& Carbon Nanotubes

Supramolecular Assemblies of Nucleoside Functionalized CarbonNanotubes: Synthesis, Film Preparation, and Properties

Alessandra Micoli,[a] Antonio Turco,[a] Elsie Araujo-Palomo,[b] Armando Encinas,[b]

Mildred Quintana,*[b] and Maurizio Prato*[a]

Abstract: Nucleoside-functionalized multi-walled carbonnanotubes (N-MWCNTs) were synthesized and characterized.A self-organization process using hydrogen bonding interac-tions was then used for the fabrication of self-assembled N-MWCNTs films free of stabilizing agents, polymers, or surfac-tants. Membranes were produced by using a simple water-dispersion-based vacuum-filtration method. Hydrogen-bondrecognition was confirmed by analysis with IR spectroscopyand TEM images. Restoration of the electronic conductionproperties in the N-MWCNTs membranes was performed byremoving the organic portion by thermal treatment underan argon atmosphere to give d-N-MWCNTs. Electrical con-ductivity and thermal gravimetric analysis (TGA) measure-

ments confirmed the efficiency of the annealing process. Fi-nally, oxidative biodegradation of the films N-MWCNTs andd-N-MWCNTs was performed by using horseradish perox-idase (HRP) and low concentrations of H2O2. Our results con-firm that functional groups play an important role in the bio-degradation of CNT by HRP: N-MWCNTs films were com-pletely biodegraded, whereas for d-N-MWCNTs films nodegradation was observed, showing that the pristine CNTundergoes minimal enzyme-catalyzed oxidation This novelmethodology offers a straightforward supramolecular strat-egy for the construction of conductive and biodegradablecarbon nanotube films.

Introduction

Carbon nanotube (CNT) processing is extremely important inthe development of new advanced materials such as flexibleelectrodes, electronic paper, antistatic coatings, protectiveclothing, high-performance composites for aircraft and auto-motive industry, and micro/nano-platforms for cell growth anddifferentiation.[1] To achieve this, it is well known that CNT han-dling must be maximized to reach optimal material propertiessuch as strength, conductivity, and biocompatibility.[2] In thisdirection, a reliable and successful strategy for the constructionof functional CNT-based architectures is the use of nucleo-base–CNT nanohybrids as building blocks in the self-assemblyof multifunctional materials.[3] The presence of the nucleobaseon the CNT surface improves the dispersibility of the nanohy-brid in polar solvents and directs the recognition through hy-drogen bonding of complementary base pairs in nonpolaraprotic solvents. To achieve this goal, two functionalization

strategies are usually employed, encompassing either a cova-lent or a supramolecular approach. Whereas the first protocolexploits the chemical reactivity of the CNT graphitic wall bygrafting the nucleobase groups directly onto the tubular sur-face,[4] the second approach is based on noncovalent recogni-tion of the base-pairing motifs involving at least two hydro-gen-bond interactions as directional and predictable noncova-lent attractive forces between complementary hydrogen donor(D) and acceptor (A) moieties.[5]

The molecular-recognition-driven assembly of CNT is mediat-ed by the quantity of functional groups covalently grafted tothe CNTs surface (cooperative effect). In particular, when theassembly is driven by thymine-derived CNTs, the self-organiza-tion through the formation of double hydrogen bonded ho-modimers results in compact 2 D or 3 D superstructures as thedegree of covalent functionalization is increased.[6] Followingthe same approach, herein, we demonstrate a versatile meth-odology for the construction of conductive CNT membranescompletely free of surfactants or polymers. This methodologyis expected to improve the properties of conductive films[7]

prepared from solutions containing stabilizing agents[8, 9] asstrong interactions between the stabilizing molecules andCNTs may significantly affect the electronic transitions,[10] re-sulting in modification of the electronic properties of the com-posite.[11] Furthermore, in some instances, the presence of sta-bilizing molecules can also affect the biocompatibility of theCNT-based composite, in particular, the use of surfactants in bi-oapplications.[12]

[a] Dr. A. Micoli, Dr. A. Turco, Prof. M. PratoCenter of Excellence for Nanostructured Materials (CENMAT)INSTM UdR di Trieste, Dipartimento di Scienze Chimiche e FarmaceuticheUniversity of TriestePiazzale Europa 1, 34127 Trieste (Italy)E-mail : prato@untis.it

[b] E. Araujo-Palomo, Prof. A. Encinas, Prof. M. QuintanaInstituto de F�sica, Universidad Aut�noma de San Luis Potos�Manuel Nava 6, Zona Universitaria 78290, San Luis Potos�, SLP (Mexico)E-mail : mildred@ifisica.uaslp.mx

Supporting information for this article is available on the WWW underhttp ://dx.doi.org/10.1002/chem.201304780.

