ARTICLE pubs.acs.org/JPCC
Programmed Dispersions of MWNTs in Aqueous Media by Coating with Photopolymerizable Synthetic Amphiphilesz Cedric Thauvin,†,^ Aurelia Perino,†,^ Emmanuel Contal,† Emmanuelle Morin,†,‡ Patrick Schultz,§ Stephane Meunier,† and Alain Wagner*,† †
Laboratory of Functional Chemo-Systems, UMR 7199, Faculte de Pharmacie, Universite de Strasbourg, 74 route du Rhin, 67401 Illkirch, France ‡ Laboratoire de Chimie Genetique, UMR 7199, Faculte de Pharmacie, Universite de Strasbourg, 74 route du Rhin, 67401 Illkirch, France § Departement de Biologie et Genomique Structurale, UMR 7104, Institut de Genetique et de Biologie Moleculaire et Cellulaire, Universite de Strasbourg, 1 rue Laurent Fries, 67404 Illkirch, France
bS Supporting Information ABSTRACT: Amphiphiles are known to be suitable molecules to disperse carbon nanotubes (CNTs) in water by shielding their highly hydrophobic surfaces. Herein, we describe tailored anionic, nonionic, and cationic photopolymerizable synthetic amphiphiles that are designed to achieve programmed pHdependent dispersions of CNTs. The reported process involves (1) the assessment of the amphiphiles’ ability to form micelles in given buffer solutions, (2) the equilibrated self-assembly of the amphiphiles into hemimicellar structures on the hydrophobic surface of the CNTs, (3) the stabilization of these labile supramolecular assemblies by photopolymerization, (4) the purification of the obtained CNTs/photopolymerized lipidic assemblies (PLA) constructs, and (5) the characterization of the CNTs/PLA constructs pH dependant dispersion. In conclusion, we demonstrate that CNTs/PLA constructs with specific pH-dependent dispersion properties can easily be prepared. The constructs appear quite robust because they can withstand several cycles of precipitation and redispersion, and interestingly, they demonstrate the same pH-dependency as the amphiphiles used in the coating process.
I. INTRODUCTION Because of their electronic, thermal, optical, and mechanical properties, carbon nanotubes (CNTs) are of a great interest in various fields, including nanoelectronics,1,2 polymer reinforcement,3-5 biosensors,6,7 and field emission devices.8 However, the stable dispersion of CNTs is very difficult to obtain because pristine CNTs aggregate due to strong van der Waals attractions.9 Thus, many studies have been conducted to improve the dispersion of CNTs in different media, especially aqueous media. They usually refer to two approaches: covalent modification10,11 of the CNTs’ surface and noncovalent complexations via adsorption of surfactants12,13 or synthetic lipids,14-17 π-π interactions with aromatic compounds,18,19 as well as coating with polymers,20-22 proteins,23,24 or DNA.25 Moreover, it has been reported that these dispersions can respond to external stimuli, such as concentration, temperature, or pH, depending on the nature of the resulting CNTs’ envelope.11,12,20,21,23,24 Our group focuses on dispersing CNTs using noncovalent interactions and especially on the adsorption of synthetic amphiphiles.26 This has a distinct advantage because the structure of CNTs remains unchanged. We recently demonstrated27 that particularly stable dispersions of single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs), and multi-walled carbon nanotubes (MWNTs) could be obtained in basic aqueous solutions, such as Tris buffer pH 8, using r 2011 American Chemical Society
the amphiphile 2 (Figure 1a), which contains a photosensitive diacetylene moiety and a polar headgroup consisting of nitrilotriacetic acid (NTA). This amphiphile possesses two key features: it self-assembles into hemimicellar structures on the surface of CNTs and it photopolymerizes upon UV irradiation.28 These assemblies are stabilized by the formation of an ene-yne backbone (Figure 1a,b), which is in contrast to the frequently reported surfactants, such as SDS or CTAB.13,26 Building on these preliminary results, we decided to assess whether various photopolymerizable amphiphiles containing different hydrophobic polar heads would self-assemble on the surface of CNTs. We also wanted to determine whether these self-assemblies would tolerate the photopolymerization conditions and how the polar heads would affect specific dispersion properties.
