pH-Responsive Mercaptoundecanoic Acid Functionalized Gold Nanoparticles and Applications in

12 Pages·2017·1.39 MB·English

nanomaterials
Article
pH-Responsive Mercaptoundecanoic Acid
Functionalized Gold Nanoparticles and
Applications in Catalysis
SiyamM.Ansar1,SaptarshiChakraborty1andChristopherL.Kitchens1,2,* ID
1 DepartmentofChemicalandBiomolecularEngineering,ClemsonUniversity,Clemson,SC29634,USA;
[email protected](S.M.A.);[email protected](S.C.)
2 InstituteofEnvironmentalToxicology(CU-ENTOX),ClemsonUniversity,509WestinghouseRoad,
Pendleton,SC29670,USA
* Correspondence:[email protected];Tel.:[phone]
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:18April2018;Accepted:15May2018;Published:17May2018
Abstract: Mercaptoundecanoicacid(MUA)functionalizedgoldnanoparticles(AuNP-MUA)were
synthesizedanddemonstratedtopossesspH-triggeredaggregationandre-dispersion,aswellas
thecapabilityofphasetransferbetweenaqueousandorganicphasesinresponsetochangesinpH.
ThepHofaggregationforAuNP-MUAisconsistentwiththepK ofMUA(pH~4)insolution,while
a
AuNP-MUAphasetransitionbetweenaqueousandorganicphasesoccursatpH~9. Theionpair
formationbetweentheaminegroupinoctadecylamine(ODA),thecarboxylategroupinMUA,andthe
hydrophobicalkylchainofODAfacilitatesthephasetransferofAuNP-MUAintoanorganicmedium.
TheAuNP-MUAwereinvestigatedasareusablecatalystinthecatalyticreductionof4-nitrophenol
byborohydride—amodelreactionforAuNPs. Itwasdeterminedthat100%MUAsurfacecoverage
completelyinhibitsthecatalyticactivityofAuNPs. Decreasingthesurfacecoveragewasshownto
increasecatalyticactivity,butthisdecreasealsoleadstodecreasedcolloidalstability,recoverability,
and reusability in subsequent reactions. At 60% MUA surface coverage, colloidal stability and
catalytic activity were achieved, but the surface coverage was insufficient to enable redispersion
following pH-induced recovery. A balance between AuNP colloidal stability, recoverability, and
catalyticactivitywithreusabilitywasachievedat 90%MUAsurfacecoverage. TheAuNP-MUA
catalystcanalsoberecoveredatdifferentpHrangesdependingontherecoverymethodemployed.
AtpH~4,protonationoftheMUAresultsinreducedsurfacechargeandaggregation. AtpH~9,
ODAwillformanion-pairwiththeMUAandinducephasetransferintoanimmiscibleorganicphase.
BoththepH-triggeredaggregation/re-dispersionandaqueous/organicphasetransfermethodswere
employedforcatalystrecoveryandreuseinsubsequentreactions. Theabilitytorecoverandreuse
theAuNP-MUAcatalystbytwodifferentmethodsanddifferentpHregimesissignificant,basedon
thefactthatnanoparticle-catalyzedreactionsmayoccurunderdifferentpHconditions.
Keywords: goldnanoparticles;MUA;aggregationandredispersion;phasetransfer;reusablecatalyst
1. Introduction
Goldnanoparticles(AuNPs)haveattractedsignificantinterestduetotheiroptical,electronic,and
chemicalproperties,whichhavedemonstratedpotentialapplicabilityinavarietyoffields,including
chemicalcatalysis[1–3]. WhileAuNPspossessinherentproperties,surfacefunctionalizationwitha
varietyofligandsaffordstheenhancementofexistingpropertiesortheintroductionofadditional
capabilitiesthatmakethefunctionalizedAuNPssuitablefornovelapplications. Forexample,AuNPs
functionalizedwithexternalstimuli-responsivemoleculespossesschemicalorphysicalpropertiesthat
Nanomaterials2018,8,339;doi:10.3390/nano8050339 www.mdpi.com/journal/nanomaterials
Nanomaterials2018,8,339 2of12
aretriggeredbylight,pH,temperature,ions,orotherstimuli,whichhaveasignificantpotentialfor
applicationsinreusablecatalysis,sensorydevises,biomedicalapplications,etc.[4–8].
AuNP functionalized with pH-responsive groups, such as carboxylic acids, sulfonates, and
amines,havebeensynthesizedandpossesspH-responsivebehaviorsinsolution[4,9–11]. Forexample,
11-mercaptoundecanoicacid(MUA)isapH-responsiveligandthatbindsstronglytoAuNPsthrough
the thiol group and effectively disperses nanoparticles in water at neutral and basic pH levels.
MUA-stabilized AuNPs (AuNP-MUA) have been synthesized, and their colloidal behavior has
been studied as a function of pH, ionic strength, and amine-induced AuNP-MUA aggregation in
water [12–14]. Su et al. synthesized the MUA-functionalized 13 nm AuNPs via ligand exchange
betweencitrateandMUA,andstudiedthecolloidalstabilityandphasebehavior[15]. Theybelieved
thattheaggregationofAuNP-MUAatpH3isgovernedbyhydrogen-bondingforcesbetweenthe
surface adsorbed MUA molecules. At pH 11, the AuNP-MUA are colloidally stable in solution
but form three-dimensional close-packed aggregates on TEM grids, due to decreased electrostatic
repulsioninteractionsbetweendeprotonatedMUAandcounter-ions(Na+)duringthesampledrying
process. Recently,Pillaietal. studiedthenanoparticlesizeeffectontheprecipitationpH(pHprec)for
AuNPsfunctionalizedwithamixedmonolayerofMUAandN,N,N-trimethyl(11-mercaptoundecyl)
ammoniumion[12]. TheyfoundthatthepHprecincreasedfrompH=5.3topH=7.3whenincreasing
thenanoparticlesizefrom4.2to11.5nm. Laaksonenetal. studiedthestabilityof2.3nmAuNP-MUA
at a set pH, using the hydroxide as base and varying the size of counter-ions, and showed that
AuNP-MUAaggregationoccurredat70–90mMforNa+,andatgreaterthan1Mforthequaternary
ammoniumcation[13]. Thesterichindrancecausedbythequaternaryammoniumadsorbedinthe
SternlayerstabilizedtheAuNP-MUAagainstaggregation. Recently,Wangetal. studiedthestability
of4–6nmAuNP-MUAtodifferentmonovalentcationsthathavedifferentpropensitiesforbridging
interactions,aswellasforconcomitantAuNP-MUAaggregation[14]. Theauthorsshowedthatthe
orderofsaltconcentrationsneededforAuNP-MUAaggregationisCsCl>>KCl>LiCl>NaCl>RbCl,
whichdoesnotcorrelatewiththesizeofthehydratedcations.
