DTIC ADA552979: Controlled Assembly of Ag Nanoparticles and Carbon Nanotube Hybrid Structures for Biosensing

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ARTICLE
pubs.acs.org/JACS
Controlled Assembly of Ag Nanoparticles and Carbon Nanotube
Hybrid Structures for Biosensing
Sangeeta Sahoo,*), ,† Sudhir Husale,*), ,‡ Shashi Karna,^ Saroj K. Nayak,z and Pulickel M. Ajayan§
†
DepartmentofMaterialsScienceandEngineering,RensselaerPolytechnicInstitute,Troy,NewYork12180,UnitedStates
‡
RowlandInstituteatHarvard,HarvardUniversity,100EdwinH.LandBoulevard,Cambridge,Massachusetts02142,UnitedStates
^
AMSRD-ARL-WM-BD,Weapons&MaterialsDirectorate,USArmyResearchLaboratory,AberdeenProvingGround,Maryland
21005-5069,UnitedStates
z
DepartmentofPhysics,AppliedPhysicsandAstronomy,RensselaerPolytechnicInstitute,Troy,NewYork12180,UnitedStates
§DepartmentofMechanicalEngineeringandMaterialsScience,RiceUniversity,6100MainStreet,Houston,Texas77005,UnitedStates
b
S SupportingInformation
ABSTRACT:Herewereportachemical-free,simple,andnovel
methodinwhichapartfromasilver-basedanodeiscontrollably
usedinastraightforwardmannertoproducesilvernanoparti-
cles(AgNPs)inordertofabricateacontrolledassemblyofAg
NPs and single walled carbon nanotube (SWCNT) hybrid
structures. The attachment and distribution of Ag NPs along
SWCNTs have been investigated and characterized by field
emission scanning electron microscopy (FESEM). We have
achievedthedecorationofSWCNTswithdifferentdensitiesof
Ag NPs by changing the deposition time, the applied voltage,
andthelocationofcarbonnanotubeswithrespecttotheanode.
Atlowvoltage,singlesilvernanoparticleissuccessfullyattached
at the open ends of SWCNTs whereas at high voltage, inter-
mediateandfullcoveragedensitiesofAgNPsareobserved.As
voltageisfurtherincreased,fractalsofAgNPsalongSWCNTs
are observed. In addition, a device based on a Ag NPs-SWNT hybrid structure is used for the label-free detection of ssDNA
moleculesimmobilizedonit.Webelievethattheproposedmethodcanbeusedtodecorateand/orassemblemetalnanoparticlesor
fractalpatternsalongSWCNTswithdifferentnovelmetalssuchasgold,silver,andcopperandcanbeexploitedinvarioussensitive
applicationsforfundamentalresearchandnanotechnology.
Metal decoration on nanotubes has opened a new avenue beusedtofabricateAgNPs-SWNT-basedhybridnanostructures
of research with significant opportunities in various havingpotentialuseinvariousnanotechnologicalapplications.
fields.1-10Onemainpurposeofthedecorationofnanotubesis Our simple method to decorate SWCNTs is depicted sche-
to make a sensitive hybrid device in which metalnanoparticles maticallyinFigure1AandexplainedinExperimentalMethods.