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Full PaperDOI: 10.1002/chem.201304780

The procedure described here involves the covalent attach-ment of each of the four nucleosides moieties (Thymidine, T;Adenosine, A; Cytidine, C; Guanosine, G) to multi-walled CNTs(MWCNTs) as supramolecular motifs, N-MWCNTs (N = A, T, G orC). The recognition between complementary base pair nanohy-brids, T/A-MWCNTs or G/C-MWCNTs systems dispersed inwater, allows the fabrication of freestanding homogeneousfilms by a simple vacuum-filtration method.[13] As depicted inScheme 1, the engineering design behind the formation and

stabilization of the film is the establishment of hydrogenbonds between the complementary sites of the nucleotides A–T and G–C (Ka = 102

m�1 and 103–105

m�1 in CHCl3, respectively).

The recognition event was monitored by IR spectroscopy andTEM images. The restoration of the electronic conductionproperties in the N-MWCNTs membranes was performed byremoving the organic portion by thermal treatment underargon, giving defunctionalized N-MWCNTs (d-N-MWCNTs), asdemonstrated by electrical conductivity and thermal gravimet-ric analysis (TGA) measurements. In addition, we demonstratethat the new films become biodegradable, in contrast to filmsformed by pristine CNTs, which are known to be potentiallytoxic.[14]

Results and Discussion

The nanohybrids were prepared by using the diazonium salt-based arylation reaction.[15] Firstly, MWCNTs with amino termi-nal groups were synthesized, NH2-MWCNTs.[6] After this, eachof the four nucleoside moieties was easily introduced on NH2-MWCNTs by nucleophilic substitution to obtain N-MWCNTs(Scheme S5 in the Supporting Information). The final loading

of nucleoside molecules attached to the MWCNTs, calculatedby the Kaiser test[3] and TGA, is reported in Table 1.

Confirmation of the covalent attachment of nucleoside moi-eties to the CNTs surface was provided by TGA coupled withonline monitoring of the volatile products by mass spectrome-try. The peaks corresponding to the ionic fragments of the nu-cleoside moieties covalently bonded to MWCNTs evolved athigher temperatures compared with free nucleoside moleculesfor all the N-MWCNTs nanohybrids (Figure S1 in the Support-

ing Information). IR spectra (Fig-ure S2 in the Supporting Infor-mation) also confirm the pres-ence of the complementary rec-ognition functions through thecharacteristic peaks of the or-ganic groups: stretching modesof NH and OH (n= 3871–3408),CH (n= 3275–2920), C=O (n=

1741–1736), C=N (n= 1739), andC=C (n= 1588–1554).

Although all the nucleobaseshave the ability to homodimer-ize, the presence of complemen-tary hydrogen-bond arrays, thatis, donor sites comprising theNH amide and NH amine, andacceptor sites comprising the Oand N functionalities, allow themto oligomerize through hydro-gen-bonding interactions.[16]

Thus, the influence of the inter-action between nucleosidegroups on the dispersibility of N-MWCNTs was compared in sol-vents possessing different polari-

ty: DMF and H2O as polar solvents, and CH2Cl2 as a noncompe-titive solvent unable to generate hydrogen-bond interactions.After 5 min of sonication of 0.1 mg of each compound in 1 mLof the respective solvent, N-MWCNTs exhibited good dispersi-bility in DMF and H2O, whereas aggregates were always ob-served in CH2Cl2 (Figure S3 in the Supporting Information). Thepresence of the ribose or deoxyribose fragment in the nucleo-side moiety covalently grafted to the CNT surface allowed thecomplete dispersion of the different N-MWCNTs derivatives in

Scheme 1. Representation of N-MWCNTs (N = Adenosine (A), Guanidine (G), Cytosine (C), and Thymidine (T)) andthe recognition processes between complementary base pairs, A/T-MWCNTs and C/G-MWCNTs.

Table 1. TGA-determined weight loss and number of nucleoside groupscovalently grafted onto the different N-MWCNTs.

