pubs.acs.org/Langmuir © 2009 American Chemical Society
Layer-by-Layer Functionalization of Carbon Nanotubes with Synthetic and Natural Polyelectrolytes Agata Zykwinska, Sadia Radji-Taleb, and Stephane Cuenot* Institut des Mat eriaux Jean Rouxel (IMN), Universit e de Nantes, 2, Rue de la Houssini ere, 44322 Nantes cedex 3, France Received July 31, 2009. Revised Manuscript Received September 18, 2009 The surface of carbon nanotubes was noncovalently modified by layer-by-layer deposition of synthetic polyelectrolytes. The efficiency of an easy functionalization process based on alternatively dipping of carbon nanotubes into solutions containing oppositely charged polyelectrolytes was demonstrated. From transmission electron microscopy (TEM) analysis, it was shown that the thickness of the adsorbed polyelectrolyte layers increases linearly with the bilayers number up to reach 6 nm. This easy functionalization covered homogeneously the whole surface of nanotubes as revealed by atomic force microscopy (AFM) images. Then, the adsorbed polyelectrolyte layers were used as anchoring ones to subsequently graft a natural biopolymer. Such postfunctionalization opens the way to design new (nano)biodevices based on carbon nanotubes.
Introduction The nanoworld has attracted a great interest because of the unique and unexpected properties of materials structured at the nanoscale and potential applications in science and technology that they may offer. Carbon nanotubes (CNTs) constitute an excellent example of such nanomaterials. They possess a large number of interesting properties ranging from their extraordinary mechanical strength to their high electrical and thermal conductivities.1,2 These unique properties enabled a wide range of applications, from novel composites3 to electronic circuits4 and sensors.5 However, for the most of applications, the chemical nature of carbon nanotubes surface remains the key factor for obtaining good performance. Indeed, as well as for nanotubes employed in electronic devices than those used as reinforcement agent in polymer matrix, the performance of these systems strongly depends on the interface between the nanotubes and the surrounding medium. In the same way, the surface chemistry of nanotubes completely controls the nanotubes dispersion in solvents or in polymer matrix. Unfortunately, pure carbon nanotubes are very difficult to disperse, and their compatibility with most polymer matrices is poor. Therefore, to achieve these applications, the surface of CNTs should first be modified in order to overcome the insolubility problems of CNTs in almost all solvents. Indeed, extended van der Waals interactions between the side walls of CNTs lead to their aggregation into insoluble bundles of different length and diameter. In order to solve such solubility problems, noncovalent functionalizations of CNTs with water-soluble polymers and *Corresponding author: e-mail
[email protected]; Ph þþ33 240376421; Fax þþ33 240373991. (1) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787– 792. (2) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (3) Dalton, A. B.; Collins, S.; Munoz, E.; Razal, J. M.; Ebron, V. H.; Ferraris, J. P.; Coleman, J. N.; Kim, B. G.; Baughman, R. H. Nature 2003, 423, 703. (4) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49–52. (5) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Science 2000, 287, 622–625. (6) O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265–271.