act as reactive sites for the adsorption of the chemical and/or At first, SWCNTswere grown on highly doped SiO /Si(100)
2
biological species. In particular, the assembly of carbon nano- wafersbyachemicalvapordepositionmethodasdescribed25and
tubes (CNTs) and silver nanoparticles (Ag NPs) has specific used26 previously in our study. Figure 1B shows the pristine
interest as a new hybrid material because its recent potential SWCNTs connected to the cathode during the nanoparticle
applications have been found useful in the study of SERS,8,11 deposition process. After an application of 500 mV potential
antimicrobial agents,12 pH sensors,13 gas sensors,14 electrical betweentheelectrodesfor19min,thedevicewastakenoutof
conductivityenhancement,15electrocatalyticactivity,16etc.Sev- theDIwater,driedundernitrogengas,andsubsequentlyimaged
eralmethodsofmetaldecorationonCNTshavebeenreported17 in FESEM. The corresponding Ag-decorated nanotubes are
and reviewed.9,18 Electrodeposition is the most popular tech- shown in Figure 1C. The images of SWCNTs before
nique used so far for the selective decoration of nanoparticles (Figure 1B) and after the deposition (Figure 1C) clearly
alongCNTs.19-24ThesemethodsdecorateCNTswithmetalsbut
requiretheuseofchemicalsand/orcatalysts.Inthiscontext,we
reportanew,simple,chemical-andcatalyst-freetechniqueprovid- Received: October16,2010
ingdifferentdensitiesofAgNPsontheSWCNTnetworkthatcan
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OCT 2010 2. REPORT TYPE 00-00-2010 to 00-00-2010
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Controlled Assembly of Ag Nanoparticles and Carbon Nanotube Hybrid
5b. GRANT NUMBER
Structures for Biosensing
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
Rensselaer Polytechnic Institute,Department of Materials Science and REPORT NUMBER
Engineering,Troy,NY,12180
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13. SUPPLEMENTARY NOTES
14. ABSTRACT
Here we report a chemical-free, simple, and novel method in which a part from a silver-based anode is
controllably used in a straightforward manner to produce silver nanoparticles (Ag NPs) in order to
fabricate a controlled assembly of Ag NPs and single walled carbon nanotube (SWCNT) hybrid structures.
The attachment and distribution of Ag NPs along SWCNTs have been investigated and characterized by
field emission scanning electron microscopy (FESEM). We have achieved the decoration of SWCNTs with
different densities of Ag NPs by changing the deposition time, the applied voltage and the location of
carbon nanotubes with respect to the anode. At low voltage, single silver nanoparticle is successfully
attached at the open ends of SWCNTs whereas at high voltage, intermediate and full coverage densities of
Ag NPs are observed. As voltage is further increased, fractals of Ag NPs along SWCNTs are observed. In
addition, a device based on a Ag NPs-SWNT hybrid structure is used for the label-free detection of ssDNA
molecules immobilized on it. We believe that the proposed method can be used to decorate and/or assemble
metal nanoparticles or fractal patterns along SWCNTs with different novel metals such as gold, silver, and
copper and can be exploited in various sensitive applications for fundamental research and
nanotechnology.
15. SUBJECT TERMS
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ARTICLE
coverage with Ag NPs is observed near the cathode region
(Figure 2E,F) facing to the anode. We have observed full
coveragewithsilverparticles,whichisequivalenttoanewclass
of hybrid materials composed of metallic nanowire and
SWCNTs.Moredensepatternswereobservedatthetipofthe
SWCNTs attached to the cathode (Figure 2E,F). Importantly,
we observe that the nanotubes that are not connected to the
electrodesremainunaffectedbythedecoration,confirmingthat
the process is directed by the electric field. Figure 2C-F
demonstrates that different particle densities can be obtained
alongtheSWCNTsbasedonthedistributionofelectricfield.
Wealsoinvestigatedtheuseofhighervoltagesonthedecora-
tion process of the SWCNT network. Higher external potential
causestheAgNP-based aggregatesto growintofractalpatterns
alongtheSWCNTs(Figure3).ThepresenceofSWCNTscanbe
observed in the fractal patterns as shown by the arrows in
Figure1. Attachmentofsilvernanoparticlesonsinglewalledcarbon Figure3B-E. Here Ag NPs formclustersof fractal patterns on
nanotubes.(A)Aschematicpresentationofsilvernanoparticle(orange theSWCNTswhichactasdividersforthegrowthpatterns,andthe
color) attachmentunderanelectric field.FESEMmicrographs repre- attachedAgNPsmightactasthepointofnucleationgrowth.Two
sentingtheSWCNTnetwork(B)beforeand(C)afterthedecoration
typesoffractalpatternswereclearlyvisibleasshowninFigure3.
with Ag NPs. Inset: The magnified image of the dotted rectangular Thefirsttypeofpatterns,showninFigure3A-C,wasobserved
portioninC.Thescalebarrepresents1μmlength.
after the application of 1 V external potential during 10 min.
demonstratetheproofoftheprincipleofthismethod,indicating Increased voltage causes a higher density of Ag NPs along the
aselectivedecorationofSWCNTswithAgNPs. SWCNT, indicating a fast and multistep process. Consistent
AsinglenanoparticleattachmentatthetipoftheSWCNTis growth of these patterns was directed along the nanotubes,
observed at 250 mV as shown by a white dotted circle in keepingthenanotubesatthecenter.
