A novel method for the functionalization of γ-irradiated single wall carbon nanotubes with DNA

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Nanotechnology 20 (2009) 445602 (7pp) doi1010880957-44842044445602

A novel method for the functionalizationof γ -irradiated single wall carbonnanotubes with DNAS P Jovanovic1 Z M Markovic1 D N Kleut1 N Z Romcevic2V S Trajkovic3 M D Dramicanin1 and B M Todorovic Markovic1

1 Vinca Institute of Nuclear Sciences POB 522 11001 Belgrade Serbia2 Institute of Physics POB 68 11001 Belgrade Serbia3 Institute of Immunology and Microbiology School of Medicine University of BelgradeDr Subotica 1 Belgrade Serbia

E-mail svetlanajovanovicvincars

Received 19 June 2009 in final form 25 August 2009Published 5 October 2009Online at stacksioporgNano20445602

AbstractIn this work we describe a novel method for highly efficient functionalization of single wallcarbon nanotubes (SWCNTs) by DNA wrapping Exposure of SWCNTs to γ -irradiation(50 kGy) has lowered by one order of magnitude the amount of single strandeddeoxyribonucleic acid (ssDNA) required for SWCNT modification The resulting hybrids ofγ -irradiated SWCNTs and ssDNA were characterized by optical absorbance spectroscopyRaman spectroscopy and Fourier transform infrared spectroscopy Atomic force microscopywas used to investigate the morphology of hybrids While γ -irradiation in three different mediahas significantly improved the process of SWCNT dispersion irradiation in ammonia was themost efficient The γ -irradiated SWCNTs functionalized with ssDNA were stabilized byelectrostatic forces This preliminary study suggests that γ -irradiation can significantly improvethe functionalization of SWCNTs with DNA

(Some figures in this article are in colour only in the electronic version)

1 Introduction

Carbon nanotubes (CNTs) are sheets of graphite that havebeen rolled into tubes [1] Because of their structureCNTs are extremely hydrophobic which is the main problemfor the application of these materials in biomedicine [2ndash6]electronics [7] and many other fields of technology [8 9]The problem of utilizing the extraordinary properties of singlewall carbon nanotubes (SWCNTs) in bulk form is not yetsolved [10ndash12] Strong intermolecular van der Waals forcesover a great contact surface are the main reason for thedifficulty in isolating SWCNTs from the bundles In order toeliminate or considerably reduce this interaction researchersuse covalent modification to attach polar functional groups tothe nanotube wall [13] Alternatively modification of CNTscan be achieved through non-covalent adsorption of variousfunctional molecules to the nanotubesrsquo sidewalls [14] Amonga vast number of polymers used for this purpose DNA is oneof the most efficient [15]

Energetic particles such as electrons [16] protons [17] andatomic ions [18] can be used to form defects in the sidewalls ofnanotubes It was noticed that during irradiation of CNTs theyundergo structural modifications bending folding kinkingradial deformation shrinkage of diameter coalescence andcutting of the electronic transport modification [19] andfield emission [20] properties of the nanotubes Alsoboth SWCNTs and multi-wall carbon nanotubes (MWCNTs)undergo the process of amorphization [21] Khare et alreported that exposure of SWCNTs to 1 MeV proton irradiationresulted in CndashH bond formation [22] Guo et al concludedthat the solubility of γ -irradiated MWCNTs in organic solventswas considerably enhanced compared to that of non-irradiatednanotubes [23] Skakalova et al found that defect formationafter γ -irradiation could lead to cross-linking of SWCNTs inbundles while electrical conductivity of irradiated nanotubeswas enhanced by factor of 45 [24] Hulman et al establishedthat the increase in G-band intensity is ascribed to a softeningof the q = 0 selection rules rather than to the initialization of

0957-448409445602+07$3000 copy 2009 IOP Publishing Ltd Printed in the UK1

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

q gt 0 phonon scattering of the double-resonance model [25]Wu et al developed a simple method for one-step decorationof MWCNTs with Ag nanoparticles via covalent grafting ofthe polymer to the surface of the MWCNTs and simultaneousreduction of Ag+ under γ -ray irradiation [26]

Owing to the close packing of SWCNTs in bundles thenanotube surface cannot be reached by modifying reagents Inorder to provide uniform modification of the nanotube wallit is necessary to split nanotubes from the bundle as a pre-treatment in functionalization of SWCNTs In this workwe used γ -irradiation as a pre-treatment in the process offunctionalization of SWCNTs with DNA In order to attachdifferent polar functional groups to the sidewalls of SWCNTswe utilized γ -irradiation of SWCNT dispersions in water a30 solution of ammonia and SWCNTs in air After γ -irradiation dried SWCNTs were functionalized with DNAAtomic force microscopy (AFM) measurements were usedto detect the size and morphology of pristine and modifiednanotubes The effects of high energy γ -irradiation on theelectronic structure of SWCNTs were studied by Ramanspectroscopy In addition we used Fourier transform infrared(FTIR) spectroscopy to examine covalent modifications of thenanotubesrsquo sidewalls At present this method is not stillefficient in the preparation of dispersions with well isolatedSWCNTs

2 Experimental details

Purified SWCNTs (95 purity) purchased from BuckyCorporation (Houston TX USA) were mixed (100 mg) with30 ml of MilliQ water or 30 ml of 30 ammonia solution Thethird sample was SWCNTs in an air environment Preparedsamples were irradiated by a γ -ray flux from 60Co nuclideswith photon energy of 13 MeV (Centre of Irradiation VincaInstitute of Nuclear Sciences) The samples were exposed toγ -irradiation at a dose of 50 kGy After irradiation treatmentsamples of nanotubes in aqueous liquids were dried in airat room temperature A solution containing 1 mg DNA(salmon sperm DNA Sigma-Aldrich St Louis MO USA)in 100 ml of distilleddeionized water was heated for 5 h at90 C to obtain single stranded DNA (ssDNA) Pristine andγ -irradiated SWCNT powder (25 mg) was added to 100 mlof the DNA solution The γ -irradiated SWCNTssDNA(γ -SWCNTssDNA) dispersions were sonicated (750 Wultrasonic bath) for 3 h After sonication all samples werecentrifuged at 4000 rpm for 1 h For each sample the well-dispersed supernatant was taken for characterization leavingbehind the precipitate

The UV spectra of nanotubessDNA dispersions werescanned within the wavelength range 200ndash500 nm using anAvantes UVndashvis spectrophotometer All UV measurementswere carried out at 20 C and automatically corrected for thesuspending medium (water) In order to determine the amountof unreacted ssDNA samples were centrifuged at 8000 rpm for30 min Clear supernatant was carefully decanted Absorptionsat λ = 264 nm were compared for stock solution of ssDNA andcollected supernatant

Concentrations of all γ SWCNTssDNA dispersions weremeasured using a gravimetric method 10 ml of eachdispersion was dried at a room temperature in the airand the mass was measured Values of concentrationswere obtained as the average concentration value of threedispersions The obtained concentrations were 60 plusmn12 mg lminus1 585 plusmn 11 mg lminus1 and 694 plusmn 21 mg lminus1 forSWCNTs irradiated in air and functionalized with ssDNA(γ -air-SWCNTssDNA) SWCNTs irradiated in water andfunctionalized with ssDNA (γ -water-SWCNTssDNA) andSWCNTs irradiated in ammonia and functionalized withssDNA (γ -ammonia-SWCNTssDNA) respectively Theconcentration of pristine SWCNTssDNA at a ratio of 25 mgSWCNTs and 1 mg ssDNA was 1551 plusmn 0 mg lminus1

Raman spectra of pristine and modified SWCNTs wereobtained with a Micro Raman Chromex 2000 using 532 nm ofa frequency doubled NdYaG laser with power of 2 mW Thespectral resolution was 4 cmminus1 Raman spectra were recordedat room temperature Samples of dried modified nanotubeswere pressed into the shallow hole of an indium substrate

For the FTIR spectroscopic analysis nanotube dispersionswere dried at 60 C Dried nanotubes were mixed withKBr powder and pellets were formed FTIR spectra weremeasured at room temperature in the spectral range from 400to 4000 cmminus1 on a Nicolet 380 FTIR spectrometer (ThermoElectron Corporation)