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biomolecules have successfully been developed.6-11 The stabilization of CNTs with attached macromolecules results from the thermodynamic preference of CNT-polymer interactions with respect to the CNT-water interactions due to suppression of the hydrophobic surface on the CNTs. Moreover, the physical adsorption of molecules onto CNTs surface allows preserving their electronic structure, contrary to the chemical adsorption, where the covalent attachment of material onto the skeleton of CNTs destructs their continuous π-electronic network, resulting in the loss of their unique electronic properties. The noncovalent functionalization of the CNT surfaces may be achieved by a simple adsorption of only one polymer (poly(vinylpyrrolidone),6 poly(styrenesulfonate),6 poly(diallyldimethylammonium chloride)7) or one biopolymer (amylose,8 chitosan,9 protein,10 DNA11) species onto CNTs. It can also be performed using an association of oppositely charged polyelectrolytes adsorbed onto the CNTs surfaces by the so-called layerby-layer (LbL) method.12,13 The resulting polyelectrolyte layers are stabilized, on the one hand, by electrostatic interactions between the two constituting charged polymers and, on the other hand, through hydrophobic interactions, π-π stacking, and van der Waals attractive forces with the CNTs surfaces. The LbL method possesses several advantages. Indeed, it allows to control deposited layer thickness, roughness, porosity, and mechanical properties by using polyelectrolytes of different natures in various conditions of pH and ionic strength.14-16 These (7) Wang, S.; Jiang, S. P.; Wang, X. Nanotechnology 2008, 19, 1–6. (8) Lii, C.; Stobinski, L.; Tomasik, P.; Liao, C. Carbohydr. Polym. 2003, 51, 93–98. (9) Zhang, M.; Smith, A.; Gorski, W. Anal. Chem. 2004, 76, 5045–5050. (10) Karajanagi, S. S.; Yang, H.; Asuri, P.; Sellitto, E.; Dordick, J. S.; Kane, R. S. Langmuir 2006, 22, 1392–1395. (11) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; McLean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338–342. (12) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210, 831–835. (13) Decher, G. Science 1997, 277, 1232–1237. (14) Ladam, G.; Schaad, P.; Voegel, J.-C.; Schaaf, P.; Decher, G.; Ciusinier, F. Langmuir 2000, 16, 1249–1255. (15) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655–6663. (16) Voigt, U.; Jaeger, W.; Findenegg, G. H.; Klitzing, R. J. Phys. Chem. B 2003, 107, 5273–5280.
Published on Web 10/12/2009
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already functionalized with synthetic polymers can further be used for the design of biocompatible and biofunctional materials. This easy process to functionalize the surface of carbon nanotubes opens the way to interesting perpectives such as the improvement of the nanotubes dispersion as well as the ability to elaborate new biodevices based on carbon nanotubes by postfunctionalization.
Experimental Section
Figure 1. Schematic representation of the polyelectrolytes used: poly(diallyldimethylammonium chloride) (A), poly(sodium 4-styrenesulfonate) (B), and ι-carrageenan (C).
different characteristics make the polyelectrolyte multilayers deposited onto CNTs attractive as reservoirs immobilizing macromolecules for biosensor purposes17 and as templates for tissue engineering.18 Two synthetic polyelectrolytes, namely, poly(diallyldimethylammonium chloride) (PDDA), a polycation, and poly(sodium 4-styrenesulfonate) (PSS), a polyanion, were successfully used in a few studies to functionalize CNTs using the LbL method.19-21 All these studies were only carried out on the CNTs grown across transmission electron microscopy (TEM) copper grids. In that case, the LbL deposition of polyelectrolytes onto well-individualized nanotubes seems to be facilitated. However, it is well-known that CNTs are present as bundles difficult to maintain separately because of strong hydrophobic interactions. Therefore, to develop new easy methods for modifying the CNTs surface remains a challenge. In the present work, we describe, for the first time, an easy functionalization method of CNTs based on alternated dipping of nanotubes into solutions containing oppositely charged polyelectrolytes. The nanotubes were noncovalently functionalized with synthetic polyelectrolytes, PSS and PDDA (Figure 1). The LbL functionalization of the CNTs with PSS and PDDA was performed with no salt added. In these conditions, these strong polyelectrolytes adopt an extended, rodlike conformation which is stabilized by the electrostatic repulsive forces. Polyelectrolyte layers formed are therefore thin, and multilayer displays a typical linear growth. In order to demonstrate the efficiency of the deposition process and the growth of homogeneous polyelectrolyte layers onto CNTs surface, the modified nanotubes were analyzed by TEM and atomic force microscopy (AFM). From TEM analysis, it was revealed that the thickness of the adsorbed polyelectrolyte layers increases linearly with the number of bilayers up to reach a plateau for a thickness of 6 nm. AFM images clearly showed that the polyelectrolyte functionalization was homogeneous over the whole surface of carbon nanotubes. Then, the well-characterized CNTs-polyelectrolyte system was used in a subsequent step as a substrate for grafting another polymer. The grafting of a natural biopolymer, ι-carrageenan, allows to demonstrate that carbon nanotubes (17) Munge, B.; Liu, G.; Collins, G.; Wang, J. Anal. Chem. 2005, 77, 4662–4666. (18) Gheith, M. K.; Sinani, V. A.; Wicksted, J. P.; Matts, R. L.; Kotov, N. A. Adv. Mater. 2005, 17, 2663–2670. (19) Artyukhin, A. B.; Bakajin, O.; Stroeve, P.; Noy, A. Langmuir 2004, 20, 1442–1448. (20) Huang, S.-C. J.; Artyukhin, A. B.; Wang, Y.; Ju, J.-W.; Stroeve, P.; Noy, A. J. Am. Chem. Soc. 2005, 127, 14176–14177. (21) Artyukhin, A. B.; Shestakov, A.; Harper, J.; Bakajin, O.; Stroeve, P.; Noy, A. J. Am. Chem. Soc. 2005, 127, 7538–7542.
