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Design and development of novel layered nanostructured hybrid materials for environmental,medical, energy and catalytic applicationsPotsi, Georgia

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Citation for published version (APA):Potsi, G. (2016). Design and development of novel layered nanostructured hybrid materials forenvironmental, medical, energy and catalytic applications. University of Groningen.

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Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays

35

CHAPTER3Towardsnovelmulti-functionalpillarednanostructures:

effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays

Multi-functionalpillaredmaterialsweresynthesizedby intercalationofcage-shaped

adamantylamine(ADMA)molecules intotheinterlayerspaceofgraphiteoxide(GO)

and aluminosilicate clays. The physicochemical and structural properties of these

hybrids,determinedbyXRD,FTIR,Raman,XPSandTEMshowthattheycanserveas

tunable hydrophobic/hydrophilic and stereospecific nanotemplates. Thus in the

ADMA-pillared clay hybrid the phyllomorphous clay provides a hydrophilic

nanoenvironmentwherelocalhydrophobicityismodulatedbythepresenceofADMA

moieties.Ontheotherhand,inADMA-pillaredGOhybrid,botharomaticringsofGO

sheetsandADMAmoleculesdefineahydrophobicnanoenvironmentwheresp3-oxo

moieties (epoxy, hydroxyl and carboxyl groups) present on GO modulate

hydrophilicity. As test applications we demonstrate that these pillared

nanostructures are capable of selective/stereospecific trapping of small

chlorophenolsorcanactascytotoxicagents.

Thischapterisbasedonthearticle:Towardsnovelmulti-functionalpillarednanostructures:

effective intercalation of adamantylamine in graphene oxide and smectite clays by K.

Spyrou*,G.Potsi*,E.K.Diamanti*etal.,AdvancedFunctionalMaterials37,(24)5841-5850,

2014.

*K. Spyrou, G. Potsi and E. K. Diamanti have contributed to this work in equal manner.

________________________________________


36

3.1.IntroductionDiamondoids have been extensively studied in recent years due to their successful

applicationindiversefieldsofnano-andbiotechnology[1]whichrelyonthephysical

and chemical properties[2, 3, 4] imparted by their unique (cage-like) structure of

tricyclic saturated hydrocarbons. Applications reported in the literature for these

“molecular diamonds” include their use as templates in nanotechnology, or as

molecular building blocks for the synthesis of novel catalysts,[5] high temperature

polymers,[6]orhybridnanostructures.[7] Inthepharmaceutical industrydiamondoids

areemployed indrugdeliveryandasdrugtargetingagents,[8]aswellas inantiviral

drugs(influenzaA)[9]andinthetreatmentofParkinsonandAlzheimer.[10]Tofurther

extendtheuseofdiamandoids,host/guestchemistrycanhelptotuneorprotectthe

properties of these molecules. In this context, adamantylamine (ADMA), an

adamantanederivativewithacovalentlyattachedaminogroup,isexpectedtobean

ideal pillaring block to be incorporated in layered host materials such as

aluminosilicate clays or graphene oxide (GO), giving rise to new hybrid

multifunctionalnanostructures.

Layered materials represent a diverse and largely untapped source of two-

dimensional (2D) nanosystems with high specific surface area and exceptional

physicochemical properties that are important for applications such as catalysis,

sensing,environmentalremediation,biotechnology,andenergystorage.[11,12,13,14,15,

16]Thenatureoftheenvironmentbetweenthe2Dnanometer-sizedsheetsregulates

the topology of the intercalated molecules and affects possible supramolecular

rearrangementsor reactions,suchasself-assemblingprocesses thatareusuallynot

easily controlled in solution.[17, 18, 19, 20] Smectite clays and graphene oxide are two

archetypical layered materials; smectite clays are minerals consisting of

aluminosilicate nanoplatelets, with a unique combination of swelling, intercalation

andionexchangepropertiesthatmakethemvaluablenanostructures[21,22,23]foruse

ascatalysts,[24, 25] templates inorganicsynthesis,[26, 27]buildingblocks forcomposite


37

materials,[17, 28, 29, 30, 31] adsorbents of inorganic and/or organic pollutants,[32, 33] as

activeagentsorexcipientsofmedicinalproducts[34,35]andasconstituentsofmodified

drug delivery systems (MDDS)[36] in pharmaceutical industry. In most cases, the

intercalationprocess isa simple ion-exchangeprocedurebetweenhydratedcations

present in thegalleriesof the clayandorganic/or inorganic cationmoieties.Unlike

the intercalation of graphite, that of smectite clays does not necessarily involve

chargetransferbetweenthehostandtheguestspecies.Ontheotherhand,graphite

oxide (GO) isanoxygen richderivativeofgraphitedecoratedwithhydroxyl,epoxy,

andcarboxylgroups(sp3-oxomoieties).[37, 38, 39]Thesefunctionalgroupsarecreated

by strongoxidationanddistributed randomlyon thebasalplanesandedgesof the

GOsheets,generatingaliphatic regions (sp3-carbonatoms).Dueto theexistenceof

suchhydrophilicmoieties,GOpresentssimilarpropertiesassmectiteclaysinthatitis

prone to swelling and intercalation. Both GO and intercalated GO are being

considered for numerous applications such as supercapacitors,[40] high mobility

transistors,[41] lithium batteries,[42] hydrogen storage, adsorption of organic

moieties[43]ortheremovalofpollutants(e.g.chlorophenols)fromaqueoussolutions;