þ
Figure 2A. Here the distance between two electrodes was set Duringthedeposition,someAg ionsalsodepositattheAg-
to100μm.Thepotentialwasappliedfor19min.Itisverylikely cathode and nucleate to form nanoparticles. These freshly
to have a single particle decoration at the free end (where the deposited Ag NPs at the Ag-cathode can be ejected back from
electricfieldishigh)oftheSWCNTwhoseoneendisclamped the cathodetorestore the anode by reversing the biasesat the
and connected to the cathode, which is described in Experi- electrodesforashortduration.WefoundthattheseAgNPscan
mentalMethods.InFigure2B,onenanotubeclearlyshowsthe alsobeutilizedtoformrapidandcontrolledassemblyoffractal
nanoparticle attachment atthe tip whereas theothernanotube patterns along and around the SWCNTs. Fractal patterns thus
doesnotshowanyattachmentofsilvernanoparticle.Thiscould formedarenotedasthesecondtypeofpatternsandareshown
beduetothebrokenconnectionofnanotubes.Heretheelectric in Figure 3D-F. These patterns were formed by reversing
field distribution between two metal electrodes in DI water the polarity of the electrodes of a sample that had undergone
influencesaswellasdirectsthenanoparticleattachmentonthe theintermediatecoveragedecorationprocessat500mVandthe
SWCNTs.Recentlywehavealsoobservedthattheelectricfield fractalformationfor10sdepositiontimewith1.5Vreversebias.
distributionbetweentwoelectrodesplayanimportantroleinthe Theseresultsdemonstratethatthepresentapproachcanbeused
growthofvariousCu Onanostructures.27 for the synthesis of Ag fractal patterns along the SWCNTs.
2
To achieve an intermediate and full coverage of the decora- FractalpatternsalongSWCNTscanhaveusefulapplicationsin
tion,wehaveincreasedthepotentialfrom250mVto500mV, SERS-basedsensorsanddetectors.
keepingthedepositiontimethesame(19min)asitwasforthe Finally,theAgNP-decoratedSWCNTswereusedtofabricate
attachmentofasingleparticle.Wehavemonitoredtheformation two-terminal electrical devices for biosensing applications
of the Ag NP assembly on SWCNTs at different places with (Figure 4A). Current-voltage (I-V) measurements were car-
respecttotheelectrodes.Becausetherearedifferentnanotubes riedoutonthehybriddevicestoprobetheelectricaldetectionof
attached at different places of the cathode, the electric field immobilizedDNAmolecules.WemeasuredtheI-Vcharacter-
distribution depends on the specific position of the SWCNTs isticsforthedevicetreatedwith(i)highsaltbuffer,(ii)DNAin
withrespecttotheanode.Notethatthecathodeandtheanode highsaltbuffer,(iii)deionizedwatertorinseandclean,and(iv)
are of rectangular shapes (300 (cid:1) 100 μm2). Here the effective DNAinhybridizationbufferinordertoruleoutanychangesin
electricfieldplaysanimportantroleontheAgNPsattachmentto itsresistancevaluesduetothesaltbuffersonly.Theblackcurve
SWCNTs. In our results, geometry of the metal pads inFigure4Drepresentstheelectricalcharacterizationoftheas-
(electrodes)andtherespectivepositionofthenanotubesenable fabricatedhybriddevicewitharesistanceof170kΩpriortoany
thedifferentparticledensitiesalongtheSWCNT.Figure2Cand treatmentwithDNAmolecules.Afterincubationwithhighsalt
2DshowsthecoveragewithafewparticlesalongtheSWCNTs. buffer only, the device was dried with nitrogen gas. The
Note that these nanotubes were selected from the particular corresponding electrical response presented by the red curve
locationwherethedensityofAgNPswaslow.Alsothereisthe showsanincreaseintheresistanceto∼190kΩ.Thesamedevice
possibilityofsemiconductingnanotubesintheSWCNTnetwork wasagain washedwithDIwater andtreated withthethiolated
thatmightshowarelativelylowerdensityofparticles. ssDNAinhighsaltbuffer.Aftera2htreatment,thedevicewas
Theabove-mentioneddevicewiththesameparametersasin driedwithnitrogengasandresistancewasmeasuredtobe∼335
the case of medium coverage with Ag NPs also shows high kΩ, as shown by the green curve. In order to determine the
coverage of particles as shown in Figure 2E and 2F. The high resistance due only to ssDNAs, the device was extensively
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Figure2. ControlleddecorationwithAgNPsonSWCNTs.(AandB)Attachmentofasinglesilvernanoparticleattheopenend(unclamped)of
SWCNTs. The circular dotted regions highlight the specific attachments. Intermediate decoration (C and D) and full coverage (E and F) with
nanoparticlesalongSWCNTs.Thescalebarrepresents1μmlength.