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Materials. Poly(diallyldimethylammonium chloride) (PDDA), weight-average molar mass Mw =100 000-200 000 g/mol, poly(sodium 4-styrenesulfonate) (PSS), Mw = 70 000 g/mol, and ι-carrageenan were purchased from Sigma-Aldrich. Arc-discharge multiwalled carbon nanotubes (MWNTs) were obtained from Materials & Electrochemical Research Corporation (MER Corp., Tucson, AZ). This raw material contained 30-40% nanotubes with 8-30 graphitic layers, a 10-25 nm diameter, and a 1-5 μm length. The remainder consisted of multilayer polygonal carbon nanoparticles as well as amorphous and graphitic carbon nanoparticles. MWNTs and polyelectrolytes were used without any further purification. Functionalization of MWNTs with Polyelectrolyte Multilayers by the LbL Method. MWNTs (1 mg) were dispersed in a 10 mM aqueous solution of an anionic polyelectrolyte, PSS (1 mL), by mild sonication for 1 h in a sonic bath. During the sonication step, the dispersion was chilled by immersion in ice water. Excess of PSS was removed by centrifugation step (10 000 rpm, 20 min). The pellet containing MWNTs-PSS was then mixed with a 10 mM aqueous solution of a cationic polyelectrolyte, PDDA (1 mL), by mild sonication in a sonic bath (1 h, icecold water). Excess of PDDA was removed by centrifugation step. The alternating dispersions of MWNTs in an anionic polyelectrolyte and a cationic polyelectrolyte (one bilayer) was repeated until 20 bilayers of PSS and PDDA were obtained. An aliquot of MWNTs functionalized with PSS-PDDA (100 μL) was removed at each bilayer formed for TEM characterization.
Biopolymer Grafting onto Polyelectrolyte-Functionalized MWNTs. MWNTs (1 mg) were dispersed in a 10 mM aqueous solution of PSS (1 mL) by mild sonication for 1 h in a sonic bath. During the sonication step, the dispersion was chilled by immersion in ice water. Excess of PSS was removed by centrifugation step (10 000 rpm, 20 min). The pellet of MWNTs-PSS was then mixed with a 10 mM aqueous solution of PDDA (1 mL) by mild sonication in a sonic bath (1 h, ice-cold water). To remove the excess of PDDA remaining after adsorption, the dispersion was centrifuged. The pellet containing MWNTs functionalized with one bilayer PSS-PDDA was further dispersed in a ι-carrageenan (1 mL) solution. The polysaccharide solution was prepared at a concentration of 1 mg/mL in 10 mM NaCl. The solution was heated to 90 °C and stirred for 30 min and then cooled at room temperature before using. An aliquot of functionalized MWNTs (100 μL) was removed for TEM characterization. Transmission Electron Microscopy. Transmission electron microscopy (TEM) characterizations were performed on a Hitachi HF2000 field emission gun (FEG) electron microscope with an electron energy loss spectroscopy (EELS) detector. A drop of suspension of MWNTs functionalized with polyelectrolyte multilayers was deposited on a copper grid (mesh 2000) without using any supporting amorphous carbon film and air-dried. The microscope was operating at an acceleration voltage of 200 kV using low-dose exposure conditions. Thickness measurements of the polyelectrolyte bilayers deposited onto MWNTs were performed using DigitalMicrograph software (Gatan Inc., Pleasanton, CA). At least 20 measurements were taken for each carbon nanotube. Average thickness per polyelectrolyte bilayer and the corresponding standard deviation were calculated using Origin software (OriginLab Corp., Northampton, MA). Langmuir 2010, 26(4), 2779–2784
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Figure 2. Representative TEM image of the pristine multiwalled carbon nanotubes.
