Article pubs.acs.org/Langmuir
Self-Assembled Multiwalled Carbon Nanotube Films Assisted by Ureidopyrimidinone-Based Multiple Hydrogen Bonds Sumin Wang,*,† Hao Guo,† Xiaomin Wang,† Qiguan Wang,*,† Jinhua Li,† and Xinhai Wang‡ †
Scientific Research Innovation Team of Solidification Theory and Functional Materials, Shaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, People’s Republic of China S Supporting Information *
ABSTRACT: Self-assembled functionalized multiwalled carbon nanotube (MWNT) films were successfully constructed, linked by a kind of strong binding strength from the self-complementary hydrogenbonding array of ureidopyrimidinone-based modules (UPM) attached. Employing the feasible reaction of isocyanate containing ureidopyrimidinone with amine modified MWNTs, the UPMs composed of ureidopyrimidinone and ureido were attached to MWNTs with the content as low as 0.6 mmol/g MWNTs. Upon multiple hydrogenbonding interactions from incorporation of the AADD (A, hydrogenbonding acceptor; D, hydrogen-bonding donor) quadruple hydrogen bonds of ureidopyrimidinone and the double hydrogen bonds of ureido group, UPM functionalized MWNTs (MWNT−UPM) can be well dispersed in the polar solvent of N,N-dimethylformamide (DMF), while they tend to self-assemble to give a self-supported film in the apolar solvent of CHCl3. In addition, by using the multiple hydrogen-bonding interactions as the driving force, the layer-by-layer (LBL) MWNT−UPM films with high coverage on solid slides can be processed. Because of the self-association of MWNT−UPM in apolar solvent, it was found that the LBL assembly of MWNT−UPM was more favorable in the polar solvent of DMF than in the apolar solvent of CHCl3. Moreover, the hydrogenbonding linked MWNT−UPM films showed good stability upon soaking in different solvents. Furthermore, the as-prepared LBL films showed electrochemical active behaviors, exhibiting a remarkable catalytic effect on the reduction of nifedipine. hydrogen-bonded LBL multilayer films from dendrimers,16 nanoparticles,17−20 chitosan, and hyaluronic acid.21 Recently, it was found that, through the formation of double hydrogen-bonded homodimers on the surface of CNTs, thymine functionalized CNTs showed self-organized behaviors in different solvents.22 Additionally, by triple complementary ADA−DAD (A, hydrogen-bonding acceptor; D, hydrogenbonding donor) hydrogen-bonding interaction, dispersion and aggregation of CNTs can be switched on and off in different conditions.23−25 Moreover, the thymine-derived CNTs can be selectively adsorbed on photo-cross-linking matrices of polystyrene polymers bearing 2,6-di(acetylamino)-4-pyridyl deposited on glass or Si.26 Unfortunately, CNT LBL multilayer films linked by such hydrogen bonds have not yet been constructed, probably because of the relatively low binding constant. In comparison with the above hydrogen bonds, the selfcomplementary AADD quadruple hydrogen-bonding motif of ureidopyrimidinone (Figure S1, Supporting Information), first
1. INTRODUCTION Because of the potential applications in fuel cells, batteries, supercapacitors, and nanoelectrodes, substantive works have been carried out on carbon nanotube (CNT) films.1 Since the enhanced solubility of CNTs was available, the wet methods for the fabrication of CNT films based on the solution coating2,3 and layer-by-layer (LBL) assembly technique have been favorably used, because of the simple, low-temperature manipulation.4 For example, by using the LBL technique, highly conductive all multiwalled carbon nanotube (MWNT) films with controllable thickness, composition ratio, and porosity were synthesized from the LBL adsorption of carboxylic acid and amine group functionalized CNTs, which exhibit behaviors similar to those of the weak polyelectrolytes.5 In addition to the electrostatic interactions, many other strong or weak molecular interactions have been widely used in the LBL assembly, such as coordination bonding,6 chargetransfer interaction,7 specific recognition,8 and hydrogen bonding.9−11 First investigated by Rubner and Stockton9 and Zhang et al.,10 the functional LBL films from hydrogen bonding have attracted more research interest, ranging from hydrogenbonding-directed electroactive,12 photochromic,13 photoreactive polyelectrolyte,14 and self-supported multilayers15 to © 2014 American Chemical Society
Received: May 17, 2014 Revised: October 6, 2014 Published: October 8, 2014 12923
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Scheme 1. Structure of MWNT−UPM (a) and Schematic Representation of Constructing LBL MWNT−UPM Multilayer Films (b) from the Ureidopyrimidinone Group Functionalized Substrate Driven by the Multiple Hydrogen Bonding Interactions
built by Meijer and coworkers,27 possessing much stronger binding strength, may be a good candidate for the fabrication of LBL films from the giant molecules. The association constant of the quadruple hydrogen system can exceed 107 mol/L in apolar solvents, benefiting from the intermolecular quadruple AADD arrays and the intramolecular hydrogen bonds (Figure S1), which endowed the ureidopyrimidinone-derived module extensive applications in constructing the complicated functional supramolecular systems.28−35 To construct structure-stable LBL CNT films, here a multiple hydrogen-bonding interaction from ureidopyrimidinone-based module (UPM) was designed and introduced into the CNT connections. First, ureidopyrimidinone was attached onto the surface of MWNTs by reaction of isocyanate containing ureidopyrimidinone with amine modified MWNTs to form UPM functionalized MWNTs (MWNT−UPM, Scheme 1a), in which the self-complementary multiply hydrogen-bonding interaction composed of the quadruple hydrogen bonds from ureidopyrimidinone array and the double hydrogen bonds from ureido group exists. Then coupled with the LBL process, self-assembled MWNT films showing good stability were prepared (Scheme 1b).
Academy of Science. Nifedipine (NIF) was purchased from Sigma. Stock solution of NIF (1 × 10−3 mol/L) was prepared by dissolving an appropriate amount of NIF in methanol (HPLC grade). 2-Amino-6(heptan-3-yl)pyrimidin-4(1H)-one, 2(6-isocyanatohexylaminocarbonylamino)-6-methyl-4[1H]-pyrimidinone, and N-[(butylamino)carbonyl]-6-methylisocytosine were synthesized according to refs 27, 36, and 37. Toluene was refluxed with Na and redistilled. CHCl3 was refluxed with CaH2 and redistilled. During the self-assembly process, Milli-Q water (18 MΩ/cm), HPLC grade of CH3OH, and N,Ndimethylformamide (DMF) were used. Instrumentation. UV−vis spectra were obtained with a Shimadzu 1901 UV−vis spectrophotometer. For the measurement of suspension of MWNTs, the sample was first ultrasonicated for 2h, followed by centrifugation at 3000 rpm for 5 min to remove the large particles. Scanning electron microscope (SEM) images were collected with an S4800 or JSM-6360LV instrument. Transmission electron microscope (TEM) images were obtained from a JEM2010 instrument. Thermogravimetric analysis (TGA) data were collected with a Q50 TGA instrument. Raman measurements were performed by using an inVia Raman spectrometer with 532 nm wavelength laser source. The surface chemistry of functionalized MWNTs was analyzed by using a PHI 5400 X-ray photoelectron spectrometer (XPS). All spectra were calibrated with the C 1s photoemission peak for sp2 hybridized carbons at 284.3 eV. Curve fitting of the photoemission spectra was done after a Shirley type background subtraction. The tensile properties of the MWNT−UPM films were determined by using a DDL300 electronic tensile testing machine. Tensile tests were conducted on 6 mm wide and 0.06 mm thick samples, and a minimum of three samples were tested. Cyclic voltammetry was carried out with the MWNT−UPM films assembled on ITO glass as working electrode, a saturated calomel electrode (SCE) as reference electrode and a Pt wire as the counter electrode. The working
2. EXPERIMENTAL SECTION Materials. All reagents and chemicals were used as received unless otherwise noted. Carboxylic acid functionalized MWNTs (MWNT− COOH) (−COOH % = 2.56 wt %, purity > 95%), which were prepared by oxidizing the pristine MWNTs by KMnO4 in H2SO4, with the external diameter of 20−30 nm and length of 10−30 μm, were obtained from Chengdu Institute of Organic Chemistry, Chinese 12924
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electrode area is 0.25 cm2, which was always kept immersed in the electrolyte solution during the data collection. Preparation of Chlorinated MWNTs. A total of 50 mg of carboxylated MWNTs (MWNT−COOH) was added to 100 mL of SOCl2 and ultrasonicated for 30 min. Afterward, the suspension was refluxed for 12 h under N2 atmosphere. After evaporating the excessive SOCl2, the chlorinated MWNTs obtained were used immediately for the next step without further purification. Preparation of Amine Functionalized MWNTs. A volume of 80 mL of 1,2-ethylenediamine was added to the above chlorinated MWNTs and ultrasonicated for 30 min. Afterward, the suspension was rigorously stirred at 80 °C for 24 h under N2 atmosphere. After filtration, the black powder was thoroughly washed with toluene, CHCl3, and DMF sequentially. Dried at 50 °C in vacuum for 24 h, the amine functionalized MWNTs (MWNT−NH2) were obtained. Preparation of Ureidopyrimidinone Functionalized MWNTs. A total of 50 mg of MWNT−NH2, 35 mg of 2(6-isocyanatohexylaminocarbonylamino)-6-methyl-4[1H]-pyrimidinone (NCO(CH2)6-UPy), and 0.1 mL of dibutyltin dilaurate (DBDTL) were added to 80 mL of dry CHCl3 and stirred at 50 °C for 24 h under N2 atmosphere. After filtration, the black powder was thoroughly washed with toluene, CHCl3, and DMF. After drying at 50 °C in vacuum for 24 h, ureidopyrimidinone functionalized MWNTs (MWNT−UPM) were obtained. Preparation of Isocyanate Terminated Substrates. After being cleaned with acetone and water, the quartz or ITO substrates were ultrasonicated in a hot piranha solution (mixture of 98% H2SO4 and 30% H2O2, v/v 7:3) for 30 min. (Caution! Piranha solution is extremely corrosive.) After being thoroughly washed with pure water and dried by N2, the hydroxyl group functionalized substrates obtained were immersed in a toluene solution (60 mL) containing 1.5 g of NCO(CH2)6NCO and 0.1 mL of DBDTL for 10 h. After washing with toluene and drying, isocyanate terminated substrates were obtained. Preparation of Ureidopyrimidinone Terminated Substrates. The isocyanate terminated substrates were immersed in 30 mL of dry CHCl3 solution containing 30 mg of 2-amino-6-(heptan-3-yl)pyrimidin-4(1H)-one and 0.1 mL of DBDTL at 50 °C for 24 h. After being thoroughly washed by toluene, CHCl3, DMF, and H2O sequentially and dried under N2, ureidopyrimidinone terminated substrates were obtained (Scheme 1b). Self-Assembly LBL Films of MWNT−UPM. To form the stable dispersion for LBL assembly process, 1 mg of the dried MWNT− UPM powders was ultrasonicated in 10 mL of DMF for 1 h. Then the ureidopyrimidinone terminated substrates were immersed into the MWNT−UPM dispersion for 10 min. After being washed with the solvent and dried with N2, the substrates adsorbed by one layer of MWNT−UPM were dipped again into the MWNT−UPM dispersion to obtain the second MWNT−UPM layer. Under the repetitive operation of such steps, the self-assembly LBL multilayer films of MWNT−UPM could be obtained in a cyclic fashion.
