DOI: 10.1002/chem.201304912
Communication
& Hydrogels
Templating the Self-Assembly of Pristine Carbon Nanostructures in Water Miriam Mba,*[a] Ana I. Jimnez,[b] and Alessandro Moretto*[a]
Chem. Eur. J. 2014, 20, 3888 – 3893
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Communication Abstract: The low solubility of carbon nanostructures (CNs) in water and the need of ordered architectures at the nanoscale level are two major challenges for materials chemistry. Here we report that a novel amino acid based low-molecular-weight gelator (LMWG) can be used to effectively disperse pristine CNs in water and to drive their ordered self-assembly into supramolecular hydrogels. A non-covalent mechanochemical approach has been used, so the p-extended system of the CNs remains intact. Optical spectroscopy and electron microscopy confirmed the effective dispersion of the CNs in water. Electron microscopy of the hydrogels showed the formation of an ordered, LMWG-assisted, self-assembled architecture. Moreover, the very same strategy allows the solubilization and self-assembly in water of a variety of hydrophobic molecules.
Carbon nanostructures are widely used in cutting edge research in fields spanning from organic electronics and materials chemistry to biomedicine. The main challenge in materials chemistry and organic electronics is the control of architectures at the nanoscale, since it is known that molecular packing and phase separation have, for example, a strong effect in the efficiency of processes, such as energy transfer or charge separation.[1] On the other hand, biomedical applications need efficient strategies that lead to water-soluble and biocompatible CNs.[2] Low-molecular-weight gelators (LMWGs) are chemical systems that self-assemble (SA) spontaneously, or through appropriate external stimuli, leading to ordered supramolecular architectures in which the solvent remains trapped, so as to generate a gel transition.[3] It has been shown that the ability of LMWGs to generate supramolecular order can be exploited to define the spatial orientation of organic chromophores (OC) or carbon nanostructures (CNs) at the nanoscale. We and others have recently demonstrated that covalent grafting of water-soluble LMWGs to OCs and CNs can be used to obtain water-soluble derivatives that upon use of external stimuli selfassemble into well-ordered nanostructures leading to functional supramolecular hydrogels.[4] However, chemical functionalization may significantly affect the properties of the pristine CNs, thus a noncovalent functionalization is of interest. Some efforts have been made in this context towards inclusion of carbon nanotubes[5] and the remarkably soluble graphene oxide[5d, 6] in supramolecular hydrogels through a noncovalent approach, little work has been made on graphene[7] and fullerene. This lack of examples may be attributed to the low solu[a] Dr. M. Mba, Dr. A. Moretto Department of Chemistry, University of Padova via Marzolo 1, 35131 Padova (Italy) E-mail: [email protected] [email protected] [b] Dr. A. I. Jimnez Instituto de Sntesis Qumica y Catlisis Homognea (ISQCH) CSIC-Universidad de Zaragoza 50009 Zaragoza (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304912. Chem. Eur. J. 2014, 20, 3888 – 3893
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bility of CNs in water, which makes difficult their manipulation in this solvent. Recently, mechanochemistry has emerged as a valuable tool for the solid-state functionalization of CNs in the absence of solvent.[8] Ball milling has been also used for solubilization in water of CNTs[9] and fullerene[10] through formation of supramolecular complexes and for the exfoliation of graphene.[11] Mechanochemistry has also several advantages from the environmental point of view, such as reduced production of chemical waste, low cost, and simplicity.[12] In this work we focused on the noncovalent functionalization of CNs and OCs with an amino acid based LMWG. Amino acids and short peptides constitute an important family of LMWGs on account of their ease of synthesis and structural tuning and, on design, biocompatibility and water solubility.[13] Since the CNs are characterized by a p-extended system, we have design a LMWG with a p-conjugated moiety (fluorenyl group) that is expected to interact through p–p stacking with CNs. A solventless process was used, in which supramolecular nanocomposites were obtained by mechanochemical methods. Noncovalent functionalization of CNs with the LMWG reported here allowed their dispersion in water and, more importantly, the controlled self-assembly into thermoreversible hydrogels. Furthermore, the systems are pH responsive as a result of the net charge of the LMWG. We designed the LMWG 1 (Figure 1),
Figure 1. Chemical structure of LMWG 1 with the the main structural motifs highlighted.
which has three main structural features that we expect to enhance the SA in water: 1) two Fmoc (9-fluorenylmethyl chloroformate) groups that support p–p interactions, 2) a backbone rich in amide groups to promote the formation of directional hydrogen bonds, and 3) two free amines located on the l-Lys side-chain to enable solubility in water and responsiveness to pH changes. Compound 1 was obtained by a three-step, rapid, and scalable synthesis (Scheme S1 in the Supporting Information). When the ionic strength of a 0.3 % aqueous solution of 1 is increased by addition of 0.1 n HCl or NaCl, a self-supporting, transparent, thermoreversible hydrogel forms after a gentle heating/cooling cycle (Figure 2, inset to A). The gel shows very good stability for weeks at ambient conditions, whereas addition 0.1 n NaOH to the hydrogels induce solutes precipitation. The supramolecular chirality was studied by electronic circular dichroism (ECD). The ECD spectrum of an aqueous solution of 1 shows a weak positive band in the Fmoc absorption region
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Figure 3. UV/Vis spectra recorded in aqueous solution of composites obtained from 1 and A) azulene, B) carotene, C) fluorescein, D) zinc tetraphenylporphyrin, and E) perylene bis(anhydride).