Atomic Force Microscopy. To analyze the surface of pure and modified carbon nanotubes, they were dispersed onto silicon substrates. These substrates were previously cleaned and treated by UV-ozone to remove any organic contaminant. The carbon nanotubes surface was imaged with a scanning probe microscope Multimode-Nanoscope IIIA (Veeco) operating in the intermittent contact mode under ambient conditions. A standard rectangular cantilever (Nanosensors NCL-W) was used for imaging, with a free resonance frequency of 156.06 kHz and a typical spring constant of about 40 N/m. For the used cantilever, a very clean resonance shape was obtained. Phase contrast images were recorded by using soft intermittent contact conditions. Such conditions correspond to a low free amplitude (A0 =30 nm) and a set-point ratio close to unity. Precisely, this ratio of the oscillation amplitude (A) to the free amplitude was fixed to 0.9. All phase images were acquired with the same cantilever under the same operating conditions. The images presented here were not filtered and shown as captured. Note that to reduce the topographical effects of the analyzed nanotubes in phase images, it is preferable to image the nanotubes with their axis parallel to the fast scan axis.
Results and Discussion LbL Assembly of PSS and PDDA onto MWNTs. The multilayers were built by alternative electrostatic deposition of oppositely charged PSS and PDDA onto MWNTs. Carbon nanotubes were used as received, without any purification step, which is known to introduce oxygen-containing groups and to imply the loss of the electronic properties of the nanotubes. The pristine carbon nanotubes have a well-defined graphitic structure and a very smooth surface as shown in Figure 2. The pristine carbon nanotubes are present as bundles stabilized by strong hydrophobic interactions. The high attractive energy between carbon nanotubes is also related to their axial geometry, which provides a large area of contacts. In order to efficiently disperse the bundles, MWNTs were initially sonicated in water in the presence of anionic PSS. Ultrasonication facilitates the separation of aggregated carbon nanotubes, thus allowing the interaction of aromatic rings of the polyelectrolyte via π-stacking with the hydrophobic nanotube surface. PSS has a high density of negatively charged sulfonate groups which serve as primers for the subsequent adsorption of the cationic PDDA. The LbL functionalization of MWNTs with PSS and PDDA was performed in Langmuir 2010, 26(4), 2779–2784
water, and no salt was added. It was previously established that at low ionic strength conditions PSS and PDDA adopt an extended, rodlike conformation which favors the formation of thin layer of ∼1 nm, and the resulting multilayer displays a typical linear growth.22-24 To probe local details of carbon nanotubes surface modifications, in particular in order to assess the homogeneity of the functionalization as well as the thickness of the coating, local surface characterization techniques have been employed for analyzing the modified nanotubes. Precisely, a detailed analysis was realized by TEM and AFM on numerous individual nanotubes. First of all, alternating deposition of PSS and PDDA onto the carbon nanotube surfaces was followed by TEM. Up to 20 PSS-PDDA bilayers were successively deposited, and typical TEM images obtained for 4 and 10 bilayers are presented in Figure 3. Polyelectrolytes adsorbed onto the carbon nanotube surfaces can be easily distinguished as thin layer that coats the graphitic structure of nanotubes. From TEM images, the thickness of the different bilayers deposited onto the carbon nanotubes surface were measured. Precisely, for each polyelectrolyte bilayer, several measurements of the bilayers thickness were taken on numerous nanotubes. In Figure 4, histograms corresponding to the distribution of the measured thicknesses for 2 and 12 PSS-PDDA bilayers are shown. The polyelectrolyte layers seem relatively homogeneous in thickness with a narrow dispersion of values. As expected, it can be seen from the histograms that the layer thickness increases with the number of deposition cycles. However, it varies from ∼1.8 to ∼3.6 nm for two PSS-PDDA bilayers and it increases up to ∼8 nm for 12 bilayers. This slight dispersion in the coating thickness can be due to many factors. Indeed, the use of carbon