-tr?mandBergenstahlwereabletoshowthatwithanoptimizedemulsificationsystemitwaspossibletoobtainstablenanodispersedformulationswithparticlesizesofabout25nm.[117]Whereitispossibleinindividualcasestofindanacceptablesolventtheemulsification±evaporationprocedurehastheadvantageinthatitshouldbepossibletoselectsuitableprotectivecolloidsandemulsifiersonthebasisofsemi-empiricalconceptsandthusallowtargetedprocessoptimization.[101,125]

3.2.NanoparticlesbytheEmulsification±DiffusionProcedurefromAmphiphilicSolution(ProcessIII)

Thedifferentprocessvariantsareallbasedupontheuseofsolventswhichareoflimitedwatermiscibilityandcapableofspontaneousemulsionformation(e.g.propylenecarbonate,benzylalcohol,ethylacetate).Thismethodthusofferstheadvantageoftheuseofpharmaceuticallyacceptablesolventsanddoesnotrequiretheuseofhigh-pressurehomogenizersfortheformationoftheo/wemulsionasthepreliminarystageofnanoparticleformation.[110,126,127]Thetrickofthisprocess,thephysico-chemicalprinciplesofwhicharestillnotfully

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REVIEW

clarified,isthatthewater-saturatedsolventphase(??polymerandactivecompound)andthesolvent-saturatedaqueousphase(??protectivecolloid),thatis,inthermodynamicequilibrium,arefirstemulsifiedbyintensivestirring.Withthesubsequentadditionofwatertothemerelymicrodis-persedo/wemulsionthediffusionequilibriumisdisturbed.Thisinducessolventdiffusionintothehomogeneousaqueousphaseatwhichpointthesolubilitylimitsofpolymerandactivecompoundarecrossedandparticleformationcom-mences(Figure13).MechanisticinvestigationswithvariationinpreparationconditionsandtheuseofPLAaspolymerandpolyvinylalcohol(PVAL)asprotectivecolloidhaveshownthateachemulsiondropletgivesamultiplicityofnano-particles.[127]

Thisremarkablefindingwasexplainedbythedevelopmentofconcentrationfluctuationsintheboundarylayerregioncausedbysolventdiffusion,whenthesolubilitylimitisnarrowlyexceededlocallyandtheprecipitationofpolymerandactivematerialisinduced.Theinteractionofthe

organic compound

polymer+wateramphiphilic+

stabilizer

+solvent(amphiphilic+

(water)

+solvent)

spontaneous emulsificationH20LM+ waterLMformation of proto-nano-particlesLMLMHOC-loaded pseudolatex by solvent diffusion (III)

Figure13.Principleofthepreparationofactive-compoundpseudolatexnanoparticlesbytheemulsification±diffusionprocess.

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protectivecolloidmoleculespresentintheaqueousphasewiththese?proto-nanoparticles∫suppressestheirfurtheragglomerationandthusdeterminestheachievableparticlesizedistributionofthenanodispersion,whichisobtainedafterremovalofthesolventbydistillation.AccordingtothemechanismdescribedinSection2.6itwouldalsobeconceiv-ablethatwhenthesolubilitylimitiscrossedduringinter-diffusionofthetwophases,activecompoundparticleswhichareinhibitedingrowthbyadsorbingprotectivecolloids,andthuscolloidallystabilized,formcontinuously.Aprocessvariantwhichallowsafurthersimplificationoftheprocedurewasreportedrecently.[128]Herethedilutionstepwithwaterandtheseparationofthesolventiscombinedbysteamdistillation.Theprocesswastestedwithaseriesofbiode-gradableandnon-degradablepolymers.

Afurthervarianthasbeenusedwhichalsomanageswithoutahomogenizationstep,sinceemulsionformationagainoccursspontaneouslyasapreliminarystageinnano-particleformation.However,thisso-calledSESD(sponta-neousemulsificationsolventdiffusion)process[122,129]suffersfromthedisadvantagethatamphiphilicsolventmixtureswithmethylenechlorideareusedasthehydrophobiccomponent.Technicallyapolymer/active-compoundsolution,forexamplePLGA,inacetone/methylenechlorideisaddedwithstirringtoanaqueousprotective-colloidsolution(polyvinylalcohol).Acoarse-particleo/wemulsionformsspontaneouslytheparticlesizeofwhichisrapidlyreducedbydiffusivelossoftheacetoneinthedispersedphase.AfterevaporationofthesolventPVAL-stabilizedpolymerparticleswithincorporatedactivecompoundareformedinthenanometerrange.Thesolventmixtureandthepolymer-protectivecolloidcombina-tionaresoadjustedthatclearaffinitydifferencesbetweenpolymerandprotectivecolloidforthesolventcomponentsguaranteephaseseparationandallowcolloidalstabiliza-tion.[129]Itisclearthatinviewofthecomplexityoftheindividualphysico-chemicalprocessesinvolvedineachsys-temadetailedoptimizationisrequiredtoregulatethedesiredparticlesizedistribution.InthiscontextamodifiedSESDprocesshasbeendescribedbyMurakamietal.thatusessolventmixtureswithouttheundesiredchlorohydrocar-bons.[129]However,sincewater-misciblesolventmixturesareusedtheparticleformationmechanismmustbedifferentiatedfromthatoftheSESDprocess(Section3.3.1).