recentlytheyhavealsoattractedinterestregardingtheirpotentialuseinbiomedical

applications. GO has already been studied in drug delivery formulation and

bioanalysis[44,45,46,47,48,49]aswellasforitspotentialcytotoxicaction.[50,51]Incontrast

toclays,theintercalationprocessofGOinvolvescovalentbondingofguestmolecules

to the oxygen-containing groups on the GO surfaces (nucleophilic substitution

reactions). It has been demonstrated that, under proper conditions, GO can be

exfoliatedinwaterformingcolloidalsuspensionsofsinglegrapheneoxidesheets.[52,

53]

In this study, we report on the intercalation of adamantylamine (ADMA) into two

typesoflayeredmatrices,grapheneoxideandsmectiteclay.Apartfromthedifferent

structural and geometrical characteristics (e.g. interlayer space) that might occur

using different matrices, and which in turn define stereospecific properties to the

final pillared structures, the choice of these two 2D-nanotemplates was also


38

motivatedbytheirabilitytoprovide, inconjunctionwiththeadamantanemoieties,

diverse tunable hydrophobic or hydrophilic character (environment) in the final

hybrids. Thus, in ADMA-pillared clay hybrid, smectite clay provides a hydrophilic

nanoenvironmentwherelocalhydrophobicityismodulatedbythepresenceofADMA

moietieswhile in ADMA-pillared GO hybrid, both aromatic rings of GO sheets and

ADMAmolecules define a hydrophobic nanoenvironmentwhere oxygen-containing

groupspresentonGOmodulatehydrophilicity.SincetheincorporationofADMAinto

these layered materials opens the way to diverse applications in the chemical,

pharmaceutical and electronic (sensor) industry, we decided to perform also two

representative case studies of great importance in biomedicine and environmental

remediation, namely the use of these hybrid nanostructures as antiproliferative

agents in cells and effective adsorbents for the removal of organic pollutants from

aqueoussolution.

3.2.ResultsanddiscussionThe smectite clay consists of octahedral alumina layers, each fused between two

tetrahedral silica layers. In the tetrahedral sheet Al+3 can replace Si+4 creating a

negativecharge.OccasionallyFe+3cationsarealsopresentinthetetrahedrallattice.

IntheoctahedralsheetsnegativechargingmayoccurbysubstitutionofMg+2forAl+3.

Fe+2 and Fe+3 may also part of the octahedral sheets. The negative charge of the

isomorphous substituents is compensated by hydrated cations (usually sodium)

presentintheinterlayerspace.Thesechargebalancingcationscanbereplacedwith

water soluble organic or inorganic cationic species;[21, 22, 23] this approach was

followed to intercalate ADMA in two different clay minerals, namely SWy-2

montmorillonite (product called SWy-2/ADMA) in the following) and a synthetic

trioctahedralhectorite,LaponiteRD(productcalledLap/ADMA).Initiallytheterminal

amine groups of adamantylamine had to be protonated to form a cationic

adamantanespecies.Theintercalationofthepositivelychargedadamantylamine,


39

Scheme3.1.Schematicrepresentationoftheintercalationprocessofadamantylamine,after

theprotonationoftheamineendgroups,intotheinterlayerspaceofclay.Theboldlines

representthenegativelychargedclayplatelets.

dispersedinaqueoussolution,isachievedbyion-exchangeaccordingtothereaction.

TheprocessisillustratedinScheme3.1:

Contrary to clay minerals, GO does not contain charge balancing cations, thus

intercalation isobtainedbygrafting to functional groupson theGOsurfaceand/or

adsorption of molecules held between the basal planes by van der Waals

interactions.ADMAwasintercalatedinoxidizedgraphiteassketchedinScheme3.2;

theproduct iscalledGO/ADMAinthefollowing.WhenADMA,dissolved indistilled

deionizedwater,wasaddedtoawaterdispersionofGOanimmediateflocculationof

GO particles was observed. This phenomenon is induced by the insertion of

adamantylamine in the GO galleries through covalent bonding via the amine

functionality of the adamantane derivative (see XPS data below). The amine end

groupsinteractviaaringopeningreactionoftheepoxidegroupsofGO.[52,53,54]


40

Scheme3.2.SchematicrepresentationofthesyntheticprocedureofGO/ADMA.