Atomic force microscopy (AFM) measurements wereperformed using a Quesant microscope operating in tappingmode in air at room temperature In tapping mode thecantilever oscillates close to resonance and the tip only slightlytouches the surface [27] Diluted dispersions of nanotubeswere deposited on a mica substrate and imaged after dryingStandard silicon tips (Nano and More Wetzlar Germany) witha force constant of 40 N mminus1 were used The accuracy ofthe AFM determination of the mean diameter was improvedby deconvolution The freshly cleaved mica has a very smallroughness (mean roughness 012 nm) favouring the formationof aggregates during drying of the thin layer of colloid due tocapillary forces

In order to explore the stabilization mechanism we added1 NaCl to each nanotube-based dispersion

3 Results and discussion

31 UV spectroscopy

Figure 1 shows the UV absorption spectra of γ SWCNTndashssDNA complexes A well known property of ssDNA is itsstrong absorption at 264 nm Accordingly SWCNTsssDNAdisplayed one broad band at 266 nm Nanotube functionaliza-tion caused shifting of the ssDNA band from 264 to 252 nm(for γ -water-SWCNTssDNA) or 245 nm for γ -ammonia-SWCNTssDNA and γ -air-SWCNTssDNA Therefore it isclear that ssDNA forms stable complexes with γ -SWCNTs

Absorption measurements have shown that clear su-pernatant of γ -water-SWCNTssDNA γ -air-SWCNTssDNAand γ -ammonia-SWCNTssDNA contains 54 48 and 44of unreacted ssDNA SWCNTs irradiated in ammonia interactmost efficiently with ssDNA compared to other samples

2

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 1 UV spectra of (1) ssDNA (2) pristine SWCNTsfunctionalized with ssDNA (3) γ -air-SWCNTs functionalized withssDNA (4) γ -water-SWCNTs functionalized with ssDNA(5) γ -ammonia-SWCNTs functionalized with ssDNA The spectrahave been vertically displaced for clarity but are otherwise plotted onthe same vertical scale

32 Raman spectroscopy

Raman spectroscopy is a powerful tool for investigationof the vibration properties and electronic structure ofSWCNTs [28 29] In figure 2 Raman spectra ofpristine γ -irradiated and ssDNA-functionalized SWCNTsare presented The laser energy was 241 eV at whichdominantly semiconducting nanotubes are resonant For532 nm excitation RBM signals in the region of 130ndash150150ndash215 and 230ndash300 cmminus1 originate from the van Hoveelectronic transitions of semiconducting SWCNTs and metallicSWCNTs respectively In the 1230ndash1750 cmminus1 region twoRaman bands are observed a relatively broad band nearsim1300 cmminus1 (D-band) and a strong band with structure inthe 1550ndash1600 cmminus1 (G-band) [30] The D-band is knownas the disorder or defect mode because a defect is requiredfor the elastic scattering Therefore the appearance of the D-band indicates the presence of defects in the sidewalls of theSWCNTs A minor increase in the intensity of the D-band wasobserved after γ -irradiation The G-band at 1590 cmminus1 is atangential shear mode of carbon atoms that corresponds to thestretching mode in the graphite plane

The positions of the D- and G-bands and the IDIG ratioare presented in table 1 As can be seen from the table theRaman spectra of SWCNTs irradiated in three different mediashow a slight increase in the IDIG intensity These resultsimplicate the presence of defects and sp3 carbon atoms in thesidewalls of SWCNTs The intensity of the D-band furtherincreased with the DNA functionalization of γ -irradiatednanotubes (table 1) indicating that the polymer backbone iswrapped around nanotubes rather than inserted into defects inthe nanotube wall This is consistent with the fact that a highlydisordered and reactive coiled structure is a common feature ofssDNA obtained upon DNA denaturation [31]

It has previously been reported that γ -irradiation cancause perforation of the sidewalls of SWCNTs Focused

Figure 2 Raman spectra of SWCNTs γ -irradiated at a dose of50 kGy in different media (1) Raman spectrum of pristine SWCNTs(2) pristine SWCNTsssDNA (3) γ -water-SWCNTsssDNA(4) γ -ammonia-SWCNTsssDNA and (5) γ -air-SWCNTsssDNAThe spectra have been vertically displaced for clarity but areotherwise plotted on the same vertical scale

Table 1 Positions of corresponding peaks (D and G) and the ratioIDIG of pristine irradiated and DNA-functionalized carbonnanotubes (E = 24 eV)

IDIG D (cmminus1) G (cmminus1)

Pristine SWCNTs 0118 1349 15883Pristine SWCNTsssDNA 0120 1344 1588γ -water-SWCNTs 0146 13450 15864γ -ammonia-SWCNTs 0130 13373 15864γ -air-SWCNTs 0141 13462 15851γ -water-SWCNTsssDNA 0151 1358 1587γ -ammonia-SWCNTsssDNA 0141 1338 15828γ -air-SWCNTsssDNA 0144 13463 1590

electronic beams knock out carbon atoms from the nanotubewalls leading to defect formation on the surface of thenanotubes [32] Telling et al [33] have theoretically discussedthe stabilization of the products of γ -irradiation on a graphitelattice suggesting that vacancies produced by γ -rays arestabilized by creating pentagonndashheptagon defects and pushingone carbon atom out of the graphene plane

Our data obtained by Raman spectroscopy are consistentwith the ability of γ -irradiation to form defects on the sidewallsof CNTs Furthermore the adsorption of ssDNA moleculesto the sidewall of irradiated SWCNTs increased the degreeof disorder at the nanotube surface These results are ingood agreement with UV measurements (figure 1) and FTIR(presented below)

33 FTIR spectroscopy

In order to determine structural changes in SWCNT moleculesafter γ -irradiation and functionalization with ssDNA weused FTIR spectroscopy For determination of ssDNAmolecules by FTIR spectroscopy the most important region is800ndash1800 cmminus1 As shown in figure 3 the main absorptionbands of DNA are at 966 cmminus1 corresponding to the CndashC structure of the ssDNA backbone 1060 cmminus1 of the PndashO

3

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 3 FTIR spectroscopy of pristine SWCNTs (1) pristineSWCNTsssDNA (2) ssDNA (3) γ -air-SWCNTsssDNA (4)γ -ammonia-SWCNTsssDNA (5) and γ -water-SWCNTsssDNA (6)The spectra have been vertically displaced for clarity but areotherwise plotted on the same vertical scale

or CndashO backbone stretch 1219 cmminus1 for antisymmetric PO2

stretching 1278 1379 1422 and 1480 cmminus1 of base sugarmoieties 1536 cmminus1 of cytosine and 1696 cmminus1 of C=C andC=N stretching in base planes (curve 3) Curve 1 (FTIRspectrum of pristine SWCNTs) shows peaks at 1607 cmminus1

which stem from aromatic C=C stretching bonds as well asat 2850 and 2920 cmminus1 originating from CndashH bonds [34] ThessDNA wrapping and γ -irradiation induce dramatic changesin the infrared spectra of pristine SWCNTs Curve 2 (pristineSWCNTssDNA) shows peaks at 1060 1219 and 1696 cmminus1

which stem from ssDNA as well as small peaks at 2850and 2920 cmminus1 which originate from CndashH bonds The FTIRspectrum of functionalized pristine SWCNTs did not showthe presence of amino groups Spectra 4 5 and 6 (FTIRspectra of SWCNTs irradiated in three different media) exhibitbroad bands around 650ndash680 cmminus1 which can be interpreted ascharacteristic for the amine bond (NndashH) Strong bands at 10441385 and 1621 cmminus1 in spectra of γ -water-SWCNTssDNA(curve 6) are evidence of adsorption of ssDNA on the sidewallsof the SWCNTs A broad band at 3404 cmminus1 correspondsto OndashH stretching in the γ -water-SWCNTs The spectrumof γ -air-SWCNTssDNA (curve 4) shows bands at 1056 and1643 cmminus1 which also indicate attachment of ssDNA toSWCNTs These results provide evidence for the interactionof ssDNA with the sidewalls of γ -irradiated SWCNTs