Though AuNP-MUA aggregation and redispersion in water has been explored before,
thepH-triggeredAuNP-MUAphasetransferbetweenthewaterandorganicphases(withoutaggregation)
andreuseincatalysishasbeennotreported.Whileunderstoodphenomenologically,thereisafundamental
tradeoffbetweencolloidalstabilityandcatalyticactivity,whichisdrivenbynanoparticleligandsurface
passivation.Thisunderstandingisintegraltothedesignofcolloidalcatalystswithsufficientactivityand
theabilitytoberecoveredandreusedinsubsequentreactions.Ourapproachistousestimuli-responsive
surfacefunctionalgroupsforthecatalystrecoveryandreuse; however,thechallengeistobalancethe
degree of surface coverage where higher passivation promotes colloidal stability and preservation of
thenanoparticlecatalyst,butalsoinhibitsactivity.Recently,westudiedthecatalyticactivityofthiolated
polyethylene glycol (PEG) ligands with varying chain lengths and surface coverage for the catalytic
4-nitrophenolreductionreaction[16].Ourresultsdemonstratedaninversecorrelationbetweencatalytic
activityandPEGsurfacecoverageontheAuNPs.
In this work, we perform an in-depth study of pH-triggered AuNP-MUA aggregation and
redispersion,aswellasAuNPphasetransferbetweenwaterandorganicphases. Thisphasebehavior
isthencoupledwithapplicationasarecoverableandreusablecolloidalcatalyst. Ourresultsshowthat
MUAprovidespH-responsivedispersibilityandphasetransferabilitybetweenaqueousandorganic
media,withtheadditionofapH-responsivephasetransferfacilitator. TheactivityofAuNP-MUA
inthecatalyzedreductionof4-nitrophenol(4-NP)to4-aminophenol(4-AP)bysodiumborohydride
(NaBH )wasexplored. AuNP-MUAarecatalyticallyactivetowardsthereductionof4-NPto4-APat
4
lowerMUAsurfacecoverage;however,lowsurfacecoveragealsoresultsindecreasedrecoveryand
reusability. WehaveexploredthistradeoffforAuNP-MUAanddemonstratedtheabilitytoachieve
pH-triggeredAuNP-MUAphasetransferbetweenthewaterandorganicphase(withoutaggregation)
andreuse,withoutlossincatalyticactivity.
Nanomaterials2018,8,339 3of12
2. MaterialsandMethods
2.1. ChemicalsandEquipment
Toluene was purchased from Alfa Aesar (Tewksbury, MA, USA). All other chemicals were
acquiredfromSigmaAldrich(St. Louis,MO,USA).Nofurtherpurificationwasconductedonthe
chemicals. Ultra-pureMilli-Qwater(resistivity18.2MΩ.cm)wasusedforallsynthesisandreactions.
ApHmeter(sympHonySB90M5,VWRInternational,Radnor,PA,USA)wasusedtomeasurepH.
UV-VIS spectra were acquired on a UV-VIS Spectrometer (Varian Cary 50, Agilent Technologies,
SantaClara,CA,USA).
2.2. MercaptoundecanoicAcid(MUA)FunctionalizedAuNPSynthesis
Borohydride reduction was employed to synthesize citrate-stabilized 5 nm diameter AuNPs.
Thecitratereductionmethodwasusedforsynthesizing13and45nmdiameterAuNPs[17–19].Amixture
of0.5mMHAuCl (50mL)and0.5mMtrisodiumcitratewasmadeinaconicalflask.Asolutionof0.1M
4
sodiumborohydride(1.5mLofice-cold,freshlyprepared)wassubsequentlyaddeddropwiseunder
constantstirring. Stirringwascontinuedforanadditionalhour. For13-nmAuNPs,150mLof1mM
HAuCl aqueoussolutionwasheatedwhilegentlystirring. Whenthesolutionbeginstoboil,5.0mL
4
of120-mMcitrateinH Owasadded,andtheresultingsolutionwasstirredat400rpmfor15minas
2
thecolorofthesolutionchangedfromcolorlesstored. Two-stepnanoparticleseededgrowthmethod
wasusedtosynthesizethecitrate-capped45-nmAuNPs[19].Inbrief,10mLofas-synthesized13AuNP
wasaddedto150mLofboilingsolutioncontaining0.6mMHAuCl followedbyadditionof1%w/w
4,
aqueoustrisodiumcitrate(21.7µmol,1.3mL).Themixturewasheatedfor30minundervigorousstirring.
Citrate-stabilizedAuNPswereligandexchangedwiththiolatedMUAtogenerateAuNP-MUA.
Atotal of 1.3 mM MUA (30 mL, dilute NaOH) and 10 mL of as-synthesized citrate-AuNP were
incubatedfor24h. AuNP-MUAwaswashedbyrepeatedcentrifugalprecipitationandre-dispersion
threetimeswithH O,toremoveexcessMUA.
2
2.3. Thermogravimetric(TGA)Analysis
ThequantityofMUAgraftedtoAuNPwasmeasuredthroughTGA(SDTQ600,TAInstruments,
NewCastle,DE,USA).OnaTGApan(alumina),50mLpurifiedAuNP-MUAwasreduceddownto
60µLbyrepeatedcentrifugation(14,500rpm,1h)anddeposited.Waterwasremovedinitiallybyholding
theTGAtemperatureat100◦Cfor15min. Atemperaturerampof10◦C/minwasappliedtillafinal
temperatureof600◦Cwasachievedandthetemperaturewasheldfor15min(N purge,20mL/min).
2
2.4. 4-NitrophenolReductionCatalysis
Time-resolvedUV-VISspectrawasacquiredina4mLquartzcell(VarianCary50spectrophotometer).
AuNP-MUA(1mL),H O(0.9mL),and0.2mM4-NP(1mL)weremixedinthequartzcell. Changein
2
intensityof4-NPpeakat400nmasfunctionoftime(individualspectrawereacquiredevery0.2min)was
usedtotrackreactionprogress.
2.5. TransmissionElectronMicroscopy(TEM)Analysis
Hitachi9500(300kV,Hitachi,Schaumburg,IL,USA)wasusedtoacquirehigh-resolutionTEM
imagesofAuNPs,andImageJsizeanalysiswasconductedontheimages. Then10µLofAuNPwas
dropcastona300meshCugrids(Formvarcoated)andallowedtodry. TEMgridsweresubsequently
storedinadesiccatorforcompleteremovalofsolvent.
2.6. DynamicLightScattering(DLS)Measurements
DLS measurements were made on five-times-diluted as-prepared AuNPs at 25 ◦C (Malvern
instrumentZetasizerNanoseries,Westborough,MA,USA).Thesolutionswereadjustedtothedesired
Nanomaterials2018,8,339 4of12
pH with either 0.1 M HCl or 0.1 M NaOH solutions, and their hydrodynamic diameters (number
averaged)andζpotentialsweremeasured.
Nanomaterials 2018, 8, x FOR PEER REVIEW 4 of 12
3. Redsueslitrseda npdHD wisitchu sesitihoenr 0.1 M HCl or 0.1 M NaOH solutions, and their hydrodynamic diameters
(number averaged) and ζ potentials were measured.