Figure 2. A) ECD spectra of LMWG 1 in aqueous solution and as a hydrogel; inset: vials containing the solution and the corresponding hydrogel. B) TEM image of hydrogels from 1. C) AFM image and height profile of the selected fiber; inset: TEM image of a single fiber.
(250–300 nm) indicating no specific structuration in solution (Figure 2 A). Upon hydrogel formation, an intense positive band appears at 265 nm along with Cotton effects of minor intensities. This indicates that the Fmoc chromophore is involved into chiral supramolecular structures, in agreement with the single helical fibers detected by transmission electron microscopy (TEM) and atomic force microscopy (AFM). TEM analysis of the xerogel (using uranyl acetate negative stain) revealed the formation of a compact network of helical nanofibers up to 10 mm long and about 60 nm wide (Figure 2 B). AFM confirmed the formation of helical nanofibers with a regular pitch of about 20 nm (Figure 2 C and TEM of a single fiber shown in inset). Field emission scanning electron microscopy (FESEM) of a lyophilized gel indicated the existence of a dense 3D-fibrillar network (see Figure S1 in the Supporting Information) also in this physical state. To demonstrate the potentiality of 1 to solubilize hydrophobic molecules, we first tested its ability to solubilize in water a series of pristine organic chromophores, azulene, carotene, fluorescein, zinc tetraphenylporphyrin and perylene bis(anhydride), which are known for their electronic properties, but only sparingly soluble in water (fluorescein and perylene bis(anhydride) are partially soluble only in basic water). Few reports describe the solubilization of organic chromophores through formation of noncovalent complexes; in most of them an ultrasonication protocol is used.[14] In this work the supramolecular composites were obtained by manual grinding in agate mortar for 10 min, the resulting solid was suspended in water, and the insoluble aggregates were separated by using high speed centrifugation followed by filtration (45 mm membrane) giving in all cases colored supernatants (Figure 3 and inset). UV/Vis spectra of the supernatants showed the typical Chem. Eur. J. 2014, 20, 3888 – 3893
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absorption profiles of the corresponding chromophores, indicating that the grinding approach was successful for the creation of a stable supramolecular complex/composite between the chromophores and the LMWG 1. The best weight ratio for water dissolution of 1/OC was found to be 4:1. To determine the real amount of OC in solution, the solutions were first lyophilized and then separated into the original components by flash chromatography. We found that 100 mg of 1 were able to solubilize 11 mg of azulene, 12 mg of carotene, 14 mg of fluorescein, 7 mg of tetraphenyl zinc porphyrin, and 10 mg of perylene bis(anhydride), generating the corresponding aqueous solutions with a concentration of 0.3, 0.3, 0.4, 0.2 and 0.5 mg mL 1, respectively. Most likely, p–p stacking between the Fmoc group and the p-cloud of the chromophore is the main interaction. Moreover, after addition of HCl 1 n and gentle heating, all the solutions generated hydrogels upon cooling to room temperature (for the overall TEM characterizations see Figures S2–S4 in the Supporting Information). It is worth noting that even slight modifications on the chemical structure of 1 leads to a loss of the solubilization properties described above. We started our studies on CNs with fullerene C60. The solubilization of fullerene in water was achieved as described above. To study how the 1:C60 ratio could impact the amount of solubilized C60 and the self-assembling process, we performed a series of experiments with different ratios ranging from 4:1, 2:1, and 1:2, As expected, at 1:2 LMWG 1/fullerene ratio the composites were only slightly soluble in water, but otherwise transparent amber solutions were obtained. UV/Vis spectra of the solutions clearly showed the absorption profile reported for dispersed fullerene nanoparticles in water, with absorption maxima at 341 and 434 nm, with a weak band at 622 nm (Figure 4 A), indicating that C60 is chemically intact.[15] The lyophilized solutions obtained with 2:1 and 4:1 ratio were analyzed by thermogravimetry. The composites showed three main weight losses at 121, 194 and a last one from 469 to 501 8C (TGA analysis in air). The first two losses are attributed to the LMWG 1, while the third
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Figure 4. A) CD spectra of 1–fullerene composite in aqueous solution and in the gel state; UV/Vis spectra of the 1–fullerene composite in aqueous solution. B) TEM details of the 1–fullerene composite in water. C) TEM details of the fibrillar network of the 1–fullerene hydrogel. D) AFM image of the 1-fullerene hydrogel. E) FESEM of lyophilized 1-fullerene hydrogel. Inset in B) and C): vials containing 1-C60 solution and the corresponding hydrogel.