Afurthervariantfortheformationofnanodispersedpseudolatexdispersions,possiblyloadedwithactivecom-pound,throughanemulsionphaseasintermediatestageistheso-calledsalting-outprocess.[110,130]Theprocessisbasedupontheabilityofelectrolytes(forexampleNaCl,MgCl2,CaCl2[110,131])orsaccharose[132]tosalt-outacetonefromanaqueoussolution.Theactivecompound/polymersolutioninacetoneisinitiallyemulsifiedintheaqueouselectrolyteorsugarsolutioninthepresenceofaprotectivecolloidandthendiffusionofacetoneintotheaqueousphase–withsimulta-neousformationofnanoparticles–isinducedbytheadditionofwater.Heretooboundarysurfaceturbulence(initiatedbyacetonediffusion)ormechanismsasdescribedinSection2.6(precipitationduringtheinterdiffusionofthephases)canexplainthenanoparticleformation.However,therehavebeennodetailedinvestigationsonthemechanismofparticle4344

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formationtodate.Theserelationshipshaveprovedtobeextremelycomplex.Thus,incorporationoftheactivecom-poundintothenanoparticleisinfluencedconsiderablybythesalting-outcomponents.[110,133,134]HithertoonlytheprotectivecolloidsPVAL,polyvinylpyrrolidone(PVP),andhydroxy-ethylcellulosehavedemonstratedanadequateeffective-ness.[110]Thesolventandthesalting-outcomponentsareseparatedbydistillationorcross-currentfiltration

3.3.NanoparticleFormationbySolvent-DisplacementProcessesfromHydrophilicSolution

Theindustrialadvantagesoftheprocessesdiscussedatthispointrestupontheuseofwatermiscible,toxicologicallyacceptablesolvents,(e.g.acetone,short-chainalcohols).Themethodsweredescribedbothforthepreparationofnano-dispersedpseudolatextransportformsoflipophilicactivecompoundsandforthepreparationofpurenanohydrosolsofactivecompoundsandeffectmaterialswhicharepoorlysolubleorinsolubleinwater.

3.3.1.PseudolatexSystems(ProcessIV)

ThemodifiedSESDprocess(describedinSection3.2)representsaspecialcaseofpseudolatexformationbywayofatransientemulsionstage.[129]Thesolventmixtureconsistsoftwowater-misciblesolvents(acetone/ethanol)withdifferentaffinitiesforthepolymerandtheprotectivecolloid.IntheexampleusedthepolymerPLGAhasahigheraffinityforacetone,whilsttheprotectivecolloidPVALismoresolubleinethanolormethanol.Afive-stagemodelwassuggestedforthemechanismofparticleformation.[129]AftermixingthePLGAsolutionwiththeaqueousPVALsolutionrapiddiffusionofthealcoholcomponentfirstleadstoparticlesizereductionofthetransientemulsionintermediatewhichisformedaccord-ingtotheMarangoniEffect.[135]ThepreferreddiffusionofthealcoholcomponentisexplainedbytheloweraffinityofthealcoholforPLGAinthedispersedphase.ThelikewiseoccurringacetonediffusionleadstoacollapseofthePVALprotectivecolloidintheboundarylayer,accompaniedbyaPLGAcondensationintheincreasinglyacetone-depleteddispersedphase.Evenundermildstirringconditionsthisspontaneousparticleformationprocessalsoleadstonano-particulatepseudolatexdispersions(Figure14).Towhatextentthisnovelprocesscanbeextendedtootherpolymer/protective-colloidcombinationscannotyetbeassessed.Clearly,experiencewithpolymerincorporationoflipophilicactivecompoundsiscurrentlynotavailable.