The success of the intercalation reaction can be proven by X-ray diffraction,which

allowsestimatingtheinterlayerspacingbetweenGOorclaysheets.TheXRDpatterns

of GO/ADMA and SWy-2/ADMA are displayed in Figure 3.1. The insertion of

adamantylamine between the aluminosilicate and graphene oxide layers increases

the interlayer distance.More specifically, for intercalation in SWy-2 clay, the basal

d001-spacing,whichis12.4±0.3Åintheinitialmontmorilloniteclay,becomes15.6±0.4

Åafterthemodification;thiscorrespondstoaninterlayerseparationΔ=15.6-9.6=6

Å,where9.6Årepresentsthethicknessofaclaylayer.[20]Thisvalueisinaccordance

with the size of the adamantylamine molecule if we assume an orientation

perpendicular to thealuminosilicateplatelets.[55] In the caseofGO/ADMA, the001

diffractionpeak,centredat~12oinpristineGO,shiftstoloweranglescorresponding

toad001-spacingof10.4±0.3Å.Takingintoaccountthethicknessofagrapheneoxide

layer (6.1Å),[56] this corresponds toan interlayer separationΔ=10.4–6.1=4.3Å

occupied by the ADMA pillaring moieties. This value implies that the adamantane

derivativemustadoptaninclinedorientationintheGOinterlayerspace.[57,58,59]


41

5 10 15 20

5 10 15 20

GO/ADMA

GO

SWy/ADMA

2Theta(ο)

d=7.4Å

d=9.9Å

15.6Å

12.4Å

SWy

B

Intens

ity(a

.u.)

A

Figure3.1.ComparisonoftheX-raydiffractionpatternsofthehybridpillaredsystems(A)

SWy-2/ADMAand(B)GO/ADMAwiththoseofthepristinelayeredmatrices(SWy-2andGO).

Theresultsforlaponite(Figure3.2)aresimilartothoseformontmorillonite:thebasal

d001-spacing,whichis12.0±0.3Åintheinitiallaponiteclay,becomes15.5±0.4Åafter

theintercalationofadamantylamine;thiscorrespondstoaninterlayerseparationof

15.5Å-9.6Å=5.9Å.Thisvalueisinagreementwiththesizeoftheadamantylamine

molecule if we assume an orientation perpendicular to that of the aluminosilicate

plateletsassketchedinScheme3.1.


42

2 4 6 8 10 12 14

Lap/ADMA

LapIn

tens

ity (a

.u)

2T heta 0

d=12Å

d=15 Å

Figure3.2.XRDpatternsofthehybridLap/ADMAincomparisonwithpristineLaponite

An additional tool for the characterization of hybrid pillared materials is FTIR

spectroscopy, which can confirm the successful incorporation of the adamantane

derivativeinthelayeredmatrices.Figure3.3displaystheFTIRspectraofSWy-2and

GO before and after the intercalation process. In the case of SWy-2/ADMA, the

spectrumdisplaysallthecharacteristicbandsarisingfromaluminosilicateclayat465

cm-1 (Si-O-SiandSi-Obendingvibrations),524cm-1 (Si-O-Sibending),778cm-1 (Si-O

deformation), 797 cm-1 (Si-O and Si-O-Al stretching), 884 cm-1 (Al-Fe-OH

deformation), 918 cm-1 (Al-OH-Al bending), 1047 cm-1 (Si-O-Si stretching) and 1639

cm-1(H2Obending).[60, 61] Inaddition,thepresenceofadamantylamineinthehybrid

materialisrevealedbythebandscentredat2863cm-1and2921cm-1corresponding

to stretching vibrations of C-H, as well as by the peak at 1520 cm-1, due to N-H

vibrations.[19,62]InthecaseofGO/ADMAthesamepeaksoriginatingfromtheADMA

moleculesarealsopresentinthespectrum,togetherwithcharacteristicbandsarising

fromGOat3410cm-1(hydroxylstretchingvibrationofC-OHgroups),1621cm-1(C=O

stretchingvibrationsofthe-COOHgroups),1396cm-1(O-HdeformationsoftheC-OH

groups)and1062cm-1(C-Ostretchingvibrations).[63]


43

Figure3.3.FTIRspectraofthehybridpillaredsystemsSWy-2/ADMAandGO/ADMA;theFTIR

spectraofpristineSWy-2andGOareplottedforcomparison.

The FTIR spectrum of Lap/ADMA shown in Figure 3.4 displays in addition to the

characteristic bands arising from the aluminosilicate clay a peak at approximately

1515 cm-1, due to N-H vibrations as well as the bands centred at 2863 cm-1 and

2921cm-1 corresponding to stretching vibrations of C-H; this testifies to the

intercalationoftheadamantylaminemoietiesinbetweentheclaysheets.