Compound Nucleoside groups[mmol g�1][a]

% wt loss (TGA)

T-MWCNTs 584 11A-MWCNTs 602 13C-MWCNTs 654 10G-MWCNTs 506 10

[a] The amount of thymidine, adenosine, cytosine, and guanosine groups,respectively, was calculated as the difference between the Kaiser testbefore and after the covalent attachments of nucleoside groups.

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the polar solvents DMF and H2O in concentrations of0.1 mg mL�1.

To verify the selective molecular recognition by the comple-mentary nucleobases T–A and C–G on N-MWCNTs, equimolaramounts (in term of nucleoside functionalization) of T-MWCNTs (7.0 mg) and A-MWCNTs (6.8 mg) nanoybrids, and ina different set of experiments, C-MWCNTs (5.4 mg) and G-MWCNTs (7.0 mg) nanohybrids were dispersed by sonicationin DMF, H2O or CH2Cl2 (70 mL). Each solution was stirred over-night at room temperature and the recognition between com-plementary N-MWCNTs in different solvents was analyzed byTEM. As expected, aggregation in CH2Cl2 was always observed,whereas good dispersibility in polar solvents like DMF and H2Owas obtained, as depicted in Figure 1.

The formation of hydrogen bonds was assessed by means ofIR spectroscopy. More precisely, we observed a general shift offrequencies towards lower values and the formation of wid-ened and augmented intense bands. The IR spectra of thethree black solids, recovered by filtration from the three differ-ent solvents, show a redshift of the C=O stretching frequency.The characteristic peak at 1733 cm�1 in DMF, moves to1638 cm�1 in H2O, and to 1558 cm�1 in CH2Cl2 for the T/A-MWCNTs recognition system, Figure 2 a, whereas for the C/G-

MWCNTs the stretching mode of the C=O group shifts tolower frequencies, Figure 2 b, from 1644 cm�1 in DMF, to1633 cm�1 in H2O, and to 1574 cm�1 in CH2Cl2. These resultsconfirm that aggregation is guided by hydrogen-bond recogni-tion in noncompetitive solvents.

There are different direct techniques for growing CNT filmswith high purity and relative absence of tube bundles.[1a] How-ever, films prepared from solutions are attractive as they canbe cost-effectively scaled to large areas and are compatiblewith a wide variety of substrates.[8, 9] Thus, after the completecharacterization of the nanohybrids and the verification of themolecular recognition between complementary N-MWCNTs,we fabricated self-assembled freestanding membranes byusing a simple dispersion-based vacuum-filtration method. Forthis, equimolar amounts of T-MWCNTs and A-MWCNTs, and ina different batch, C-MWCNTs and G-MWCNTs, were first dis-persed in water, and then the suspensions were filteredthrough a Millipore membrane (VSWP, 0.025 mm). Water waschosen as a solvent to produce N-MWCNTs dispersions withconcentration of 0.01 mg mL�1 to prevent aggregation, therebyfavoring formation of homogeneous films. Then, the mem-brane filters were degraded in acetone to obtain the blackfilms of N-MWCNTs (Figure 3 a). The films were devoid oftraces of filter, as observed by SEM images (Figure 3 b) and cor-roborated by Raman spectra (Figure S4 in the Supporting Infor-mation). The membranes were allowed to dry under vacuumovernight. It is important to note that the dispersion and filtra-tion of pristine- (p-MWCNTs) or NH2-MWCNTs produces pow-ders as they dry, whereas the filtration of only one type of N-MWCNT results brittle films that break into pieces upon con-tact.

Electrical conductivity measurements were performed atroom temperature on the samples by using a four-probe con-figuration with a Jandel resistivity meter. The measurementswere repeated on three different places in each sample, andcurrent sweeps were performed with different ranges up to100 mA. The N-MWCNTs annealing samples, d-N-MWCNTs,were prepared following thermal treatment at 350 8C for20 min in an inert atmosphere of argon to measure the trans-port properties of a p-MWCNTs membrane. The successful de-functionalization of the N-MWCNTs to give d-N-MWCNT mem-branes was verified by TGA as shown in Figure 4.