II. EXPERIMENTAL METHODS A. Material. Arc-discharge MWNTs were purchased from n-Tec, Norway. All lipids are the result of a peptidic coupling between commercially available 10,12-pentacosadiynoic acid and different amines. Synthesis details are given in the Supporting Received: January 3, 2011 Published: March 29, 2011 7319
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Scheme 1. Chemical Structures of Amphiphiles Used in This Studya
a
Figure 1. Photopolymerization of the hemimicellar assemblies on the surface of CNTs. (a) The synthetic amphiphile 2 and the resulting polymer (the NTA polar heads are shown in blue, and the cross-link is in red). (b) Schematic representation of the process to obtain CNTs coated with photopolymerized CNTs in a hemimicellar arrangement. 1: equilibrated formation of the hemimicellar structure on the CNTs’ surface by micellar amphiphile adsorption. 2: photopolymerization of the structures around the nanotubes freezes the equilibrium (the crosslink is symbolized as red dots). 3: centrifugation separates the surplus of polymerized micelles from the coated CNTs.
Table 1. Ability of Each Amphiphile to Form Micelles in the Four Buffers As Assessed by DLS formation of micelles in the four buffers (pH) amphiphiles
acetate (4.7)
HEPES (7.4)
Tris (8)
Tris (11)
1
no
no
no
yes
2 3
no yes
yes yes
yes yes
yes yes
4
yes
no
no
no
Information. Tris buffers were prepared from tris(hydroxymethyl)aminomethane (100 mM) and sodium chloride (150 mM), the pH of the solution being adjusted at 8 or 11 with concentrated hydrochloric acid. HEPES buffer was prepared from HEPES salt (100 mM), the pH of the solution being adjusted at 7.4 with an aqueous solution of 1M NaOH. PBS buffer was prepared from Na2HPO4 (100 mM), NaH2PO4 (100 mM), and sodium chloride (150 mM). Acetate buffer was prepared from sodium acetate (100 mM), the pH of the solution being adjusted at 4.7 with concentrated acetic acid. B. Apparatus. A Bandelin Sonorex, RK100H sonicator bath was used at 80 W for the sonication steps. A Cross-Linker BioLink 254 from Fischer Bioblock was used at 48 W, 254 nm for the photopolymerization steps. A BR4i centrifuge from JOUAN with
The number of steps and overall yields are indicated on the arrows.
a AB1.14 fixed-angle rotor was used for the centrifugation steps (maximal force = 14 000 g). TEM images of coated CNTs have been done with a Philips CM 120 microscope where 10 μL of the sample is disposed on a Cu/Rh grid covered with carbon, dried, and stained with uranyl acetate (2%). DLS analyses have been done with a Nano-ZS Malvern apparatus. C. General CNT Coating Process. Amphiphiles (6 mg) and MWNTs (36 mg), purchased from n-Tec company, were sonicated in a bath sonicator (80 W) for 30 min in 2 mL of buffer, which enabled the formation of micelles as defined in Table 1: 1 in Tris buffer pH 11, 2 in Tris buffer pH 8, 3 in HEPES buffer pH 7.4, and 4 in acetate buffer pH 4.7. The different samples were then irradiated under a UV light (254 nm, 48 W) for 10 h.