one includes also weight loss by degradation of C60 (see Figure S5 in the Supporting Information). From the analysis emerge that using a 4:1 ratio we were able to solubilize 0.8 mg of C60 per mL of water, a solubility equal to that obtained, for example, for the known C60/cyclodextrin complexes.[16] A confirmation of the interactions between 1 and C60 comes from the CD signals observed, although extremely weak, in the absorption region of the “achiral” fullerene (Figure 4 A). Most likely, the interactions are due to p–p stacking of the Fmoc moieties with the p-cloud of C60. TEM analysis of the solution showed the presence of C60 aggregates characterized by a spherical shape with diameters ranging from 20 to 50 nm (Figure 4 B). Dynamic light scattering (DLS) measurements confirmed the presence of nanoparticles with an average hydrodynamic diameter of 40 nm (see Figure S6 in the Supporting Information). When an aqueous solution containing 4 mg of 1C60 composite per mL was acidified to pH 4 and heated, a transparent, gold-brown hydrogel was formed upon standing. TEM images of the gel showed a network of tapes in which C60 nanoparticles are mainly ordered in a pearl-collar-like arrangement alongside the tapes formed by the LMWG 1 (Figure 4 C). Upon gelation, enhanced Cotton effects are observed in the C60 absorption region of the CD spectrum, suggesting that fullerene is involved in an ordered chiral supramolecular structure (Figure 4 A). The inclusion/absorption of the above-described spherical aggregate in the fibrillar network was further confirmed by AFM (Figure 4 D). Moreover by FESEM analysis a large number of spherical entities were detected above the surface of the 1–fullerene lyophilized composite (Figure 4 E). We then considered the challenging multiwalled carbon nanotube (MWCNT) and graphite materials. Following a similar procedure to that used for C60 (but excluding the membrane filtration step), we were able to obtain aqueous dispersions in Chem. Eur. J. 2014, 20, 3888 – 3893
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Figure 5. A) TEM images of aqueous dispersions for 1-MWCNTs; insets: TEM detail and vials showing aqueous dispersions of 1-MWCNTs and the precipitate upon addition of NaOH. B) and C) TEM images of hydrogel from 1MWCNTs (B concentrated sample, C diluted sample); inset to B) and C): vials showing hydrogel from 1-MWCNTs. D) and E) TEM images of aqueous dispersions for 1–graphite. F) TEM images of hydrogel from 1–graphite. Inset in E) and F): vials containing 1-graphite composite solution and the corresponding hydrogel.
both cases, black for the 1–MWCNT composite and grey for the 1–graphite composite, which could be stored for months at rt without observing any precipitation (inset to Figure 5 E, F). The TGA analysis carried out for the lyophilized solutions indicated that 1.5 mg mL 1 of MWCNTs and 0.8 mg mL 1 of graphite were achieved in solution (Figures S7 and S8 in the Supporting Information). The UV/Vis spectra of the aqueous solution of 1–MWCNT composite showed a broad absorption from the UV to the NIR region (see Figure S9 in the Supporting Information), typical of MWCNTs. The TEM analysis of the aqueous dispersions revealed the presence of disaggregated and debundled MWCNTs (Figure 4 A). The dispersed MWCNTs show an external diameter of 22 nm and an internal one of 6 nm, while commercial MWCNTs used in this study are reported to have an outer diameter in the range of 6–13 nm, and 2–6 nm for the inner diameter. These data suggest the presence of an external coating. Some applications of MWCNTs in gene and drug delivery and smart sensors require the control on dispersion/aggregation through external stimuli.[17] Since, in this case, the solubility of the composite relies on the charged ammonium group of 1, we hypothesised that the dispersion/aggregation of MWCNTs in water could be triggered by pH changes. Indeed, in neutral and acidic water the amino group is protonated and the MWCNTs are homogenously dispersed. In contrast, addition of NaOH induces deprotonation and loss of charge, so the nanocomposites precipitate out of the solution (inset to Figure 5 A). In the case of 1–graphite composites, the TEM analysis of the aqueous dispersions revealed the presence of graphite flakes (Figure 5 D and E). The fact that graphite flakes with different tonalities of grey are observed may indicate that partial exfoliation (different number of graphene layers) has occurred. Indeed, wet ball-milling in the presence of N,N-dimethylformamide (DMF)[11b] or melamine[11a] has been
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Communication used for the exfoliation of graphite. Raman spectroscopy gave us further information. The Raman spectra of the composite 1–MWCNT and untreated MWCNTs are similar, indicating no degradation of the tubes during the grinding process (see Figure S10 in the Supporting Information). In the case of the 1–graphite composite the D and D’ bands, arising from defects,[18] show increased intensity when compared to untreated madagascar graphite (see Figure S11 in the Supporting Information). The D’ peak is clearly distinguish indicating a moderate defect concentration. The ratio I(D)/I(D’) can be used to probe the nature of these defects.[19] In the present case, the small I(D)/ I(D’) ratio indicates that these defects are mainly due to boundaries and not to sp3 defects arising from functionalization. We therefore hypothesized that the D and D’ bands observed in the spectra arise from flake edges.