Figures 5 a and b compare the measurements obtained in N-MWCNTs and d-N-MWCNTs, along with the correspondinglinear fits, which provide the sample resistance. In all cases, thesamples show a linear IV curve consistent with ohmic behavior.Moreover, these measurements show that for the d-N-MWCNTfilms the removal of the functional groups reduces the electri-cal resistance significantly, an observation that is consistentwith an enhancement of the conductivity. From the measuredvalues of the electrical resistance R, the bulk resistivity 1, sheetresistance Rs, and electrical conductivity s, have been deter-mined. Values are given in Figure 5. From these values it canbe seen that the removal of the organic portion increases theconductivity of the d-N-MWCNTs membranes by an order ofmagnitude. The conductivity values for A/T-MWCNTs and C/G-MWCNTs membranes are 34.8 and 29.5 S m�1, respectively.

Figure 1. TEM images of T/A-MWCNTs in a) DMF, b) H2O, and c) CH2Cl2 andTEM images of C/G-MWCNTs in d) DMF, e) H2O, and f) CH2Cl2. Insets showthe dispersibility of N/N-MWCNTs in the different solvents.

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These values are considerably higher than those reported forMWCNT films prepared by using poly(p-phenylene sulfide) asstabilizer, 0.01 S m�1,[9f] and MWNT films prepared in poly(vinyli-dene fluoride), 2 S m�1.[9g]

Finally, we have compared the biodegradation of both N-MWCNT and d-N-MWCNTs. First, we incubated small frag-

ments of the membranes (ap-proximately 2 mm per side) forthree days in Bovine Serum (CalfSerum) at room temperature. Aclear difference was noticed inthe morphology of the filmsafter incubation. A strong inter-action with the biologicalmedium was observed for N-MWCNT membranes. An evidentswelling of the films was ob-served, whereas d-N-MWCNTsmembranes remained stable asa compact film, Figure 6. Wethen performed the oxidativedegradation of the membranesby using horseradish peroxidase(HRP) and low concentrations ofH2O2.[17] TEM was used to exam-ine aliquots taken from the sam-ples after incubation at 3, 5, 7,and 18 days. For d-N-MWCNTs,the membrane remained un-changed during the entire incu-bation period, and TEM analysisdid not show any CNTs in the so-lution at any time (Figure S6 inthe Supporting Information).Conversely, a substantial degra-dation of the N-MWCNTs filmwas observed. The incubationsolution gradually changed toa black dispersion. TEM analysisat three days showed smallmembrane fragments, Figure 7 a.Over the next few days, individu-al CNTs were observed, Fig-ure 7 b and c. At 18 days, debris

was found on the TEM grid, Figure 7 d. Our results confirm pre-vious studies, where it was demonstrated that functionalgroups play an important role in the biodegradation of CNT by

Figure 2. IR spectra of a) A/T-MWCNTs and b) C/G-MWCNTs recovered by filtration from DMF (upper), H2O(middle), and CH2Cl2 (bottom).

Figure 3. a) Image of freestanding A/T-MWCNTs membrane and b) SEM mi-crograph of A/T-MWCNTs membrane. Figure 4. TGA of p-MWCNTs, A/T-MWCNTs, and d-A/T-MWCNTs.

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HRP and corroborate that pristine CNT suffers minimal enzyme-catalyzed oxidation.[17]

Conclusion

MWCNTs conductive films completely free of stabilizing agents,such as surfactants or polymers, were produced by a self-or-ganization process using hydrogen-bond interactions. MWCNTswere functionalized with one of the four nucleoside bases(Thymidine, T; Adenosine, A; Cytidine, C; Guanosine, G) assupramolecular motifs. The functionalization of N-MWCNTsand the directional recognition by hydrogen-bond interactionsbetween complementary nucleoside pairs were followed bythe Kaiser test, IR spectroscopy, TEM, and TGA, all of whichconfirmed the directional, reliable, and predictable noncova-lent attractive forces between complementary hydrogen donor

(D) and acceptor (A) moieties. The produced N-MWCNTs mem-branes showed a linear IV curve consistent with ohmic behav-ior. The thermal treatment in an inert atmosphere, affording d-N-MWCNTs, significantly decreased the electrical resistance, anobservation consistent with the enhancement of the conduc-tivity by an order of magnitude when compared with N-MWCNTs. The covalent attachment of the nucleosides in N-MWCNTs promotes their oxidative degradation by HRP/H2O2.Importantly, d-N-MWCNTs remain as a stable film under thesame incubation conditions, demonstrating that functionalgroups are essential for CNT biodegradation. By using a combi-nation of covalent/noncovalent interactions, we were able totune CNTs properties such as biocompatibility, degradability,and conductivity. Methodologies such as the one reportedhere are expected to impact the design of new advanced ma-terials.