Scheme 2. Synthetic Route of MWNT−UPM
Figure 1 shows the FT-IR spectra of MWNT−NH2 (Figure 1a), MWNT−UPM (Figure 1b), and the reference compound
Figure 1. FT-IR spectra of MWNT−NH2 (a), MWNT−UPM (b), and UPy−R (c). Inset shows the molecular structure of UPy−R.
N-[(butylamino)carbonyl]-6-methylisocytosine (UPy−R) (Figure 1c) with the structure as shown in the inset. From the spectrum of MWNT−NH2 shown in Figure 1a, the characteristic absorption band at 1635 cm−1 assigned to the stretching vibration of CO group of amide (amide I band) and the band at 1592 cm−1 assigned to the bending vibration of N−H group of the amide (amide II band) confirm the presence of the amide functional group in MWNT−NH2. In addition, the absorption bands located at 3462, 3131, and 3403 cm−1 in Figure 1a attributed to characteristic N−H stretching vibration of the primary amine and amide group respectively illustrate the presence of the saturated primary amine in MWNT−NH2. In the case of MWNT−UPM (Figure 1b), the absorption of stretching vibration corresponding to N−H was integrated into one broad peak at 3437 cm−1 because of the reaction of primary amine on MWNT−NH2 with NCO(CH2)6UPy. Meanwhile, a new absorption band at 1650 cm−1 was found for MWNT− UPM, which was assigned to stretching vibrations of CC and CN of ureidopyrimidinone. This showed the successful attachment of the ureidopyrimidinone group on the MWNTs. Finally, compared to MWNT−NH2, the characteristic stretching vibration peak of CO (1630 cm−1) and bending vibration peak of N−H (1587 cm−1) in amide groups of MWNT−UPM showed a shift to lower wavenumbers under the hydrogenbonding interactions with which they were involved. Compound UPy−R, always existing as dimers in the solid state because of the formation of hydrogen bonding, shows the
3. RESULTS AND DISCUSSION Preparation and Characterization of MWNT−UPM. A three-step reaction strategy was employed to prepare UPM functionalized MWNTs, as shown in Scheme 2. First, carboxylated MWNTs were chlorinated by refluxing with SOCl2, and then amine groups were introduced on the walls of MWNTs by reaction of chlorinated MWNTs with excessive 1,2-ethylenediamine to form MWNT−NH2. After the feasible reaction of isocyanate group of NCO(CH2)6UPy with the amine group on MWNT−NH2, the UPM functionalized MWNTs were obtained, where ureidopyrimidinone groups were attached to the surface of MWNTs spaced by the long alkyl chain. Such a structural design endows the resultant MWNT−UPM good solubility in organic solvents. It was found that there were no sediments in MWNT−UPM/DMF dispersion stored for 6 months. 12925
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Figure 2. XPS survey scans of chemically modified MWNTs (a). C 1s (b) and O 1s (c) XPS spectra of MWNT−COOH. N 1s (d), C 1s (e), and O 1s (f) XPS spectra of MWNT−NH2. N 1s (g), C 1s (h), and O 1s (i) XPS spectra of MWNT−UPM.
spectrum of MWNT−NH2 (Figure 2d) also showed the presence of amide and amine on MWNT.39 For the MWNT−UPM, the XPS spectra (Figure 2g−i) showed relatively different features compared with those of MWNT−NH2, because of hydrogen bonds introduced by ureido and pyrimidinone groups. The N 1s peak at 399.8 eV attributed to nitrogen in amide of MWNT−NH2 (Figure 2d) was shifted to 399.3 eV in the case of MWNT−UPM (Figure 2g). Meanwhile, the O 1s peak at 531.7 eV attributed to oxygen in amide group of MWNT−NH2 (Figure 2f) was shifted to higher binding energy at 532.3 eV in the case of MWNT−UPM (Figure 2i). This is because of the strong hydrogen-bonding interactions introduced by the ureido groups which changed the electronic state and the binding energy of the involved atoms in MWNT−NH2. Hydrogen bonds are composed of a hydrogen atom that serves as a bridge between two electronegative atoms called hydrogen donor and hydrogen acceptor, making the hydrogen acceptor of oxygen more electropositive and the hydrogen donor of nitrogen more electronegative.40,41 Additionally, the C 1s peak of amide group centered at 288.0 eV for MWNT−NH2 (Figure 2e) was also shifted to higher binding energy at 288.6 eV in the case of MWNT−UPM (Figure 2h). This is due to the charge transfer from the carbon of −NH−CO to the bond near hydrogen acceptor in the formation of hydrogen bonding. Moreover, the presence of a new N 1s peak at 400.4 eV in the XPS spectra of MWNT−UPM (Figure 2g) was assigned to −N of pyrimidinone which serves as a hydrogen acceptor in the formation of hydrogen bonding.41 In addition, from the quantitative estimation of UPM groups on MWNTs by TGA analysis (Figure S2), it showed that the number of initial carboxylic acid moieties was 0.6 mmol/g of MWNTs, which could be totally converted to UPM groups
similar absorption bands with that of MWNT−UPM for the amide groups located at 1630 and 1583 cm−1, which further confirms the presence of the strong hydrogen-bonding interactions in MWNT−UPM. To verify the results from FT-IR, XPS analysis (Figure 2) of MWNT−COOH, MWNT−NH2, and MWNT−UPM was performed. As shown from the survey spectra in Figure 2a, N 1s features cannot be found in the raw material of MWNT− COOH, aside from the O 1s and C 1s photoemission peaks centered at 285.0 and 533.0 eV. However, the XPS survey spectra of MWNT−NH2 and MWNT−UPM showed the N 1s features centered at 400.0 eV besides C 1s (285.0 eV) and O 1s (533.0 eV) features. For the C 1s XPS spectrum of MWNT−COOH (Figure 2b), the peaks located at 284.3 and 285.1 eV are attributed to sp2 and sp3 hybridized carbons, respectively. The peak with the higher binding energy at 288.3 eV shown in Figure 2b was attributed to the carbons of carboxylic acid on MWNT− COOH, accompanied by the O 1s features at 531.7 and 533.1 eV (Figure 2c) which were attributed to the oxygen of −CO and −OH, respectively.38 After attachment by amine group, the C 1s XPS feature at 288.3 eV assigned to the carbons of carboxylic acid on MWNT−COOH (Figure 2b) was shifted to lower binding energy at 288.0 eV in the case of MWNT−NH2 (Figure 2e), which was assigned to carbons of amide (−NH− CO) group.39 Accompanied by the conversion of carboxylic acid to amide, the O 1s XPS peak at 533.1 eV assigned to −OH on MWNT−COOH (Figure 2c) disappeared in the case of MWNT−NH2 (Figure 2f). Meanwhile, the peak at 286.5 eV different from the amide features in the C 1s XPS spectrum of MWNT−NH2 (Figure 2e) was attributed to the −CH2−NH2 groups.5,39 In addition, the peak located at 399.8 eV in the N 1s 12926
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addition of polar solvent such as DMF and dimethyl sulfoxide (DMSO). As shown in Figure S4, after adding 4 mL of DMSO to the badly dispersed system (Figure S4a) of 1 mg of MWNT−UPM in 20 mL of CHCl3, the large aggregates of MWNT−UPM were well dispersed (Figure S4b) with the aid of ultrasonication. This showed that the conventional hydrogen bonding between polar solvent and MWNT−UPM can weaken the multiple hydrogen bonding between MWNT−UPM, which is favorable for the dispersion of MWNT−UPM aggregates. TEM studies of pristine MWNTs dispersion in DMF (Figure 3c) and CHCl3 (Figure 3f) were also carried out as control experiments. Because in the polar solvent of DMF, the multiple−hydrogen-bonding interactions between MWNT− UPM bundles were significantly weakened under the interactions between MWNT−UPM and solvent in contrast to pristine MWNTs, which results in the significantly unbundled MWNTs networks in the former (Figure 3b) compared with pristine MWNTs (Figure 3c). However, the presence of strong multiple hydrogen-bonding interactions between MWNT−UPM compared with the weak interactions between pristine MWNTs in the apolar solvent of CHCl3, larger aggregates of MWNTs (Figure 3e) were observed for MWNT−UPM than the latter (Figure 3f). The control experiments also prove that it is the multiple-hydrogen-bonding interaction that leads to the different assembly behaviors of MWNT−UPM in different media. Since the absorption properties of CNTs can be strongly influenced by the degree of bundling,43,44 the assembly