Fessietal.showedforthefirsttimethatinsteadofasolventmixturetheuseofonlyonesolventwithunlimitedwatermiscibilitycanalsoleadtothespontaneousformationofnanoparticulatepseudolatexdispersions.[136]Normallyace-tone,ethanol,[110,136±138]orTHF[121]areused.InanalogytothemodifiedSESDprocessthepolymersolution,whichpossiblyalsocontainsactivecompoundandotheradditives,ismixedwiththeaqueousprotectivecolloid(PVAL)solution.Thespontaneousparticleformationisagainexplainedbyboun-Angew.Chem.Int.Ed.2001,40,4330±4361

REVIEW

administration.[207]Furthermore,thereareexperimentalin-dicationsthatnanoparticlescanbeabsorbedthroughtheGALTsystem(gut-associatedlymphoidtissue.)[209]Afurtheradvantageofnanodispersedactivecompoundformulations(<200nm)isthatsterilefiltrationtechniquescanbeusedwithoutthepreparationsbeingsubjectedtothermalstress.[130]Referenceismadeheretotheextensiveliteratureregardingfurtherdetailsonthestatusofdevelopmentandthebio-pharmaceuticalpropertiesofnanodispersedformulationsbaseduponmillingandhomogenizationprocedures.[106]

Incontrast,therehavetodatebeenonlyisolatedreportsonthepreparationandpropertiesofnanodispersedformulationsofpoorlysolubleactivecompoundsbyprecipitationreactions.Thus,afterextensiveinvestigationsSuckerandco-work-ers[154,175]wereabletoprepareX-rayamorphousnanodis-persedhydrosolswithpoorlysolubleactivematerials(S<10mgmLà1inwater)bycontrolledprecipitationfromalcoholsolutionwithwaterinastaticmixer(seeSection3.2).Tostabilizethenanoparticlesinsitutheprecipitationreactionwascarriedoutinaqueoussolutionsofdifferentgelatintypes(electrostericstabilization)orpoloxamers(stericstabiliza-tion).Thehydrosolsobtainedweretransformedbyspraydryingintostorable,redispersibledrypowderstopreventparticlegrowth(Ostwaldripening).Forthispurposelactoseormannitolwereaddedassprayingaids.Withtheactivecompoundsisrapidin,beclomethasonedipropionate,andcyclosporin,nanodispersedformulationswithparticlesizesintherangeof200nmwereobtained.Indetailedinvestiga-tionswithredispersedcyclosporinhydrosolsinanimals,concentrationsoftheactivecompoundsimilartothosefoundaftertheinjectionofmicellarsolutionsweredetectedindifferenttissuetypesafterintravenousinjection.[154,175]

WiththeactivesteroidbudesonideMatijevic

?andRuchrecentlydrewattentiontotherarelyexploredindustrialpotentialofthepreparationofmicro-tonanodispersedactive-compoundhydrosolsbyaprecipitationreactionfromhydrophilicsolution.[157]

4.1.2.WaterInsolubleActiveMaterials

Themostextensiveexperienceontheimprovementofperoralbioavailabilityofinsolubleactivecompoundsbyhydrosolformationcurrentlylieswithsyntheticcarotenoids.Thecarotenoidsconstituteaclassofcolorpigmentswide-spreadinnaturewithyellowtoredcolortonenuan-ces.[113,210,211]Allthesecompoundsarecharacterizedbyastructuralelementconsistingofapolyenechainbuiltupfromfourisoprenylunitsthedifferentsubstitutionpatternsofwhichatthechainterminiallowanextraordinarilylargebreadthofvariationwithinthecompoundclass.Todayabout600carotenoidsareknown.[212]Thebestknownrepresenta-tive,b-carotene,wasfirstisolatedfromcarrotsin1831andhasbeenproducedindustriallysince1954.[213]Sincethensyntheticroutesforawholeseriesofcarotenoidshavebeendevelopedandconvertedintoindustrialproductionprocesses;[214]aselectionisillustratedinFigure18.4350

J.RiegerandD.Horn

β -carotene

C40H56lycopene

C40H56OHOHHOHOlutein

C40H56O2zeaxanthin

C40H56O2OOOHHOOOcanthaxanthin

C40H52O2astaxanthin

C40H52O4Figure18.Selectionofcarotenoidsofphysiologicalimportanceandofinterestasfoodstuffcoloringsandforanimalnutrition.Allthecompoundsareinsolubleinwater.Aqueousapplicationsthereforerequirenano-dispersedformulationsforoptimizationofthebioavailabilityandthecoloristicproperties.

Inadditiontothedescribedcolorthephysiologicalfunctionofcarotenoidsisofconsiderableinterest.Bestknownistheactionofb-caroteneasprovitaminA.[211]Thepracticaluseofnature-identicalsynthesisproductsasactivecompoundsinpharmaceuticalsandcosmeticsorascoloringagentsinthefoodstuffsandanimalfeedsiscomplicated,however,bytheinsolubilityinwaterandpoorsolubilityinfatsandoilstypicalofthecompoundclassasawhole.[114,115,210,211]Thereforetheconversionofthecrystallinesynthesisproductintonano-dispersedformulationsisanimportantrequirement,espe-ciallyfortheplethoraofformsofadministrationofthecarotenoidsinaqueousmedia.CurrentlydifferentrecipevariantsofthemixedchamberprocessdescribedinSec-tion3.3.2areusedwidelyfortheproductionofdiversecarotenoidpreparationswithhighcolorintensityandhighbioavailability.[152,153]Figure19givesanoverview.