Experimental Section

Materials

All reagents and solvents were obtained from commercial suppliers(Sigma–Aldrich) and were used without further purification.MWCNTs 7000 series were purchased from Nanocyl (lot MWMP031105, www.nanocyl.com).

Characterization techniques

Thermogravimetric analyses were recorded on a TGA Q500 (TA In-struments) using a flowing nitrogen atmosphere. The sampleswere equilibrated (1 mg per sample) at 100 8C for 20 min and thenheated at a rate of 10 8C min�1 up to 1000 8C. Thermogravimetricanalysis–mass spectrometry (TGA-MS) experiments were carriedout on a ThermoStar Mass Spectrometer (Pfeiffer Vacuum) coupledto a Q500 thermogravimetric analyzer (TA Instruments) using

Figure 5. Comparison of the measured resistance of a) A/T-MWCNTs and d-A/T-MWCNTs and b) C/G-MWCNTs and d-C/G-MWCNTs, along with the cor-responding linear fits. c) The table shows the calculated values of the bulkresistivity 1, sheet resistance Rs, and electrical conductivity s.

Figure 6. Illustration of N-MWCNTs and d-N-MWCNTs membranes incubatedin Bovine Serum for three days and ultrasonicated for 1 min.

Figure 7. TEM images of N-MWCNTs at 3, 5, 7, and 18 days (a, b, c, and d, re-spectively) during the degradation process by HRP/H2O2.

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a flowing He atmosphere by equilibrating the samples (approxi-mately 5 mg per sample) at 100 8C, and then heating at a rate of20 8C min�1 up to 800 8C. Infrared analyses were performed byusing a Fourier-transform infrared (FT-IR) spectrometer (PerkinElmer2000). The samples were prepared as KBr pellets. Transmissionelectron microscopy analyses were performed on a TEM PhilipsEM208 using an accelerating voltage of 100 kV. The samples wereprepared by dropping an aliquot of the dispersions in the specifiedsolvent on a TEM grid (200 mesh, Nichel, carbon only). For scan-ning electron microscopy measurements, the samples were sput-ter-coated with gold in an Edwards S150 A apparatus (EdwardsHigh Vacuum, Crawley, West Sussex, UK), and examined witha Leica Stereoscan 430i scanning electron microscope (Leica Cam-bridge Ltd. , Cambridge, United Kingdom). Raman spectra were re-corded with an Invia Renishaw microspectrometer equipped witha He–Ne laser at 633 nm using the 100 X objective. Samples wereprepared by drop casting of the dispersion on silicon oxide surfa-ces (Si-Mat silicon wafers, CZ) and the solvent was allowed to evap-orate. The thickness was obtained from three repeated measuresby using a high-accuracy digimatic micrometer (Mitutoyo, MD-H25). The sheet resistance and bulk resistivity values were recordedwith a Jandel resistivity meter (RM3000) with a four-point probehead (probe spacing of 0.635 mm, needles of tungsten carbide) byrepeating the measurements at three different points of the mem-branes.

Preparation of the N-MWCNTs

In a typical experiment, a solution of the nucleoside moiety(46.7 mmol, see the Supporting Information) in anhydrous DMF(1 mL) was added to a well-dispersed solution of NH2-MWCNTs(30 mg) in anhydrous DMF (5 mL) and TEA (triethylamine; 4 mL,28.1 mmol). The resultant mixture was stirred at 90 8C for 48 hunder an argon atmosphere. The black suspension was filtered,washed with DMF, methanol, and dichloromethane and finallydried under vacuum.

Acknowledgements

This work was supported by the Italian Ministry of EducationMIUR (cofin Prot. 2010N3T9M4 and Firb RBAP11C58Y), the Eu-ropean Union through the ERC Advanced Grant “Carbonano-bridge”. Thanks are given to CONACYT through the projectsCB-166014, CB-105568 and PROMEP/103.5/12/3953. E.A.P.thanks CONACYT for the scholarship 328587.

Keywords: biodegradable · carbon nanotubes · conductivefilms · covalent functionalization · nucleosides · supramolecularassemblies

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Received: December 5, 2013

Published online on March 19, 2014

Chem. Eur. J. 2014, 20, 5397 – 5402 www.chemeurj.org � 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim5402

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