behaviors of MWNT−UPM were also investigated by UV− vis absorption spectroscopy. Because of the significant conglomeration from the multiple hydrogen bonding, the UV−vis spectrum of MWNT−UPM dispersed in CHCl3 only exhibited a featureless absorption in the region of 255−400 nm (Figure S5a). However, the UV−vis spectrum of MWNT− UPM in DMF showed a distinct absorption peak at 286 nm (Figure S5b), which was attributed to the π−π* transition of MWNT−UPM. This is because the dissociation of the MWNT−UPM aggregates makes MWNT−UPM more active to the specific band of UV−vis light. Furthermore, the featureless absorption of MWNT−UPM aggregates in CHCl3 can be significantly changed by introduction of CH3OH. As shown in Figure S6, upon addition of CH3OH in the MWNT− UPM/CHCl3 dispersion, a new peak at 283 nm attributed to the π−π* transition of MWNT−UPM appeared. Moreover, with the increase of the amount of CH3OH, the intensity of the peak at 283 nm was gradually increased. This is because the addition of CH3OH can weaken the hydrogen-bonding interactions among MWNT−UPM and lead to dissociation of the aggregates. As a control experiment, the same amount of CH3OH added in the pristine MWNT/CHCl3 dispersion cannot change the shape of the UV−vis absorption spectra (Figure S7), where almost no hydrogen-bonding interactions between pristine MWNT and CH3OH can be used to break the aggregation of MWNTs. The similar results were found in the case of adding DMF to MWNT−UPM/CHCl3 dispersion. (Figures S8 and S9). Interestingly, such multiply hydrogen-bonding interactions between MWNT−UPM molecules were so strong that a selfsupported MWNTs film (Figure 3g) can be formed by vacuum filtration of the MWNT−UPM/CHCl3 dispersion through a Millipore membrane according to a reported method.45 From the SEM images shown in Figure 3h, the MWNTs in the resultant films were evenly and densely connected under the
according to synthetic route shown in Scheme 2. Moreover, as shown in the Raman spectra of MWNT−COOH, MWNT− NH2, and MWNT−UPM (Figure S3), the D- to G-band intensity ratio (ID/IG) for MWNT−COOH is ca. 1.4, which is typical for the class of carbon nanomaterials.42 The ID/IG values for MWNT−UPM and MWNT−NH2 are ca. 1.2 and 1.1, respectively, both close to that of MWNT−COOH, implying that the fundamental structure of MWNTs almost remains intact upon the functionalization according to synthetic route shown in Scheme 2. Hydrogen Bonding Association of MWNT−UPM in Different Solvents. It is well-known that the association of the dispersed molecules bearing multiple hydrogen bonds can be significantly influenced by the solvents used, which leads to the different assembly behaviors of such molecules in different media. Therefore, the hydrogen-bonding association of MWNT−UPM molecules was analyzed from the formed morphologies of MWNT−UPM dispersion in different solvents by TEM images. As illustrated from the TEM images shown in Figure 3a and b, the MWNT−UPM dispersed in the polar
Figure 3. TEM images of MWNT−UPM/DMF (a), pristine MWNTs/DMF (c), MWNT−UPM/CHCl3 (d), pristine MWNTs/ CHCl3 (f). High resolution TEM images of MWNT−UPM/DMF (b) and MWNT−UPM/CHCl3 (e). Photo (g) and SEM (h) images of the self-supported film of MWNT−UPM.
solvent of DMF was significantly unbundled and uniformly distributed. However, from the images shown in Figure 3d and e, the MWNT−UPM dispersed in the apolar solvent of CHCl3 was significantly aggregated and unevenly connected. This is because the conventional hydrogen-bonding interactions between the polar groups in DMF and the ureidopyrimidinone as well as ureido groups in MWNT−UPM hindered the typical association of MWNT−UPM by the multiple hydrogen bonding, which makes MWNT−UPM individually dispersed in DMF. While, in the case of MWNT−UPM dispersed in CHCl3, the strength of the interactions between MWNT− UPM and solvent is weak. Therefore, the larger MWNT−UPM aggregates in CHCl3 were clearly observed, which was mainly due to the recognition and the association of MWNT−UPM molecules by the multiple hydrogen bonding. Furthermore, the aggregates of MWNT−UPM in CHCl3 could be dissociated by 12927
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interactions of multiple hydrogen bonding from the UPM, which endowed the system relatively high flexibility. The MWNT−UPM buckypaper showed an electrical conductivity value of 10 S/cm, which is relatively lower than the reported values of MWNT−NH2 (25 ± 1 S/cm) and MWNT−COOH (26 ± 2 S/cm) buckypapers prepared by vacuum filtration of their aqueous dispersion assisted by Triton X-100. The decrease of the conductivity may result from the presence of organic groups on MWNT−UPM.46 The film tensile strength and modulus are 35.8 ± 5 MPa and 2.1 ± 0.3 GPa, respectively, which showed the same order of magnitude as the nitric acid treated single-walled carbon nanotube buckypapers.47 Additionally, in the case of the pristine MWNTs, the self-supported MWNT buckypaper cannot be obtained after vacuum filtration of the MWNT/CHCl3 dispersion (Figure S10), where the interactions mainly from the disordered bundling between MWNTs were relatively weak compared to those in the MWNT−UPM/CHCl3 system. Fabrication of LBL Multilayer Films of MWNT−UPM. To further find the contribution of such multiple hydrogen bonding to the formation of MWNT films, the LBL process of the MWNT−UPM was experimentally tried. As shown from Scheme 1, the driving force for the fabrication of the MWNT LBL films mainly resulted from the multiple hydrogen-bonding interactions between MWNT−UPM itself, which acts as both hydrogen-bond donor and hydrogen-bond acceptor, because of the self-complementary characteristic of the quadruple hydrogen bonds of ureidopyrimidinone and the double hydrogen bonds of ureido group. For the fabrication of LBL films of MWNT−UPM, the surface of the substrates should be modified by the ureidopyrimidinone group first. The introduction of the terminal ureidopyrimidinone group on the substrates provides binding sites that can form hydrogen-bonding interaction with MWNT−UPM. The fabrication route for ureidopyrimidinone terminated substrate is illustrated in Scheme 1. First, the hydroxyl group functionalized substrate was reacted with excess NCO(CH2)6NCO to form a isocyanate terminated substrate. Then ureidopyrimidinone terminated substrate was prepared by the reaction of 2-amino-6-(heptan-3-yl)pyrimidin-4(1H)one with the above isocyanate terminated substrate in CHCl3. The XPS analysis of C 1s and N 1s features (Figure S11) suggests the presence of sp3 hybridized carbons (285.3 eV in C 1s XPS) and amide groups associated with hydrogen bonding (288.9 eV in C 1s XPS and 399.4 eV in N 1s XPS), which showed the successful attachment of ureidopyrimidinone group to the quartz slide. In addition, UV−vis spectrum of the slide (Figure S12a) showed a π−π* absorption band of pyrimidinone ring centered at 265 nm, which further supports the XPS results. From the UV−vis absorption spectra of the ureidopyrimidinone modified quartz slides repetitively dipped into the MWNT−UPM/DMF dispersion (Figure 4), well-resolved bands centered at 260 nm, in agreement with the π−π* absorption of MWNT−UPM, were found. Just deposited twice, the quartz substrate significantly darkened, because of the strong absorption of the deposited MWNTs in the visible region,48 which showed it was an effective way for the MWNTs LBL film formation by using the multiple hydrogen bonding as the driving force. In addition, from the inset illustrated in Figure 4, it was shown that the absorbance centered at 260 nm and the optional point at 400 nm both increase linearly with the increase of the number of deposited layers. It suggested that the
Figure 4. UV−vis absorption spectra of MWNT−UPM multilayers on quartz slides with an increasing number of layers. Inset: absorbance at 260 and 400 nm with the increasing number of layers. The absorbance of ureidopyrimidinone terminated quartz slide was subtracted as baseline. Solvent, DMF; assembly time, 10 min/layer.