washedwithDIwateranddriedwithnitrogengas.Notethatafter
thecleaningonlythiolatedDNAshouldbepresentonthedevice,
whereasthenonspecificallyboundDNAaswellasthebuffersalts
shouldberemoved.Inthiscasewefoundtheresistancevaluetobe
around220kΩ.Weassumedthatthisvaluerepresentsthedevice
resistancewithonlyssDNAattachedviatheAgNPstothenanotube.
Additionalexperimentswereperformedwith1xSSC(saline-
sodiumcitrate)hybridizationbuffer.Thedevicewasincubatedin
this buffer for2h anddried with nitrogen gas.The changesin
electricalresistanceareshownbythegraycurveindicating(698
kΩ) a further increase in resistance as compared to the green
curve measured with ssDNA under high salt buffer conditions.
The device was rinsed carefully twice with DI water and dried
with nitrogen gas. The measured electrical response on the
washeddeviceisshownbythecyancurve.Measuredresistance
ofthedevicewas∼226kΩ,whichisveryclosetotheresistance
ofthedevicewithssDNAsattachedviaAgNPsaloneabove(220
kΩ, blue curve), confirming the label-free detection of ssDNA
moleculesimmobilizedonthishybriddevice.
In conclusion, we have shown a controlled and selective
decoration process of SWCNTs with Ag NPs by introducing a
self-sacrificiallayer-basedtechniqueanduseditforthelabel-free Figure 3. High coverage with Ag NPs along the SWCNTs. (A-C)
detection of ssDNA molecules accurately under different buffer FESEMimagesrepresentingfractalnanopatternformationwithAgNPs
alongtheSWCNT.(DandF)FESEMmicrographsoftwo-dimensional
conditions. The approach represents a simple and catalyst- and
fractalpatterns.ThearrowsindicatethepositionofSWCNTsattached
chemical-freeelectrodepostionofAgNPsonSWCNTs,leadingto tothepatterns.Thescalebarrepresents1μmlength.
aclean,activesurfaceatapredefinedsubstratelocationthatcanbe
potentiallyusedinawidevarietyofsensoranddetectorapplica-
tions.Suchhybrid/fullydecoratedSWCNTscanbeusedasmetal network, electrical pads were patterned by conventional lithography
nanowires for applications in biosensing or fractal formations. followedbythemetallizationusingane-beamdepositionsystem.The
Furtherinvestigationsofsuchnanobiohybriddevicescoupledwith electrodesareofrectangularshapewitharea∼300(cid:1)100μm2,thickness
microfluidicsmayprovidemoresensitiveandspecificapproaches ∼100 nm. As in our previous study, we used deionized (DI) water
forDNAhybridizationdetectionandrealtimedetectionofsingle (resistivity∼18MΩ-cm,5mL)asanelectrolytefortheelectrodeposi-
basepairmismatchesinliquidenvironments. tionprocess.Inthismethod,oneendoftheSWCNTswereelectrically
contactedbya100nmsilverlayerontopofanadhesionlayerof5nmTi.