nanotubes without purification and the presence of small defects onto their surfaces may limit the ability of extended polyelectrolytes to form a homogeneous coating. Additionally, it can be thought that if the deposition of the first PSS layer was not efficient enough to cover the whole nanotube surface, in a subsequent step, PDDA could be adsorbed not only onto the first PSS layer but also onto the nanotube surface not yet functionalized. Indeed, the possibility of interaction between less hydrophobic PDDA and carbon nanotubes was already suggested by Yang et al.25 The following alternating adsorptions of PSS and PDDA will lead to inhomogeneous thickness coating, which increases with the number of deposited layers. Furthermore, although electrostatic interactions between oppositely charged polyelectrolytes play a major role during the LbL assembly, additional contributions such as van der Waals interactions, hydrogen bonds, and hydrophobic attractions can also contribute substantially. In particular, hydrophobic interactions were revealed as essential for multilayer formation.26 The presence of interactions other than electrostatic may therefore prevent from tight adsorption and lead to a more looplike conformation, which increases a local disorder that contributes to the inhomogeneous growth observed. Moreover, TEM images of the polyelectrolyte functionalized carbon nanotubes do not show any helical wrapping of adsorbed polyelectrolytes onto the carbon nanotube surfaces. Indeed, high chain stiffness of both polyelectrolytes makes the wrapping phenomenon very unlikely. (22) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153–8160. (23) Lvov, Y. M.; Lu, Z.; Schenkman, J. B.; Zu, X.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 1073–4080. (24) Grosberg, A. I. U.; Khokhlov, K. R. Statistical Physics of Macromolecules; AIP: New York, 1994. (25) Yang, D.-Q.; Rochette, J.-F.; Sacher, E. J. Phys. Chem. B 2005, 109, 4481– 4484. (26) Kotov, N. A. Nanostruct. Mater. 1999, 12, 789–796.
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Figure 3. Representative TEM images of the multiwalled carbon nanotubes after coating with 4 (A) and 10 (B) alternating PSS-PDDA bilayers.
Figure 4. Distribution of the thickness measured (from TEM analysis) for 2 (A) and 12 (B) PSS-PDDA bilayers adsorbed onto multiwalled carbon nanotubes.
Figure 5. Average thickness of PSS-PDDA bilayers deposited onto carbon nanotubes as a function of the bilayers number.
It can also be thought that the polymer chains behave as if they were on a flat surface when adsorbing on the surface of MWNTs (10-25 nm of diameter), which may prevent from proper wrapping. The average thickness and its corresponding standard error were calculated for each PSS-PDDA bilayer from the thickness distributions and are shown in Figure 5. An increase in thickness of ∼1.2 nm per bilayer was observed until eight PSS-PDDA bilayers. Further incubation of carbon nanotubes with polyelectrolytes did not lead to an increase of thickness, and a plateau value of ∼6 nm was observed. Similar results were reported for PSS and PDDA adsorbed onto silicon wafers, where the thickness of 10 polyelectrolyte bilayers, in the absence of salt added, was ∼6 nm.23 It can be thought that the rapid setting of the equilibrium thickness is due to the interpenetration of adjacent 2782 DOI: 10.1021/la902818h
Figure 6. AFM phase contrast images (2 μm width) of an unmodified carbon nanotube (a) and a functionalized nanotube by two PSS-PDDA bilayers (b) deposited onto silicon substrate. The same experimental conditions and the same phase angle scale were used.
polyelectrolyte species and counterbalancing of charges.13,27,28 According to Arys et al.,29 three mechanisms govern growth and structuring of polyelectrolyte in the case of films: (i) adsorption of the polyelectrolyte through electrostatic balance, (ii) diffusion of the polyelectrolyte into the previously adsorbed film, and (iii) surface-constrained complexation between the polyanion and the polycation resulting from their mixing due to diffusion. It can be (27) Schmitt, J.; Gr€unewald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; L€osche, M. Macromolecules 1993, 26, 7058–7063. (28) L€osche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893–8906. (29) Arys, X.; Laschewsky, A.; Jonas, A. M. Macromolecules 2001, 34, 3318– 3330.