[20] The solutions containing MWCNTs and graphite formed stable hydrogels when a hot acidified solution was left to cool down to room temperature. The sol–gel transition temperature was determined for all of the hydrogels synthesized in this work (see the Supporting Information). The hydrogels of 1– MWCNTs are more stable than those formed from 1 only (Tsol–gel = (81 2) 8C for 1–MWCNTs versus Tsol–gel = (63 2) 8C for 1), suggesting an effective inclusion of MWCNTs into the fibers. TEM images of the Figure 6. AFM of A) hydrogel from 1, B) hydrogel from 1–C , C) hydrogel from 1–graph60 1-MWCNT composites showed a dense fibrillar net- ite, and d) hydrogel from 1-MWCNT. FESEM of E) lyophilized hydrogel from 1–graphite work (Figure 5 B). A closer look at the network, in and F) lyophilized hydrogel from 1–MWCNT. a more diluted deposition, showed that it is formed by rods with lengths of 2–3 mm (Figure 5 C). The abtions or dispersions in water, but also to the formation of hysence of negative staining led us to consider that these rods drogels. In these hydrogels the CNs are not simply embedded, may be made up of MWCNTs covered by the self-assembled but they form well-ordered nanoarchitectures. As compared to LMWG 1. Analogously, for the 1–graphite composite, a packed other strategies, our approach shows several advantages: it fibrillar network with embedded graphite nanoflakes was obhas a broad applicability; it avoids covalent functionalization served (Figure 5 F). and preserves the physicochemical properties of the pristine Figure 6 A–D compares the AFM studies carried out on filmmaterials; it avoids the use of organic solvents, such as 1,2-dicoated samples of the hydrogels for 1, 1–fullerene, 1–graphite, chlorobenzene, dimethylformamide or N-cyclohexyl-2-pyrrolidiand 1–MWCNTs respectively. In particular 1 displays an organnone commonly used to process CNs; and it avoids the use of ized short cylindrical-like compact morphology that is in part high temperatures and extensive sonication. We are currently conserved (but less ordered) in the 1–fullerene hydrogel. In extending this approach to the design of soft functional matecontrast, the 1–graphite composite shows a morphology decorials for applications in organic electronics. We would like to rated with aligned flat areas most likely originated from the control the morphology of bulk hetero-junction solar cells and presence of graphite flakes, whereas the 1-MWCNT composite construct supramolecular light-harvesting antennae with exhibits an ordered stick-like morphology perhaps originated a well-defined position of the chromophores. from 1–MWCNTs self-organization. Finally, the FESEM analysis of the lyophilized gels confirms that a flat morphology occurred also in this solid state for 1–graphite (Figure 6 E) comAcknowledgements posite and confirm the ensued inclusion of MWCNTs into the fibrillar network (Figure 6 F). The authors are grateful to Dr. F. Caicci (University of Padova) In summary, we designed a new amino acid based hydrogeland Dr. S. Silvestrini (University of Padova) for TEM and Raman ator, easy to synthesize on a large scale, and that is able to support. A.M. and M.M. are grateful to Prof. Fernando Formagform supramolecular composites spanning from pristine OC to gio and Prof. Michele Maggini (both, University of Padova) for CNs by means of mechanochemical methods. The noncovalent helpful discussion. MIUR (PRIN project 2010NRREPL), University functionalization of non-water soluble organic chromophores of Padova (PRAT C91J11003560001, PRAT CPDA119117, and and pristine carbon nanostructures led not only to stable soluChem. Eur. J. 2014, 20, 3888 – 3893
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Communication Progetto Strategico HELIOS STPD08RCX5), Ministerio de Ciencia e Innovacin-FEDER (grant CTQ2010-17436), and Gobierno de Aragn-FSE (group E40) are acknowledged for financial support. Keywords: carbon nanostructures · fullerenes · hydrogels · mechanochemistry · self-assembly [1] E. Moulin, J.-J. Cid, N. Giuseppone, Adv. Mater. 2013, 25, 477 – 487. [2] a) R. Partha, J. L. Conyers, Int. J. Nanomed. 2009, 4, 261 – 275; b) M. Prato, K. Kostarelos, A. Bianco, Acc. Chem. Res. 2008, 41, 60 – 68. [3] a) X. Yang, G. Zhang, D. Zhang, J. Mater. Chem. 2012, 22, 38 – 50; b) L. A. Estroff, A. D. Hamilton, Chem. Rev. 2004, 104, 1201 – 1217; c) M. George, R. G. Weiss, Acc. Chem. Res. 2006, 39, 489 – 497; d) D. J. Abdallah, R. G. Weiss, Adv. Mater. 2000, 12, 1237 – 1247; e) P. Terech, R. G. Weiss, Chem. Rev. 1997, 97, 3133 – 3160. [4] M. Mba, A. Moretto, L. Armelao, M. Crisma, C. Toniolo, M. Maggini, Chem. Eur. J. 2011, 17, 2044 – 2047. [5] a) Z. Tan, S. Ohara, M. Naito, H. Abe, Adv. Mater. 2011, 23, 4053 – 4057; b) T. Kar, S. K. Mandal, P. K. Das, Chem. Commun. 2012, 48, 8389 – 8391; c) M. S. Arnold, M. O. Guler, M. C. Hersam, S. I. Stupp, Langmuir 2005, 21, 4705 – 4709; d) S. Roy, A. Baral, A. Banerjee, Chem. Eur. J. 2013, 19, 14950 – 14957. [6] Q.-Y. Cheng, D. Zhou, Y. Gao, Q. Chen, Z. Zhang, B.-H. Han, Langmuir 2012, 28, 3005 – 3010. [7] B. Adhikari, A. Banerjee, Soft Matter 2011, 7, 9259 – 9266. [8] a) S.-E. Zhu, F. Li, G.-W. Wang, Chem. Soc. Rev. 2013, 42, 7535 – 7570; b) N. Rubio, C. Fabbro, M. A. Herrero, A. de la Hoz, M. Meneghetti, J. L. G. Fierro, M. Prato, E. Vazquez, Small 2011, 7, 665 – 674. [9] a) A. Ikeda, Y. Tanaka, K. Nobusawa, J.-I. Kikuchi, Langmuir 2007, 23, 10913 – 10915; b) A. Ikeda, T. Hamano, K. Hayashi, J.-i. Kikuchi, Org. Lett. 2006, 8, 1153 – 1156; c) A. Ikeda, K. Hayashi, T. Konishi, J.-i. Kikuchi, Chem. Commun. 2004, 1334 – 1335. [10] a) A. Ikeda, T. Genmoto, N. Maekubo, J.-i. Kikuchi, M. Akiyama, T. Mochizuki, S. Kotani, T. Konishi, Chem. Lett. 2010, 39, 1256 – 1257; b) T. Furuishi, Y. Ohmachi, T. Fukami, H. Nagase, T. Suzuki, T. Endo, H. Ueda, K. Tomono, J. Inclusion Phenom. Macrocyclic Chem. 2010, 67, 233 – 239;
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Received: December 16, 2013 Published online on March 18, 2014
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& Hydrogels
Templating the Self-Assembly of Pristine Carbon Nanostructures in Water Miriam Mba,*[a] Ana I. Jimnez,[b] and Alessandro Moretto*[a]
Chem. Eur. J. 2014, 20, 3888 – 3893
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Communication Abstract: The low solubility of carbon nanostructures (CNs) in water and the need of ordered architectures at the nanoscale level are two major challenges for materials chemistry. Here we report that a novel amino acid based low-molecular-weight gelator (LMWG) can be used to effectively disperse pristine CNs in water and to drive their ordered self-assembly into supramolecular hydrogels. A non-covalent mechanochemical approach has been used, so the p-extended system of the CNs remains intact. Optical spectroscopy and electron microscopy confirmed the effective dispersion of the CNs in water. Electron microscopy of the hydrogels showed the formation of an ordered, LMWG-assisted, self-assembled architecture. Moreover, the very same strategy allows the solubilization and self-assembly in water of a variety of hydrophobic molecules.