500 1000 1500 3000 3500

500 1000 1500 3000 3500

S Wy-2

Wavenumbers (cm-1)

Abs

orba

nce(arb.units)

S W y-2/ADMA

G O

G O /ADMA


44

500 1000 1500 2000 2500 3000 3500

LapA

bsor

banc

e (a

.u)

Wavenumbers(cm-1)

Lap/ADMA

Figure3.4.FTIRspectraofLap/ADMAandpristineLaponite

In the case of GO/ADMA Raman measurements were performed as well and are

presentedinFigure3.5.Ramanspectroscopyiswidelyusedinthecharacterizationof

carbon systems because it provides information about their structure. The Raman

spectrum of the graphite used as starting material includes the G peak at

approximately ~1580 cm-1, as well a very weak 2D band located at ~ 1353 cm-1,

caused by the tangential E2g in-plane vibration mode of the graphite lattice and

secondorderboundaryphonons(A1gbreathingmode)respectively.[64]The2Dbandis

associated with defects leading to sp3 carbon atoms.[65] The GO sample exhibits

strongfluorescenceandtherefore itsweakRamanspectrumcouldnotbeobserved

whenexcitingat532nm.Toovercomethisproblem,asampleobtainedbydeposition

of GO sheets on a gold substrate, as described elsewhere,[53] was used. After the

chemicaloxidationofgraphitetheGbandisbroadenedandshiftedslightlyto1594

cm-1,while theDbandat1363cm-1becomesprominentdue todefectscreatedby

theattachmentofoxygen-containinggroups (sp3-oxomoieties) to thecarbonbasal

planes.[66]Thedegreeofdisorderinthecarbonflakescanbeexpressedbytheratio

betweenintensitiesoftheGandDbands.InthecaseofGO,theID/IGratiois1.06


45

Figure3.5.Ramanspectraofpristinegraphite,GOandGO/ADMAexcitedat532nm.

pointing to the creation of sp3 domains in the oxidation treatment. After the

intercalationof theorganicmoieties that ratio remainsalmost thesame.Thisgives

furthersupporttothecovalentbondingoftheadamantanederivativestotheepoxy

groups on the GO surface since such a bonding does not decrease the number of

aromaticcarbon-carbonbonds.

Toverifythepresenceand integrityof theadamantylaminecycloalkaneswithinthe

layerednanostructures,aswellastoanalysethechemicalenvironmentofADMA,we

employedX-ray photoelectron spectroscopy (XPS). TheXPS spectrumofGO/ADMA

reveals inC1score level region(Figure3.6 left) thecharacteristiccontributionofC-

C/C-HbondsoftheGOlatticeandoftheadamantanecyclohexaneringcentredata

binding energy of 285.0 eV; these bonds contribute 54.3% of the total carbon 1s

intensity.Thepeakat286.3eVisduetoC-O/C-Nbondsandrepresents23.8%ofthe

totalcarbon1sintensity.Finallytwocomponentsat287.9eVand289.6eVare

1000 1200 1400 1600

GO/ADMA

G Band

Inte

nsity

(a.u

.)

Raman Shift (cm-1)

D Band

GO

Graphite


46

Figure3.6.XPSspectraoftheC1s(left)andN1s(right)corelevelregionsoftheGO/ADMA

hybrid.

attributed to carbonyl (C=O) and carboxyl (O-C=O) groups respectively, which are

created on the basal planes and at the boarders of the carbon sheets by the acid

treatment. TheN1s core level regionof theXPS spectrumofGO/ADMA,plotted in

Figure3.6(right)exhibitstwomainpeaks,locatedat399.8eVand401.5eV.

These peaks arise from the amine end groups of ADMA chemically grafted to the

epoxy groups of GO,[67, 68] and from protonated amines ionically bonded to the

carboxylicacidanionicgroupsofGO,[69]respectivelyasisshowninFigure3.7where

theN1sXPSspectraofpristineGOandGO/ADMA,arecompared.Thespectrumof

GO/ADMA exhibits two main peaks, located at 399.8 eV and 401.5 eV in binding

energy. They arise from the amine end groups of ADMA chemically grafted to the

epoxy groups of GO[36] and from protonated amines ionically bonded to the

carboxylic acid anionic groups of GO, respectively. In fact, the corresponding

spectrum of pristine ADMA is deconvoluted into two photoelectron peaks, one at

400.1 eV, which is attributed to the amine groups of the organic molecule and a

secondoneat401.5eVduetoprotonatedamines.Afterintercalationnoaminepeak

isfoundbutacontributiontypicalofC-N-Cbondsappearsat399.8eV,indicatingthe

successfulfunctionalizationofADMAintheGOgalleries,whiletheprotonatedgroups

remainatthesamebindingenergyvalue.


47

Figure3.7.ComparisonbetweentheN1scorelevelphotoemissionspectraof

adamantylamineandGO/ADMAhybrid(ongoldsubstrates)

XPSmeasurements were also performed for the SWy-2/ADMA hybrid system. The

survey spectra of SWy-2 and SWy-2/ADMA are shown in Figure 3.8. Characteristic

photoelectronandAugerpeaksofO,Si,Al,andMgareclearlydistinguishableinthe

spectrumofpristinemontmorilloniteclay.Asmallcarbonpeakappears,mainlydue

to adventitious carbon always present on the outer surface of air-exposed[70, 71]

materials but alsodue to soil organicmatter present innatural clayminerals.[70, 72]

After the insertion of ADMA a pronounced increase in the intensity of the carbon

signal points to the presence of adamantylamine inside the clay galleries.