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Figure 7. (A) TEM image of ι-carrageenan grafted on a carbon nanotube functionalized with one PSS-PDDA bilayer. (B) Distribution of the thickness measured for the carrageenan-polyelectrolytes complex adsorbed onto multiwalled carbon nanotubes.
thought that similar phenomena occur during multilayers formation onto the colloidal surfaces. In the present study, the interpenetration of the polyelectrolytes onto the carbon nanotube surfaces can be confirmed by the fact that PSS and PDDA are supposed to produce each a layer of ∼1 nm thick whereas an increase in thickness of only 1.2 nm per PSS-PDDA bilayer was observed (Figure 5). High-resolution TEM study is complemented by AFM for checking the homogeneity of the polyelectrolyte functionalization over the whole surface of nanotubes. Indeed, although TEM is a particularly well-suited technique to measure the polyelectrolyte layers thickness and the coating seems homogeneously distributed along the nanotubes, AFM was used to investigate the homogeneity of the surface modification on individual nanotubes. The surface of pure and functionalized carbon nanotubes, previously deposited onto silicon substrates, was imaged by AFM operating in intermittent contact mode. Precisely, all phase contrast images were realized by using the same soft conditions. Indeed, during the past 10 years, numerous works have pointed out that the tip-sample interaction regimes strongly depend on the experimental parameters.30 Under soft conditions, mainly surface properties are probed whereas hard conditions induce an important contribution of volume properties. In our experiments, the contrast of phase images comes essentially from the dissipated energy in the tip-surface interaction per oscillation cycle. Figure 6a shows the phase contrast obtained for a pure carbon nanotube (20.3 nm of diameter) lying on a silicon substrate. As expected, no clear contrast difference between the unmodified nanotube and the substrate was evidenced. The dissipated energy in the tip-nanotube interaction is not significantly different than that measured in the tip-substrate interaction. This result is also confirmed by the high hardness of both materials. The same experimental conditions were then used to image polyelectrolyte functionalized nanotubes. For these modified nanotubes, phase contrast images clearly revealed a phase shift between nanotubes and the substrate (Figure 6b). In Figure 6, the same phase angle scale was used for the two phase images. Precisely, a higher phase shift (appearing in white in Figure 6b) was observed on the functionalized nanotube (11.5 nm of diameter) by two PSSPDDA bilayers with respect to the substrate. As the phase angle is directly related to the dissipated energy, the observed phase contrast indicates that more energy is dissipated on the nanotube.31-33 Bodiguel et al.34 have recently shown that the dissipation measured on polymer layers mainly originates in their (30) Bar, G.; Thomann, Y.; Brandsch, R.; Cantow, H.-J.; Whangbo, M.-H. Langmuir 1997, 13, 3807–3812.
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high adhesion properties, which are related to the softness of the material. As the same experimental conditions were rigorously used for the two phase images (Figure 6), the contrast difference between them can only be attributed to the polyelectrolyte layers adsorbed onto carbon nanotubes. The signature of these polyelectrolyte layers also reveals that these layers are adsorbed over the whole surface of the nanotube (not only at the nanotube ends). Biopolymer Grafting onto Polyelectrolyte Functionalized MWNTs. In order to demonstrate that carbon nanotubes functionalized with synthetic polymers can further be used as departure matrix for design of other materials, especially those that are biocompatible and/or biofunctional, a natural anionic polymer, namely ι-carrageenan, was subsequently deposited onto already functionalized carbon nanotube surfaces. This anionic sulfated polysaccharide is recovered after extraction from red seaweeds. It possesses a linear galactan backbone composed of alternating 3-linked β-D-galactopyranose and 4-linked 3,6-anhydrogalactose (Figure 1).35 A very interesting characteristic of ι-carrageenan is its capacity to adopt a structured helical conformation at room temperature. The conformation of ι-carrageenan strongly depends on the ionic strength and the temperature of the solution. A reversible helix-random coil conformational transition is observed upon heating in the presence of salt.36,37 In the present study, ι-carrageenan solution was prepared in the presence of NaCl salt, which is known to induce a helix conformation of ι-carrageenan chains.38 Recently, Schoeler et al.39 have demonstrated that the helical conformation of ι-carrageenan chains was preserved during the LbL film formation with oppositely charged synthetic polyelectrolyte, poly(allylamine hydrochloride). The authors reported that the presence of ordered helical structures results in thick films with high mechanical properties.39 Thus, taking into account the previous studies, ι-carrageenan appears as a good candidate for further CNTs functionalization. (31) Cleveland, J. P.; Anczykowski, B.; Schmid, A. E.; Elings, V. B. Appl. Phys. Lett. 1998, 72, 2613–2615. (32) Garcia, R.; Tamayo, J.; Calleja, M.; Garcia, F. Appl. Phys. A: Mater. Sci. Process. 1998, A66, S309. (33) Nony, L.; Boisgard, R.; Aime, J.-P. J. Chem. Phys. 1999, 111, 1615. (34) Bodiguel, H.; Montes, H.; Fretigny, C. Rev. Sci. Instrum. 2004, 75, 2529– 2535. (35) Knutsen, S. H.; Myslabodski, D. E.; Larsen, B.; Usov, A. I. Bot. Mar. 1994, 37, 163–169. (36) Morris, E. R.; Rees, D. A.; Robinson, G. J. Mol. Biol. 1980, 138, 349–362. (37) Tako, M.; Nakamura, S.; Khoda, Y. Carbohydr. Res. 1987, 161, 247–255. (38) Grinberg, V. Y.; Grinberg, N. V.; Usov, A. I.; Shusharina, N. P.; Khoklov, A. R.; de Kruif, K. G. Biomacromolecules 2001, 2, 864–873. (39) Schoeler, B.; Delorme, N.; Doench, I.; Sukhorukov, G. B.; Fery, A.; Glinel, K. Biomacromolecules 2006, 7, 2065–2071.