Carbon nanostructures are widely used in cutting edge research in fields spanning from organic electronics and materials chemistry to biomedicine. The main challenge in materials chemistry and organic electronics is the control of architectures at the nanoscale, since it is known that molecular packing and phase separation have, for example, a strong effect in the efficiency of processes, such as energy transfer or charge separation.[1] On the other hand, biomedical applications need efficient strategies that lead to water-soluble and biocompatible CNs.[2] Low-molecular-weight gelators (LMWGs) are chemical systems that self-assemble (SA) spontaneously, or through appropriate external stimuli, leading to ordered supramolecular architectures in which the solvent remains trapped, so as to generate a gel transition.[3] It has been shown that the ability of LMWGs to generate supramolecular order can be exploited to define the spatial orientation of organic chromophores (OC) or carbon nanostructures (CNs) at the nanoscale. We and others have recently demonstrated that covalent grafting of water-soluble LMWGs to OCs and CNs can be used to obtain water-soluble derivatives that upon use of external stimuli selfassemble into well-ordered nanostructures leading to functional supramolecular hydrogels.[4] However, chemical functionalization may significantly affect the properties of the pristine CNs, thus a noncovalent functionalization is of interest. Some efforts have been made in this context towards inclusion of carbon nanotubes[5] and the remarkably soluble graphene oxide[5d, 6] in supramolecular hydrogels through a noncovalent approach, little work has been made on graphene[7] and fullerene. This lack of examples may be attributed to the low solu[a] Dr. M. Mba, Dr. A. Moretto Department of Chemistry, University of Padova via Marzolo 1, 35131 Padova (Italy) E-mail: [email protected] [email protected] [b] Dr. A. I. Jimnez Instituto de Sntesis Qumica y Catlisis Homognea (ISQCH) CSIC-Universidad de Zaragoza 50009 Zaragoza (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304912. Chem. Eur. J. 2014, 20, 3888 – 3893
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bility of CNs in water, which makes difficult their manipulation in this solvent. Recently, mechanochemistry has emerged as a valuable tool for the solid-state functionalization of CNs in the absence of solvent.[8] Ball milling has been also used for solubilization in water of CNTs[9] and fullerene[10] through formation of supramolecular complexes and for the exfoliation of graphene.[11] Mechanochemistry has also several advantages from the environmental point of view, such as reduced production of chemical waste, low cost, and simplicity.[12] In this work we focused on the noncovalent functionalization of CNs and OCs with an amino acid based LMWG. Amino acids and short peptides constitute an important family of LMWGs on account of their ease of synthesis and structural tuning and, on design, biocompatibility and water solubility.[13] Since the CNs are characterized by a p-extended system, we have design a LMWG with a p-conjugated moiety (fluorenyl group) that is expected to interact through p–p stacking with CNs. A solventless process was used, in which supramolecular nanocomposites were obtained by mechanochemical methods. Noncovalent functionalization of CNs with the LMWG reported here allowed their dispersion in water and, more importantly, the controlled self-assembly into thermoreversible hydrogels. Furthermore, the systems are pH responsive as a result of the net charge of the LMWG. We designed the LMWG 1 (Figure 1),
Figure 1. Chemical structure of LMWG 1 with the the main structural motifs highlighted.
which has three main structural features that we expect to enhance the SA in water: 1) two Fmoc (9-fluorenylmethyl chloroformate) groups that support p–p interactions, 2) a backbone rich in amide groups to promote the formation of directional hydrogen bonds, and 3) two free amines located on the l-Lys side-chain to enable solubility in water and responsiveness to pH changes. Compound 1 was obtained by a three-step, rapid, and scalable synthesis (Scheme S1 in the Supporting Information). When the ionic strength of a 0.3 % aqueous solution of 1 is increased by addition of 0.1 n HCl or NaCl, a self-supporting, transparent, thermoreversible hydrogel forms after a gentle heating/cooling cycle (Figure 2, inset to A). The gel shows very good stability for weeks at ambient conditions, whereas addition 0.1 n NaOH to the hydrogels induce solutes precipitation. The supramolecular chirality was studied by electronic circular dichroism (ECD). The ECD spectrum of an aqueous solution of 1 shows a weak positive band in the Fmoc absorption region
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Figure 3. UV/Vis spectra recorded in aqueous solution of composites obtained from 1 and A) azulene, B) carotene, C) fluorescein, D) zinc tetraphenylporphyrin, and E) perylene bis(anhydride).
