University of Groningen
Design and development of novel layered nanostructured hybrid materials for environmental,medical, energy and catalytic applicationsPotsi, Georgia
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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.
________________________________________
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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,
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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]
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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]
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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.
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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]
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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.
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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.
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
49
Figure3.9.ComparisonbetweentheC1sandSi2pcorelevelregionsoftheXPSspectraof
pristinemontmorilloniteclay,SWy-2,andofthehybridobtainedafterintercalationof
adamantylamine,SWy-2/ADMA.TheratiosbetweentheC1sandtheSi2pspectralintensities
areindicated.
Figure3.10.XPSspectrumoftheN1scorelevelregionofSWy-2/ADMA.
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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.
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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).
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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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).
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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( ).
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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.
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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)
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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.
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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.
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
62
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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).
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TableS1.Molecularvolume(Å3)Chemicalstructureandspacefillingsurfaceofdifferent
phenols(calculatedwithChemDraw7.0)
Towardsnovelmulti-functionalpillarednanostructures:Effectiveintercalationofadamantylamineingrapheneoxideandsmectiteclays
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