Figure19.Preparativevariantsofthemixing-chamberprocessforthepreparationofnanodispersed,water-compatiblecarotenoidformulations.

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Dependingonthecompositionoftheactive-compoundsolutionproducedbytemperatureshock,nanodispersedhydrosols,emulsions,andmicellarsolutionsareproducedfordifferentuses.[152]Theeffectofparticlesizeandtheequallytechnicallycontrollablesupramolecularstructureofthedis-persedphaseonthecolorarediscussedindetailinSec-tion4.3.Moreover,thereareincreasingindicationsthatbiologicalabsorptionisinfluencedequallyextensivelybythesupramolecularstructureoftheparticles.Infeedingexperimentswithcalvesandratsitwaspossibletodemon-stratebymeasurementoftheblood-levelvaluesorthevitaminAliver-storagevaluesthatthebiologicalabsorptionofb-carotenehydrosolsincreasessignificantlywithdecreasingparticlesize(seeFigure20).[153,215]Undercomparableexper-

Figure20.Influenceofparticlesizeonthebioavailabilityofb-carotenehydrosolduringoraluptakebycalveswithminimalb-carotenestatus.Atthecross-overpointthefeedingtothetwocalfcohortswascontinuedafterexchangeofformulationsIandII.

imentalconditionsmilledcrystallineproductsinthemicro-meterparticlesizerangewerepracticallynotabsorbed(Figure21).Inotherareasofanimalnutrition,carotenoidsinnanodispersedformulationsshowanincreasedbioavail-ability.Thusnanoparticulatehydrosolsfromcanthaxanthinarewidelyusedinpoultrybreedingforanaturalegg-yolkpigmentation.[115]Analogouslyastaxanthin,thecoloringagentwhichgivesfishandseafoodareddishcolor,isusedasa

Figure21.Timecourseoftheb-carotenebloodlevelaftersingleoraladministrationofb-carotenetocalveswithminimalb-carotenestatus.Comparisonofnanodispersedformulationswithamicrodispersedcrystal-lizate.

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REVIEW

nanodispersed,amorphoushydrosolformulationintroutandsalmonfarming.[115]

Inalltheapplicationsdescribedthenanoprecipitatesareusedasgranulateswhichareobtainedbyspraydryingprocedures.Intheseformulationstheactivecompoundispresentinconcentrationsof5±10%.Theremainingconstit-uentsarecomposedofprotectivecolloids(e.g.gelatin,polysaccharides)andotheradditives(emulsifiers,antioxi-dants)typicalforprecipitationprocessesaswellasfurtheradditives(sugar,starch)whichenablespraydryingandwhichalsocontributetostoragestabilityandredispersibilityintheadministrationmedium.[152,153]Themodeofactionoftheproteinsandpolysaccharidesusedasprotectivecolloidsaredescribedindetailinref.[178].DetailsontheirfunctioninnanodispersedcarotenoidformulationsarediscussedbyHornetal.[152,153,216]

Inadditiontophysiologicalpropertiesofdifferentcarote-noidhydrosolsdescribedalready–thecoloringaspects,ofinteresttothefoodstuffsindustry,arediscussedmorecloselyinSection4.3–thepointersforfurther,health-promotingactionsofcarotenoidsarecurrentlyincreasing.Inanalogytotheprotectiveactionintheplantkingdom,whereinthephotosynthesissystemcarotenoidsprotectchlorophyllfromattackbyoxygenradicalsandsingletoxygen,[217]similareffectsarealsoproposedinotherorganismsincludinghumans.Thusinpatientswithlowb-carotenoidstatustherearesignsthattherisktohealthfromprostatecancercanbereducedbyb-carotenesupplementation.[218]Theprotectiveactionofb-caroteneagainstUV-inducedskinchangeshasbeenestablishedbyBiesalskietal.[219]Othercarotenoidswhicharecurrentlythesubjectofintenseinvestigationfortheirhealth-promotingeffectsarelycopene,[220]theredcolorantintomatoes,andluteinandzeaxanthin.[221]Recentresultssuggestthatahighlycopeneintakebytheconsumptionoftomatoesalsoreducestheriskofprostatecancer.[222]Luteinandzeaxanthinaretheonlycarotenoidstobeconcentratedintheyellowspotoftheretinaandinotheroculartissuesandcouldbeinvolvedinthepreventionofmaculadegenerationassociatedwithage[223]andareductionintheriskofcataractformation.[224]

Thesearejustafewexamplesofthefunctionofcarotenoidsintheareaofhealthandnutritionwhichinthefuturecouldextendconsiderablytheuseofthenature-identicalactivecompounds.Nanodispersedformulatedcarotenoidscouldthusrepresentimportantexamplesforeffect-optimizednutraceuticals,thatis,nature-identicalactivematerialswhichmanifesttheirhealth-promoting,prophylacticactionintheboundarybetweennutrientsandpharmaceuticals.