Citrate-capped AuNPs with three different sizes (5, 13, and 45 nm) were first synthesized by
the borohydride and citrate reduction methods [17–19]. Transmission electron microscopy (TEM)
3. Results and Discussion
showsthattheaveragesizesofas-synthesizedAuNPsare4.6±1.9,13.4±1.1,and45.9±5.9nmin
Citrate-capped AuNPs with three different sizes (5, 13, and 45 nm) were first synthesized by the
diameter(SupplementaryMaterials,FigureS1). MUA-stabilizedAuNPswerepreparedbyaligand
borohydride and citrate reduction methods [17–19]. Transmission electron microscopy (TEM) shows
exchange reaction between citrate-stabilized AuNPs and the MUA in dilute KOH. Dynamic light
that the average sizes of as-synthesized AuNPs are 4.6 ± 1.9, 13.4 ± 1.1, and 45.9 ± 5.9 nm in diameter
scattering(DLS)dataforAuNPsbeforeandafterMUAfunctionalizationdemonstratethecolloidal
(Supplementary Materials, Figure S1). MUA-stabilized AuNPs were prepared by a ligand exchange
stabilityofnanoparticlesinadiluteKOHsolution(SeeSupplementaryMaterials). TheUV-VISspectra
reaction between citrate-stabilized AuNPs and the MUA in dilute KOH. Dynamic light scattering
of Au(DNLPSs)- dMatUa Afore AxhuiNbiPts abecfhoarer aacntde raifstteirc MloUcaAl ifzuendctsiounraflaizceatipolna sdmemoonnrsetrsaotne athnec eco(lLloSidPaRl )staabbsiloitryp otifo n at
510–5n6a0nnopma,rtcioclnefsi rimn ian gditlhuetes tKabOiHlit ysoolfuttihoen b(aSseiec mSuepdpiluemme(nStuarpyp lMemateenritaalsry). MThaet eUriVal-sV,IFSi gsupreectSra2 )o.fM UA
stronAgluyNbPisn-MdsUAw ietxhhiAbiut Na cPhsartahcrteoruisgtihc lcoocavlaizleend tsubrofancde ipnlgasmofonth reestohniaonlcteo (LtShPeRg) aobldsorsputriofanc aet, 5y1i0e–lding
a pH5-6r0e snpmo,n csoivnefir–mCinOgO thHe sgtarobuilipty aotf tthhee bdaissicta ml eednidum. (FSiugpuprleem1eAntasrhyo wMsatetrhiealsp, HFi-gruersep So2n)s. iMveUnAe ss of
strongly binds with AuNPs through covalent bonding of the thiol to the gold surface, yielding a pH-
AuNP-MUA, undergoing reversible aggregation/precipitation and re-dispersion at an acidic and
responsive –COOH group at the distal end. Figure 1A shows the pH-responsiveness of AuNP-MUA,
basicpH,respectively. Wine-redcolorsolution(leftvial)indicateswell-dispersednanoparticlesinbasic
undergoing reversible aggregation/precipitation and re-dispersion at an acidic and basic pH,
medium. In acidic medium (dilute HCl is used to adjust pH with mild stirring), the nanoparticles
respectively. Wine-red color solution (left vial) indicates well-dispersed nanoparticles in basic
aggregateimmediatelyandprecipitateoveranhourofincubation,leadingtothecompletesettlingof
medium. In acidic medium (dilute HCl is used to adjust pH with mild stirring), the nanoparticles
nanoparticles(rightvial). ThestrongLSPRpeakisusedtomonitortheaggregationandredispersionof
aggregate immediately and precipitate over an hour of incubation, leading to the complete settling
AuNoPfs bnaynUopVa-rVtiIcSless p(ercigtrhot scvoiapl)y. (TShuep pstlreomnge nLtaSrPyR Mpaeatekr iiasl su,sFeidg utroe mS3o)n.iTtohre tchoem apglgerteegdatisioanp paenadr ance
oftherepdeisapkearstioann oafc AiduiNcpPsH byin UdVic-aVtIeSs stpheacttraogscgorpegy a(Steudppnlaenmoepnatarrtyic Mleastceorimalsp, lFeitgeulyres Se3t)t.l eTdheo cuotm. Cploemte plete
re-disdpisearpsipoenaroafncthe eofp trheec pipeiatka taetd anA aucNidPics p(Hdi ilnudteicKatOesH thiast uagsgedregfoartepdH naandojpuasrttmicleens tcwomitphlemteillyd ssettitrlerdin g)is
evideonutt.b Cyotmhpelceotem rep-dleitseperrescioonv eorfy thoef ptrheeciLpSitPatRedp AeaukNaPts 5(d2i6luntem KaOnHd ias bussoedrb faonr cpeHo afdajupsptmroexnimt waittehl y1.1,
mild stirring) is evident by the complete recovery of the LSPR peak at 526 nm and absorbance of
accounting for dilution. Figure 1B shows the reversibility of the AuNP-MUA (13 nm particles)
approximately 1.1, accounting for dilution. Figure 1B shows the reversibility of the AuNP-MUA (13
aggregation/re-dispersionprocessforseveralcycles.OthersizesofAuNP-MUA(5and45nmparticles)
nm particles) aggregation/re-dispersion process for several cycles. Other sizes of AuNP-MUA (5 and
exhibitthesamereversibleaggregationandre-dispersion(datanotshown).
45 nm particles) exhibit the same reversible aggregation and re-dispersion (data not shown).

Figure 1. (A) Photographs showing the reversibility of 13-nm mercaptoundecanoic acid
Figure1.(A)Photographsshowingthereversibilityof13-nmmercaptoundecanoicacidfunctionalized
functionalized gold nanoparticles (AuNP-MUA) clustering/re-dispersion by changing the pH of the
goldnanoparticles(AuNP-MUA)clustering/re-dispersionbychangingthepHofthemedium.Theleft
medium. The left vial (a) contains well-dispersed AuNP-MUA at a basic pH, and the right vial (b)
vial(a)containswell-dispersedAuNP-MUAatabasicpH,andtherightvial(b)containsaggregatedand
contains aggregated and settled AuNP-MUA at an acidic pH; (B) Plot showing the pH-triggered
settlerdevAeursNibPil-iMty UofA agagtraengaatcioidn iacnpdH re;-(dBi)spPelorstiosnh omwoinnigtotrhede pbyH t-hteri lgogcearliezdedr esvuerfrascibe ipliltaysmofoang rgerseognaatnicoen and
re-dis(pLeSrPsRio) npemako ninitteonrseidtyb ayt 5th25e nlomc afloirz e1d3-nsumr fAauceNpPl-aMsmUAon; arneds o(Cn)a nnocrem(LalSizPeRd) UpVea-VkIiSn atebnsosritbyanacte5 p25eankm for
13-nmraAtiou NoPf -aMggUrAeg;aatnedd (aCn)dn uonrmagaglrizeegdateUdV -AVuINSPab-MsoUrbAa nacse ap efaukncrtaiotino ooff aagqgureeoguast edphaansde upnHa.g gTrheeg ated
AuNPab-MsoUrbAancaes faorf uunnc-atigognreogfataeqdu 5e,o 1u3s, apnhda s4e5 pnHm .dTiahmeeatbers oArubNanPcse wfoerreu mne-aagsugrreedg aatte wda5v,e1le3n,gatnhsd o4f5 nm
diame5t2e2r, A52u5N, aPnsdw 5e5r1e nmme,a sruesrpeedctaitvwelya,v aenledn gththe saobfso5r2b2a,n5c2e5 ,foarn dag5g5r1egnamte,dr e5s, p1e3c, tiavnedly 4,5a nAdutNhPesa bwseorreb ance
measured at wavelengths of 562, 595, and 725 nm, respectively.
foraggregated5,13,and45AuNPsweremeasuredatwavelengthsof562,595,and725nm,respectively.