Quantitatively,theincorporationofADMAintheclayisdeterminedbymeasuringthe


48

Figure3.8.XPSsurveyspectraofthepristinemontmorilloniteclay,SWy-2,andthehybrid

obtainedafterintercalationofadamantylamine,SWy-2/ADMA.

ratiobetweenthecarbon1sandthesilicon2p core levelphotoemission intensities

before and after the intercalation process. The C1s / Si2p intensity ratio increased

from1.42±0.06before the incorporationofadamantylamine to2.79±0.11after the

introductionofADMA,asshowninFigure3.9.

ThepresenceoftheamineterminalgroupsonadamantaneisdeducedfromtheN1s

photoelectron spectrum shown in Figure 3.10.More specifically theN1s core level

spectrum is deconvoluted into two peaks, one located at 401.5 eV due to the

protonated amines of adamantine molecules intercalated into the SWy-2 galleries

(64.3%of the totalN1s intensity) andone centred at 399.5 eV attributed to non-

protonated amine groups (35.7 % of the total N1s intensity) of adamantane that

interacts through hydrogen and van der Waals bonding with the aluminosilicate

surfacesand/orotherintercalatedadamantanemolecules.


49

Figure3.9.ComparisonbetweentheC1sandSi2pcorelevelregionsoftheXPSspectraof

pristinemontmorilloniteclay,SWy-2,andofthehybridobtainedafterintercalationof

adamantylamine,SWy-2/ADMA.TheratiosbetweentheC1sandtheSi2pspectralintensities

areindicated.

Figure3.10.XPSspectrumoftheN1scorelevelregionofSWy-2/ADMA.


50

To estimate the amount of adamantylamine introduced in GO and in clay,

thermogravimetric(TGA)anddifferentialthermalanalysis(DTA)measurementswere

performedonthetwopillaredhybridsaswellasonthepristinematerials.Basedon

these data, theweight loss observed in the temperature range 280–400 °C,which

originates from the combustionof the aminogroupsof adamantylamine,[73, 74]was

calculatedtobeequalto8and4wt%forGO/ADMAandSWy-2/ADMA,respectively.

Moreover, in thecaseofSWy-2/ADMA,between400°Cand700°Canotherweight

loss of 8.5 wt%, due to combustion of the cycloalkane of adamantylamine, is

observed.Basedontheseweightlosses,weestimatethattheamountofintercalated

adamantylamineintheSWy-2/ADMAhybridcorrespondsto~12.5wt%.Inthecaseof

GO/ADMA the secondweight loss cannotbedistinguished from the combustionof

thegrapheneoxidelayers,whichtakesplaceinthissametemperaturerange.

Inmoredetailthethermogravimetric(TGA)anddifferentialthermalanalysis(DTA)of

puregraphite,grapheneoxide,andthetwocompositenanostructuresaredisplayed

inFigures3.11a,bandc.TheDTAcurveofgraphite(topleft)exhibitsanexothermic

peakat700oCduetothecombustionofcarbonlayers,whileinthecaseofgraphene

oxidewehavethepresenceoftwoexothermicpeakscentredat250°Cand500°C,

whichcorrespondtoa30wt%anda50wt%weight loss, respectively.Thesepeaks

areattributed to the removalofoxygen-containing functionalgroups, createdafter

the acid treatment of graphite (first peak), and to carbon combustion (second

peak).[52] In the caseofGO/ADMA, theDTA curve exhibits one exothermic peak at

~201 °C with 30 wt% weight loss, ascribed to the removal of oxygen-containing

functionalgroups,whilethecombustionofthecarbonlayersappearsat540°C.The

weight loss observed in the temperature range 280–400 °C originates from the

combustionof theaminogroupsofadamantylamineand represents~8wt%of the

totalmassofthematerial.


51

Figure3.11.DTAandTGAcurvesofpristinegraphite(a),GO(b)andoftheGO/ADMA

hybrid(c)

100 200 300 400 500 600 700 800 900

-70

-60

-50

-40

-30

-20

-10

0

10

Temperature (OC)

DTA

(arb

.uni

ts)

GRAPHITE

0

20

40

60

80

100

% T

G750 oC

100 200 300 400 500 600 700 800-100

-50

0

50

100

150

200

Temperature (oC)

DTA

(arb

.uni

ts)

0

20

40

60

80

100500 oC

230 oC

% T

G

GO

0 100 200 300 400 500 600 700 800

-40

-20

0

20

40

60

Temperature (oC)

DTA

(arb

.uni

ts)

GO/ADMA

202

540

9%

0

20

40

60

80

100

TG %

a

b

c


52

Thermogravimetric (TGA) and differential thermal analysis (DTA) of Lap/ADMA are

displayedinFigure3.12b.Inthethermogravimetricanalysisofthishybridsystemwe

observeat100°Caweight lossofabout6.5wt%,correspondingto theremovalof

physisorbedwater.Above100°Candupto400°Caweightlossof12wt%duetothe

calcination of the amino groups of adamantylamine can be seen. Finally, between

400 °C and 750 °C another weight loss of 8wt% is observed, which is due to

combustion of the cycloalkane of adamantylamine. The two thermogravimetric

weight losses between that range (400 °C and 750 °C) are probably due to

adamantylaminemoleculeswhich residewithdifferentorientation in the interlayer

spaceofthelaponiteclay.