similar amount of MWNT−UPM was deposited on the solid substrate for every assembly cycle, which showed the characteristic of the stepwise and regular process. From the comparison of UV−vis absorbance for the single layer MWNT−UPM assembled on quartz slide (Figure S13), the LBL assembly of MWNT−UPM was more favorable in the polar solvent of DMF than in the apolar solvent of CHCl3. This is because the significant self-association and the limited dispersion of MWNT−UPM in the apolar solvent of CHCl3 makes the MWNT−UPM bundling difficult to be uncoupled to incorporate with the hydrogen-bonding modules on the substrate slides. Also, the controlled experiments under the same conditions showed that the MWNT LBL films cannot be obtained if pristine MWNTs or ureido group functionalized MWNTs are used as the building block (Figure S14), which proves the necessity of using driving forces with stronger hydrogen-bonding association for the LBL process. The presence of hydrogen-bonding interactions in the linkage of MWNT−UPM LBL films can be seen from XPS analysis. The binding energy of O 1s for the carbonyl groups of the five-layer MWNT−UPM film is 0.5 eV higher than that of MWNT−NH2 (531.7 eV) (Figure S15a), because the oxygen in the carbonyl groups in the MWNT−UPM film became more electropositive as the strong hydrogen bonding formed. In addition, the lower binding energy of 399.2 eV for N 1s in amide of the LBL MWNT−UPM film (Figure S15b) relative to MWNT−NH2 at 399.8 eV, and a higher binding energy for the N 1s of −N of pyrimidinone at 400.3 eV in the LBL MWNT−UPM film (Figure S15b) relative to the reported value of 397.1 eV41 also indicated the presence of hydrogenbonding interaction. Encouragingly, the LBL films of MWNT−UPM linked by such strong hydrogen-bonding interactions showed good stability upon soaking in different solvents. For a five-layer MWNT−UPM film on quartz slides, the stability of the multilayer system was tested by placing the MWNT−UPM film into different solvents such as H2O, DMF, and CHCl3 at room temperature for 8 h. It was found that the UV−vis spectra of the film showed almost the same features as those before soaking (Figure S16). In addition, the absorbance at 260 nm attributed to the π−π* transition of MWNT−UPM illustrated no sharp decrease, which showed that the MWNTs within the LBL films can be hardly peeled off from the surface of the functionalized quartz slides, because of the strong multiple 12928
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spectra shown in Figure 4. Moreover, the presence of a great quantity of nanoscale pores (see the arrows in Figure 5b) formed in these LBL films of MWNT−UPM may endow them good electrochemical activities because of the enlarged specific surface area.5,49 With the number of MWNT−UPM layers increased, the sheet resistance of quartz slides was found to decrease. As shown in Figure S19, it can be as low as 1 × 10−5 Ω/sq, for the quartz slides with five-layer LBL films of MWNT−UPM, due to the densely packing and the interconnected networks of MWNTs, which is equivalent to that of all carbon nanotube LBL films from electrostatic adsorption of functionalized MWNTs.5 Electrocatalytic Effect on Nifedipine. To find the potential application of such electroactive films, the electrocatalytic effect of LBL MWNT−UPM films on nifedipine was examined. As a calcium channel blocker, nifedipine (NIF) with the structure as shown in Figure S20 has been widely used in the treatment of angina pectoris, arterial hypertension, and various cardiovascular diseases. However, its overdose is toxic in nature and may cause severe dizziness, pounding heartbeats, nausea, vomiting, and so forth. Therefore, from a health concern perspective, a low cost, sensitive, and easy monitor of NIF is of great importance.50 Figure 6A shows the cyclic voltammograms of bare ITO (a) and five-layer MWNT−UPM/ ITO (b) electrodes toward NIF (1 × 10−4 mol/L) in 0.1 mol/L NH4Cl−NH3 (pH = 9.0) buffer solution. As shown from Figure 6A, the five-layer MWNT−UPM/ITO electrodes exhibited an enhanced catalytic effect on reduction of NIF. Compared with the bare ITO, the reduction peak potential of NIF was positively shifted from −0.94 to −0.87 V in the case of MWNT−UPM/ITO because of the enhanced electronic transport of ITO by MWNTs. Moreover, due to the presence of more electrochemical active compounds in the case of MWNT−UPM/ITO, the reduction current was increased four times as against that of bare ITO. The electrocatalytic activity of MWNT−UPM/ITO toward NIF is satisfactory, although it is relatively lower than the β-cyclodextrin (β-CD) modified MWNT paste electrode which can increase the current intensity of NIF solution by 6-fold because of the synergistic effects from MWNT and β-CD.50 In addition, from Figure S21, it can be observed that with increasing scan rate the reduction peak of NIF for MWNT−UPM/ITO was more negatively
hydrogen-bonding interactions between UPMs on MWNTs. Moreover, the five-layer MWNT−UPM films on quartz slides showed the satisfactory stability to the mild ultrasonication of 70 W for 1 min in H2O (Figure S17), just like the five-bilayer MWNT films linked by electrostatic interactions prepared according to the reported method5 (Figure S18). The good stability of the multilayer film upon soaking and ultrasonication is very useful for its potential applications in fuel cells, photoelectrochemical cells, and biosensors. Scanning electron microscopy was monitored to characterize the surface morphologies of the LBL thin films of MWNT− UPM self-assembled from the hydrogen bonding, as shown in Figure 5. From the SEM image of a five-layer LBL film of
Figure 5. SEM images of the surface of five-layer (a−c) and three-layer (d) LBL MWNT−UPM films on quartz slides.
MWNT−UPM (Figure 5a−c), the MWNTs were densely packed and uniformly distributed on the quartz slide. This is mainly because the multiply hydrogen bonding from the UPM arrays on MWNTs makes them significantly debundled and tightly packed. In addition, the coverage of the solid plates was increased with the increase of the assembly cycle. From the SEM image of a five-layer LBL film of MWNT−UPM (Figure 5c), the morphology with the higher-density coverage was found compared to that of the three-layer LBL film (Figure 5d), which is consistent with the results analyzed by the UV−vis
Figure 6. (A) Cyclic voltammogram of bare ITO (a) and five-layer MWNT−UPM/ITO (b) for NIF (1 × 10−4 mol/L) in NH4Cl−NH3 (pH = 9.0) buffer solution. (B) Cyclic voltammogram of five-layer MWNT−UPM/ITO for NIF solutions with different concentrations (mol/L): (a) 5 × 10−6, (b) 1 × 10−5, (c) 5 × 10−5, (d) 1 × 10−4, (e) 2 × 10−4, and (f) 3 × 10−4. Inset: the linear relationship of peak current (Ip) to the concentration of NIF. Scan rate: 50 mV/s. 12929
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Notes
shifted, along with the increasing peak current. The peak current of NIT was found to be proportional to square root of scan rate in the range 10−500 mV/s (Figure S22), which reveals the reduction of NIF on the MWNT−UPM/ITO should be a diffusion controlled process. The cyclic voltammogramm response of the five-layer MWNT−UPM/ITO in NH4Cl−NH3 (pH = 9.0) buffer solution containing different concentrations of NIF is shown in Figure 6B. The calibration curve in inset of Figure 6B shows the peak current increased lineally with concentration of NIF increased from 5 × 10−6 to 2 × 10−4 mol/L at the scan rate of 50 mV/s, which follows the equation as Ip (mA) = 1.996C (mM) + 54.7, where Ip is redox peak current and C is the concentration of NIF. This paved the way for the fabrication of biosensors based on the LBL MWNT−UPM films.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 21103133); the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry; the Natural Science Foundation of Shaanxi Province (No. 2013JM6012); Shaanxi Provincial Education Department Program (No.2013JK0928).
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4. CONCLUSION In summary, based on the self-complementary multiple hydrogen-bonding interactions, the modified MWNTs functionalized by ureidopyrimidinone-based modules (UPMs) showed a strong trend to self-assemble together in the apolar solvent and to be well dispersed in the polar solvent, which can be used to prepare the self-supported MWNT films assisted by a vacuum filtration method and the superthin MWNT films on solid substrates from the LBL process, respectively. This research will enrich the assembly fields of CNTs. The LBL MWNT−UPM films with high coverage on solid slides showed satisfactory stability upon soaking in different solvents, as well as good electrocatalytic behaviors, which can be applied to design ideal electrode materials for fuel cells, photoelectrochemical cells, and biosensors.