Nanomaterials2018,8,339 5of12
ThepHofAuNP-MUAaggregationwasdeterminedfromaUV-VISabsorbancetitrationcurve
(Figure1C)obtainedbymonitoringthepeakmaximumabsorbanceforaggregatedandun-aggregated
Nanomaterials 2018, 8, x FOR PEER REVIEW 5 of 12
peaksatdifferentpHvalues(SupplementaryMaterials,FigureS4). TheLSPRpeakat510–560nmfor
well-dispeTrhsee pdHA oufN APusNpPe-MakUsAh iafgtsgrteogaatihoing hwears wdeatveremleinnegdth f,roamnd a dUeVc-rVeaISs easbsionrbinantecne stiittryataisonth ceurAveu NPs
aggre(gFaigtuerae n1dCs) eottblteai[n2e0d,2 1b]y. mThoeniptoHrinogf tAheu NpPea-kM UmAaxiamgugmre gaabtsioornba(npcHe for) aisggdreetgeartmedi naendd fruonm- the
agg
inflecatgiognrepgaotiendt poefaakss iagtm doififderaelnfit tpoHf tvhaeluaebs s(oSrubpapnlecme,eynitealrdy iMngatperHialvsa, lFuigeusroef S44.)3. ,T4h.5e ,LaSnPdR 4p.9eafko rat5 nm,
510–560 nm for well-dispersed AuNPs peak shifts to a higher wavelength, and decreases in intensity
13 nm, and 45 nm particles, respectively. Pillai et al. also observed similar trend for a MUA and
N,N,Nas- ttrhiem AeuthNyPls( 1a1g-gmreegractaep atnodu snedttelec y[2l)0,a2m1].m Tohne ipuHm oifo AnumNiPx-eMdUmAo angoglraeygeartiofunn (pctHioagng)a ilsiz deedte4rm.2–in1e1d.5 nm
from the inflection point of a sigmoidal fit of the absorbance, yielding pH values of 4.3, 4.5, and 4.9
AuNPs[12]. Also,Wangetal. reportedthatthepK valueofMUAboundtoAuNPsincreaseswith
for 5 nm, 13 nm, and 45 nm particles, respectively. Paillai et al. also observed similar trend for a MUA
increasing nanoparticle size from 4.1 to 7.2 nm [22]. Therefore, it is clear that as the particle size
and N,N,N-trimethyl (11-mercaptoundecyl) ammonium ion mixed monolayer functionalized 4.2–
increa1s1e.5s n(nma nAoupNaPrtsi c[1le2]c. uArlvsoa,t Wuraenrge detu acle. sre),ptohretedde tphraot ttohne aptKioa vnaolufet hoef M–CUOAO bHougnrdo tuop AounNtPhse inncarneoaspeasr ticle
surfacweitihs iinnchriebaistiendg ,ndaunoeptaorttihclee sstirzoe nfrgomele 4c.1tr toos 7ta.2t incmre [p22u]l.s Tiohnersebfoertew, iet eisn ctlheaer ctharabt oasx ythlea tpeairotinclse. sIinzeo ther
wordisn,caretaasegs iv(neannpopHarvtiaclleu ec,utrhveatfurraec trieodnucoefs)–, CtOheO −de(pcroomtonpaatrieodn toof –tCheO O–CHO)OoHn tghreouApu NonP tshuer face
increansaensoapsatrhtieclne asnuorfpacaer tiisc lienhsiibzeitedde,c dreuaes teos ,thweh sitcrhoncgo rerleecstproonstdatsict oretphuelpsiHons beintwcreeeans ethwe ictahrbnoaxnyolaptaer ticle
agg
size. Fiounrst.h Ienr omthoerre ,witorsdhso, ualtd a bgeivneno tpeHd vthaalutet,h tehew faravcetlieonn gotfh –CanOdOi−n (tceonmspitayreodf ttoh e–CLOSPORHp) oenak thoef ApulaNsmP onic
nanopsuarrftaicclee sinicsrevaesreys saesn tshieti vneantooptahretidcliee lseizcter idcepcrreoapseesr,t iweshoicfht hcoerlroecspalonendvs itroo nthme epnHtaoggf itnhcerenaasne owpiathrt icles
nanoparticle size. Furthermore, it should be noted that the wavelength and intensity of the LSPR peak
andtheinterparticleinteraction(particlespacing)ofnanoparticles[23,24]. Thus,todelineatethese
of plasmonic nanoparticles is very sensitive to the dielectric properties of the local environment of
effects,measurementoftheapparentdiffusioncoefficientandhydrodynamicdiameterbyDLScan
the nanoparticles and the interparticle interaction (particle spacing) of nanoparticles [23,24]. Thus, to
complementtheUV-VISasaninsitumeasurementofnanoparticleaggregationasafunctionofpH.
delineate these effects, measurement of the apparent diffusion coefficient and hydrodynamic
Tdhiaemoentesre tbpy HDLoSf cAanu NcoPm-MpleUmAenatg tghree UgaVt-iVonISd aes taenrm ini nsietud mbyeatshuereDmLeSntt iotrf antaionnopcaurrtivcele, aagsgerveigdaetinocne dby
increaassi na gfuhnycdtiroond oyfn paHm. icdiameter(Figure2A),commencesatapHofabout4.1forallthethreesizesof
AuNPs-MTUhAe o.nOsuetr pDHL oSf rAesuuNltPs-MfoUrApH agagtreognasteiotno fdeAteurNmPin-eMd UbyA thage gDrLeSg atittiroantioanre cucorvnes, iasste envtid(wenictehdin the
samebpyH incurneaistsin)gw hiytdhrothdeyndaamtaic odbiatmaienteerd (Ffrigoumre t2hAe),U cVom-VmISentcietsr aatti oa npHm oeft haboodut( F4i.1g uforre a1llC th).e Athsrecea n be
seensinizeFsi gofu AreuN2BP,s-aMtUhAig. hOeurrp DHLS( >re5s)u,ltths efoζr -ppHot eatn otinasleti sofh AiguhNlyP-nMeUgaAt iavgegrdeugeatitoon tahree cdoenpsrisotteonnt ated
(within the same pH units) with the data obtained from the UV-VIS titration method (Figure 1C). As
carboxylategroupofMUA,whichprovideselectrostaticrepulsionbetweenAuNP-MUAandthus
can be seen in Figure 2B, at higher pH (>5), the ζ-potential is highly negative due to the deprotonated
colloidalstability. Themagnitudeoftheζ-potentialiscommonlyusedasthemeasureofcolloidal
carboxylate group of MUA, which provides electrostatic repulsion between AuNP-MUA and thus
stability, neglecting steric contributions [25–27]. Once the pH decreases below 5, the ζ-potential
colloidal stability. The magnitude of the ζ-potential is commonly used as the measure of colloidal
dramatically decreases, due to the protonation of the carboxylate groups over the pH range from
stability, neglecting steric contributions [25–27]. Once the pH decreases below 5, the ζ-potential
5to3d.rAamsaatriceaslulyl td,ethcreeaesleesc,t drouset atot itchree ppruotlosinoantiobne towf teheen canrabnooxyplaarteti gclreosupdse ocrveear stehse, peHve rnantugae lflryomle a5d toin gto
nanop3.a Artsic ale raesguglrt,e gthaet ioelnec.trTohsteatdice crreepauslisniognm baetgwneietund neanoofptahreticAleus NdPec-rMeaUseAs, ζe-vpeonttuenaltliya lleaasdaingfu tnoc tion
of pHnainnodpiacratticelse tahgagtretghaetioonn. sTehteo dfecargegasriengga mtioagnn,iwtuidteh oaf nthein AcrueNaPse-MdUhAy dζr-poodtyenntaiaml aics ad fiuamncetitoenr oaft pH
4.1, opcHcu irnsdaictataesζ t-hpaot tethnet ioanlsoetf o~f− a2g0grmegVatifoonr, waliltht haen sinizceresaosefdp hayrdtircoldesy.naTmhicu sd,iatmheeteζr- paot tpeHn ti4a.1l, data
occurs at a ζ-potential of ~ −20 mV for all the sizes of particles. Thus, the ζ-potential data indicate that
indicate that the AuNP-MUA aggregate with an approximately 50% reduction of surface charge.