Here, SWCNTs were used as the template (defined as the cathode
’EXPERIMENTALMETHODS electrode) for the deposition, and an additional silver contact pad,
definedastheanodeelectrode,servedasthedepotfortheAgNPs.Note
Experimental Details. The size of the substrates was ∼8 (cid:1) 8 thatthesilveranodeusedasasacrificiallayer,andonlyasmallportion,
mm2. At a specific location, defined by the density of the SWCNT wascontributingtothemetaldecoration(Figure1A),andwefoundthat
C dx.doi.org/10.1021/ja1093327|J.Am.Chem.Soc.XXXX,XXX,000–000
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Figure4. BiosensingwithaAgNPs-decoratedSWCNThybriddevice.(A)FESEMmicrographofaAgNPs-SWCNT-basedhybridtwo-terminal
electricaldevice.(B)ZoomedimageoftheselectedportionindicatedbywhitedottedsquareregioninA.(C)SchematicsofimmobilizedssDNAonAg
NPs-SWCNT-basedhybriddevices.(D)Thecurrent-voltage(I-V)characteristicsofthedeviceunderspecificbiologicalconditions(asindicatedin
thegraph).Thescalebarrepresents1μmlength.
thismetallayerwasmorethansufficientforthecoatingwithAgNPsat 5 nm Ti layer. The Keithley SMU was used to measure the I-V
differentdensitiesontheSWCNTnetwork. characteristicsthroughthedevice.Anewlyfabricatedhybriddevicewas
Electrodeposition was carried out by applying a potential between incubatedwithadropofssDNAmoleculesfor2h.Specificimmobiliza-
twoelectrodesasexplainedabove.Thesizeoftheelectrodesandthe tionofthiolatedssDNA(sequence:50thiol_TCATACAGCTAGATA
distance between the two electrodes were kept constant while the ACCAAAGA)wascarriedoutinhighsaltbuffer,1MNaCl,phosphate
potentialwasvariedfortheresultspresentedinthisstudy.AKarl-Suss buffer 10 mM, pH 7.4. In the incubation step, enough humidity was
micromanipulatoraccompaniedbyaKeithley4200sourcemeasureunit maintainedtopreventthedropfromdrying.
(SMU) was used for micromanipulation, contacting electrodes, and
controllingvoltagesforthesynthesisandalsotomeasurethecurrent-
voltagecharacteristicsinthisstudy.Priortoinducinganyvoltage,the ’ASSOCIATEDCONTENT
micromanipulatorprobeswereconnectedtothepadsofthesamplethat
b
wasimmersedinDIwaterinordertoconnectcathodeandanode. S SupportingInformation. Atablepresentingthedecora-
Underanelectricfieldinwateratthepositiveterminalanode,silver tiontimeforformingvariousdensitiesofAgNpsalongSWCNTs
atoms move out from the metal as Ag ions, leaving electrons in the andanAFMimageofasinglenanoparticle-decoratedSWCNT.
electrode(Figure1A).Thedifferenceinpotentialbetweentheanode ThismaterialisavailablefreeofchargeviatheInternetathttp://
andcathodegivesthedrivingforcetotheseionstoflowfromanodeto pubs.acs.org.
cathode.TheAgionsreleasedfromthesilveranodeaccumulateonthe
SWCNTsconnectedtothecathodewherereductionoccurs.Releaseof ’AUTHORINFORMATION
particlesfromthepositiveterminaldependsontheappliedpotentialand
thusitaffectsthecoveragewithAgNPsontheSWCNTs. CorrespondingAuthor
DuringSWCNTdecorationwithAgNPs,wehaveobservedthatthe [email protected];[email protected]
coverageofparticlesdependsonthedistancebetweenthetwoelectro-
AuthorContributions
des, applied bias voltage, deposition time, and the location of the
SWCNTsattachedtothecathodewithrespecttotheanode.Different )Theseauthorscontributedequallytothiswork.
potentialswereappliedtoachieveaverylowdensity(oneparticle)toa
veryhighdensityofnanoparticles.Rapidself-assemblyofAgNPsintoa
fractalpatternwasobservedathighvoltages.
’ACKNOWLEDGMENT
Biosensing Device Fabrication. The silver cathode already
attached to one end of the hybrid structure was used as one of the FinancialsupportwasreceivedfromRPI,theIFCNewYorkat
metal contacts. The other contact was made by e-beam lithography RPI,ArmyResearchLab,andtheNSFNSECtopursuethiswork
followedbyane-beamdepositionofa100nmAulayerontopofthe andishighlyappreciated.
D dx.doi.org/10.1021/ja1093327|J.Am.Chem.Soc.XXXX,XXX,000–000
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ARTICLE
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