III. RESULTS AND DISCUSSION Four specific amphiphiles (Scheme 1) were designed and synthesized. They were all obtained from commercially available 10,12-pentacosadiynoic acid (PCDA) and consequently bear the same hydrophobic photopolymerizable tail that is able to interact with the CNTs' surface. The different functions of these polar heads—malonic (1), NTA (2), PEG-8 (3), and amine (4)29— were chosen for their different pH-dependent properties in aqueous media. We thus expected that the resulting constructs, generically called carbon nanotubes/photopolymerized lipidic assemblies (CNTs/PLA), will show pH-dependent solubilities, which we would be able to correlate to the lipid polar head function. We previously assumed that the formation of micelles is a key step for the production of CNTs/PLA constructs26 and, therefore, evaluated the ability of each amphiphile to form micelles in four aqueous solvents: acetate buffer pH 4.7, HEPES buffer pH 7.4, Tris buffer pH 8, and Tris buffer pH 11. These different buffers cover a wide range on the aqueous pH scale. Three milligrams of each amphiphile was dissolved in 1 mL of buffer and sonicated for 30 min in a bath sonicator (80 W). Samples were then submitted to dynamic light scattering (DLS) analysis. Amphiphiles were considered to have formed micelles only when particles with a diameter around 10 nm were observed (Table 1). As expected, amphiphiles 1, 2, and 4 only form micelles when their polar heads are in their ionic form: above pH 11 for 1, above pH 7.4 for 2, and below pH 4.7 for 4. In contrast, amphiphile 3 forms micelles in all of the buffers, independent of the pH value. We then subjected these amphiphiles to a CNT coating process following a systematic scheme, setting the ratio of amphiphile to CNTs to 1/6, as recommended in a previous study.27 Characterization of the resulting dispersions of CNTs/ 7320
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Figure 3. Dispersions of MWNTs by coating with amphiphiles (a) 1, (b) 2, (c) 3, and (d) 4 in (A) acetate buffer pH 4.7, (B) HEPES buffer pH 7.4,29 (C) Tris buffer pH 8, and (D) Tris buffer pH 11 after (left) 5 min, (middle) 1 day, and (right) 1 month.
Figure 2. TEM pictures of CNTs/PLA constructs obtained from amphiphiles. (a) 1 in Tris buffer pH 11 (scale bar = 0.5 μm). (b) 2 in Tris buffer pH 8 (scale bar = 0.2 μm). Close-up views of (c) 1 in Tris buffer pH 11, (d) 2 in Tris buffer pH 8, (e) 3 in HEPES buffer pH 7.4, and (f) 4 in acetate buffer pH 4.7 (scales bars = 50 nm). The CNTs' concentration is 18 mg/mL.
PLA constructs was carried out by transmission electron microscopy (TEM) (Figure 2). The TEM pictures (Figure 2a,b) provide representative examples of the state of CNTs' dispersion in the various samples. They show that CNTs are mainly dispersed as individual entities with a length of several micrometers. Moreover, at high resolution, each sample shows that isolated MWNTs are completely covered by regular striations perpendicular to the nanotube axes, which correspond to the self-organized amphiphiles. This observation proves that the CNT coating process, previously reported only for lipid 2, could be generalized to amphiphiles bearing various polar heads. This standardized procedure thus appears amenable to the preparation of a wide range of CNTs/ PLA constructs without any optimization. However, as previously described, the initial amphiphile/CNTs ratio might require some adjustment, especially when changing from one type of CNT to another. Control experiments showed that, in the absence of amphiphiles, CNTs precipitate directly after the sonication step. To correlate the pH-dependent ability of amphiphiles to form micelles (Table 1) and the stable dispersion of CNTs, each of
these four CNTs/PLA samples was divided into four equal volumes and centrifuged for 90 min at 14 000 g to remove the original buffer. Moreover, this centrifugation step allowed the elimination of the excess of micelles, as confirmed by DLS analysis of the supernatant, which revealed these photopolymerised micelles to have a similar size distribution to the original micelles. Final concentrations of MWNTs reach 4.5 mg/mL. The resulting aggregated CNTs/PLA constructs were then dispersed in the four buffers (2 mL), as previously described, and incubated for different lengths of time to assess the stability of the dispersions (Figure 3). The vials presented in Figure 3 correspond to all possible combinations of CNTs/PLA constructs in the different buffers after incubation for 5 min, 1 day, and 1 month. After 5 min, black dispersions are observed with amphiphiles 1 and 2 in neutral and basic media, with 3 in all media and with 4 only in the