Figure 2. A) ECD spectra of LMWG 1 in aqueous solution and as a hydrogel; inset: vials containing the solution and the corresponding hydrogel. B) TEM image of hydrogels from 1. C) AFM image and height profile of the selected fiber; inset: TEM image of a single fiber.
(250–300 nm) indicating no specific structuration in solution (Figure 2 A). Upon hydrogel formation, an intense positive band appears at 265 nm along with Cotton effects of minor intensities. This indicates that the Fmoc chromophore is involved into chiral supramolecular structures, in agreement with the single helical fibers detected by transmission electron microscopy (TEM) and atomic force microscopy (AFM). TEM analysis of the xerogel (using uranyl acetate negative stain) revealed the formation of a compact network of helical nanofibers up to 10 mm long and about 60 nm wide (Figure 2 B). AFM confirmed the formation of helical nanofibers with a regular pitch of about 20 nm (Figure 2 C and TEM of a single fiber shown in inset). Field emission scanning electron microscopy (FESEM) of a lyophilized gel indicated the existence of a dense 3D-fibrillar network (see Figure S1 in the Supporting Information) also in this physical state. To demonstrate the potentiality of 1 to solubilize hydrophobic molecules, we first tested its ability to solubilize in water a series of pristine organic chromophores, azulene, carotene, fluorescein, zinc tetraphenylporphyrin and perylene bis(anhydride), which are known for their electronic properties, but only sparingly soluble in water (fluorescein and perylene bis(anhydride) are partially soluble only in basic water). Few reports describe the solubilization of organic chromophores through formation of noncovalent complexes; in most of them an ultrasonication protocol is used.[14] In this work the supramolecular composites were obtained by manual grinding in agate mortar for 10 min, the resulting solid was suspended in water, and the insoluble aggregates were separated by using high speed centrifugation followed by filtration (45 mm membrane) giving in all cases colored supernatants (Figure 3 and inset). UV/Vis spectra of the supernatants showed the typical Chem. Eur. J. 2014, 20, 3888 – 3893
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absorption profiles of the corresponding chromophores, indicating that the grinding approach was successful for the creation of a stable supramolecular complex/composite between the chromophores and the LMWG 1. The best weight ratio for water dissolution of 1/OC was found to be 4:1. To determine the real amount of OC in solution, the solutions were first lyophilized and then separated into the original components by flash chromatography. We found that 100 mg of 1 were able to solubilize 11 mg of azulene, 12 mg of carotene, 14 mg of fluorescein, 7 mg of tetraphenyl zinc porphyrin, and 10 mg of perylene bis(anhydride), generating the corresponding aqueous solutions with a concentration of 0.3, 0.3, 0.4, 0.2 and 0.5 mg mL 1, respectively. Most likely, p–p stacking between the Fmoc group and the p-cloud of the chromophore is the main interaction. Moreover, after addition of HCl 1 n and gentle heating, all the solutions generated hydrogels upon cooling to room temperature (for the overall TEM characterizations see Figures S2–S4 in the Supporting Information). It is worth noting that even slight modifications on the chemical structure of 1 leads to a loss of the solubilization properties described above. We started our studies on CNs with fullerene C60. The solubilization of fullerene in water was achieved as described above. To study how the 1:C60 ratio could impact the amount of solubilized C60 and the self-assembling process, we performed a series of experiments with different ratios ranging from 4:1, 2:1, and 1:2, As expected, at 1:2 LMWG 1/fullerene ratio the composites were only slightly soluble in water, but otherwise transparent amber solutions were obtained. UV/Vis spectra of the solutions clearly showed the absorption profile reported for dispersed fullerene nanoparticles in water, with absorption maxima at 341 and 434 nm, with a weak band at 622 nm (Figure 4 A), indicating that C60 is chemically intact.[15] The lyophilized solutions obtained with 2:1 and 4:1 ratio were analyzed by thermogravimetry. The composites showed three main weight losses at 121, 194 and a last one from 469 to 501 8C (TGA analysis in air). The first two losses are attributed to the LMWG 1, while the third
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Figure 4. A) CD spectra of 1–fullerene composite in aqueous solution and in the gel state; UV/Vis spectra of the 1–fullerene composite in aqueous solution. B) TEM details of the 1–fullerene composite in water. C) TEM details of the fibrillar network of the 1–fullerene hydrogel. D) AFM image of the 1-fullerene hydrogel. E) FESEM of lyophilized 1-fullerene hydrogel. Inset in B) and C): vials containing 1-C60 solution and the corresponding hydrogel.