34 Atomic force microscopy

The morphology of the different dispersions of γ -irradiatedSWCNTs functionalized with ssDNA was examined by AFMThe lengths and diameters of γ -SWCNTssDNA hybrids weredetermined using the Quesant SMP program

The AFM image in figure 4 shows a wide area of ssDNAThe agglomerated ssDNA molecules can be seen as ellipticaland spherical structures on the mica substrate Most of thespherical structures have nearly the same diameter around25 nm

Figure 4 Top view AFM image of a dried aqueous solution ofssDNA Elliptical and spherical structures were recognized as theassembly of ssDNA molecules

The γ -ammonia-SWCNTs are dispersed in tetrahydrofu-ran (THF) and imaged by AFM (figures 5(a) and (b)) As canbe seen on the AFM image bundles of γ -irradiated SWCNTsare very straight with one branch at the end The average di-ameter of SWCNT bundles was 44 nm The surface of theSWCNTs was not uniform and not completely smooth Theprocess of γ -irradiation has improved the solubility of nan-otubes in THF due to a major reduction in the size of the bun-dles compared to pristine nanotubes dissolved in THF [35]

AFM images of pristine and γ -irradiated SWCNTsfunctionalized with ssDNA are shown in figures 5(c)ndash(j) It canbe seen that SWCNTs have a rugged wavy surface probablyas a consequence of adsorption of ssDNA Large scale AFMimages of all specimens (figures 5(a) (c) (e) (g) (i)) showa lot of bundles of SWCNTs Figure 5(d) shows entangledbundles of DNA-functionalized pristine nanotubes with anaverage bundle diameter of 65 nm Figure 5(f) is a higherresolution image of a single γ -water-SWCNTssDNA structurewith a length of 815 nm and diameter of 30ndash45 nm Thetwo spherical structures detected on the surface of bundle ofSWCNT are probably the aggregates of ssDNA moleculesformed during sonication A representative image of SWCNTsirradiated in air and dispersed with ssDNA is shown infigure 5(h) The average diameter of these nanotube bundles is71 nm and the length is 1200 nm The DNA-wrapped SWCNTbundles irradiated in ammonia are shown in figures 5(i) and (j)The nanotube bundle shown had a diameter of 172 nm and alength of 650 nm Also a spherical agglomerate of ssDNAmolecules can be observed at one end of the bundle Based ondata obtained from the large scale AFM image (figure 5(a)) itwas determined that the lengths of pristine SWCNT bundleswere in the range from 2ndash5 μm while the length of γ -irradiated SWCNT bundles functionalized with DNA was lessthan 2 μm

Therefore the AFM study has shown that γ -irradiationreduces the length and diameter of the SWCNT bundlesobtained after subsequent functionalization with DNA Thisseems particularly true for SWCNTs irradiated in the presenceof ammonia which had the smallest size

4

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 5 (a) Large scale AFM image of γ -ammonia-SWCNTs dissolved in THF (b) Representative region of γ -ammonia-SWCNTsdissolved in THF (c) Large scale AFM image of pristine SWCNTs functionalized by ssDNA (d) Representative region of pristineSWCNTsssDNA (e) Large scale AFM image of ssDNASWCNTs irradiated in deionized water (f) Representative region ofssDNASWCNTs irradiated in deionized water (g) Large scale AFM image of ssDNASWCNTs irradiated in air (h) Representative region ofssDNASWCNTs irradiated in air (i) Large scale AFM image of ssDNASWCNTs irradiated in a 30 solution of ammonia (j)Representative region of SWCNTs irradiated in a 30 solution of ammonia and functionalized by ssDNA The irradiation dose was 50 kGy

35 Mechanism of stabilization

It is known that molecules of an amphiphilic polymer canbe wrapped around CNTs and form hybrids that are stablein an aqueous environment [36] DNA molecules are also

able to form complexes with SWCNTs [37] thus improvingsolubilization and the process of isolation of single nanotubesfrom a bundle In our experiment the amount of ssDNAnecessary for functionalization of SWCNTs was considerablysmaller (25 times lower) than in previous studies [37ndash40]

5

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 6 Suggested model of the mechanism for DNA-mediatedstabilization of SWCNTs irradiated in air (compound 1) Theintermolecular interaction between γ -air-SWCNTs and ssDNAresults in a formation of hybrid γ -air-SWCNTsssDNA(compound 2)

In order to promote formation of hybrid SWCNTsssDNAwe used γ -irradiation as a pre-treatment for functionalizationwith ssDNA Gamma irradiation causes radiolysis of watermolecules which leads to the formation of free radical(Hmiddot middotOH) and molecular (H2 H2O2) species [41] At lowdoses and room temperature the main radiolytic productof the radiolysis of liquid ammonia is ammonium azide(NH4N3) [42] but stable products such as N2H4 NHminus

2 and Nminus3

can also be formed [43] Molecules present in air undergo γ -ray irradiation and form reactive radical species and molecules(NO NO2) [44]

We applied γ -irradiation that was of low energy (50 kGy)in comparison with other studies [23 24] The reason forchoosing this energy was to conserve the pristine structureof the nanotubes Utilizing three different media as theenvironment for γ -irradiation of SWCNTs we covalentlyattached amino and hydroxyl groups depending on themedium Due to their great penetrating power γ -photonscause the binding of a polar functional group from theirradiated medium to the wall of the SWCNTs Theattached covalent groups cause a weakening of van der Waalsinteractions between CNTs in a bundle The subsequentreduction of hydrophobic forces in the bundles increases theefficiency of separation and isolation of single nanotubes

In the figure 6 we presented a possible mechanism forssDNA-mediated stabilization of SWCNTs irradiated in airWe assumed that ssDNA was wrapped around the SWCNTsaccording to the mechanism described above Covalentlyattached polar groups (hydroxyl and amino groups) arepresent at the sidewalls of the SWCNTs while ssDNAcovering the surface of SWCNTs forms π -interactions Ina double stranded DNA nucleotides from one chain areconnected with those from other chain by carbonylndashaminebonds Denatured ssDNA has many reactive groups that may

strongly interact with groups at the surface of γ -irradiatednanotubes (amine carboxyl carbonyl) Furthermore π -interactions are established between the nanotube surfaceand oligonucleotides Apparently the number of amine andcarbonyl bonds attached to a nanotube sidewall depends onthe medium in which the SCWNTs are irradiated Thelargest number of bonds is found in γ -ammonia-SWCNTs andthe smallest in γ -water-SWCNTs Accordingly the highestconcentration of nanotubes was observed in γ -ammonia-SWCNTssDNA suspension

The mechanism of stabilization of γ -SWCNTsssDNAwas tested by increasing the ionic strength of the dispersionsIt was noticed that the addition of 1 sodium chloride resultedin a precipitation of both pristine and irradiated ssDNA-functionalized SWCNTs which occurred within a few hoursFurther addition of sodium chloride accelerated precipitationThese results are consistent with the electrostatic mechanismof stabilization

4 Conclusion

In this study we investigated the effects of γ -irradiationon the efficiency of dispersion of SWCNTs by ssDNASWCNTs were irradiated with dose of 50 kGy in three mediawater ammonia and air Raman analysis has shown that γ -irradiation caused a slight increase in the IDIG ratio probablyreflecting the increase of disorder in the SWCNT structureFTIR spectroscopy has verified the disorder of the SWCNTsdetecting the presence of amino and carbonyl groups at thesidewalls of the SWCNTs AFM analysis has shown that γ -irradiation increases solubility and the reduction of size andlength of nanotube bundles Based on solubility tests themost efficient medium for the formation of reactive amine andcarbonyl groups is γ -irradiation of SWCNTs in a 30 solutionof ammonia The greatest advantage of this novel process forthe production of SWCNTssDNA dispersion was the use ofssDNA in a significantly smaller quantity At the moment thedescribed procedure is still unable to separate SWCNTs frombundles in significant amounts

Acknowledgment

This research was supported by the Ministry of Science of theRepublic of Serbia (project no 145073)