the AuNP-MUA aggregate with an approximately 50% reduction of surface charge. Indeed, the ζ-
Indeed,theζ-potentialisnotquiteequivalenttothesurfacechargeonAuNPs;also,theζ-potential
potential is not quite equivalent to the surface charge on AuNPs; also, the ζ-potential of AuNP-MUA
ofAuNP-MUAisdependentnotonlyonthepK ofsurface-adsorbedMUA,butalsoontheMUA
is dependent not only on the pKa of surface-adsoarbed MUA, but also on the MUA packing density
packingdensityandthesurroundingenvironment.
and the surrounding environment.

Figure 2. (A) Hydrodynamic diameter of the AuNP-MUA as a function of pH; and (B) ζ-potential of
Figure2.(A)HydrodynamicdiameteroftheAuNP-MUAasafunctionofpH;and(B)ζ-potentialof
the AuNP-MUA as a function of pH. The red line indicates the onset of AuNP-MUA aggregation
theAuNP-MUAasafunctionofpH.TheredlineindicatestheonsetofAuNP-MUAaggregationbased
based on the hydrodynamic diameter data from figure A.
onthehydrodynamicdiameterdatafromfigureA.
Nanomaterials2018,8,339 6of12
Therefore,theζ-potentialandhydrodynamicdataalsoconfirmthatthepH ofAuNP-MUAis
agg
~4.1,whichiscomparablewithpK ≈4.8forMUAinsolution[28].However,thepH forAuNP-MUA
a agg
is about two pH unit smaller than the reported pK value for MUA adsorbed on AuNPs [22,29].
a
Recently,Charronetal. reportedthepK valueofMUAadsorbedonto5nmAuNPbytitratingwith
a
NaOH(acid-basetitrationmethod)[29]. TheyreportedthepK valueofMUAadsorbedonAuNPis
a
around7,whichsuggestsapK abouttwopHunitshigherthanthatoftheunboundMUA.Wangetal.
a
studiedthedissociationbehaviorofAuNP-tetheredMUAasafunctionofpH,usinganacid-base
(orpotentiometric)titrationmethod[22]. TheyalsoobservedsimilarphenomenaforthepK ofMUA
a
boundto7.2nmAuNPsincreasedto~8.3,whichissignificantlyhigherthanthatofMUAinsolution
(pK ≈4.8)[22].
a
Direct transfer of nanoparticles from aqueous to organic phases is frequently employed in
nanoparticlesynthesisandpurificationapplications[30,31]. Insomecolloidalnanoparticlecatalytic
applications,phasetransferofnanoparticlesbetweentwoimmiscibleliquidsisextremelyadvantageous
fortherecyclingandreuseofcatalysts,duetotheavoidanceofirreversiblenanoparticleaggregation.
To date, many methods used to modulate nanoparticle phase transfer have been developed, such
ashost–guestinteractions[32,33],electrostaticinteractions[34–36],covalentmodifications[37],and
ligandexchanges[38–42]. Itmustbementionedthatwithmanyofthesemethods,reversiblephase
transferisnotachieved;however,forcertainapplications,irreversiblephasetransferispreferred. Here
wedemonstratereversiblepH-triggeredphasetransferof13nmAuNP-MUAbetweentheaqueous
andorganicphases. AuNP-MUAinanaqueousphasearetransferredintoaCHCl layer,byreducing
3
theaqueouslayerpHfrom11.0to8.0with0.1MHClandvigorousmixingfor2min(Figure3A).
The phase transfer occurs only in the presence of octadecylamine (ODA), which acts as the phase
transferringagentwhentheODAisprotonated(charged)atpHbelowthepK andisdeprotonated
a
at pH above the pK . In short, a pH of 1.5 mL of AuNP-MUA (pH = 11) was adjusted to 8.0 with
a
HCl, andsubsequentlyvigorouslyagitatedwith1.5mLofchloroformcontaining~1mgODAfor
2min. ThenecessityofODAasaphase-transferringagentisdemonstratedwithacontrolexperiment
whereAuNP-MUAaggregatesonthevialsurfaceandwater-chloroforminterfacewhenacidicpH
is employed without using ODA. (Supplementary Materials, Figure S5). Phase transfer between
aqueousandorganicphasesisreversibleforatleastfourcycles(Figure3B),asindicatedbymonitoring
theLSPRpeakofAuNP-MUAintheaqueouslayer(SupplementaryMaterials,FigureS6). ThepH
forthephasetransition(pH )of8.7wasdeterminedfromtheinflectionpointofasigmoidalfit
trans
of the percentage of AuNP-MUA transferred from aqueous to organic layers as a function of pH,
determined from the LSPR peak absorbance in the aqueous layer (Figure 3C and Supplementary
Materials,FigureS7). ThetransferfromaqueoustoorganicphaseoccurswhenthepHoftheaqueous
layer is below the pK of the amine headgroup in ODA (~10.6) and above the pK of MUA (~4.5)
a a
(Figure 3C). The phase transfer into the organic phase is due to the ion-pair formation between a
negatively-charged carboxylate group (above pH ~4) and the positively-charged amine group of
ODA (below pH 10.6). The long hydrophobic alkyl chain of ODA makes the AuNP-MUA more
hydrophobic via its ion-pair formation. AuNP-MUA (1.8 nm diameter) phase transfer to organic
phasebybindingtohighly-hydrophobiccationicmolecules,suchastetraoctylammonium,hasbeen
reported previously [43]. Recently, Yuan et al. demonstrated a phase transfer cycle (aqueous →
organic → aqueous) where glutathione functionalized Ag, Au, Cu, and Pt nanoparticles (<2 nm
diameter)havebeentransferredintotolueneorhexaneviaelectrostaticinteractionbetweennegatively
chargedcarboxylategroupsonmetalnanoparticlesandpositivelychargedcetyltrimethylammonium
(CTA+,hydrophobic)[34]. TheremovalofCTA+fromthenanoparticlebyformingahydrophobicsalt
betweentetramethylammoniumdecanoateandCTA+ restoresthenegativechargeonthenanoparticle
surface,andreturnsthenanoparticlesbacktotheaqueousphase. Inthiswork,wehaveshownthat
theAuNP-MUAcaneasilyandreversiblyseparatefromanaqueousphasebyeitheraggregationor
phaseseparationmethods.