one includes also weight loss by degradation of C60 (see Figure S5 in the Supporting Information). From the analysis emerge that using a 4:1 ratio we were able to solubilize 0.8 mg of C60 per mL of water, a solubility equal to that obtained, for example, for the known C60/cyclodextrin complexes.[16] A confirmation of the interactions between 1 and C60 comes from the CD signals observed, although extremely weak, in the absorption region of the “achiral” fullerene (Figure 4 A). Most likely, the interactions are due to p–p stacking of the Fmoc moieties with the p-cloud of C60. TEM analysis of the solution showed the presence of C60 aggregates characterized by a spherical shape with diameters ranging from 20 to 50 nm (Figure 4 B). Dynamic light scattering (DLS) measurements confirmed the presence of nanoparticles with an average hydrodynamic diameter of 40 nm (see Figure S6 in the Supporting Information). When an aqueous solution containing 4 mg of 1C60 composite per mL was acidified to pH 4 and heated, a transparent, gold-brown hydrogel was formed upon standing. TEM images of the gel showed a network of tapes in which C60 nanoparticles are mainly ordered in a pearl-collar-like arrangement alongside the tapes formed by the LMWG 1 (Figure 4 C). Upon gelation, enhanced Cotton effects are observed in the C60 absorption region of the CD spectrum, suggesting that fullerene is involved in an ordered chiral supramolecular structure (Figure 4 A). The inclusion/absorption of the above-described spherical aggregate in the fibrillar network was further confirmed by AFM (Figure 4 D). Moreover by FESEM analysis a large number of spherical entities were detected above the surface of the 1–fullerene lyophilized composite (Figure 4 E). We then considered the challenging multiwalled carbon nanotube (MWCNT) and graphite materials. Following a similar procedure to that used for C60 (but excluding the membrane filtration step), we were able to obtain aqueous dispersions in Chem. Eur. J. 2014, 20, 3888 – 3893
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Figure 5. A) TEM images of aqueous dispersions for 1-MWCNTs; insets: TEM detail and vials showing aqueous dispersions of 1-MWCNTs and the precipitate upon addition of NaOH. B) and C) TEM images of hydrogel from 1MWCNTs (B concentrated sample, C diluted sample); inset to B) and C): vials showing hydrogel from 1-MWCNTs. D) and E) TEM images of aqueous dispersions for 1–graphite. F) TEM images of hydrogel from 1–graphite. Inset in E) and F): vials containing 1-graphite composite solution and the corresponding hydrogel.
both cases, black for the 1–MWCNT composite and grey for the 1–graphite composite, which could be stored for months at rt without observing any precipitation (inset to Figure 5 E, F). The TGA analysis carried out for the lyophilized solutions indicated that 1.5 mg mL 1 of MWCNTs and 0.8 mg mL 1 of graphite were achieved in solution (Figures S7 and S8 in the Supporting Information). The UV/Vis spectra of the aqueous solution of 1–MWCNT composite showed a broad absorption from the UV to the NIR region (see Figure S9 in the Supporting Information), typical of MWCNTs. The TEM analysis of the aqueous dispersions revealed the presence of disaggregated and debundled MWCNTs (Figure 4 A). The dispersed MWCNTs show an external diameter of 22 nm and an internal one of 6 nm, while commercial MWCNTs used in this study are reported to have an outer diameter in the range of 6–13 nm, and 2–6 nm for the inner diameter. These data suggest the presence of an external coating. Some applications of MWCNTs in gene and drug delivery and smart sensors require the control on dispersion/aggregation through external stimuli.[17] Since, in this case, the solubility of the composite relies on the charged ammonium group of 1, we hypothesised that the dispersion/aggregation of MWCNTs in water could be triggered by pH changes. Indeed, in neutral and acidic water the amino group is protonated and the MWCNTs are homogenously dispersed. In contrast, addition of NaOH induces deprotonation and loss of charge, so the nanocomposites precipitate out of the solution (inset to Figure 5 A). In the case of 1–graphite composites, the TEM analysis of the aqueous dispersions revealed the presence of graphite flakes (Figure 5 D and E). The fact that graphite flakes with different tonalities of grey are observed may indicate that partial exfoliation (different number of graphene layers) has occurred. Indeed, wet ball-milling in the presence of N,N-dimethylformamide (DMF)[11b] or melamine[11a] has been
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Communication used for the exfoliation of graphite. Raman spectroscopy gave us further information. The Raman spectra of the composite 1–MWCNT and untreated MWCNTs are similar, indicating no degradation of the tubes during the grinding process (see Figure S10 in the Supporting Information). In the case of the 1–graphite composite the D and D’ bands, arising from defects,[18] show increased intensity when compared to untreated madagascar graphite (see Figure S11 in the Supporting Information). The D’ peak is clearly distinguish indicating a moderate defect concentration. The ratio I(D)/I(D’) can be used to probe the nature of these defects.[19] In the present case, the small I(D)/ I(D’) ratio indicates that these defects are mainly due to boundaries and not to sp3 defects arising from functionalization. We therefore hypothesized that the D and D’ bands observed in the spectra arise from flake edges.