References

[1] Thostenson E Ren Z and Chou T 2001 Comput Sci Technol61 1899

[2] Lacerda L Bianco A Prato M and Kostarelos K 2006 AdvDrug Deliv Rev 58 1460

[3] Bianco A Kostarelos K and Prato M 2005 Curr Opin ChemBiol 9 674

[4] Yang W Thordarson P Gooding J Ringer S and Braet F 2007Nanotechnology 18 412001

[5] Foldvari M and Bagonluri M 2008 Biol Med 4 173[6] Hampel S et al 2008 Nanomedicine 3 175[7] Ahammad A Lee J and Rahman M 2009 Sensors 9 2289[8] Zhang S and Kumar S 2008 Small 4 1270

6

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 1 UV spectra of (1) ssDNA (2) pristine SWCNTsfunctionalized with ssDNA (3) γ -air-SWCNTs functionalized withssDNA (4) γ -water-SWCNTs functionalized with ssDNA(5) γ -ammonia-SWCNTs functionalized with ssDNA The spectrahave been vertically displaced for clarity but are otherwise plotted onthe same vertical scale

32 Raman spectroscopy

Raman spectroscopy is a powerful tool for investigationof the vibration properties and electronic structure ofSWCNTs [28 29] In figure 2 Raman spectra ofpristine γ -irradiated and ssDNA-functionalized SWCNTsare presented The laser energy was 241 eV at whichdominantly semiconducting nanotubes are resonant For532 nm excitation RBM signals in the region of 130ndash150150ndash215 and 230ndash300 cmminus1 originate from the van Hoveelectronic transitions of semiconducting SWCNTs and metallicSWCNTs respectively In the 1230ndash1750 cmminus1 region twoRaman bands are observed a relatively broad band nearsim1300 cmminus1 (D-band) and a strong band with structure inthe 1550ndash1600 cmminus1 (G-band) [30] The D-band is knownas the disorder or defect mode because a defect is requiredfor the elastic scattering Therefore the appearance of the D-band indicates the presence of defects in the sidewalls of theSWCNTs A minor increase in the intensity of the D-band wasobserved after γ -irradiation The G-band at 1590 cmminus1 is atangential shear mode of carbon atoms that corresponds to thestretching mode in the graphite plane

The positions of the D- and G-bands and the IDIG ratioare presented in table 1 As can be seen from the table theRaman spectra of SWCNTs irradiated in three different mediashow a slight increase in the IDIG intensity These resultsimplicate the presence of defects and sp3 carbon atoms in thesidewalls of SWCNTs The intensity of the D-band furtherincreased with the DNA functionalization of γ -irradiatednanotubes (table 1) indicating that the polymer backbone iswrapped around nanotubes rather than inserted into defects inthe nanotube wall This is consistent with the fact that a highlydisordered and reactive coiled structure is a common feature ofssDNA obtained upon DNA denaturation [31]

It has previously been reported that γ -irradiation cancause perforation of the sidewalls of SWCNTs Focused

Figure 2 Raman spectra of SWCNTs γ -irradiated at a dose of50 kGy in different media (1) Raman spectrum of pristine SWCNTs(2) pristine SWCNTsssDNA (3) γ -water-SWCNTsssDNA(4) γ -ammonia-SWCNTsssDNA and (5) γ -air-SWCNTsssDNAThe spectra have been vertically displaced for clarity but areotherwise plotted on the same vertical scale

Table 1 Positions of corresponding peaks (D and G) and the ratioIDIG of pristine irradiated and DNA-functionalized carbonnanotubes (E = 24 eV)

IDIG D (cmminus1) G (cmminus1)

Pristine SWCNTs 0118 1349 15883Pristine SWCNTsssDNA 0120 1344 1588γ -water-SWCNTs 0146 13450 15864γ -ammonia-SWCNTs 0130 13373 15864γ -air-SWCNTs 0141 13462 15851γ -water-SWCNTsssDNA 0151 1358 1587γ -ammonia-SWCNTsssDNA 0141 1338 15828γ -air-SWCNTsssDNA 0144 13463 1590

electronic beams knock out carbon atoms from the nanotubewalls leading to defect formation on the surface of thenanotubes [32] Telling et al [33] have theoretically discussedthe stabilization of the products of γ -irradiation on a graphitelattice suggesting that vacancies produced by γ -rays arestabilized by creating pentagonndashheptagon defects and pushingone carbon atom out of the graphene plane

Our data obtained by Raman spectroscopy are consistentwith the ability of γ -irradiation to form defects on the sidewallsof CNTs Furthermore the adsorption of ssDNA moleculesto the sidewall of irradiated SWCNTs increased the degreeof disorder at the nanotube surface These results are ingood agreement with UV measurements (figure 1) and FTIR(presented below)

33 FTIR spectroscopy

In order to determine structural changes in SWCNT moleculesafter γ -irradiation and functionalization with ssDNA weused FTIR spectroscopy For determination of ssDNAmolecules by FTIR spectroscopy the most important region is800ndash1800 cmminus1 As shown in figure 3 the main absorptionbands of DNA are at 966 cmminus1 corresponding to the CndashC structure of the ssDNA backbone 1060 cmminus1 of the PndashO

3

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 3 FTIR spectroscopy of pristine SWCNTs (1) pristineSWCNTsssDNA (2) ssDNA (3) γ -air-SWCNTsssDNA (4)γ -ammonia-SWCNTsssDNA (5) and γ -water-SWCNTsssDNA (6)The spectra have been vertically displaced for clarity but areotherwise plotted on the same vertical scale

or CndashO backbone stretch 1219 cmminus1 for antisymmetric PO2

stretching 1278 1379 1422 and 1480 cmminus1 of base sugarmoieties 1536 cmminus1 of cytosine and 1696 cmminus1 of C=C andC=N stretching in base planes (curve 3) Curve 1 (FTIRspectrum of pristine SWCNTs) shows peaks at 1607 cmminus1

which stem from aromatic C=C stretching bonds as well asat 2850 and 2920 cmminus1 originating from CndashH bonds [34] ThessDNA wrapping and γ -irradiation induce dramatic changesin the infrared spectra of pristine SWCNTs Curve 2 (pristineSWCNTssDNA) shows peaks at 1060 1219 and 1696 cmminus1

which stem from ssDNA as well as small peaks at 2850and 2920 cmminus1 which originate from CndashH bonds The FTIRspectrum of functionalized pristine SWCNTs did not showthe presence of amino groups Spectra 4 5 and 6 (FTIRspectra of SWCNTs irradiated in three different media) exhibitbroad bands around 650ndash680 cmminus1 which can be interpreted ascharacteristic for the amine bond (NndashH) Strong bands at 10441385 and 1621 cmminus1 in spectra of γ -water-SWCNTssDNA(curve 6) are evidence of adsorption of ssDNA on the sidewallsof the SWCNTs A broad band at 3404 cmminus1 correspondsto OndashH stretching in the γ -water-SWCNTs The spectrumof γ -air-SWCNTssDNA (curve 4) shows bands at 1056 and1643 cmminus1 which also indicate attachment of ssDNA toSWCNTs These results provide evidence for the interactionof ssDNA with the sidewalls of γ -irradiated SWCNTs

34 Atomic force microscopy

The morphology of the different dispersions of γ -irradiatedSWCNTs functionalized with ssDNA was examined by AFMThe lengths and diameters of γ -SWCNTssDNA hybrids weredetermined using the Quesant SMP program

The AFM image in figure 4 shows a wide area of ssDNAThe agglomerated ssDNA molecules can be seen as ellipticaland spherical structures on the mica substrate Most of thespherical structures have nearly the same diameter around25 nm

Figure 4 Top view AFM image of a dried aqueous solution ofssDNA Elliptical and spherical structures were recognized as theassembly of ssDNA molecules

The γ -ammonia-SWCNTs are dispersed in tetrahydrofu-ran (THF) and imaged by AFM (figures 5(a) and (b)) As canbe seen on the AFM image bundles of γ -irradiated SWCNTsare very straight with one branch at the end The average di-ameter of SWCNT bundles was 44 nm The surface of theSWCNTs was not uniform and not completely smooth Theprocess of γ -irradiation has improved the solubility of nan-otubes in THF due to a major reduction in the size of the bun-dles compared to pristine nanotubes dissolved in THF [35]