UndeniableproofforthesuccessfulintercalationofADMAintheinterlayerspaceof

the montmorillonite clay mineral comes from the High-Resolution Transmission

ElectronMicroscopyimages.Figure3.13showsHRTEMimagesoftheSWy-2/ADMA

hybrid system. From the TEM image of ADMA-intercalated SWy-2 at low

magnification (Figure3.13a) the inter sheet separationcanbeestimated,while the

highdegreeof stackingof the clayplatelets is also confirmed. Thebasal spacing is

measured to be approximately 1.5 nm in agreement with the XRD findings.

Moreover,intheTEMimageathighermagnificationshownatFigure3.13b,aswell

as in the zoomed-in image shown in Figure 3.13 d, intercalated ADMA layers

(indicated by yellow arrows) can be recognized between clay layers, which are

composedofanoctahedral layer(pinkarrow)sandwichedbetweentwotetrahedral

layers (blue arrows), as expected for a 2:1 layered silicate likemontmorillonite (an

octahedralaluminalayerfusedbetweentwotetrahedralsilicalayers).


53

Figure3.12.DTAandTGAcurvesof(a)SWy-2/ADMAand(b)DTAandTGAcurvesof

Lap/ADMA.

Figure3.13.(a)TEMimageofADMA-intercalatedSWy-2atlowmagnification.Basalspacing

ismeasuredtobeapproximately1.5nm,consistentwiththeXRDmeasurement.(b)TEM

imageathighermagnification,withazoomed-inimageshownin(d).IntercalatedADMA

layer(indicatedbyyellowarrows)canberecognizedbetweenclaylayerswhichare

composed(c)oftetrahedrallayer(bluearrow)plusoctahedrallayer(pinkarrow)plus

tetrahedrallayer(bluearrow).


54

Nitrogenadsorption-desorptionmeasurementsat77K (-196.14 oC)wereperformed

on both GO/ADMA and SWy-2/ADMA in order to reveal the creation of pillared

structures.Table3.1reportsthespecific-surface-areasobtainedthroughBETanalysis

(SBET) for the pristine layered matrices, GO and SWy-2, and for the corresponding

pillared hybrid nanostructures. The specific surface area of pure GO (~9m2 g-1) is

small,indicatingthatN2canonlyadsorbontheexternalsurfacesofGOandreduced

GO while the interlayer space of GO is inaccessible. However, for the GO/ADMA

hybridthespecificsurfaceareaisfourtimeslarger(37m2g-1)thanforGO,supporting

the hypothesis that adamantylamine acts as pillaring species. In fact, this result

impliesthatnitrogenadsorbsnotonlyontheexternalsurfacebutalso inthepores

createdby theADMApillars.The resultsweremorepronounced in thecaseof the

montmorillonitehybrid,whichshowedasignificantlyincreasedBETsurfacearea(135

m2g-1)comparedtopristinemontmorilloniteclay.

Table3.1. Specific surfaceareavalues forGOandSWy-2and finalhybridsGO/ADMAand

SWy-2/ADMA.

SpecificSurfaceArea(SBET)

(m2g-1)

GO 9

GO/ADMA 37

SWy-2 65

SWy-2/ADMA 135


55

In view of possible applications for these new pillared hybrids,we testedwhether

SWy-2/ADMA and GO/ADMA are capable of adsorbing organic pollutants (phenol

derivatives) from solution. In Figure 3.14 the adsorption isotherms of 2.4.6-

trichlorophenol(TCP),2.4-dichlorophenol(DCP)andpentachlorophenol(PCP),inthe

startingmaterial(SWy-2)andintheSWY-2/ADMAhybridareshown.Theadsorption

isothermspresentaplateauat increasedchlorophenol:clayratioswhichdefinesthe

maximumadsorptioncapacityofthematerial,listedinTable3.2.Asevidentfromthe

isotherms,TCPandDCPphenolsadsorbintheinterlayerspaceofthepristineclayas

wellasintheporesofSWy-2/ADMA,whiletheadsorptionofPCPremainedzeroinall

cases.

Figure3.14.(A)Langmuiradsorptionisothermsof2,4-DCPonpristineSWy-2clay(n)and

SWy-2/ADMA(p),(B)Adsorptionisothermsof2,4,6-TCPonpristineSWy-2clay(£)and

SWy-2/ADMA(r),(C)AdsorptionisothermsofPCPonpristineSWy-2clay(S)and

SWy-2/ADMA( ).


56

Table 3.2. Maximum adsorption capacity for chlorophenol [mM per mg] by SWy-2

montmorilloniteclay,GOandtheirderivativesSWy-2/ADMAandGO/ADMA.

Chlorophenol SWy-2 SWy-2/ADMA GO GO/ADMA

2,4-DCP 20×10-4 27×10-4 0 32×10-4

2,4,6-TCP 2×10-4 15×10-4 0 1.3×10-4

PCP 0 0 0 0

From Table 3.2 one notices the adsorption of the various phenols followed a

consistenttrendforallmaterials.