[20] The solutions containing MWCNTs and graphite formed stable hydrogels when a hot acidified solution was left to cool down to room temperature. The sol–gel transition temperature was determined for all of the hydrogels synthesized in this work (see the Supporting Information). The hydrogels of 1– MWCNTs are more stable than those formed from 1 only (Tsol–gel = (81 2) 8C for 1–MWCNTs versus Tsol–gel = (63 2) 8C for 1), suggesting an effective inclusion of MWCNTs into the fibers. TEM images of the Figure 6. AFM of A) hydrogel from 1, B) hydrogel from 1–C , C) hydrogel from 1–graph60 1-MWCNT composites showed a dense fibrillar net- ite, and d) hydrogel from 1-MWCNT. FESEM of E) lyophilized hydrogel from 1–graphite work (Figure 5 B). A closer look at the network, in and F) lyophilized hydrogel from 1–MWCNT. a more diluted deposition, showed that it is formed by rods with lengths of 2–3 mm (Figure 5 C). The abtions or dispersions in water, but also to the formation of hysence of negative staining led us to consider that these rods drogels. In these hydrogels the CNs are not simply embedded, may be made up of MWCNTs covered by the self-assembled but they form well-ordered nanoarchitectures. As compared to LMWG 1. Analogously, for the 1–graphite composite, a packed other strategies, our approach shows several advantages: it fibrillar network with embedded graphite nanoflakes was obhas a broad applicability; it avoids covalent functionalization served (Figure 5 F). and preserves the physicochemical properties of the pristine Figure 6 A–D compares the AFM studies carried out on filmmaterials; it avoids the use of organic solvents, such as 1,2-dicoated samples of the hydrogels for 1, 1–fullerene, 1–graphite, chlorobenzene, dimethylformamide or N-cyclohexyl-2-pyrrolidiand 1–MWCNTs respectively. In particular 1 displays an organnone commonly used to process CNs; and it avoids the use of ized short cylindrical-like compact morphology that is in part high temperatures and extensive sonication. We are currently conserved (but less ordered) in the 1–fullerene hydrogel. In extending this approach to the design of soft functional matecontrast, the 1–graphite composite shows a morphology decorials for applications in organic electronics. We would like to rated with aligned flat areas most likely originated from the control the morphology of bulk hetero-junction solar cells and presence of graphite flakes, whereas the 1-MWCNT composite construct supramolecular light-harvesting antennae with exhibits an ordered stick-like morphology perhaps originated a well-defined position of the chromophores. from 1–MWCNTs self-organization. Finally, the FESEM analysis of the lyophilized gels confirms that a flat morphology occurred also in this solid state for 1–graphite (Figure 6 E) comAcknowledgements posite and confirm the ensued inclusion of MWCNTs into the fibrillar network (Figure 6 F). The authors are grateful to Dr. F. Caicci (University of Padova) In summary, we designed a new amino acid based hydrogeland Dr. S. Silvestrini (University of Padova) for TEM and Raman ator, easy to synthesize on a large scale, and that is able to support. A.M. and M.M. are grateful to Prof. Fernando Formagform supramolecular composites spanning from pristine OC to gio and Prof. Michele Maggini (both, University of Padova) for CNs by means of mechanochemical methods. The noncovalent helpful discussion. MIUR (PRIN project 2010NRREPL), University functionalization of non-water soluble organic chromophores of Padova (PRAT C91J11003560001, PRAT CPDA119117, and and pristine carbon nanostructures led not only to stable soluChem. Eur. J. 2014, 20, 3888 – 3893
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Communication Progetto Strategico HELIOS STPD08RCX5), Ministerio de Ciencia e Innovacin-FEDER (grant CTQ2010-17436), and Gobierno de Aragn-FSE (group E40) are acknowledged for financial support. Keywords: carbon nanostructures · fullerenes · hydrogels · mechanochemistry · self-assembly [1] E. Moulin, J.-J. Cid, N. Giuseppone, Adv. Mater. 2013, 25, 477 – 487. [2] a) R. Partha, J. L. Conyers, Int. J. Nanomed. 2009, 4, 261 – 275; b) M. Prato, K. Kostarelos, A. Bianco, Acc. Chem. Res. 2008, 41, 60 – 68. [3] a) X. Yang, G. Zhang, D. Zhang, J. Mater. Chem. 2012, 22, 38 – 50; b) L. A. Estroff, A. D. Hamilton, Chem. Rev. 2004, 104, 1201 – 1217; c) M. George, R. G. Weiss, Acc. Chem. Res. 2006, 39, 489 – 497; d) D. J. Abdallah, R. G. Weiss, Adv. Mater. 2000, 12, 1237 – 1247; e) P. Terech, R. G. Weiss, Chem. Rev. 1997, 97, 3133 – 3160. [4] M. Mba, A. Moretto, L. Armelao, M. Crisma, C. Toniolo, M. Maggini, Chem. Eur. J. 2011, 17, 2044 – 2047. [5] a) Z. Tan, S. Ohara, M. Naito, H. Abe, Adv. Mater. 2011, 23, 4053 – 4057; b) T. Kar, S. K. Mandal, P. K. Das, Chem. Commun. 2012, 48, 8389 – 8391; c) M. S. Arnold, M. O. Guler, M. C. Hersam, S. I. Stupp, Langmuir 2005, 21, 4705 – 4709; d) S. Roy, A. Baral, A. Banerjee, Chem. Eur. J. 2013, 19, 14950 – 14957. [6] Q.-Y. Cheng, D. Zhou, Y. Gao, Q. Chen, Z. Zhang, B.-H. Han, Langmuir 2012, 28, 3005 – 3010. [7] B. Adhikari, A. Banerjee, Soft Matter 2011, 7, 9259 – 9266. [8] a) S.-E. Zhu, F. Li, G.-W. Wang, Chem. Soc. Rev. 2013, 42, 7535 – 7570; b) N. Rubio, C. Fabbro, M. A. Herrero, A. de la Hoz, M. Meneghetti, J. L. G. Fierro, M. Prato, E. Vazquez, Small 2011, 7, 665 – 674. [9] a) A. Ikeda, Y. Tanaka, K. Nobusawa, J.-I. Kikuchi, Langmuir 2007, 23, 10913 – 10915; b) A. Ikeda, T. Hamano, K. Hayashi, J.-i. Kikuchi, Org. Lett. 2006, 8, 1153 – 1156; c) A. Ikeda, K. Hayashi, T. Konishi, J.-i. Kikuchi, Chem. Commun. 2004, 1334 – 1335. [10] a) A. Ikeda, T. Genmoto, N. Maekubo, J.-i. Kikuchi, M. Akiyama, T. Mochizuki, S. Kotani, T. Konishi, Chem. Lett. 2010, 39, 1256 – 1257; b) T. Furuishi, Y. Ohmachi, T. Fukami, H. Nagase, T. Suzuki, T. Endo, H. Ueda, K. Tomono, J. Inclusion Phenom. Macrocyclic Chem. 2010, 67, 233 – 239;
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Received: December 16, 2013 Published online on March 18, 2014
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