AFM images of pristine and γ -irradiated SWCNTsfunctionalized with ssDNA are shown in figures 5(c)ndash(j) It canbe seen that SWCNTs have a rugged wavy surface probablyas a consequence of adsorption of ssDNA Large scale AFMimages of all specimens (figures 5(a) (c) (e) (g) (i)) showa lot of bundles of SWCNTs Figure 5(d) shows entangledbundles of DNA-functionalized pristine nanotubes with anaverage bundle diameter of 65 nm Figure 5(f) is a higherresolution image of a single γ -water-SWCNTssDNA structurewith a length of 815 nm and diameter of 30ndash45 nm Thetwo spherical structures detected on the surface of bundle ofSWCNT are probably the aggregates of ssDNA moleculesformed during sonication A representative image of SWCNTsirradiated in air and dispersed with ssDNA is shown infigure 5(h) The average diameter of these nanotube bundles is71 nm and the length is 1200 nm The DNA-wrapped SWCNTbundles irradiated in ammonia are shown in figures 5(i) and (j)The nanotube bundle shown had a diameter of 172 nm and alength of 650 nm Also a spherical agglomerate of ssDNAmolecules can be observed at one end of the bundle Based ondata obtained from the large scale AFM image (figure 5(a)) itwas determined that the lengths of pristine SWCNT bundleswere in the range from 2ndash5 μm while the length of γ -irradiated SWCNT bundles functionalized with DNA was lessthan 2 μm

Therefore the AFM study has shown that γ -irradiationreduces the length and diameter of the SWCNT bundlesobtained after subsequent functionalization with DNA Thisseems particularly true for SWCNTs irradiated in the presenceof ammonia which had the smallest size

4

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 5 (a) Large scale AFM image of γ -ammonia-SWCNTs dissolved in THF (b) Representative region of γ -ammonia-SWCNTsdissolved in THF (c) Large scale AFM image of pristine SWCNTs functionalized by ssDNA (d) Representative region of pristineSWCNTsssDNA (e) Large scale AFM image of ssDNASWCNTs irradiated in deionized water (f) Representative region ofssDNASWCNTs irradiated in deionized water (g) Large scale AFM image of ssDNASWCNTs irradiated in air (h) Representative region ofssDNASWCNTs irradiated in air (i) Large scale AFM image of ssDNASWCNTs irradiated in a 30 solution of ammonia (j)Representative region of SWCNTs irradiated in a 30 solution of ammonia and functionalized by ssDNA The irradiation dose was 50 kGy

35 Mechanism of stabilization

It is known that molecules of an amphiphilic polymer canbe wrapped around CNTs and form hybrids that are stablein an aqueous environment [36] DNA molecules are also

able to form complexes with SWCNTs [37] thus improvingsolubilization and the process of isolation of single nanotubesfrom a bundle In our experiment the amount of ssDNAnecessary for functionalization of SWCNTs was considerablysmaller (25 times lower) than in previous studies [37ndash40]

5

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 6 Suggested model of the mechanism for DNA-mediatedstabilization of SWCNTs irradiated in air (compound 1) Theintermolecular interaction between γ -air-SWCNTs and ssDNAresults in a formation of hybrid γ -air-SWCNTsssDNA(compound 2)

In order to promote formation of hybrid SWCNTsssDNAwe used γ -irradiation as a pre-treatment for functionalizationwith ssDNA Gamma irradiation causes radiolysis of watermolecules which leads to the formation of free radical(Hmiddot middotOH) and molecular (H2 H2O2) species [41] At lowdoses and room temperature the main radiolytic productof the radiolysis of liquid ammonia is ammonium azide(NH4N3) [42] but stable products such as N2H4 NHminus

2 and Nminus3

can also be formed [43] Molecules present in air undergo γ -ray irradiation and form reactive radical species and molecules(NO NO2) [44]

We applied γ -irradiation that was of low energy (50 kGy)in comparison with other studies [23 24] The reason forchoosing this energy was to conserve the pristine structureof the nanotubes Utilizing three different media as theenvironment for γ -irradiation of SWCNTs we covalentlyattached amino and hydroxyl groups depending on themedium Due to their great penetrating power γ -photonscause the binding of a polar functional group from theirradiated medium to the wall of the SWCNTs Theattached covalent groups cause a weakening of van der Waalsinteractions between CNTs in a bundle The subsequentreduction of hydrophobic forces in the bundles increases theefficiency of separation and isolation of single nanotubes

In the figure 6 we presented a possible mechanism forssDNA-mediated stabilization of SWCNTs irradiated in airWe assumed that ssDNA was wrapped around the SWCNTsaccording to the mechanism described above Covalentlyattached polar groups (hydroxyl and amino groups) arepresent at the sidewalls of the SWCNTs while ssDNAcovering the surface of SWCNTs forms π -interactions Ina double stranded DNA nucleotides from one chain areconnected with those from other chain by carbonylndashaminebonds Denatured ssDNA has many reactive groups that may

strongly interact with groups at the surface of γ -irradiatednanotubes (amine carboxyl carbonyl) Furthermore π -interactions are established between the nanotube surfaceand oligonucleotides Apparently the number of amine andcarbonyl bonds attached to a nanotube sidewall depends onthe medium in which the SCWNTs are irradiated Thelargest number of bonds is found in γ -ammonia-SWCNTs andthe smallest in γ -water-SWCNTs Accordingly the highestconcentration of nanotubes was observed in γ -ammonia-SWCNTssDNA suspension

The mechanism of stabilization of γ -SWCNTsssDNAwas tested by increasing the ionic strength of the dispersionsIt was noticed that the addition of 1 sodium chloride resultedin a precipitation of both pristine and irradiated ssDNA-functionalized SWCNTs which occurred within a few hoursFurther addition of sodium chloride accelerated precipitationThese results are consistent with the electrostatic mechanismof stabilization

4 Conclusion

In this study we investigated the effects of γ -irradiationon the efficiency of dispersion of SWCNTs by ssDNASWCNTs were irradiated with dose of 50 kGy in three mediawater ammonia and air Raman analysis has shown that γ -irradiation caused a slight increase in the IDIG ratio probablyreflecting the increase of disorder in the SWCNT structureFTIR spectroscopy has verified the disorder of the SWCNTsdetecting the presence of amino and carbonyl groups at thesidewalls of the SWCNTs AFM analysis has shown that γ -irradiation increases solubility and the reduction of size andlength of nanotube bundles Based on solubility tests themost efficient medium for the formation of reactive amine andcarbonyl groups is γ -irradiation of SWCNTs in a 30 solutionof ammonia The greatest advantage of this novel process forthe production of SWCNTssDNA dispersion was the use ofssDNA in a significantly smaller quantity At the moment thedescribed procedure is still unable to separate SWCNTs frombundles in significant amounts

Acknowledgment

This research was supported by the Ministry of Science of theRepublic of Serbia (project no 145073)

References

[1] Thostenson E Ren Z and Chou T 2001 Comput Sci Technol61 1899

[2] Lacerda L Bianco A Prato M and Kostarelos K 2006 AdvDrug Deliv Rev 58 1460

[3] Bianco A Kostarelos K and Prato M 2005 Curr Opin ChemBiol 9 674

[4] Yang W Thordarson P Gooding J Ringer S and Braet F 2007Nanotechnology 18 412001

[5] Foldvari M and Bagonluri M 2008 Biol Med 4 173[6] Hampel S et al 2008 Nanomedicine 3 175[7] Ahammad A Lee J and Rahman M 2009 Sensors 9 2289[8] Zhang S and Kumar S 2008 Small 4 1270