TheuptakeofDCPwassignificantlyhigherthanforTCP.Strikinglytheadsorptionof

PCP remained zero in all cases; the adamantylamine-intercalated clay showed an

improvedadsorptioncapacitywithrespecttothatofthepristineclay.Indeed,SWy-

2/ADMA adsorbed significantly higher amounts of DCP for all concentrations of

addedphenolandshoweda40%enhanceduptakecomparedtothepristineclayat

thehighestphenolconcentrations,seeTable3.2.SWy-2/ADMAalsoadsorbedalmost

eighttimesmoreTCPthantheSWy-2.Comparingthethreetypesofchlorophenolwe

conclude that the higher adsorption of DCP than of TCP as well as the negligible

adsorption of PCP could be explained by the molecular size of the three phenols

(stereospecifictrapping):TCPandPCParelargersotheymightblocktheporesofthe

pillaredhybridandtherebyhampertheadsorptionoffurthermolecules(seephysical

modelbelow).

Figure 3.15presents the adsorption isotherms for TCP,DCP andPCPbyGO/ADMA

and by pristine GO. GO appears to adsorb none of the chlorophenols, whereas

GO/ADMAadsorbsselectivelyonlyDCPwhiletheadsorptionofTCPandofPCPwas

minimal.


57

Figure3.15.Adsorptionisothermsof2,4-DCPonpristineGO(n)andonGO/ADMA(£);of

2,4,6-TCPonpristineGO(�)andonGO/ADMA(l);ofPCPonpristineGO(r)andon

GO/ADMA(�).

Table 3.3.Molecular volume (Å3) chemical structure and space filling surface of different

phenolsandofADMA(calculatedwithChemDraw7.0)


58

The observed differences in the uptake of PCP, 2,4,6-TCP and 2,4-DCP can be

attributed to the difference in the molecular size of the phenols. As illustrated in

Table3.3,themolecularvolumesare60.3Å3forDCP,67.9Å3forTCPand83.2Å3for

PCP.Thus,themorebulkyPCPisadsorbedlessbythepillaredclayandnotatallby

GO/ADMA,whileDCPseemstobeabletopenetrate(stereospecifictrapping)inthe

interlayer space of both SWy-2/ADMA andGO/ADMA. This can be explained ifwe

take into account the change in the interlayer space as deduced fromXRD (Figure

3.1).TheinterlayerdistanceofthepristineSWy-2clayis2.8Åandbecomes6.0Åin

SWy-2/ADMA. On the other hand, the interlayer distance in pristine GO is 1.3 Å,

whichismuchsmallerthanthatofthepristineSWy-2clay,andbecomes4.3Åafter

modification with adamantylamine. Despite this increase, only DCP can be

accommodatedGO/ADMAandthis results in theobservedsignificantadsorptionof

DCPincontrasttoTCPandPCP(selective/stereospecificadsorption).

Anotherpossiblefutureapplicationofthepillaredlayerednanostructuresistheiruse

as cytotoxic agents. We chose laponite and intercalated laponite for these tests

becausethissyntheticclaywithsmallplateletsizeisusedinpharmaceuticalindustry

asdrugcarrier.[75]Nevertheless,thereisagapinknowledgeconcerningLaponite’sin

vitroantiproliferativeactivity.Thus,thepresentstudyevaluatedtheinvitrocytotoxic

activity of adamantylamine, laponite and of the intercalated laponite, Lap/ADMA,

againstacancercellline(A549)andanormalone(MRC-5).ParalleltoLap/ADMAwe

alsotestedthecytotoxicbehaviourofGOandGO/ADMA.TheIC50(μg/mL)valuesfor

cellproliferation(MTTassay)after48hsoftreatmentwithadamantanehybridsand

pristinematerialsareshowninTable3.4forA549andMRC-5cellsandschematically

presentedinFigure3.16.TheIC50valuesofADMA,LapandLap/ADMAforA549cells

were122±13.7μg/mL,205±20.7μg/mLand91±5.0μg/mL.ItisnoteworthythatLap

and Lap/ADMA exhibited cytotoxic activity on MRC-5 cells (normal cells) in

concentrationhigherthan250μg/mL(288±41.8μg/mLand307±32.2μg/mLforLap

and Lap/ADMA, respectively). The IC50 (μg/mL) values for cell proliferation (MTT

assay)after48hsoftreatmentwithGOandGO/ADMAforA549cellswere259±48


59

μg/mL and 192±27.6 μg/mL, respectively, whereas GO and GO/ADMA exhibit

cytotoxicactiononnormalcells(MRC-5)inconcentrationhigherthan400μg/mL(IC50

values for MRC-5 cells were 1480±39.6 μg/mL and 425±52.6 μg/mL for GO and

GO/ADMA, respectively).Whatwe observe is that Lap, Lap/ADMA, GO, GO/ADMA

andADMA exhibited antiproliferative activity against A549 cells. The controlwells,

containing different volumes of sterilized water (solvent) equal to volumes of the

solutionsadded to the testwells,didnotpresentanycytotoxicityagainstbothcell

lines (data not presented here). All substances, except ADMA, exhibited a mild

cytotoxicactivityonMRC-5cells,with IC50valuesbeinghigher than250μg/mLand

among all, Lap/ADMA is the most cytotoxic agent against the cancer cell line. In

addition,Lap/ADMAshowedasignificantlyhighercytotoxicity(p<0.05)comparedto

thatof laponite(startingmaterial)andadamantylamineagainstA459cells.Similarly

there is a statistically significant differencebetween cell lines treatedwithGOand