Nanomaterials2018,8,339 7of12
Nanomaterials 2018, 8, x FOR PEER REVIEW 7 of 12

Figure 3. (A) Photographs of the pH-triggered reversible phase transfer of 13-nm AuNP-MUA
Figure3.(A)PhotographsofthepH-triggeredreversiblephasetransferof13-nmAuNP-MUAbetween
between water and CHCl3 layers, by switching the pH. The left side vial contains well-dispersed
waterandCHCl layers,byswitchingthepH.Theleftsidevialcontainswell-dispersedAuNP-MUAin
AuNP-MUA3 in the aqueous phase (top layer) at basic pH, and the right side vial contains AuNP-
theaqueousphase(toplayer)atbasicpH,andtherightsidevialcontainsAuNP-MUAtransferredinto
MUA transferred into the CHCl3 phase (bottom) layer after adding HCl and vigorous shaking; (B)
theCHPlCotl 3shpohwaisneg( bpoHtt-tormig)gelareyde rreavfteerrsiabdled pinhgasHe Ctrlaannsfderv oigf o1r3o nums sAhuakNiPn-gM;(UBA) Pbleottwseheonw thine gwpaHte-rt raingdg ered
reversoirbglaenpich pahsaester, abnys mfeornoitfo1ri3ngn mtheA AuuNNPP--MMUUAA LbSePtwR peeenakt hinetewnsaitteyr aat n52d5 onrmg awnaicveplehnagsteh, ibny aqmuoenouitso ring
theApuhNasPe-;M anUdA (CL) SabPsRorpbaenacke ionft AenusNitPy-MatU5A2 5inn amquweoauvse plehnasget hati 5n25a qnume o(luefst spchalaes)e v;earnsuds (tChe) paHbs aonrdb ance
of AupNerPce-MntaUgAe tirnanasqfeur eoofu AsupNhPa-sMeUaAt 5f2r5omn man (laeqfutesocuasl et)o vae rCsHuCslt3h leaypeHr aas nad fupnecrtcioenn toafg epHtr. aTnhsef er of
AuNPp-eMrceUnAtagfero omf tarnanasqfeur ewouass tcoalcauClaHteCd lbyl atyaekrinags tahefu anbcstoirobnanocfep oHf .tTheh eApueNrPce-MntUaAge (oinf tarqaunesofuesr was
3
medium) at pH 11.0 as 0%. The red color solid curve represents sigmoidal fitting of the experimental
calculatedbytakingtheabsorbanceoftheAuNP-MUA(inaqueousmedium)atpH11.0as0%.Thered
data.
colorsolidcurverepresentssigmoidalfittingoftheexperimentaldata.
The ability to reversibly induce AuNP-MUA separation and re-dispersion is only half of the
Teqhueaatiboinl iftoyr tcoolrloeivdearl sciabtlaylyisnisd; uitc meuAstu aNlsPo -pMosUseAsss ceaptaalryatitcio anctiavnitdy. rTeh-de icsaptaelrystiioc nacitsivoitny loyf h13a lnfmo f the
equatAiounNfPo-rMcUoAllo widaas ltecsatetadl ywsiitsh; tihtem 4-unsittraolpshoepnools (s4e-sNsPc)a rteadluyctitcioanc btiyv bitoyr.oThyhdercidaet,a wlyhtiiccha icst aiv ciotmymofo1n3 nm
AuNPm-ModUelA rewacatisonte fsotre dligwanitdh-mthoed4if-inedit rAoupNhePns o[4l4(–44-N7].P F)igreudreu c4tAio anndb ySubpoprolehmyednrtiadrey, Mwahtiecrhialiss Faigcuorme mon
modeSl8r eshacotwio tnhefo trimlieg-arensdo-lvmeodd UifiVe-dVIAS uspNePctsra[ 4o4f– a4 74]-.NFPi gruedreuc4tAiona nredacStuiopnp claetmaleynzetdar byyM AautNerPia, lass Fai gure
function of MUA surface coverage. The MUA surface coverage on the AuNPs was controlled by
S8 show the time-resolved UV-VIS spectra of a 4-NP reduction reaction catalyzed by AuNP, as a
stoichiometry—mixing different concentrations of MUA with AuNPs during the ligand exchange
function of MUA surface coverage. The MUA surface coverage on the AuNPs was controlled by
process. MUA surface coverage on AuNP for 1 mM MUA with the AuNPs sample was determined
stoichiometry—mixing different concentrations of MUA with AuNPs during the ligand exchange
by thermogravimetric analysis (TGA) (Supplementary Materials, Figure S9). The percentage weight
process. MUAsurfacecoverageonAuNPfor1mMMUAwiththeAuNPssamplewasdeterminedby
loss of MUA adsorbed onto AuNPs is 3.7%, corresponding to the MUA monolayer packing density
thermoong ArauvNimPse otrfi c4.a5n6 amlyosleiscu(TleGs/Anm)(2S (uSepep SleumppelnetmareyntMarayt Meraiatelsr,iaFlsig),u wrehiSc9h) .isT choemppearrcaebnlet atgo epwreeviigohustllyo ssof
MUAreapdosrotrebde dMoUnAto pAacukNinPgs disen3s.7it%y ,ocno rAreusNpoPns d(5in.7g0 tomtohleecMuleUs/Anmm2o) n[4o8la].y MerUpAac skuinrfgacdee cnosviteyraogne Aatu NPs
of4.5d6ifmfeorelenct ucloensc/ennmtra2ti(oSnese oSfu pMpUleAm ienn atanr yAMuNaPte rliigaalsn)d, wexhcihcahnigse cwomasp daeratebrlmeitnoedp ruesviinogu sthlye r4e.5p6o rted
MUAmpoalecckuilnegs/ndme2n msiotynoolanyeAr upNacPkisng(5 d.7e0nsmityo. lTehceu leessti/mnamte2d) s[u4r8fa].ceM coUveArasguesr foanc eAucNovPesr aargee 0%at, 3d0i%ff,e rent
conce6n0t%ra, t1io0n0%s,o fanMdU 1A00i%n afonr A0u, N2.5P, l5ig.0a, n1d0.0e,x cahndan 2g5e.0w µaMsd MetUerAm iinn etdheu sliignagndth eex4c.h5a6nmgeo rleeaccutlieosn/, nm2
respectively (See SI). Figure 4B shows the kinetics of the reaction monitored in situ using time-
monolayer packing density. The estimated surface coverages on AuNPs are 0%, 30%, 60%, 100%,
resolved UV-VIS spectroscopy, via changes in intensity of the 4-NP peak at 400 nm [44–47]. No
and 100% for 0, 2.5, 5.0, 10.0, and 25.0 µM MUA in the ligand exchange reaction, respectively
reaction was observed for the 100% MUA surface coverage, which is expected due to complete thiol
(SeeSupplementaryMaterials). Figure4Bshowsthekineticsofthereactionmonitoredinsituusing
binding to all catalytic sites. At surface coverages below 100%, the AuNP-MUA are active in
time-resolved UV-VIS spectroscopy, via changes in intensity of the 4-NP peak at 400 nm [44–47].