6

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 3 FTIR spectroscopy of pristine SWCNTs (1) pristineSWCNTsssDNA (2) ssDNA (3) γ -air-SWCNTsssDNA (4)γ -ammonia-SWCNTsssDNA (5) and γ -water-SWCNTsssDNA (6)The spectra have been vertically displaced for clarity but areotherwise plotted on the same vertical scale

or CndashO backbone stretch 1219 cmminus1 for antisymmetric PO2

stretching 1278 1379 1422 and 1480 cmminus1 of base sugarmoieties 1536 cmminus1 of cytosine and 1696 cmminus1 of C=C andC=N stretching in base planes (curve 3) Curve 1 (FTIRspectrum of pristine SWCNTs) shows peaks at 1607 cmminus1

which stem from aromatic C=C stretching bonds as well asat 2850 and 2920 cmminus1 originating from CndashH bonds [34] ThessDNA wrapping and γ -irradiation induce dramatic changesin the infrared spectra of pristine SWCNTs Curve 2 (pristineSWCNTssDNA) shows peaks at 1060 1219 and 1696 cmminus1

which stem from ssDNA as well as small peaks at 2850and 2920 cmminus1 which originate from CndashH bonds The FTIRspectrum of functionalized pristine SWCNTs did not showthe presence of amino groups Spectra 4 5 and 6 (FTIRspectra of SWCNTs irradiated in three different media) exhibitbroad bands around 650ndash680 cmminus1 which can be interpreted ascharacteristic for the amine bond (NndashH) Strong bands at 10441385 and 1621 cmminus1 in spectra of γ -water-SWCNTssDNA(curve 6) are evidence of adsorption of ssDNA on the sidewallsof the SWCNTs A broad band at 3404 cmminus1 correspondsto OndashH stretching in the γ -water-SWCNTs The spectrumof γ -air-SWCNTssDNA (curve 4) shows bands at 1056 and1643 cmminus1 which also indicate attachment of ssDNA toSWCNTs These results provide evidence for the interactionof ssDNA with the sidewalls of γ -irradiated SWCNTs

34 Atomic force microscopy

The morphology of the different dispersions of γ -irradiatedSWCNTs functionalized with ssDNA was examined by AFMThe lengths and diameters of γ -SWCNTssDNA hybrids weredetermined using the Quesant SMP program

The AFM image in figure 4 shows a wide area of ssDNAThe agglomerated ssDNA molecules can be seen as ellipticaland spherical structures on the mica substrate Most of thespherical structures have nearly the same diameter around25 nm

Figure 4 Top view AFM image of a dried aqueous solution ofssDNA Elliptical and spherical structures were recognized as theassembly of ssDNA molecules

The γ -ammonia-SWCNTs are dispersed in tetrahydrofu-ran (THF) and imaged by AFM (figures 5(a) and (b)) As canbe seen on the AFM image bundles of γ -irradiated SWCNTsare very straight with one branch at the end The average di-ameter of SWCNT bundles was 44 nm The surface of theSWCNTs was not uniform and not completely smooth Theprocess of γ -irradiation has improved the solubility of nan-otubes in THF due to a major reduction in the size of the bun-dles compared to pristine nanotubes dissolved in THF [35]

AFM images of pristine and γ -irradiated SWCNTsfunctionalized with ssDNA are shown in figures 5(c)ndash(j) It canbe seen that SWCNTs have a rugged wavy surface probablyas a consequence of adsorption of ssDNA Large scale AFMimages of all specimens (figures 5(a) (c) (e) (g) (i)) showa lot of bundles of SWCNTs Figure 5(d) shows entangledbundles of DNA-functionalized pristine nanotubes with anaverage bundle diameter of 65 nm Figure 5(f) is a higherresolution image of a single γ -water-SWCNTssDNA structurewith a length of 815 nm and diameter of 30ndash45 nm Thetwo spherical structures detected on the surface of bundle ofSWCNT are probably the aggregates of ssDNA moleculesformed during sonication A representative image of SWCNTsirradiated in air and dispersed with ssDNA is shown infigure 5(h) The average diameter of these nanotube bundles is71 nm and the length is 1200 nm The DNA-wrapped SWCNTbundles irradiated in ammonia are shown in figures 5(i) and (j)The nanotube bundle shown had a diameter of 172 nm and alength of 650 nm Also a spherical agglomerate of ssDNAmolecules can be observed at one end of the bundle Based ondata obtained from the large scale AFM image (figure 5(a)) itwas determined that the lengths of pristine SWCNT bundleswere in the range from 2ndash5 μm while the length of γ -irradiated SWCNT bundles functionalized with DNA was lessthan 2 μm

Therefore the AFM study has shown that γ -irradiationreduces the length and diameter of the SWCNT bundlesobtained after subsequent functionalization with DNA Thisseems particularly true for SWCNTs irradiated in the presenceof ammonia which had the smallest size

4

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 5 (a) Large scale AFM image of γ -ammonia-SWCNTs dissolved in THF (b) Representative region of γ -ammonia-SWCNTsdissolved in THF (c) Large scale AFM image of pristine SWCNTs functionalized by ssDNA (d) Representative region of pristineSWCNTsssDNA (e) Large scale AFM image of ssDNASWCNTs irradiated in deionized water (f) Representative region ofssDNASWCNTs irradiated in deionized water (g) Large scale AFM image of ssDNASWCNTs irradiated in air (h) Representative region ofssDNASWCNTs irradiated in air (i) Large scale AFM image of ssDNASWCNTs irradiated in a 30 solution of ammonia (j)Representative region of SWCNTs irradiated in a 30 solution of ammonia and functionalized by ssDNA The irradiation dose was 50 kGy

35 Mechanism of stabilization

It is known that molecules of an amphiphilic polymer canbe wrapped around CNTs and form hybrids that are stablein an aqueous environment [36] DNA molecules are also

able to form complexes with SWCNTs [37] thus improvingsolubilization and the process of isolation of single nanotubesfrom a bundle In our experiment the amount of ssDNAnecessary for functionalization of SWCNTs was considerablysmaller (25 times lower) than in previous studies [37ndash40]

5

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 6 Suggested model of the mechanism for DNA-mediatedstabilization of SWCNTs irradiated in air (compound 1) Theintermolecular interaction between γ -air-SWCNTs and ssDNAresults in a formation of hybrid γ -air-SWCNTsssDNA(compound 2)

In order to promote formation of hybrid SWCNTsssDNAwe used γ -irradiation as a pre-treatment for functionalizationwith ssDNA Gamma irradiation causes radiolysis of watermolecules which leads to the formation of free radical(Hmiddot middotOH) and molecular (H2 H2O2) species [41] At lowdoses and room temperature the main radiolytic productof the radiolysis of liquid ammonia is ammonium azide(NH4N3) [42] but stable products such as N2H4 NHminus

2 and Nminus3

can also be formed [43] Molecules present in air undergo γ -ray irradiation and form reactive radical species and molecules(NO NO2) [44]

We applied γ -irradiation that was of low energy (50 kGy)in comparison with other studies [23 24] The reason forchoosing this energy was to conserve the pristine structureof the nanotubes Utilizing three different media as theenvironment for γ -irradiation of SWCNTs we covalentlyattached amino and hydroxyl groups depending on themedium Due to their great penetrating power γ -photonscause the binding of a polar functional group from theirradiated medium to the wall of the SWCNTs Theattached covalent groups cause a weakening of van der Waalsinteractions between CNTs in a bundle The subsequentreduction of hydrophobic forces in the bundles increases theefficiency of separation and isolation of single nanotubes

In the figure 6 we presented a possible mechanism forssDNA-mediated stabilization of SWCNTs irradiated in airWe assumed that ssDNA was wrapped around the SWCNTsaccording to the mechanism described above Covalentlyattached polar groups (hydroxyl and amino groups) arepresent at the sidewalls of the SWCNTs while ssDNAcovering the surface of SWCNTs forms π -interactions Ina double stranded DNA nucleotides from one chain areconnected with those from other chain by carbonylndashaminebonds Denatured ssDNA has many reactive groups that may

strongly interact with groups at the surface of γ -irradiatednanotubes (amine carboxyl carbonyl) Furthermore π -interactions are established between the nanotube surfaceand oligonucleotides Apparently the number of amine andcarbonyl bonds attached to a nanotube sidewall depends onthe medium in which the SCWNTs are irradiated Thelargest number of bonds is found in γ -ammonia-SWCNTs andthe smallest in γ -water-SWCNTs Accordingly the highestconcentration of nanotubes was observed in γ -ammonia-SWCNTssDNA suspension