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Therefore, the biopolymer was grafted on CNTs functionalized with one PSS-PDDA bilayer, which allows sufficient dispersion of CNTs. In order to avoid the counterbalancing of charges and consequently to optimize the postfunctionalization, only one PSS-PDDA bilayer was previously adsorbed on the nanotube surfaces. The representative TEM image of the new complex formed is presented in Figure 7a. Although the majority of the functionalized nanotubes were then covered by ι-carrageenan chains, these chains were not always homogeneously distributed along the nanotubes. Precisely, carrageenan macromolecules did not always cover the whole surface of modified nanotubes. The histogram obtained from the TEM analysis of several carrageenan-carbon nanotubes shows clearly the presence of two populations of different thicknesses (Figure 7b). The first population of ∼1 nm thick corresponds most likely to one PSS-PDDA bilayer that was first deposited onto the carbon nanotube surfaces, whereas the second population of ∼5 nm in thickness should correspond to grafted ι-carrageenan chains. The ability to realize a postfunctionalization over the polyelectrolyte-carbon nanotubes complex paves the way to create, in the future, new biodevices based on carbon nanotubes.
Conclusion In this work, an easy process to modify the chemical nature of carbon nanotubes was proposed. Layer-by-layer functionalization of the carbon nanotubes surface was realized by dipping alternatively the carbon nanotubes into aqueous solutions containing oppositely charges polyelectrolytes. By this method, the nanotubes were noncovalently functionalized with synthetic polyelectrolyte layers, PSS and PDDA. From TEM analysis realized
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on the functionalized nanotubes, a linear increase in the thickness of adsorbed bilayers as a function of the bilayer number was observed up to reach a thickness of about 6 nm. This equilibrium thickness, mainly due to the interpenetration of polyelectrolyte layers and counterbalancing of charges, was not significantly exceeded by increasing the number of bilayers. AFM phase contrast images also confirmed that the polyelectrolyte layers were homogeneously adsorbed over the whole nanotubes surface. From an experimental point of view, AFM operating in intermittent contact mode with soft conditions allows to distinguish polymer modified nanotubes from unmodified nanotubes as well as to check the homogeneity of the functionalized layers along the nanotubes. Then, the adsorbed polyelectrolyte layers can be used as an anchoring layer to a subsequent easy functionalization with different biomacromolecules. To demonstrate this ability, a natural biopolymer (ι-carrageenan) was successfully grafted onto already functionalized carbon nanotube surfaces with one PSS-PDDA bilayer. The noncovalent functionalization of the carbon nanotubes surface (keeping their outstanding physical properties) by polyelectrolyte multilayers offers an interesting approach to easy modify the hydrophobic nature of nanotubes and thus to improve their dispersion. Moreover, from the polyelectrolyte layers, new biodevices or nanosensors based carbon nanotubes could be easily designed by a postfunctionalization. Acknowledgment. A.Z. and S.C. acknowledge the “Agence Nationale de la Recherche” for financial support in the frame of “ANR06-Jeunes Chercheurs-n°24” program. The authors acknowledge Eric Gautron for his help and practical advice during TEM manipulations.
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