Inviewofthisexperiencewithcompoundsofthecarote-noidclassitmayalsobepossibletoprecipitatepoorlysolublehydrophobicactivecompoundsasnanodispersionsfromhydrophilicsolutionwithappropriatetemperaturecontrol,contrarytotheoccasionallyexpressedopinion.[104]However,ithastobepresumedthatforanunderstandingofparticleformationinthesesystemstheclassicalconceptsonnuclea-tionandgrowthastheyhavebeenusedsuccessfullyfortheunderstandingandprocessdevelopmentofnormalcrystal-lizationandprecipitationreactions[163±166,168,225]playarathersubordinateroleinviewoftheextremelyhighsupersatura-4351

REVIEW

tionswhichareachievable,andspinodalseparationphenom-enapossiblydominateintheparticleformationprocess.

Nanoprecipitationprocessesinassociationwiththeexplo-rationofsuitablenaturalandsyntheticadditiveswithhighactive-compoundaffinityofferattractiveprospectsforthedevelopmentofnanodispersedpharmaceuticalformula-tionsoflipophilicactivecompoundswhichcouldprovideindustrial-pharmaceuticalresearchanddevelopmentwithnewstimuli.

4.2.NanodispersedSystemsfortheTargetedandControlledReleaseofPharmaceuticallyActiveCompounds

Ineverydayindustrial-pharmaceuticalparlancethetermnanoparticlehascometobeusedtomeannanodispersedpolymericsupportsystemsforactivecom-pounds.[107,109,151,226±228]Theydifferfundamentallyfromthenanosuspensions[106]andhydrosols[154,175]discussedinSec-tion3inwhichthepolymersassumethefunctionofsurface-activeprotectivecolloidsforthecolloidalandpossiblyalsothechemicalstabilizationofthenanodispersedactive-com-poundphase.

Nanoparticlesthusrepresentanalternativetootheractive-compoundtransportformssuchasmicroemulsions,[229]nio-somes,[230]liposomes,[227,231]orotherso-calledSLNs(solidlipidnanoparticles).[208,232,233]Oneofthereasonsforthesearchforalternative,polymer-basedtransportsystemsisthatanincreasedstabilityofthenanodispersedformulationoftheactivematerialduringstorageandadministrationappearsdesirable.[151,234]Moreover,thetargetedfunctionalizationofthepolymermatrixorthesolid-particlesurfaceappearsmorereadilyrealizableforthecontrolledandtargetedreleaseoftheactivecompound.

SincethepioneeringworkofBirrenbachandSpeiser[235]andCouvreurandco-workers[236]thesearchforbiocompatiblepolymermaterialsandmethodsfortheirnanodispersedformulationhasleadtoafloodofpublications(seeSection3).Inparticular,thebiopharmaceutical,physiological,andtherapeuticpropertiesofsuchsystemshavebeeninvestigatedinextensivestudies,includingclinicaltrials.Criticalevalua-tionoftheresultshavebeenpresentedrecentlyinre-views[208,228,234]andmonographs,[106,151]andalreadybelongtopharmaceutical-technologytextbookknowledge.[107]Conse-quently,adetaileddiscussionissuperfluousatthispointespeciallysincenanodimensioningonlyindirectlyaffectsthepropertiesoftheactivecompound.Insummaryitmustbepointedout,however,thatinspiteoftheconsiderableresearcheffortwhichhasbeenexpendedandtheencouragingresultsincertainareas,nanoparticlesassupportsystemsforactivepharmaceuticalcompoundshaveasyetfoundnouseinmedicalpractice.[208,228]Thereasonsgivenforthisincludeunansweredquestionsonlong-termstabilityandonthecytotoxicityofpolymersandtheirdegradationproducts–andthusobstaclestomedicalregistration/legislation–aswellasindustrialproductionaspectssuchasthetransferofthelaboratorymethodsdescribedtocompetitiveproductionprocesses.[151,208]4352

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4.3.OpticalandElectroopticalPropertiesofNanodispersedOrganicPigmentsandDyestuffs

Crystalsoforganicmoleculesarefindingnumeroustech-nicalapplicationsaschromophoricdispersioncolloids.[105]Amongstthemostimportantapplicationsarethecoloringoflacquers,printinginks,colortoners,andalargenumberofindustrialplastics.[101]Themostimportantclassofpigmentsincludethephthalocyanins(blue),perylenes(red),andazodyes(yellow).Specialdemandsareplacedonpigmentswhichareusedinthecoloringofdrinksandfoodstuffs;thecarotenoidsasnature-identicalpigmentsarealsomentionedinthiscontext.Thebroadfieldofrelevanttechnicalaspectsofrespectivepigmentformulationscannotbeundertakenhere.[101,105]Boththecoloringproperties(colorstrength,colorshade,transparency)andtheflowpropertiesofpigmentedsystemsimportantforprocessingareinfluencedquitesignifi-cantlybytheparticlesizeandparticleshapeatagivenpigmentconcentration.[237]