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ASSOCIATED CONTENT
S Supporting Information *
Structure of AADD quadruple hydrogen-bonding motif of ureidopyrimidinone and NIF; TGA thermograms of MWNT− COOH and MWNT−UPM; photo images of the broken film prepared by filtration of MWNT/CHCl3 dispersion; Raman spectra of MWNT−COOH, MWNT−NH2, and MWNT− UPM; photos of the ultrasonically dispersed system of MWNT−UPM in CHCl3 by addition of DMSO; UV−vis absorption spectra of MWNT and MWNT−UPM dispersion in different solvents; XPS spectra of ureidopyrimidinone terminated quartz slides, five-layer MWNT−UPM films on quartz slides and the peak fitting results; UV−vis spectra of singlelayer MWNT−UPM films on quartz slides prepared from dispersion of DMF and CHCl3 respectivley; UV−vis spectra of ureidopyrimidinone terminated quartz slides after being assembled from dispersion of pristine MWNTs and ureido group functionalized MWNTs in DMF; UV−vis spectra of fivelayer MWNT−UPM films upon soaking into H2O, DMF, and CHCl3 as well as mild ultrasonication; linear relationship of peak current of NIT reduction to the square root of scan rate. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
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AUTHOR INFORMATION
Corresponding Authors
*(Q.G.W.) Email: [email protected]. Tel/Fax: +86 29 86173324. *(S.M.W.) E-mail: [email protected]. Tel/Fax: +86 29 86173324 12930
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Self-Assembled Multiwalled Carbon Nanotube Films Assisted by Ureidopyrimidinone-Based Multiple Hydrogen Bonds Sumin Wang,*,† Hao Guo,† Xiaomin Wang,† Qiguan Wang,*,† Jinhua Li,† and Xinhai Wang‡ †
Scientific Research Innovation Team of Solidification Theory and Functional Materials, Shaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, People’s Republic of China S Supporting Information *
ABSTRACT: Self-assembled functionalized multiwalled carbon nanotube (MWNT) films were successfully constructed, linked by a kind of strong binding strength from the self-complementary hydrogenbonding array of ureidopyrimidinone-based modules (UPM) attached. Employing the feasible reaction of isocyanate containing ureidopyrimidinone with amine modified MWNTs, the UPMs composed of ureidopyrimidinone and ureido were attached to MWNTs with the content as low as 0.6 mmol/g MWNTs. Upon multiple hydrogenbonding interactions from incorporation of the AADD (A, hydrogenbonding acceptor; D, hydrogen-bonding donor) quadruple hydrogen bonds of ureidopyrimidinone and the double hydrogen bonds of ureido group, UPM functionalized MWNTs (MWNT−UPM) can be well dispersed in the polar solvent of N,N-dimethylformamide (DMF), while they tend to self-assemble to give a self-supported film in the apolar solvent of CHCl3. In addition, by using the multiple hydrogen-bonding interactions as the driving force, the layer-by-layer (LBL) MWNT−UPM films with high coverage on solid slides can be processed. Because of the self-association of MWNT−UPM in apolar solvent, it was found that the LBL assembly of MWNT−UPM was more favorable in the polar solvent of DMF than in the apolar solvent of CHCl3. Moreover, the hydrogenbonding linked MWNT−UPM films showed good stability upon soaking in different solvents. Furthermore, the as-prepared LBL films showed electrochemical active behaviors, exhibiting a remarkable catalytic effect on the reduction of nifedipine. hydrogen-bonded LBL multilayer films from dendrimers,16 nanoparticles,17−20 chitosan, and hyaluronic acid.21 Recently, it was found that, through the formation of double hydrogen-bonded homodimers on the surface of CNTs, thymine functionalized CNTs showed self-organized behaviors in different solvents.22 Additionally, by triple complementary ADA−DAD (A, hydrogen-bonding acceptor; D, hydrogenbonding donor) hydrogen-bonding interaction, dispersion and aggregation of CNTs can be switched on and off in different conditions.23−25 Moreover, the thymine-derived CNTs can be selectively adsorbed on photo-cross-linking matrices of polystyrene polymers bearing 2,6-di(acetylamino)-4-pyridyl deposited on glass or Si.26 Unfortunately, CNT LBL multilayer films linked by such hydrogen bonds have not yet been constructed, probably because of the relatively low binding constant. In comparison with the above hydrogen bonds, the selfcomplementary AADD quadruple hydrogen-bonding motif of ureidopyrimidinone (Figure S1, Supporting Information), first
1. INTRODUCTION Because of the potential applications in fuel cells, batteries, supercapacitors, and nanoelectrodes, substantive works have been carried out on carbon nanotube (CNT) films.1 Since the enhanced solubility of CNTs was available, the wet methods for the fabrication of CNT films based on the solution coating2,3 and layer-by-layer (LBL) assembly technique have been favorably used, because of the simple, low-temperature manipulation.4 For example, by using the LBL technique, highly conductive all multiwalled carbon nanotube (MWNT) films with controllable thickness, composition ratio, and porosity were synthesized from the LBL adsorption of carboxylic acid and amine group functionalized CNTs, which exhibit behaviors similar to those of the weak polyelectrolytes.5 In addition to the electrostatic interactions, many other strong or weak molecular interactions have been widely used in the LBL assembly, such as coordination bonding,6 chargetransfer interaction,7 specific recognition,8 and hydrogen bonding.9−11 First investigated by Rubner and Stockton9 and Zhang et al.,10 the functional LBL films from hydrogen bonding have attracted more research interest, ranging from hydrogenbonding-directed electroactive,12 photochromic,13 photoreactive polyelectrolyte,14 and self-supported multilayers15 to © 2014 American Chemical Society
Received: May 17, 2014 Revised: October 6, 2014 Published: October 8, 2014 12923
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Scheme 1. Structure of MWNT−UPM (a) and Schematic Representation of Constructing LBL MWNT−UPM Multilayer Films (b) from the Ureidopyrimidinone Group Functionalized Substrate Driven by the Multiple Hydrogen Bonding Interactions
built by Meijer and coworkers,27 possessing much stronger binding strength, may be a good candidate for the fabrication of LBL films from the giant molecules. The association constant of the quadruple hydrogen system can exceed 107 mol/L in apolar solvents, benefiting from the intermolecular quadruple AADD arrays and the intramolecular hydrogen bonds (Figure S1), which endowed the ureidopyrimidinone-derived module extensive applications in constructing the complicated functional supramolecular systems.28−35 To construct structure-stable LBL CNT films, here a multiple hydrogen-bonding interaction from ureidopyrimidinone-based module (UPM) was designed and introduced into the CNT connections. First, ureidopyrimidinone was attached onto the surface of MWNTs by reaction of isocyanate containing ureidopyrimidinone with amine modified MWNTs to form UPM functionalized MWNTs (MWNT−UPM, Scheme 1a), in which the self-complementary multiply hydrogen-bonding interaction composed of the quadruple hydrogen bonds from ureidopyrimidinone array and the double hydrogen bonds from ureido group exists. Then coupled with the LBL process, self-assembled MWNT films showing good stability were prepared (Scheme 1b).
Academy of Science. Nifedipine (NIF) was purchased from Sigma. Stock solution of NIF (1 × 10−3 mol/L) was prepared by dissolving an appropriate amount of NIF in methanol (HPLC grade). 2-Amino-6(heptan-3-yl)pyrimidin-4(1H)-one, 2(6-isocyanatohexylaminocarbonylamino)-6-methyl-4[1H]-pyrimidinone, and N-[(butylamino)carbonyl]-6-methylisocytosine were synthesized according to refs 27, 36, and 37. Toluene was refluxed with Na and redistilled. CHCl3 was refluxed with CaH2 and redistilled. During the self-assembly process, Milli-Q water (18 MΩ/cm), HPLC grade of CH3OH, and N,Ndimethylformamide (DMF) were used. Instrumentation. UV−vis spectra were obtained with a Shimadzu 1901 UV−vis spectrophotometer. For the measurement of suspension of MWNTs, the sample was first ultrasonicated for 2h, followed by centrifugation at 3000 rpm for 5 min to remove the large particles. Scanning electron microscope (SEM) images were collected with an S4800 or JSM-6360LV instrument. Transmission electron microscope (TEM) images were obtained from a JEM2010 instrument. Thermogravimetric analysis (TGA) data were collected with a Q50 TGA instrument. Raman measurements were performed by using an inVia Raman spectrometer with 532 nm wavelength laser source. The surface chemistry of functionalized MWNTs was analyzed by using a PHI 5400 X-ray photoelectron spectrometer (XPS). All spectra were calibrated with the C 1s photoemission peak for sp2 hybridized carbons at 284.3 eV. Curve fitting of the photoemission spectra was done after a Shirley type background subtraction. The tensile properties of the MWNT−UPM films were determined by using a DDL300 electronic tensile testing machine. Tensile tests were conducted on 6 mm wide and 0.06 mm thick samples, and a minimum of three samples were tested. Cyclic voltammetry was carried out with the MWNT−UPM films assembled on ITO glass as working electrode, a saturated calomel electrode (SCE) as reference electrode and a Pt wire as the counter electrode. The working
2. EXPERIMENTAL SECTION Materials. All reagents and chemicals were used as received unless otherwise noted. Carboxylic acid functionalized MWNTs (MWNT− COOH) (−COOH % = 2.56 wt %, purity > 95%), which were prepared by oxidizing the pristine MWNTs by KMnO4 in H2SO4, with the external diameter of 20−30 nm and length of 10−30 μm, were obtained from Chengdu Institute of Organic Chemistry, Chinese 12924
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electrode area is 0.25 cm2, which was always kept immersed in the electrolyte solution during the data collection. Preparation of Chlorinated MWNTs. A total of 50 mg of carboxylated MWNTs (MWNT−COOH) was added to 100 mL of SOCl2 and ultrasonicated for 30 min. Afterward, the suspension was refluxed for 12 h under N2 atmosphere. After evaporating the excessive SOCl2, the chlorinated MWNTs obtained were used immediately for the next step without further purification. Preparation of Amine Functionalized MWNTs. A volume of 80 mL of 1,2-ethylenediamine was added to the above chlorinated MWNTs and ultrasonicated for 30 min. Afterward, the suspension was rigorously stirred at 80 °C for 24 h under N2 atmosphere. After filtration, the black powder was thoroughly washed with toluene, CHCl3, and DMF sequentially. Dried at 50 °C in vacuum for 24 h, the amine functionalized MWNTs (MWNT−NH2) were obtained. Preparation of Ureidopyrimidinone Functionalized MWNTs. A total of 50 mg of MWNT−NH2, 35 mg of 2(6-isocyanatohexylaminocarbonylamino)-6-methyl-4[1H]-pyrimidinone (NCO(CH2)6-UPy), and 0.1 mL of dibutyltin dilaurate (DBDTL) were added to 80 mL of dry CHCl3 and stirred at 50 °C for 24 h under N2 atmosphere. After filtration, the black powder was thoroughly washed with toluene, CHCl3, and DMF. After drying at 50 °C in vacuum for 24 h, ureidopyrimidinone functionalized MWNTs (MWNT−UPM) were obtained. Preparation of Isocyanate Terminated Substrates. After being cleaned with acetone and water, the quartz or ITO substrates were ultrasonicated in a hot piranha solution (mixture of 98% H2SO4 and 30% H2O2, v/v 7:3) for 30 min. (Caution! Piranha solution is extremely corrosive.) After being thoroughly washed with pure water and dried by N2, the hydroxyl group functionalized substrates obtained were immersed in a toluene solution (60 mL) containing 1.5 g of NCO(CH2)6NCO and 0.1 mL of DBDTL for 10 h. After washing with toluene and drying, isocyanate terminated substrates were obtained. Preparation of Ureidopyrimidinone Terminated Substrates. The isocyanate terminated substrates were immersed in 30 mL of dry CHCl3 solution containing 30 mg of 2-amino-6-(heptan-3-yl)pyrimidin-4(1H)-one and 0.1 mL of DBDTL at 50 °C for 24 h. After being thoroughly washed by toluene, CHCl3, DMF, and H2O sequentially and dried under N2, ureidopyrimidinone terminated substrates were obtained (Scheme 1b). Self-Assembly LBL Films of MWNT−UPM. To form the stable dispersion for LBL assembly process, 1 mg of the dried MWNT− UPM powders was ultrasonicated in 10 mL of DMF for 1 h. Then the ureidopyrimidinone terminated substrates were immersed into the MWNT−UPM dispersion for 10 min. After being washed with the solvent and dried with N2, the substrates adsorbed by one layer of MWNT−UPM were dipped again into the MWNT−UPM dispersion to obtain the second MWNT−UPM layer. Under the repetitive operation of such steps, the self-assembly LBL multilayer films of MWNT−UPM could be obtained in a cyclic fashion.