catalyzing the 4-NP reduction. Furthermore, an induction time was observed for 60% of the MUA
Noresaucrtfiaocne wcoavseroabgsee rsavmedplfeo r(Ftihgeur1e0 04B%). MInUduActsiounr faticmeec oisv egreangeera,lwly hoicbhseirsveedx pinec tliegdanddu estatobicliozmedp lete
thiolcboilnlodiidnagl ctaotaallylsctsa, taanlydt iocccsuitress d.uAe ttos umrfaascs etrcaonvsfeerra rgeesisstbaenlcoew of1fe0r0e%d ,btyh teheA luigNanPd- M[4U6,4A9]a orre saloctwiv ein
catalyszuirnfagcet hrees4tr-uNctPurree dduuec ttioo nad.sFourbrethd erremacotarnet,sa [n50i–n5d2u]. cAti osinmtiilamr ephweansomobenseornv heads fboeren6 0o%bseorfvtehde inM UA
surfacoeurc opvreiorra gweosrakm foprl eth(Fe icgautarely4tiBc )a.cIntidviutyc toiof nthtiiomlaeteisd gPeEnGe rfaulnlyctoiobnsaelrivzeedd AinulNigPasn, dwshtearbei liinzcerdeacsoeldlo idal
induction time coincided with increased surface coverage[16].
catalysts, and occurs due to mass transfer resistance offered by the ligand [46,49] or slow surface
restructureduetoadsorbedreactants[50–52]. Asimilarphenomenonhasbeenobservedinourprior

workforthecatalyticactivityofthiolatedPEGfunctionalizedAuNPs,whereincreasedinductiontime
coincidedwithincreasedsurfacecoverage[16].
Nanomaterials2018,8,339 8of12
Nanomaterials 2018, 8, x FOR PEER REVIEW 8 of 12
Nanomaterials 2018, 8, x FOR PEER REVIEW 8 of 12

FigurFeig4u.re 4C. aCtaatlaylytitcic aaccttiivviittyy ofo AfuANuPN-MPU-MA UasA funacstiofun nocf tMioUnAo pfacMkiUngA depnascitkyi onng AdueNnPssit. y(Ao) nTimAeu-NPs.
(A)Tirmeseo-lrveesdo lvUeVd-VUISV -VspISectsrpae cotfr a4o-nfi4tr-onpithreonpohl e(n4o-Nl(P4) -NrePd)urcetidoun ctrieoanctrioena ctciaotnalycazetadl ybzye dAbuyNAPsu NPs
functionalized with 0 µM MUA; (B) Time-resolved UV-VIS spectra of 4-nitrophenol reduction
functFioignuarliez 4e.d Cwatiatlhyt0icµ aMctivMityU oAf ;A(BuN)TPi-mMeU-Are saos lfvuendctUionV -oVf IMSUspAe pctarcakionfg4 d-nenitsriotyp ohne nAoulNrePds.u (cAti)o Tnimreea-ction
reaction catalyzed by AuNPs and functionalized with 10 µM MUA; and (C) The progress of the
catalyrzeseodlvbeyd AUuNV-PVsISa nsdpfeucntrcat ioonf al4iz-neidtrowpihthen1o0l µ(M4-NMPU) Are;danucdti(oCn) Trehaectpiorong rceastsaloyfztehde rbeya ctAiounNtPrsa cked
reaction tracked by the change in 4-NP absorbance peak at 400 nm over the time.
bythfeucnhctainongaeliizned4 -NwPitha b0s oµrMba nMcUeAp;e a(Bk)a tT4im00e-nremsoolvveedr tUhVe-tVimIS es.pectra of 4-nitrophenol reduction
reaction catalyzed by AuNPs and functionalized with 10 µM MUA; and (C) The progress of the
While catalytic activity was observed with 0%, 30%, and 60% MUA surface coverage, the
reaction tracked by the change in 4-NP absorbance peak at 400 nm over the time.
Wdehcirleeacseadta slyutrifcacaec tciovviteyrawgea sdoidb sneortv perdowviditeh s0u%ff,ic3i0en%t ,caonlldoi6d0a%l stMabUilAitys. uArsfa scuechco, vtheer aAgueN,tPhse cdoeuclrde ased
not be recovered and re-dispersed by aggregation/re-dispersion or phase transfer methods following
surfacecoWvehrailge ecdatiadlyntoict pacrotivviidtye wsuafsf icoibesnetrvcoedll owidiathl s0ta%b,i l3it0y%.A, asnsdu c6h0,%th eMAUuAN sPusrfcaocuel dconvoetrabgeer, etchoev ered
the catalytic reaction. However, the pH-triggered reversible phase transfer and aggregation/re-
andrde-edcirsepaesersde sdubrfyacaeg cgorvegeraatgioen d/irde -ndoits pperorsviiodne osurfpfihciaesnet tcroalnlosifderalm steatbhiolidtys. fAolsl oswucihn,g ththe eAcuaNtaPlys tcicourelda ction.
dispersion of AuNP-MUA was achieved with 60% surface coverage in the absence of the reaction
Howenvoet rb,et hreecpoHve-rterdig agnedre rde-rdeivspeersrsibedle bpyh aagsgertergaantsiofenr/raen-ddisapgegrsrieogna otiro pnh/arsee- dtriasnpsefresri omneothfoAdus NfoPllo-MwiUnAg was
(Supplementary Materials, Figure S10). In order to enhance the colloidal stability, the surface
achievtheed cwatitahly6ti0c% resaucrtfiaocne. cHoovwereavgeer,i nthteh epaHb-steringcgeeroefdt hreevreearscitbiolen p(Shuapsep ltermanesnfetra raynMd aatgegrrieaglsa,tFioing/urere- S10).
coverage was increased to 90% by increasing the MUA concentration to 7.5 µM. Ninety percent of
dispersion of AuNP-MUA was achieved with 60% surface coverage in the absence of the reaction
In order to enhance the colloidal stability, the surface coverage was increased to 90% by increasing
surface coverage on AuNP is catalytically active, despite longer induction times on the order of 20
(Supplementary Materials, Figure S10). In order to enhance the colloidal stability, the surface
theMmUiAn (cFoignucreen 5t)r.a Mtioonret iom7p.5orµtaMnt.lyN, i9n0e%ty ofp seurcrfeancte ocfovseurrafgaec eAcuoNvPe-rMagUeAo nwaAsu sNucPceisssfcualtlayl yretcicoavlelyredac tive,
coverage was increased to 90% by increasing the MUA concentration to 7.5 µM. Ninety percent of
despiateftelor nthgee fririsnt dreuaccttiioonn ctyimclee,s aonnd rtehueseodr dine ra osefc2o0ndm cianta(lFyitgicu cryecl5e) .byM bootrhe aigmgpreograttainontl/yr,e-9d0i%speorfsisoun rface
surface coverage on AuNP is catalytically active, despite longer induction times on the order of 20
coveraangde pAhuaNseP t-rManUsfAer wmaesthsoudcsc. eIsns ftuhell yserceocnodv ecyrecdle,a tfhteer ctahtaelyfitrisc tinredauccttiioonn ctyimclees, wanedrer ienucsreeadseind,a bsuetc ond
min (Figure 5). More importantly, 90% of surface coverage AuNP-MUA was successfully recovered
cataly1t0ic0%c y4c-lneitrboyphbeontohl caogngvreergsaattiioonn /wraes-d misapinetrasiinoend.a Tnhde prahtae sceontrsatannstf eisr ombteatihnoedd sb.y Ifnitttihnge stheceo dnadtac ycle,
after the first reaction cycle, and reused in a second catalytic cycle by both aggregation/re-dispersion
from Figure 5 to pseudo-first-order reaction kinetics with respect to 4-nitrophenol, and the rate
the caatnadl ypthicasien dtruanctsifoenr mtiemtheosdws. eIrne thinec sreecaosnedd ,cybculet, 1t0h0e %cat4a-lnyitticro ipndhuenctoiolnc otinmveesr swaetrioe ninwcraeassmeda, ibnutat ined.