Accordingtothecurrentstateoftheartthetechnicaldemandsplacedonthefinenessofthepigmentanddisper-sion-dyestuffformulationsusedinpracticearefulfilledbydryandwetmillingprocesses.[101]Usuallydispersioncolloidswithameanparticlesizerangeofaround1mmareobtained.Workwiththeaimofafurtherreductioninparticlesizedowntothenanometerregioniscurrentlynotofimmediateinterestfromthepracticalviewpoint.Usuallythemilledproductsobtainedaresubsequentlysubjectedtoaso-calledformulationorfinishingprocesstoobtainaproductasuniformaspossibleinparticlesizeandshapebycontrolledOstwaldripeningormodificationtransformationinorganicsolvents.Morere-centlyspecificadditiveswhichbyreasonoftheirmolecularstructurepreferentiallyoccupyindividualgrowthfacesandthusenableaspecificcontrolofgrowthkineticsandthusparticlehabithavebeenusedtocontrolthisrecrystallizationprocess.[238]Empiricalmethodsofscreeningarebeingre-placedtoanincreasingextentbystrategicmethodsofcrystalengineeringwiththeuseofmorepowerfulcomputers.Thuswithorganicmoleculesitisinsomecasesalreadypossibletopredicttheirprobablepackingdensityinthesolid,theirorientationinthecrystallattice,andtheirinteractionenergies.[239,240]However,itistodatestillnotpossibletopredictquantitativelytheparticlesizedependencyoftheabsorptionbandstructure.Theamountofpertinentexper-imentalworkisequallysparse,however.Thushypsochromicshiftsoftheabsorptionbandshavebeenobservedwithultrathinepitacticlayers(1±100nmlayerthickness)ofperylenes,phthalocyanines,andotherpigments,andinter-pretedasquantumsizeeffects.[202]

Otherwiseonlyisolatedreportsoninvestigationsintoparticlesizedependenciesofopticalconstantsofmolecularcrystalshaveappeared.[241±244]IntheworkofNakanishiandco-workersthechoiceofsystemsinvestigatedwasaimedprimarilyatthepotentialfieldsofapplicationsoforganicnanocrystalsintheareaofmicroelectronicsandphotonicsasalternativestoinorganicsystems.[245,246]Thus,onthebasisofthemethodsdescribedinSection3.3.2dispersedsystemsintheparticlesizerangedownto20nmwereobtainedbynanoprecipitationinwaterofdifferentclassesofcompounds

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OrganicNanoparticles

(pseudoisocyanines,merocyanines,perylenes,polydiacety-lenes)fromhydrophilicsolution(ethanol,acetone,THF).[174,245]Intheinvestigationsontheperylenesystemashiftintheabsorptionbandmaximumofabout30nmwasfoundonareductioninparticlesizefrom200nmto50nm.[242]Recentlybandshiftsofupto30nmhavealsobeenobservedwithdiphenyl-naphthyl-pyrazoline(DPNP)nanocrystals(400nmto20nm).[244]Inexplainingthesefindingsaconfine-menteffectwasexcluded,however,becauseoftheobservedparticlesizerange,andareductioninintermolecularinter-actionwithdecreasingparticlesizethroughcrystal-latticedeformationwasdiscussedasthecause.Ininvestigationsonthepolydiacetylenesystemablueshiftoftheabsorptionmaximumofabout15nmwasalsofoundwithdecreasingparticlesize(150nmto70nm)andsimilarlyinterpret-ed.[243,247]Foracompleteexplanationoftheexperimentalresults,possiblyalsowithinthecontextofconfinementeffects,anextensionoftheinvestigationstoparticlesizesto10nmandbelowappearsnecessary.Apreparativeaccesstothisparticlesizerangewasrecentlydemonstratedbytheprecip-itationoforganicmolecularcrystalsininorganicsol±geltemplates.[248]Thesameappliesinthisrespecttoapossibleincreaseinthethirdordernon-linearopticalsusceptibil-ity,c(3),byconfinementeffectsinnanodispersedparticu-latesystems.[246,247,249]Inorganicsystemstheexpectationsreferinparticulartothepolydiacetyleneclassofcom-pounds.[243,247]However,anexperimentalconfirmationofcorrespondingtheoreticalpredictionsiscurrentlyunavail-able.[246,247,249]