Scheme 2. Synthetic Route of MWNT−UPM
Figure 1 shows the FT-IR spectra of MWNT−NH2 (Figure 1a), MWNT−UPM (Figure 1b), and the reference compound
Figure 1. FT-IR spectra of MWNT−NH2 (a), MWNT−UPM (b), and UPy−R (c). Inset shows the molecular structure of UPy−R.
N-[(butylamino)carbonyl]-6-methylisocytosine (UPy−R) (Figure 1c) with the structure as shown in the inset. From the spectrum of MWNT−NH2 shown in Figure 1a, the characteristic absorption band at 1635 cm−1 assigned to the stretching vibration of CO group of amide (amide I band) and the band at 1592 cm−1 assigned to the bending vibration of N−H group of the amide (amide II band) confirm the presence of the amide functional group in MWNT−NH2. In addition, the absorption bands located at 3462, 3131, and 3403 cm−1 in Figure 1a attributed to characteristic N−H stretching vibration of the primary amine and amide group respectively illustrate the presence of the saturated primary amine in MWNT−NH2. In the case of MWNT−UPM (Figure 1b), the absorption of stretching vibration corresponding to N−H was integrated into one broad peak at 3437 cm−1 because of the reaction of primary amine on MWNT−NH2 with NCO(CH2)6UPy. Meanwhile, a new absorption band at 1650 cm−1 was found for MWNT− UPM, which was assigned to stretching vibrations of CC and CN of ureidopyrimidinone. This showed the successful attachment of the ureidopyrimidinone group on the MWNTs. Finally, compared to MWNT−NH2, the characteristic stretching vibration peak of CO (1630 cm−1) and bending vibration peak of N−H (1587 cm−1) in amide groups of MWNT−UPM showed a shift to lower wavenumbers under the hydrogenbonding interactions with which they were involved. Compound UPy−R, always existing as dimers in the solid state because of the formation of hydrogen bonding, shows the
3. RESULTS AND DISCUSSION Preparation and Characterization of MWNT−UPM. A three-step reaction strategy was employed to prepare UPM functionalized MWNTs, as shown in Scheme 2. First, carboxylated MWNTs were chlorinated by refluxing with SOCl2, and then amine groups were introduced on the walls of MWNTs by reaction of chlorinated MWNTs with excessive 1,2-ethylenediamine to form MWNT−NH2. After the feasible reaction of isocyanate group of NCO(CH2)6UPy with the amine group on MWNT−NH2, the UPM functionalized MWNTs were obtained, where ureidopyrimidinone groups were attached to the surface of MWNTs spaced by the long alkyl chain. Such a structural design endows the resultant MWNT−UPM good solubility in organic solvents. It was found that there were no sediments in MWNT−UPM/DMF dispersion stored for 6 months. 12925
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Figure 2. XPS survey scans of chemically modified MWNTs (a). C 1s (b) and O 1s (c) XPS spectra of MWNT−COOH. N 1s (d), C 1s (e), and O 1s (f) XPS spectra of MWNT−NH2. N 1s (g), C 1s (h), and O 1s (i) XPS spectra of MWNT−UPM.
spectrum of MWNT−NH2 (Figure 2d) also showed the presence of amide and amine on MWNT.39 For the MWNT−UPM, the XPS spectra (Figure 2g−i) showed relatively different features compared with those of MWNT−NH2, because of hydrogen bonds introduced by ureido and pyrimidinone groups. The N 1s peak at 399.8 eV attributed to nitrogen in amide of MWNT−NH2 (Figure 2d) was shifted to 399.3 eV in the case of MWNT−UPM (Figure 2g). Meanwhile, the O 1s peak at 531.7 eV attributed to oxygen in amide group of MWNT−NH2 (Figure 2f) was shifted to higher binding energy at 532.3 eV in the case of MWNT−UPM (Figure 2i). This is because of the strong hydrogen-bonding interactions introduced by the ureido groups which changed the electronic state and the binding energy of the involved atoms in MWNT−NH2. Hydrogen bonds are composed of a hydrogen atom that serves as a bridge between two electronegative atoms called hydrogen donor and hydrogen acceptor, making the hydrogen acceptor of oxygen more electropositive and the hydrogen donor of nitrogen more electronegative.40,41 Additionally, the C 1s peak of amide group centered at 288.0 eV for MWNT−NH2 (Figure 2e) was also shifted to higher binding energy at 288.6 eV in the case of MWNT−UPM (Figure 2h). This is due to the charge transfer from the carbon of −NH−CO to the bond near hydrogen acceptor in the formation of hydrogen bonding. Moreover, the presence of a new N 1s peak at 400.4 eV in the XPS spectra of MWNT−UPM (Figure 2g) was assigned to −N of pyrimidinone which serves as a hydrogen acceptor in the formation of hydrogen bonding.41 In addition, from the quantitative estimation of UPM groups on MWNTs by TGA analysis (Figure S2), it showed that the number of initial carboxylic acid moieties was 0.6 mmol/g of MWNTs, which could be totally converted to UPM groups
similar absorption bands with that of MWNT−UPM for the amide groups located at 1630 and 1583 cm−1, which further confirms the presence of the strong hydrogen-bonding interactions in MWNT−UPM. To verify the results from FT-IR, XPS analysis (Figure 2) of MWNT−COOH, MWNT−NH2, and MWNT−UPM was performed. As shown from the survey spectra in Figure 2a, N 1s features cannot be found in the raw material of MWNT− COOH, aside from the O 1s and C 1s photoemission peaks centered at 285.0 and 533.0 eV. However, the XPS survey spectra of MWNT−NH2 and MWNT−UPM showed the N 1s features centered at 400.0 eV besides C 1s (285.0 eV) and O 1s (533.0 eV) features. For the C 1s XPS spectrum of MWNT−COOH (Figure 2b), the peaks located at 284.3 and 285.1 eV are attributed to sp2 and sp3 hybridized carbons, respectively. The peak with the higher binding energy at 288.3 eV shown in Figure 2b was attributed to the carbons of carboxylic acid on MWNT− COOH, accompanied by the O 1s features at 531.7 and 533.1 eV (Figure 2c) which were attributed to the oxygen of −CO and −OH, respectively.38 After attachment by amine group, the C 1s XPS feature at 288.3 eV assigned to the carbons of carboxylic acid on MWNT−COOH (Figure 2b) was shifted to lower binding energy at 288.0 eV in the case of MWNT−NH2 (Figure 2e), which was assigned to carbons of amide (−NH− CO) group.39 Accompanied by the conversion of carboxylic acid to amide, the O 1s XPS peak at 533.1 eV assigned to −OH on MWNT−COOH (Figure 2c) disappeared in the case of MWNT−NH2 (Figure 2f). Meanwhile, the peak at 286.5 eV different from the amide features in the C 1s XPS spectrum of MWNT−NH2 (Figure 2e) was attributed to the −CH2−NH2 groups.5,39 In addition, the peak located at 399.8 eV in the N 1s 12926
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addition of polar solvent such as DMF and dimethyl sulfoxide (DMSO). As shown in Figure S4, after adding 4 mL of DMSO to the badly dispersed system (Figure S4a) of 1 mg of MWNT−UPM in 20 mL of CHCl3, the large aggregates of MWNT−UPM were well dispersed (Figure S4b) with the aid of ultrasonication. This showed that the conventional hydrogen bonding between polar solvent and MWNT−UPM can weaken the multiple hydrogen bonding between MWNT−UPM, which is favorable for the dispersion of MWNT−UPM aggregates. TEM studies of pristine MWNTs dispersion in DMF (Figure 3c) and CHCl3 (Figure 3f) were also carried out as control experiments. Because in the polar solvent of DMF, the multiple−hydrogen-bonding interactions between MWNT− UPM bundles were significantly weakened under the interactions between MWNT−UPM and solvent in contrast to pristine MWNTs, which results in the significantly unbundled MWNTs networks in the former (Figure 3b) compared with pristine MWNTs (Figure 3c). However, the presence of strong multiple hydrogen-bonding interactions between MWNT−UPM compared with the weak interactions between pristine MWNTs in the apolar solvent of CHCl3, larger aggregates of MWNTs (Figure 3e) were observed for MWNT−UPM than the latter (Figure 3f). The control experiments also prove that it is the multiple-hydrogen-bonding interaction that leads to the different assembly behaviors of MWNT−UPM in different media. Since the absorption properties of CNTs can be strongly influenced by the degree of bundling,43,44 the assembly