constant is indicative of catalytic activity (Supplementary Materials, Figure S11). The reaction rate
Ther1a0te0%co 4n-sntiatnrotpisheonbotal icnoendvebrysaftiitotinn gwtahs emdaaitnatafirnoemd. FTihgeu rreat5e tcoonpssteaundt ois- foirbstta-oinredde rbrye faictttiinogn tkhien edtaictas with
constants for the catalysts recovered by aggregation/re-dispersion method are 0.29 ± 0.04 and 0.20 ±
respefcrtotmo 4F-ingiutrroep 5h eton opl,seaunddot-hfiersrta-oterdceorn rsetaacnttioisn inkdiniectaitcisv ewoitfhc aretasplyetcitc taoc t4iv-nitiytro(Spuhpenpolel,m aenndt atrhye Mraattee rials,
0.06 min−1 for the first and second cycles, respectively, and the rate constants for the phase transfer
FigurceoSn1s1ta)n.tT ihse inredaiccatitoivne roaft ecactoanlysttiacn atcstfivoirtyth (eSucpatpalleymstesnrteacryo vMeraetderibaylsa, gFgigruegrea tSi1o1n)/. rTeh-de irsepaecrtisoionn ramtee thod
method are 0.31 ± 0.03 and 0.23 ± 0.05 min−1 for the first and second cycles, respectively.
are0.2co9n±sta0n.0ts4 faonr dth0e. 2c0at±aly0s.0ts6 rmecionv−e1refodr btyh eagfigrrsetgaantdiosne/creo-nddiscpyecrlseios,nr meseptehcotdiv aerlye, 0a.n29d ±t h0e.0r4a taencdo 0n.s2t0a ±n tsfor
Unfortunately, the catalytic activity was lost for the third catalytic cycle, due to the irreversible
theph0aa.g0sg6er metrgaiannt−is1o ffneor rom ft hAeetu hfNiordPst- aMarneUd0A .s3 ed1cuo±rnidn0 .g0c y3thcaelen rsde, cr0oe.v2sp3ere±yc tpi0vr.e0ol5cyem,s asinensd−. 1thfeo rrathtee cfoirnssttaanntds sfeocro tnhde pcyhcalsees ,trraenspsfeecrt ively.
method are 0.31 ± 0.03 and 0.23 ± 0.05 min−1 for the first and second cycles, respectively.
Unfortunately,thecatalyticactivitywaslostforthethirdcatalyticcycle,duetotheirreversibleaggregation
Unfortunately, the catalytic activity was lost for the third catalytic cycle, due to the irreversible
ofAuNP-MUAduringtherecoveryprocesses.
aggregation of AuNP-MUA during the recovery processes.

Figure 5. Recovery and reuse of AuNP-MUA with 90% surface coverage in catalysis by using (A) pH-
triggered aggregation/redispersion method and (B) pH-triggered phase transformation metho d.

Figure 5. Recovery and reuse of AuNP-MUA with 90% surface coverage in catalysis by using (A) pH-
Figure 5. Recovery and reuse of AuNP-MUA with 90% surface coverage in catalysis by using
triggered aggregation/redispersion method and (B) pH-triggered phase transformation method.
(A)pH-triggeredaggregation/redispersionmethodand(B)pH-triggeredphasetransformationmethod.
Nanomaterials2018,8,339 9of12
4. Conclusions
WehavedemonstratedthatpHcontrolsthedispersionofMUA-functionalizedAuNPswhere
reversibleaggregationandredispersioninanaqueousphaseisachievedaroundpH4.1orthepK of
a
MUA.Furthermore,reversiblephasetransferbetweenaqueousandorganicphases(tolueneorCHCl )
3
canbeachievedwiththeuseofanamine-containingphasetransferagent(ODA)atpH8.7orthepK
a
oftheamine,whereanionpairformationinducesphasetransfertochloroform. Thecatalyticactivity
ofAuNPsfunctionalizedwithdifferentsurfacecoveragesofMUAwerestudied. Completeinhibition
ofcatalyticactivitywasobservedat100%surfacecoverageofMUA.AuNPswith60%andlessMUA
surfacecoveragewerecolloidallystableandcatalyticallyactive,butpossessedpoorrecoverabilityand
reusabilityfollowingthereactions. Inthissystem,thereisatradeoffbetweencolloidalstabilityand
catalyticactivity,whichscalewithsurfacecoverage. Surfacecoverageof90%MUAwasfoundtobean
optimallevelofcoveragewherecatalyticactivitywasobserved,aswellastheabilitytorecoverand
reusefortwocatalyticcycles.Thecatalystrecoverybyaggregation/re-dispersionandaqueous/organic
phasetransfermethodswereachievedatpHs4.1and8.7,respectively. Thefundamentalinsightfrom
this work allows for the understanding and designing the reusable colloidal metal nanoparticle
catalystswithdifferentsurfacefunctionalitiesandcatalyzingthereactionatdifferentpHconditions.
SupplementaryMaterials:Thefollowingareavailableonlineathttp://www.mdpi.com/2079-4991/8/5/339/s1.
FigureS1:TEMimagesofAuNPs,TableS1:HydrodynamicdiameterandZetapotentialofAuNPsbeforeand
afterMUAfunctionalization,FigureS2:UV-VISspectraofMUAfunctionalizationAuNPswithdiametersof5nm,
13nm,and45nm,FigureS3:UV-VISabsorptionspectraofAuNP-MUArecordedatacidicandbasicpHvalues.
FigureS4:UV-VISabsorptionspectraofAuNP-MUArecordedatdifferentpHvaluesvariedfrom9to3,Figure
S5:ControlexperimenttoshowthatODAfacilitatesthephasetransferofAuNP-MUA,FigureS6:pH-triggered
reversiblephasetransferofAuNP-MUAstudiedusingUV-VISspectroscopy,FigureS7:Theonsetofphasetransfer
ofAuNP-MUAfromaqueoustoCHCl3phase,FigureS8: Time-resolvedUV-VISspectraforcatalyticreaction
ofAuNPsasafunctionofMUAsurfacecoverage,FigureS9:TGAcurveforAuNP-MUA,FigureS10:Abilityto
recovertheAuNPsfunctionalizedwithMUAdifferentsurfacecoverages,FigureS11:Fittingtheabsorbancedata
topseudo-first-orderreactionkinetics.
AuthorContributions: S.M.A.andS.C.conceivedanddesignedtheexperiments,performedtheexperiments,
andanalyzedthedata.C.L.K.istheprincipleinvestigatorwhoseinputsandrevisionsmadethisprojectpossible.
Acknowledgments:WethankNationalScienceFoundationforthesupportthroughgrantNo.CBET-1057633.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.
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