The mechanism of stabilization of γ -SWCNTsssDNAwas tested by increasing the ionic strength of the dispersionsIt was noticed that the addition of 1 sodium chloride resultedin a precipitation of both pristine and irradiated ssDNA-functionalized SWCNTs which occurred within a few hoursFurther addition of sodium chloride accelerated precipitationThese results are consistent with the electrostatic mechanismof stabilization

4 Conclusion

In this study we investigated the effects of γ -irradiationon the efficiency of dispersion of SWCNTs by ssDNASWCNTs were irradiated with dose of 50 kGy in three mediawater ammonia and air Raman analysis has shown that γ -irradiation caused a slight increase in the IDIG ratio probablyreflecting the increase of disorder in the SWCNT structureFTIR spectroscopy has verified the disorder of the SWCNTsdetecting the presence of amino and carbonyl groups at thesidewalls of the SWCNTs AFM analysis has shown that γ -irradiation increases solubility and the reduction of size andlength of nanotube bundles Based on solubility tests themost efficient medium for the formation of reactive amine andcarbonyl groups is γ -irradiation of SWCNTs in a 30 solutionof ammonia The greatest advantage of this novel process forthe production of SWCNTssDNA dispersion was the use ofssDNA in a significantly smaller quantity At the moment thedescribed procedure is still unable to separate SWCNTs frombundles in significant amounts

Acknowledgment

This research was supported by the Ministry of Science of theRepublic of Serbia (project no 145073)

References

[1] Thostenson E Ren Z and Chou T 2001 Comput Sci Technol61 1899

[2] Lacerda L Bianco A Prato M and Kostarelos K 2006 AdvDrug Deliv Rev 58 1460

[3] Bianco A Kostarelos K and Prato M 2005 Curr Opin ChemBiol 9 674

[4] Yang W Thordarson P Gooding J Ringer S and Braet F 2007Nanotechnology 18 412001

[5] Foldvari M and Bagonluri M 2008 Biol Med 4 173[6] Hampel S et al 2008 Nanomedicine 3 175[7] Ahammad A Lee J and Rahman M 2009 Sensors 9 2289[8] Zhang S and Kumar S 2008 Small 4 1270

6

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 5 (a) Large scale AFM image of γ -ammonia-SWCNTs dissolved in THF (b) Representative region of γ -ammonia-SWCNTsdissolved in THF (c) Large scale AFM image of pristine SWCNTs functionalized by ssDNA (d) Representative region of pristineSWCNTsssDNA (e) Large scale AFM image of ssDNASWCNTs irradiated in deionized water (f) Representative region ofssDNASWCNTs irradiated in deionized water (g) Large scale AFM image of ssDNASWCNTs irradiated in air (h) Representative region ofssDNASWCNTs irradiated in air (i) Large scale AFM image of ssDNASWCNTs irradiated in a 30 solution of ammonia (j)Representative region of SWCNTs irradiated in a 30 solution of ammonia and functionalized by ssDNA The irradiation dose was 50 kGy

35 Mechanism of stabilization

It is known that molecules of an amphiphilic polymer canbe wrapped around CNTs and form hybrids that are stablein an aqueous environment [36] DNA molecules are also

able to form complexes with SWCNTs [37] thus improvingsolubilization and the process of isolation of single nanotubesfrom a bundle In our experiment the amount of ssDNAnecessary for functionalization of SWCNTs was considerablysmaller (25 times lower) than in previous studies [37ndash40]

5

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 6 Suggested model of the mechanism for DNA-mediatedstabilization of SWCNTs irradiated in air (compound 1) Theintermolecular interaction between γ -air-SWCNTs and ssDNAresults in a formation of hybrid γ -air-SWCNTsssDNA(compound 2)

In order to promote formation of hybrid SWCNTsssDNAwe used γ -irradiation as a pre-treatment for functionalizationwith ssDNA Gamma irradiation causes radiolysis of watermolecules which leads to the formation of free radical(Hmiddot middotOH) and molecular (H2 H2O2) species [41] At lowdoses and room temperature the main radiolytic productof the radiolysis of liquid ammonia is ammonium azide(NH4N3) [42] but stable products such as N2H4 NHminus

2 and Nminus3

can also be formed [43] Molecules present in air undergo γ -ray irradiation and form reactive radical species and molecules(NO NO2) [44]

We applied γ -irradiation that was of low energy (50 kGy)in comparison with other studies [23 24] The reason forchoosing this energy was to conserve the pristine structureof the nanotubes Utilizing three different media as theenvironment for γ -irradiation of SWCNTs we covalentlyattached amino and hydroxyl groups depending on themedium Due to their great penetrating power γ -photonscause the binding of a polar functional group from theirradiated medium to the wall of the SWCNTs Theattached covalent groups cause a weakening of van der Waalsinteractions between CNTs in a bundle The subsequentreduction of hydrophobic forces in the bundles increases theefficiency of separation and isolation of single nanotubes

In the figure 6 we presented a possible mechanism forssDNA-mediated stabilization of SWCNTs irradiated in airWe assumed that ssDNA was wrapped around the SWCNTsaccording to the mechanism described above Covalentlyattached polar groups (hydroxyl and amino groups) arepresent at the sidewalls of the SWCNTs while ssDNAcovering the surface of SWCNTs forms π -interactions Ina double stranded DNA nucleotides from one chain areconnected with those from other chain by carbonylndashaminebonds Denatured ssDNA has many reactive groups that may

strongly interact with groups at the surface of γ -irradiatednanotubes (amine carboxyl carbonyl) Furthermore π -interactions are established between the nanotube surfaceand oligonucleotides Apparently the number of amine andcarbonyl bonds attached to a nanotube sidewall depends onthe medium in which the SCWNTs are irradiated Thelargest number of bonds is found in γ -ammonia-SWCNTs andthe smallest in γ -water-SWCNTs Accordingly the highestconcentration of nanotubes was observed in γ -ammonia-SWCNTssDNA suspension

The mechanism of stabilization of γ -SWCNTsssDNAwas tested by increasing the ionic strength of the dispersionsIt was noticed that the addition of 1 sodium chloride resultedin a precipitation of both pristine and irradiated ssDNA-functionalized SWCNTs which occurred within a few hoursFurther addition of sodium chloride accelerated precipitationThese results are consistent with the electrostatic mechanismof stabilization

4 Conclusion

In this study we investigated the effects of γ -irradiationon the efficiency of dispersion of SWCNTs by ssDNASWCNTs were irradiated with dose of 50 kGy in three mediawater ammonia and air Raman analysis has shown that γ -irradiation caused a slight increase in the IDIG ratio probablyreflecting the increase of disorder in the SWCNT structureFTIR spectroscopy has verified the disorder of the SWCNTsdetecting the presence of amino and carbonyl groups at thesidewalls of the SWCNTs AFM analysis has shown that γ -irradiation increases solubility and the reduction of size andlength of nanotube bundles Based on solubility tests themost efficient medium for the formation of reactive amine andcarbonyl groups is γ -irradiation of SWCNTs in a 30 solutionof ammonia The greatest advantage of this novel process forthe production of SWCNTssDNA dispersion was the use ofssDNA in a significantly smaller quantity At the moment thedescribed procedure is still unable to separate SWCNTs frombundles in significant amounts

Acknowledgment

This research was supported by the Ministry of Science of theRepublic of Serbia (project no 145073)

References

[1] Thostenson E Ren Z and Chou T 2001 Comput Sci Technol61 1899

[2] Lacerda L Bianco A Prato M and Kostarelos K 2006 AdvDrug Deliv Rev 58 1460

[3] Bianco A Kostarelos K and Prato M 2005 Curr Opin ChemBiol 9 674

[4] Yang W Thordarson P Gooding J Ringer S and Braet F 2007Nanotechnology 18 412001

[5] Foldvari M and Bagonluri M 2008 Biol Med 4 173[6] Hampel S et al 2008 Nanomedicine 3 175[7] Ahammad A Lee J and Rahman M 2009 Sensors 9 2289[8] Zhang S and Kumar S 2008 Small 4 1270