acidic medium. However, it is observed that a very light gray dispersion of 1 and a gray dispersion of 2 occur in acidic medium, and the stability of the dispersion of 4 is reduced as the pH is increased. The same dispersion pattern is observed in the vials after 1 day and after 1 month, except for 1 in neutral and Tris pH 8 buffers. Thus, these results show that the dispersion of the CNTs/PLA constructs is correlated with the ability of amphiphiles to form micelles, as presented in Table 1. Moreover, these dispersions are very stable because they are still observed after a few months. The only notable exception is obtained with amphiphile 1 in Tris buffer pH 11 where the dispersion seems to slowly precipitate, although the amphiphile has been proved to form micelles. Our results suggest that, when amphiphiles form organized structures, such as micelles, further stabilization of the system is obtained by the arrangement of the amphiphiles on the CNTs’ surface (Figure 1b, step 1). This can be explained by the highly hydrophobic property of the CNTs’ surface, which drives the amphiphile organization from a micellar structure to a hemimicellar structure in order to maximize the CNTs' coverage, thus shielding the hydrophobic surface from water. Photopolymerization then freezes the equilibrium (Figure 1b, step 2) to afford stable CNTs/PLA constructs. In contrast, if the amphiphiles do not self-organize themselves, the net energy gain seems insufficient to drive the system toward the highly ordered CNTs/PLA constructs observed. Finally, to evaluate the reversibility of the pH-dependent properties of the CNTs/PLA constructs, a sample obtained from amphiphile 4 solubilized in acetate buffer pH 4.7 was engaged in five successive cycles of (1) centrifugation at 14 000 g for 7321
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The Journal of Physical Chemistry C 20 min to remove acetate buffer pH 4.7, (2) sonication in Tris buffer pH 11, (3) centrifugation at 14 000 g for 3 min to remove Tris buffer pH 11, and (4) dispersion in the acetate buffer pH 4.7. The buffers were chosen according to the observations in Figure 3. At the end of this process, a stable dispersion of MWNTs was still observed after 1 day, demonstrating that changing the pH of the medium does not alter the pH-dependent properties of the CNTs/PLA constructs.
IV. CONCLUSION In summary, four synthetic amphiphiles containing the same photopolymerizable lipophilic tail, but with different hydrophilic polar heads, were designed and synthesized for the pH-dependent dispersion of MWNTs. Following self-assembly on the hydrophobic surface of MWNTs, these four amphiphiles formed hemimicellar structures that were stabilized by photopolymerization. The resulting CNTs/PLA constructs formed stable dispersions of MWNTs in aqueous media, which can be reversibly switched between a dispersed and an aggregated state by changing the pH. This leads to interesting opportunities to incorporate CNTs into different water-based chemical environments and to develop novel supramolecular systems for programmed CNT dispersion responding to an external stimulus. Moreover, we demonstrated that these CNTs/PLA constructs have the same pH dependency as the amphiphiles used in the coating process. The described CNT coating process relies on the photopolymerization of self-organized amphiphiles on the CNTs’ hydrophobic surface. The resulting properties of the final CNTs/PLA constructs can be easily derived from the ability of the amphiphile to form micelles. This procedure appears appropriate for producing coated nanotubes with specific solubility properties. It radically differs from the traditional polymer coating approaches that were laborious to perform and required thorough characterization. Indeed, our straightforward synthesis of amphiphiles enables the preparation of an almost infinite variety of trigger responsive nanotube dispersions. The ability to precisely specify the condition of CNT dispersion/aggregation might be of particular interest for creating ordered CNT-based nanostructures and CNT-based nanoarchitectures for sensors, optical, or surface-coating applications. ’ ASSOCIATED CONTENT
bS
Supporting Information. Protocols for the syntheses of amphiphiles 1, 2, 3, and 4. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: þ33 368 85 42 97. Fax: þ33 368 85 43 06. E-mail:
[email protected]. Author Contributions ^
These authors contributed equally.
’ ACKNOWLEDGMENT This research was supported by CHANEL Research and Technology Group. We thank Jean-Roger Desmurs for helpful discussions and Dr. Louise Chisholm for editing the manuscript.
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