Particlesizeeffectsarealsoreceivingincreasingatten-tion[250]inconnectionwiththewidespreaduseoforganicphotoconductorsforxerographiccopyingprocesses.[201]Intheopticalproductionofagraphicelectrostatic-chargepatternonareversiblychargeablecarrier,materialsarerequiredwhichathighquantumyieldpossesshighphotoconductivityinthevisiblespectralregion–andmorerecentlyforuseindiodelaserprintersinthenearinfraredregiontoo.Forthispurposeorganicsemiconductorpigmentssuchasphthalocyanines,perylenes,andazopigmentsareconvertedintonanodispersedsystems(20±500nm)bycomplexmillingprocessesandincorporatedintopolymericsupportmaterialsinhighcon-centrations.[199,201,250]However,heretootherelationshipsbetweentheparticlesizedistributionandthesupramolecularparticlestructureandthetechnicallyrelevantphysicalpropertiessuchasphotoconductivityandchargetransporthavestillbeenlittleresearched–inspiteofthehightechnicalstateofdevelopment–sincethepreviouslydescribedmor-phologicalparametersarepoorlycontrollablebymillingprocesses.[199,200,201,250,251]

Inthiscontextparticlesizeeffectsaresignificantfortworeasons:ontheonehandtheproductionofthechargecarriersattheboundarysurfaceoftheopticallyactivepigmentparticlestakesplacebytheinteractionwiththepolymermatrix,withadditivechargecarriers,oralsowithadsorbedoxygenorwater.[200,201,250]Atagivenpigmentvolumeconcentrationthesizeofthephotoopticallyactivesurfaceincreaseswithdecreasingparticlesize,whichisimportantespeciallyinfieldstrengthsof<10Vmmà1.[250]Ontheotherhandthephotoconductivityofthematerial,especiallywith

Angew.Chem.Int.Ed.2001,40,4330±4361

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inertpolymersupportsisdeterminedtoaconsiderableextentbythepercolationstructure,thatis,themeanparticledistanceandtheconnectivityoftheparticleconfiguration.[200,250]

Whereastheimportanttechnologicaladvanceshavehith-ertobeenbaseduponempiricaldevelopmentwork,[201]withtheexampleoftitanylphthalocyanin(TiOPc),anespeciallyefficientrepresentativeoforganicphotoconductors,afirststeptowardsasystematictreatmentofthisproblemhasbeenmademorerecently.[199]ThesuccessfulpreparationofaTiOPc/BBL(BBL??poly(benzobisimidazobenzophenan-throne))nanocompositewithspherical(d??100nm),X-rayamorphousTiOPcparticleswasachievedbyinsituhydrolysisofamolecularlydispersed,solidsolutionofaLewisacidcomplexofTiOPcinapolymermatrix(BBL).BysubsequentshapingofthefilmsinCHCl3,vapor-phasetransformationintothecrystallineb-modificationwithrod-shapedpigmentcrystalstookplace.

Inadditiontothecriticalinfluenceofcrystalstructureontheabsorptionprocessandthusonthefrequencydependencyofthephotoconductivitywithorganicmolecularcrystals,[201]inthecaseofTiOPcaprogressiveshort-waveshiftofthecharacteristicQbands(700±900nm)oftheb-modificationwasobservedwithcontrolledreductioninparticlesize.[199,252]Thephotoconductivityfoundinthiswavelengthregionwithnanocompositescontainingtheequallyphotoconducting(450±650nm)BBLsupportmaterialwithcrystallineTiOPclaymorethantwoordersofmagnitudeabovethatofcomparablematerialswithamorphousparticles.[199,251]

Theprocessdescribedforthepreparationofnanodispersedcompositesoforganicphotoconductorswithbroadspectralsensitivityappearsnotonlyinterestingfromthepointofviewofbasicresearchonxerography,butitcouldalsoopenupnewindustrialmethodsforthepreparationofphotoactivemateri-alswithpossibleusesinthestorageofsolarenergyonthebasisofphotovoltaicsandphotoconductivity.[199,253]

TheclassicalparticlesizeeffectswhicharequantitativelydescribablebytheMietheoryinthecaseofisometricparticlesmustbedifferentiatedinprinciplefromthepreviouslydescribedopticalparticlesizeeffects,whicharebaseduponachangeintheopticalconstantsinnanodimensionalcrystalswithhighintermolecularinteraction.[254]Accordingly,atagivenchemicalandcrystallographicstructureandhencefixedopticalconstantstheparticlesizedeterminestoalargeextenttheopticalpurityandtransparencyofapigmentedsystemthroughthesize-dependentbalanceofabsorptionandscat-tering.AspecialcaseariseswithsystemswithX-rayamorphousparticlesrecentlypreparedbythedescribedprecipitationreactions.Asisshownbelowwiththeexampleofthecarotenoids,differentsupramolecularshort-rangeorderstructurescanbeobtainedinatargetedmannerbyvariationintheprecipitationconditions,whichatstrongintermoleculardipoleinteractionleadstomarkedchangesintheabsorptionspectrumandthusthecoloristicproperties.Figure22sum-marizestheeffectsofthedifferentmolecular,supramolecular,andsolid-statestructureonthecoloristicpropertiesofananodispersedsystem.Inthecaseofb-carotenethechangeinparticlesizeandthesupramolecularstructure,particularlythetransitionfromthecrystallinetotheamorphousstate,hasadramaticeffectonthestructureoftheabsorptionspectrum.