6

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

Figure 6 Suggested model of the mechanism for DNA-mediatedstabilization of SWCNTs irradiated in air (compound 1) Theintermolecular interaction between γ -air-SWCNTs and ssDNAresults in a formation of hybrid γ -air-SWCNTsssDNA(compound 2)

In order to promote formation of hybrid SWCNTsssDNAwe used γ -irradiation as a pre-treatment for functionalizationwith ssDNA Gamma irradiation causes radiolysis of watermolecules which leads to the formation of free radical(Hmiddot middotOH) and molecular (H2 H2O2) species [41] At lowdoses and room temperature the main radiolytic productof the radiolysis of liquid ammonia is ammonium azide(NH4N3) [42] but stable products such as N2H4 NHminus

2 and Nminus3

can also be formed [43] Molecules present in air undergo γ -ray irradiation and form reactive radical species and molecules(NO NO2) [44]

We applied γ -irradiation that was of low energy (50 kGy)in comparison with other studies [23 24] The reason forchoosing this energy was to conserve the pristine structureof the nanotubes Utilizing three different media as theenvironment for γ -irradiation of SWCNTs we covalentlyattached amino and hydroxyl groups depending on themedium Due to their great penetrating power γ -photonscause the binding of a polar functional group from theirradiated medium to the wall of the SWCNTs Theattached covalent groups cause a weakening of van der Waalsinteractions between CNTs in a bundle The subsequentreduction of hydrophobic forces in the bundles increases theefficiency of separation and isolation of single nanotubes

In the figure 6 we presented a possible mechanism forssDNA-mediated stabilization of SWCNTs irradiated in airWe assumed that ssDNA was wrapped around the SWCNTsaccording to the mechanism described above Covalentlyattached polar groups (hydroxyl and amino groups) arepresent at the sidewalls of the SWCNTs while ssDNAcovering the surface of SWCNTs forms π -interactions Ina double stranded DNA nucleotides from one chain areconnected with those from other chain by carbonylndashaminebonds Denatured ssDNA has many reactive groups that may

strongly interact with groups at the surface of γ -irradiatednanotubes (amine carboxyl carbonyl) Furthermore π -interactions are established between the nanotube surfaceand oligonucleotides Apparently the number of amine andcarbonyl bonds attached to a nanotube sidewall depends onthe medium in which the SCWNTs are irradiated Thelargest number of bonds is found in γ -ammonia-SWCNTs andthe smallest in γ -water-SWCNTs Accordingly the highestconcentration of nanotubes was observed in γ -ammonia-SWCNTssDNA suspension

The mechanism of stabilization of γ -SWCNTsssDNAwas tested by increasing the ionic strength of the dispersionsIt was noticed that the addition of 1 sodium chloride resultedin a precipitation of both pristine and irradiated ssDNA-functionalized SWCNTs which occurred within a few hoursFurther addition of sodium chloride accelerated precipitationThese results are consistent with the electrostatic mechanismof stabilization

4 Conclusion

In this study we investigated the effects of γ -irradiationon the efficiency of dispersion of SWCNTs by ssDNASWCNTs were irradiated with dose of 50 kGy in three mediawater ammonia and air Raman analysis has shown that γ -irradiation caused a slight increase in the IDIG ratio probablyreflecting the increase of disorder in the SWCNT structureFTIR spectroscopy has verified the disorder of the SWCNTsdetecting the presence of amino and carbonyl groups at thesidewalls of the SWCNTs AFM analysis has shown that γ -irradiation increases solubility and the reduction of size andlength of nanotube bundles Based on solubility tests themost efficient medium for the formation of reactive amine andcarbonyl groups is γ -irradiation of SWCNTs in a 30 solutionof ammonia The greatest advantage of this novel process forthe production of SWCNTssDNA dispersion was the use ofssDNA in a significantly smaller quantity At the moment thedescribed procedure is still unable to separate SWCNTs frombundles in significant amounts

Acknowledgment

This research was supported by the Ministry of Science of theRepublic of Serbia (project no 145073)

References

[1] Thostenson E Ren Z and Chou T 2001 Comput Sci Technol61 1899

[2] Lacerda L Bianco A Prato M and Kostarelos K 2006 AdvDrug Deliv Rev 58 1460

[3] Bianco A Kostarelos K and Prato M 2005 Curr Opin ChemBiol 9 674

[4] Yang W Thordarson P Gooding J Ringer S and Braet F 2007Nanotechnology 18 412001

[5] Foldvari M and Bagonluri M 2008 Biol Med 4 173[6] Hampel S et al 2008 Nanomedicine 3 175[7] Ahammad A Lee J and Rahman M 2009 Sensors 9 2289[8] Zhang S and Kumar S 2008 Small 4 1270

6

Nanotechnology 20 (2009) 445602 S P Jovanovic et al

[9] Hafner J H Cheung C L Woolley A T and Lieber C M 2001Prog Biophys Mol Biol 77 73

[10] Treacy M M J Ebbesen T W and Gibson T M 1996 Nature381 680

[11] Wong E W Sheehan P E and Lieber C M 1997 Science277 1971

[12] Yu F Files B P Arepalli S and Ruoff R S 2000 Phys Rev Lett84 5552

[13] Tasis D Tagmatarchis N Georgakilas V and Prato M 2003ChemmdashEur J 9 4000

[14] Tasis D Tagmatarchis N Bianco A and Prato M 2006 ChemRev J 106 1105

[15] Lee H K and Mijovic J 2009 Polymer 50 881[16] Kiang C H Goddard W A III Beyers R and Bethune D S 1996

J Phys Chem 100 3749[17] Basiuk V A Kobayashi K Kaneko T Negishi Y Basiuk E V

and Saniger-Blesa J M 2002 Nano Lett 2 789[18] Suzuki M Ishibashi K Toratani K Tsuya D and

Aoyagi Y 2002 Appl Phys Lett 81 2273[19] Terrones M Terrones H Banhart F Charlier J C and

Ajayan P M 2000 Science 288 1226[20] Ahn K S Kim J S Kim C O and Hong J P 2003 Carbon

41 2481[21] Brzhezinskaya M M Baitinger E M and Shnitov V V 2004

Physica B 348 95[22] Khare B Meyyappan M Moore M H Wilhite P

Imanaka H and Chen B 2003 Nano Lett 3 643[23] Guo J Li Y Wu S and Li W 2005 Nanotechnology 16 2385[24] Skakalova V Dettlaff-Weglikowska U and Roth S 2004

Diamond Relat Mater 13 296[25] Hulman M Skakalova V Roth S and Kuzmany H 2005 J Appl

Phys 98 024311[26] Wu W T Shi L Wang Y Pang W and Zhu Q 2008

Nanotechnology 19 125607

[27] Todorovic Markovic B Jovanovic S Jokanovic VDramicanin M and Markovic Z 2008 Appl Surf Sci255 3283

[28] Kavan L Rapta P Dunsch L Bronikowski M J Willis P andSmalley R E 2001 J Phys Chem B 105 10764

[29] Alvarez L Righi A Guillard T Rols S Anglaret ELaplaze D and Sauvajol J L 2000 Chem Phys Lett 316 186

[30] Kim U J Furtado C A Lui X Chen G and Eklund P C 2005J Am Chem Soc 127 15437

[31] httpwwwscribdcomdoc288488Lecture-on-DNA[32] Kaing C H Goddard W Beyers R and Bethune D 1996 J Phys

Chem 100 3749[33] Telling R H Ewels C P El-Barbary A A and Heggie M I 2003

Nat Mater 2 333[34] Lide D R 2003ndash2004 CRC Handbook of Chemistry and Physics

(Boca Raton FL CRC Press) pp 9-86ndash90[35] Markovic Z Jovanovic S Kleut D Romcevic N Jokanovic V

Trajkovic V and Todorovic Markovic B 2009 Appl Surf Sci255 6359

[36] OrsquoConnell M J Boul P Ericson L M Huffaman C Wang YHazor E Kuper C Tour J Ausman K D and Smalley R E2001 Chem Phys Lett 342 265

[37] Zheng M Jagota A Semke E D Diner B A Mclean R SLustig S R Richardson R E and Tassi N G 2003 Nat Mater2 338

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