behaviors of MWNT−UPM were also investigated by UV− vis absorption spectroscopy. Because of the significant conglomeration from the multiple hydrogen bonding, the UV−vis spectrum of MWNT−UPM dispersed in CHCl3 only exhibited a featureless absorption in the region of 255−400 nm (Figure S5a). However, the UV−vis spectrum of MWNT− UPM in DMF showed a distinct absorption peak at 286 nm (Figure S5b), which was attributed to the π−π* transition of MWNT−UPM. This is because the dissociation of the MWNT−UPM aggregates makes MWNT−UPM more active to the specific band of UV−vis light. Furthermore, the featureless absorption of MWNT−UPM aggregates in CHCl3 can be significantly changed by introduction of CH3OH. As shown in Figure S6, upon addition of CH3OH in the MWNT− UPM/CHCl3 dispersion, a new peak at 283 nm attributed to the π−π* transition of MWNT−UPM appeared. Moreover, with the increase of the amount of CH3OH, the intensity of the peak at 283 nm was gradually increased. This is because the addition of CH3OH can weaken the hydrogen-bonding interactions among MWNT−UPM and lead to dissociation of the aggregates. As a control experiment, the same amount of CH3OH added in the pristine MWNT/CHCl3 dispersion cannot change the shape of the UV−vis absorption spectra (Figure S7), where almost no hydrogen-bonding interactions between pristine MWNT and CH3OH can be used to break the aggregation of MWNTs. The similar results were found in the case of adding DMF to MWNT−UPM/CHCl3 dispersion. (Figures S8 and S9). Interestingly, such multiply hydrogen-bonding interactions between MWNT−UPM molecules were so strong that a selfsupported MWNTs film (Figure 3g) can be formed by vacuum filtration of the MWNT−UPM/CHCl3 dispersion through a Millipore membrane according to a reported method.45 From the SEM images shown in Figure 3h, the MWNTs in the resultant films were evenly and densely connected under the
according to synthetic route shown in Scheme 2. Moreover, as shown in the Raman spectra of MWNT−COOH, MWNT− NH2, and MWNT−UPM (Figure S3), the D- to G-band intensity ratio (ID/IG) for MWNT−COOH is ca. 1.4, which is typical for the class of carbon nanomaterials.42 The ID/IG values for MWNT−UPM and MWNT−NH2 are ca. 1.2 and 1.1, respectively, both close to that of MWNT−COOH, implying that the fundamental structure of MWNTs almost remains intact upon the functionalization according to synthetic route shown in Scheme 2. Hydrogen Bonding Association of MWNT−UPM in Different Solvents. It is well-known that the association of the dispersed molecules bearing multiple hydrogen bonds can be significantly influenced by the solvents used, which leads to the different assembly behaviors of such molecules in different media. Therefore, the hydrogen-bonding association of MWNT−UPM molecules was analyzed from the formed morphologies of MWNT−UPM dispersion in different solvents by TEM images. As illustrated from the TEM images shown in Figure 3a and b, the MWNT−UPM dispersed in the polar
Figure 3. TEM images of MWNT−UPM/DMF (a), pristine MWNTs/DMF (c), MWNT−UPM/CHCl3 (d), pristine MWNTs/ CHCl3 (f). High resolution TEM images of MWNT−UPM/DMF (b) and MWNT−UPM/CHCl3 (e). Photo (g) and SEM (h) images of the self-supported film of MWNT−UPM.
solvent of DMF was significantly unbundled and uniformly distributed. However, from the images shown in Figure 3d and e, the MWNT−UPM dispersed in the apolar solvent of CHCl3 was significantly aggregated and unevenly connected. This is because the conventional hydrogen-bonding interactions between the polar groups in DMF and the ureidopyrimidinone as well as ureido groups in MWNT−UPM hindered the typical association of MWNT−UPM by the multiple hydrogen bonding, which makes MWNT−UPM individually dispersed in DMF. While, in the case of MWNT−UPM dispersed in CHCl3, the strength of the interactions between MWNT− UPM and solvent is weak. Therefore, the larger MWNT−UPM aggregates in CHCl3 were clearly observed, which was mainly due to the recognition and the association of MWNT−UPM molecules by the multiple hydrogen bonding. Furthermore, the aggregates of MWNT−UPM in CHCl3 could be dissociated by 12927
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interactions of multiple hydrogen bonding from the UPM, which endowed the system relatively high flexibility. The MWNT−UPM buckypaper showed an electrical conductivity value of 10 S/cm, which is relatively lower than the reported values of MWNT−NH2 (25 ± 1 S/cm) and MWNT−COOH (26 ± 2 S/cm) buckypapers prepared by vacuum filtration of their aqueous dispersion assisted by Triton X-100. The decrease of the conductivity may result from the presence of organic groups on MWNT−UPM.46 The film tensile strength and modulus are 35.8 ± 5 MPa and 2.1 ± 0.3 GPa, respectively, which showed the same order of magnitude as the nitric acid treated single-walled carbon nanotube buckypapers.47 Additionally, in the case of the pristine MWNTs, the self-supported MWNT buckypaper cannot be obtained after vacuum filtration of the MWNT/CHCl3 dispersion (Figure S10), where the interactions mainly from the disordered bundling between MWNTs were relatively weak compared to those in the MWNT−UPM/CHCl3 system. Fabrication of LBL Multilayer Films of MWNT−UPM. To further find the contribution of such multiple hydrogen bonding to the formation of MWNT films, the LBL process of the MWNT−UPM was experimentally tried. As shown from Scheme 1, the driving force for the fabrication of the MWNT LBL films mainly resulted from the multiple hydrogen-bonding interactions between MWNT−UPM itself, which acts as both hydrogen-bond donor and hydrogen-bond acceptor, because of the self-complementary characteristic of the quadruple hydrogen bonds of ureidopyrimidinone and the double hydrogen bonds of ureido group. For the fabrication of LBL films of MWNT−UPM, the surface of the substrates should be modified by the ureidopyrimidinone group first. The introduction of the terminal ureidopyrimidinone group on the substrates provides binding sites that can form hydrogen-bonding interaction with MWNT−UPM. The fabrication route for ureidopyrimidinone terminated substrate is illustrated in Scheme 1. First, the hydroxyl group functionalized substrate was reacted with excess NCO(CH2)6NCO to form a isocyanate terminated substrate. Then ureidopyrimidinone terminated substrate was prepared by the reaction of 2-amino-6-(heptan-3-yl)pyrimidin-4(1H)one with the above isocyanate terminated substrate in CHCl3. The XPS analysis of C 1s and N 1s features (Figure S11) suggests the presence of sp3 hybridized carbons (285.3 eV in C 1s XPS) and amide groups associated with hydrogen bonding (288.9 eV in C 1s XPS and 399.4 eV in N 1s XPS), which showed the successful attachment of ureidopyrimidinone group to the quartz slide. In addition, UV−vis spectrum of the slide (Figure S12a) showed a π−π* absorption band of pyrimidinone ring centered at 265 nm, which further supports the XPS results. From the UV−vis absorption spectra of the ureidopyrimidinone modified quartz slides repetitively dipped into the MWNT−UPM/DMF dispersion (Figure 4), well-resolved bands centered at 260 nm, in agreement with the π−π* absorption of MWNT−UPM, were found. Just deposited twice, the quartz substrate significantly darkened, because of the strong absorption of the deposited MWNTs in the visible region,48 which showed it was an effective way for the MWNTs LBL film formation by using the multiple hydrogen bonding as the driving force. In addition, from the inset illustrated in Figure 4, it was shown that the absorbance centered at 260 nm and the optional point at 400 nm both increase linearly with the increase of the number of deposited layers. It suggested that the
Figure 4. UV−vis absorption spectra of MWNT−UPM multilayers on quartz slides with an increasing number of layers. Inset: absorbance at 260 and 400 nm with the increasing number of layers. The absorbance of ureidopyrimidinone terminated quartz slide was subtracted as baseline. Solvent, DMF; assembly time, 10 min/layer.