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molecular/supra-molecular structure

molecular structuresupramolecularsolubilizationaggregationabsorption/scatteringcoloristic propertiescrystal structureconfinement effects ?particle size/shapesolid-state structure

Figure22.Controlofthecoloristicpropertiesofnanodispersedcarotenoidhydrosolsbymolecular,supramolecular,andsolid-statephysicalfactors.

Figure23showsacomparisonofabsorptionspectraofdifferentb-caroteneformulationswhichallowcolor-tonenuancingfromyellow,throughorange,tored(seealsoFigure24).[152]Incomparisontotheabsorptionspectrumof

1.2molecular solutionin n-hexane

1.0hydrosol, experimentalcrystallizate, calculated0.8(H-aggregate)(J-aggregate)

40 nm50 nm0.6200 nmA157 nm0.4exp. 1 μm0.20200250300350400450500550600650700λ/ nmFigure23.UV/Visabsorptionspectraof5ppmb-carotene.Influenceofaggregatestructureandparticlesizecomparedwiththemolecularsolutioninn-hexane.

hydrosolhydrosolhydrosol

suspensioncrystalH-aggregatesH/J-aggregatesJ-aggregates

J-aggregates

oiloiloiloiloiloiloil100 nm180 nm250 nm300 nmFigure24.Influenceofparticlesizeandaggregationstructureonthecolortonenuanceofnanodispersedb-carotenehydrosols.

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amolecularsolutioninn-hexanethespectraoftheamor-phousnanoprecipitatesshowanincreasingblueshiftwithdecreasingparticlesize,whereasaredshiftwithconcomitantchangeinbandstructurecharacterizesthespectraofthecrystallinedispersioncolloids.[152]Accordingtothemostrecentexperimentalandtheoreticalinvestigationssupramo-lecularstructureandparticlesizeeffectsparticipateequallyinthedevelopmentofthecomplicatedbandstructure.[255]

Inaccordwiththecrystalstructureofb-carotene[256]theredshiftisexplainedbyaso-calledJ-aggregateinteraction(head-to-tail)ofthetwob-carotenemoleculesperunitcell.[152,257,258]Asyettherehasbeennoexactassignmentofthesuper-imposedvibrationalcouplingstructure.Thespectrumofamilledcrystallineproduct(meanparticlesizeca.1mm)andthespectraofnanodispersedsystemscalculatedbytheMietheoryonthebasisofsingle-crystalspectra[259]shownosimilaritywiththespectraoftheproductsofthesamesizeclassobtainedbynanoprecipitation.AsshowninFigure25,theseproductsarecomposedofparticleswithacore±shellstructurewhichisvisualizedelectronmicroscopicallybytheuseofspecificstainingtechniques.[152,153]Detailsofthefunctionofthegelatinusedasprotectivecolloid(typeB100,adsorbatefilmthicknessca.40nm)andotherprotectivecolloidshavebeendiscussedthoroughlyelsewhere.[152,216]

250 nm250 nmFigure25.Electronmicroscopicrepresentationofthecore-shellstructureofnanodispersedb-carotenehydrosols.Thespecificstainingoftheactive-compoundcore(left)wascarriedoutwithOsO4,thegelatinshell(right)withuranylacetate.

MorerecentX-rayandtheoreticalinvestigationsarenowprovidingthefirstcluesonthesupramolecularstructureoftheactivecompoundcoreandonthesignificanceofthecharacteristicblueshiftoftheabsorptionbandsofb-carotenenanoprecipitates.[255]Fromasimpleexcitonmodel[257,258,260]andquantum-mechanicalmolecular-modelingcalculations[261]itwaspossibletoshow[255]thatintermolecularinteractionswithinthecontextofanH-aggregatestructure(?card-stackstructure∫)[152,257,258,262]areconsistentwiththespectroscopicfindings.Hypsochromicbandshiftsof44nmand40nmwerecalculatedforanidealH-aggregateandatetrameric(2_1_1)aggregate,respectively,whichcorrespondstoasupercellformedfromtwoH-aggregates.Incontrast,abandsplitting(excitonsplitting)isexpectedforaJ-aggregate:inadditionto

Angew.Chem.Int.Ed.2001,40,4330±4361

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