GO/ADMA(p<0.05).AlsoGO/ADMAshowedalowercytotoxicity(p<0.05)compared

withGO.

Figure3.16.IC50(μg/mL)valuesofADMA,Lap/ADMAandGO/ADMAonA549andMRC-5

cells,comparedwithpristinelaponiteclay(Lap)andGO.


60

Table3.4.IC50values(μg/mL)ofADMA,Lap,Lap/ADMA,GOandGO/ADMAonA459and

MRC-5cells.

IC50(μg/mL) A549 MRC-5

ADMA 122±13.7 149±38.6

Lap 205±20.7* 288±41.8

Lap/ADMA 91±5.0*,a 307±32.2

GO 259±48* 480±39.6

GO/ADMA 192±27.6* 425±52.6

*Significant difference between cell lines; aSignificant difference between Lap and

Lap/ADMA; p < 0.05. Data are presented as mean ± σ (where σ is the standard

deviation).

3.3.ConclusionsWe successfully achieved the intercalation of adamantylamine into the interlayer

spaceoflayeredhostmaterials,namelygraphiteoxide,montmorilloniteandlaponite

clay. X-ray diffraction measurements demonstrated the successful intercalation of

adamantylamine into all three host matrixes. In the case of montmorillonite an

undeniable proof for the successful intercalation comes from TEM images. X-ray

photoelectron spectroscopy, FTIR spectroscopy as well as thermogravimetric and

differential thermal analysis illustrated the type of interactions between the host

materials and the intercalatedmolecules as well as informing on the intercalation

yield.Porosimetrymeasurementsrevealedthatpillaredstructureswerecreatedand

gavethespecificsurfaceareaofthehybridnanostructures.Thehybridnanomaterial

obtainedbyintercalationofadamantylamineintomontmorilloniteclayiscapableof

adsorbing significant quantities of organic pollutants, which entails a significant


61

potential for environmental remediation. GO/ADMA showed a stereospecificity-

determined, selective DCP uptake. Finally the hybrid nanostructures obtained by

intercalation of adamantylamine into laponite clay and graphene oxide were

investigatedfortheircytotoxicityagainstonecancercelllineandonenormalcellline.

The findings revealed thatLap/ADMAandGO/ADMApresented improvedcytotoxic

activity on A549 cells, whilst the cytotoxicity towardsMRC-5 cells (normal cells) is

maintained or only slightly increased as compared to Lap and GO, respectively.

Possibly,thesecombinationsactsynergisticallybycombiningtwodifferentmolecular

pathways,leadingtohighercytotoxicity.Furtherbiologicalinvestigationisneededto

elucidatetherolesofLap,GO,ADMAandtheircombinationsoncancerandnormal

celllines.Weestablishedacontrollableandreproduciblemethodforthesynthesisof

multi-functional materials with potential for exploitation in the fields of

environmentalremediationandbiomedicalapplications.


62

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68

AppendixAdsorptionofchlorophenols

The left panel of Figure S9 presents UV-Vis spectra for 2,4,6-TCP and 2,4-DCP and

PCP.Quantificationofthechlorophenolswasdoneusingthepeakat280nmfor2,4-

DCP,thepeakat290nmfor2,4,6-TCPandtheoneat210nmforPCP,asmarkedby

arrows. By comparing these UV-Vis spectra for solutions with known phenol

concentrationswiththecorrespondingspectrarecordedafteradsorptiononpristine

SWy-2 and on the two types of SWy-2/ADMA hybrids – see examples in the right

panelofFigureS1-wecouldcalculatetheconcentrationofchlorophenoladsorbedin

eachcaseandcompiletheadsorptionisotherms.

FigureS1.Leftpanel:UV-Visspectraof20μM2.4.6-trichlorophenol(TCP,redcurve),2.4-

dichlorophenol(DCP,blackcurve)andpentachlorophenol(PCP,greencurve)inMetOH:H2O

70:30v/v.Thearrowsmarkthepeaksusedforquantitativeanalysisofadsorption.Right

panel:UV-visspectraof2.4.6-trichlorophenol(TCP)and2.4-dichlorophenol(DCP)adsorbed

onSWy-2/ADMApreparedwith1.5and3.0CEC).


69

TableS1.Molecularvolume(Å3)Chemicalstructureandspacefillingsurfaceofdifferent

phenols(calculatedwithChemDraw7.0)


70

university of groningen design and development of novel ... · layered materials represent a...

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