similar amount of MWNT−UPM was deposited on the solid substrate for every assembly cycle, which showed the characteristic of the stepwise and regular process. From the comparison of UV−vis absorbance for the single layer MWNT−UPM assembled on quartz slide (Figure S13), the LBL assembly of MWNT−UPM was more favorable in the polar solvent of DMF than in the apolar solvent of CHCl3. This is because the significant self-association and the limited dispersion of MWNT−UPM in the apolar solvent of CHCl3 makes the MWNT−UPM bundling difficult to be uncoupled to incorporate with the hydrogen-bonding modules on the substrate slides. Also, the controlled experiments under the same conditions showed that the MWNT LBL films cannot be obtained if pristine MWNTs or ureido group functionalized MWNTs are used as the building block (Figure S14), which proves the necessity of using driving forces with stronger hydrogen-bonding association for the LBL process. The presence of hydrogen-bonding interactions in the linkage of MWNT−UPM LBL films can be seen from XPS analysis. The binding energy of O 1s for the carbonyl groups of the five-layer MWNT−UPM film is 0.5 eV higher than that of MWNT−NH2 (531.7 eV) (Figure S15a), because the oxygen in the carbonyl groups in the MWNT−UPM film became more electropositive as the strong hydrogen bonding formed. In addition, the lower binding energy of 399.2 eV for N 1s in amide of the LBL MWNT−UPM film (Figure S15b) relative to MWNT−NH2 at 399.8 eV, and a higher binding energy for the N 1s of −N of pyrimidinone at 400.3 eV in the LBL MWNT−UPM film (Figure S15b) relative to the reported value of 397.1 eV41 also indicated the presence of hydrogenbonding interaction. Encouragingly, the LBL films of MWNT−UPM linked by such strong hydrogen-bonding interactions showed good stability upon soaking in different solvents. For a five-layer MWNT−UPM film on quartz slides, the stability of the multilayer system was tested by placing the MWNT−UPM film into different solvents such as H2O, DMF, and CHCl3 at room temperature for 8 h. It was found that the UV−vis spectra of the film showed almost the same features as those before soaking (Figure S16). In addition, the absorbance at 260 nm attributed to the π−π* transition of MWNT−UPM illustrated no sharp decrease, which showed that the MWNTs within the LBL films can be hardly peeled off from the surface of the functionalized quartz slides, because of the strong multiple 12928
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spectra shown in Figure 4. Moreover, the presence of a great quantity of nanoscale pores (see the arrows in Figure 5b) formed in these LBL films of MWNT−UPM may endow them good electrochemical activities because of the enlarged specific surface area.5,49 With the number of MWNT−UPM layers increased, the sheet resistance of quartz slides was found to decrease. As shown in Figure S19, it can be as low as 1 × 10−5 Ω/sq, for the quartz slides with five-layer LBL films of MWNT−UPM, due to the densely packing and the interconnected networks of MWNTs, which is equivalent to that of all carbon nanotube LBL films from electrostatic adsorption of functionalized MWNTs.5 Electrocatalytic Effect on Nifedipine. To find the potential application of such electroactive films, the electrocatalytic effect of LBL MWNT−UPM films on nifedipine was examined. As a calcium channel blocker, nifedipine (NIF) with the structure as shown in Figure S20 has been widely used in the treatment of angina pectoris, arterial hypertension, and various cardiovascular diseases. However, its overdose is toxic in nature and may cause severe dizziness, pounding heartbeats, nausea, vomiting, and so forth. Therefore, from a health concern perspective, a low cost, sensitive, and easy monitor of NIF is of great importance.50 Figure 6A shows the cyclic voltammograms of bare ITO (a) and five-layer MWNT−UPM/ ITO (b) electrodes toward NIF (1 × 10−4 mol/L) in 0.1 mol/L NH4Cl−NH3 (pH = 9.0) buffer solution. As shown from Figure 6A, the five-layer MWNT−UPM/ITO electrodes exhibited an enhanced catalytic effect on reduction of NIF. Compared with the bare ITO, the reduction peak potential of NIF was positively shifted from −0.94 to −0.87 V in the case of MWNT−UPM/ITO because of the enhanced electronic transport of ITO by MWNTs. Moreover, due to the presence of more electrochemical active compounds in the case of MWNT−UPM/ITO, the reduction current was increased four times as against that of bare ITO. The electrocatalytic activity of MWNT−UPM/ITO toward NIF is satisfactory, although it is relatively lower than the β-cyclodextrin (β-CD) modified MWNT paste electrode which can increase the current intensity of NIF solution by 6-fold because of the synergistic effects from MWNT and β-CD.50 In addition, from Figure S21, it can be observed that with increasing scan rate the reduction peak of NIF for MWNT−UPM/ITO was more negatively
hydrogen-bonding interactions between UPMs on MWNTs. Moreover, the five-layer MWNT−UPM films on quartz slides showed the satisfactory stability to the mild ultrasonication of 70 W for 1 min in H2O (Figure S17), just like the five-bilayer MWNT films linked by electrostatic interactions prepared according to the reported method5 (Figure S18). The good stability of the multilayer film upon soaking and ultrasonication is very useful for its potential applications in fuel cells, photoelectrochemical cells, and biosensors. Scanning electron microscopy was monitored to characterize the surface morphologies of the LBL thin films of MWNT− UPM self-assembled from the hydrogen bonding, as shown in Figure 5. From the SEM image of a five-layer LBL film of
Figure 5. SEM images of the surface of five-layer (a−c) and three-layer (d) LBL MWNT−UPM films on quartz slides.
MWNT−UPM (Figure 5a−c), the MWNTs were densely packed and uniformly distributed on the quartz slide. This is mainly because the multiply hydrogen bonding from the UPM arrays on MWNTs makes them significantly debundled and tightly packed. In addition, the coverage of the solid plates was increased with the increase of the assembly cycle. From the SEM image of a five-layer LBL film of MWNT−UPM (Figure 5c), the morphology with the higher-density coverage was found compared to that of the three-layer LBL film (Figure 5d), which is consistent with the results analyzed by the UV−vis
Figure 6. (A) Cyclic voltammogram of bare ITO (a) and five-layer MWNT−UPM/ITO (b) for NIF (1 × 10−4 mol/L) in NH4Cl−NH3 (pH = 9.0) buffer solution. (B) Cyclic voltammogram of five-layer MWNT−UPM/ITO for NIF solutions with different concentrations (mol/L): (a) 5 × 10−6, (b) 1 × 10−5, (c) 5 × 10−5, (d) 1 × 10−4, (e) 2 × 10−4, and (f) 3 × 10−4. Inset: the linear relationship of peak current (Ip) to the concentration of NIF. Scan rate: 50 mV/s. 12929
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Notes
shifted, along with the increasing peak current. The peak current of NIT was found to be proportional to square root of scan rate in the range 10−500 mV/s (Figure S22), which reveals the reduction of NIF on the MWNT−UPM/ITO should be a diffusion controlled process. The cyclic voltammogramm response of the five-layer MWNT−UPM/ITO in NH4Cl−NH3 (pH = 9.0) buffer solution containing different concentrations of NIF is shown in Figure 6B. The calibration curve in inset of Figure 6B shows the peak current increased lineally with concentration of NIF increased from 5 × 10−6 to 2 × 10−4 mol/L at the scan rate of 50 mV/s, which follows the equation as Ip (mA) = 1.996C (mM) + 54.7, where Ip is redox peak current and C is the concentration of NIF. This paved the way for the fabrication of biosensors based on the LBL MWNT−UPM films.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 21103133); the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry; the Natural Science Foundation of Shaanxi Province (No. 2013JM6012); Shaanxi Provincial Education Department Program (No.2013JK0928).
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4. CONCLUSION In summary, based on the self-complementary multiple hydrogen-bonding interactions, the modified MWNTs functionalized by ureidopyrimidinone-based modules (UPMs) showed a strong trend to self-assemble together in the apolar solvent and to be well dispersed in the polar solvent, which can be used to prepare the self-supported MWNT films assisted by a vacuum filtration method and the superthin MWNT films on solid substrates from the LBL process, respectively. This research will enrich the assembly fields of CNTs. The LBL MWNT−UPM films with high coverage on solid slides showed satisfactory stability upon soaking in different solvents, as well as good electrocatalytic behaviors, which can be applied to design ideal electrode materials for fuel cells, photoelectrochemical cells, and biosensors.
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ASSOCIATED CONTENT
S Supporting Information *
Structure of AADD quadruple hydrogen-bonding motif of ureidopyrimidinone and NIF; TGA thermograms of MWNT− COOH and MWNT−UPM; photo images of the broken film prepared by filtration of MWNT/CHCl3 dispersion; Raman spectra of MWNT−COOH, MWNT−NH2, and MWNT− UPM; photos of the ultrasonically dispersed system of MWNT−UPM in CHCl3 by addition of DMSO; UV−vis absorption spectra of MWNT and MWNT−UPM dispersion in different solvents; XPS spectra of ureidopyrimidinone terminated quartz slides, five-layer MWNT−UPM films on quartz slides and the peak fitting results; UV−vis spectra of singlelayer MWNT−UPM films on quartz slides prepared from dispersion of DMF and CHCl3 respectivley; UV−vis spectra of ureidopyrimidinone terminated quartz slides after being assembled from dispersion of pristine MWNTs and ureido group functionalized MWNTs in DMF; UV−vis spectra of fivelayer MWNT−UPM films upon soaking into H2O, DMF, and CHCl3 as well as mild ultrasonication; linear relationship of peak current of NIT reduction to the square root of scan rate. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
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*(Q.G.W.) Email: [email protected]. Tel/Fax: +86 29 86173324. *(S.M.W.) E-mail: [email protected]. Tel/Fax: +86 29 86173324 12930
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