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J. Phys. Chem. B 2008, 112, 12934–12939
Functionalization of Multiwalled Carbon Nanotubes by Pyrene-Labeled Hydroxypropyl Cellulose Qiang Yang, Li Shuai, Jinjin Zhou, Fachuang Lu, and Xuejun Pan* Department of Biological Systems Engineering, UniVersity of WisconsinsMadison, 460 Henry Mall, Madison, Wisconsin 53706 ReceiVed: June 19, 2008; ReVised Manuscript ReceiVed: August 7, 2008
Pyrene-labeled hydroxypropyl cellulose (HPC-Py) was synthesized through a condensation reaction between hydroxypropyl cellulose (HPC) and 1-pyrenebutyric acid (Py). A hybrid (HPC/MWNTs) of the HPC-Py and multiwalled carbon nanotubes (MWNTs) was prepared through a noncovalent method. Temperature-variable UV-vis spectra indicated that the HPC-Py had a lower critical solution temperature of about 44 °C in water. 1H NMR, UV-vis, Raman, and fluorescence spectra were used to systematically investigate the π-π stacking interaction between the HPC-Py and MWNTs. Dispersion experiments showed the HPC/MWNTs hybrids could be well dispersed in water and many organic solvents. 1. Introduction Carbon nanotubes (CNTs) have good electronic and mechanical properties and are considered as ideal starting materials for fabricating functionalized CNT-polymer composites which have many potential applications, such as fillers in polymer systems, molecular tanks, and biosensors.1a However, pristine CNTs are insoluble in all organic solvents and aqueous solutions. They can be dispersed in some solvents by sonication, but the dispersion is not stable and precipitation immediately occurs when the sonication stops. The lack of solubility and poor dispersibility of CNTs in most solvents or polymeric systems has impeded their applications. Therefore, solubilizaton or dispersibilization of CNTs is critical, and this field is rapidly expanding. For example, CNT-polymer composites require the individualization and uniform dispersion of CNTs in a polymer matrix and good compatibility with the host matrix.1b Solubility or dispersibility is essential and critical for the directed assembly of CNTs into functional molecular devices and composites.1c Solubilization by wrapping with biocompatible materials makes CNTs biologically relevant, and they can be used as potential carriers that transport and deliver bioactive components into cells.1d On the other hand, considerable progress has been made in improving the dispersibility of carbon nanotubes by surface functionalization or modification, such as the covalent attachment of chemical groups through reactions onto the π-conjugated skeleton of CNTs or the noncovalent adsorption or wrapping of various functional molecules, which were extensively covered and discussed in two excellent reviews.1a,c Of surface modification strategies, noncovalent functionalization of the CNTs by π-π stacking interaction has been proved to be one of the most promising methods because it does not disrupt the sp2 structure and conjugation of the CNTs.2 Pyrene and its derivatives have been widely used to functionalize the CNTs because of their ability to interact with the CNTs via π-π stacking.3 CNT-polysaccharide composites have recently attracted more attention because of their potential applications in biosensors. It has been reported that CNTs were functionalized with starch,4 * To whom correspondence should be addressed. Phone: (608) 262-4951. Fax: (608) 262-1228. E-mail:
[email protected].
TABLE 1: DS and Solubilitya of HPC-Py under Different Experimental Conditions entry
Rb
DSc
water
ethanol
THF
1 2 3 4 5
0.60 0.30 0.20 0.10 0.02
0.08 0.06 0.05 0.03 0.01
+ + + +
+ + + + +
+ + + + +
a Solubility: -, insoluble; +, soluble. b R (molar feed ratio) ) MPy/HPC (mM/g). c DS ) degree of substitution, calculated from the 1H NMR spectrum by comparing the pyrene proton signals to the glucose unit proton signals.
dextran,5 and schizophyllan6 through physical adsorption or π-π stacking interaction. Cellulose, a natural polymer of D-glucose building units, is the most abundant and sustainable polysaccharide on earth.7 Composites of cellulose and carbon nanotubes have been used to construct lithium ion batteries and supercapacitors.8,9 However, different from the polysaccharides mentioned above, pristine cellulose is insoluble in most solvents except for ionic liquids due to its strong hydrogen-bonded supramolecular structure.10 Therefore, a major challenge in fabricating the composites of cellulose and the CNTs is to improve the solubility of cellulose and the compatibility between cellulose and the CNTs. To make cellulose soluble in solvents and improve its properties and performance, varied cellulose derivatives have been synthesized. Linear hydroxypropyl cellulose (HPC) is one of the most commonly used commercial cellulose derivatives with good solubility in most common solvents. The composite of HPC and single-walled carbon nanotubes has been used as the nanochemical sensor to detect various chemical gases.11 To our knowledge, a hybrid material composed of multiwalled carbon nanotubes (MWNTs) and pyrene-labeled HPC (HPCPy) has not been reported. In this work, we synthesized HPCPy and then prepared the HPC/MWNT hybrid by a noncovalent functionalization method. 2. Experimental Section 2.1. Materials. MWNTs (o.d. 20-50 nm, wall thickness 1-2 nm, length 0.5-2 µm), HPC powder (average Mw ) 80 000,
10.1021/jp805424f CCC: $40.75 2008 American Chemical Society Published on Web 09/23/2008
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J. Phys. Chem. B, Vol. 112, No. 41, 2008 12935
Figure 1. 2D HSQC NMR spectrum of HPC-Py in CDCl3.
SCHEME 1: Synthesis Route of HPC-Py
average Mn ) 10 000, 20 mesh particle size), 4-(dimethylamino)pyridine (DMAP), N,N-dicyclohexylcarbodiimide (DCC), and 1-pyrenebutyric acid (Py) were obtained from SigmaAldrich (St. Louis, MO). Solvents of tetrahydrofuran (THF), N,N-dimethylformamide (DMF), and n-hexane were purchased from Fisher Scientific (Pittsburgh, PA) and dried with 4 Å molecular sieves before use. 2.2. Synthesis of HPC-Py. Typically, 1.527 g of HPC, 92.6 mg of DMAP (0.76 mmol), 0.1564 g of DCC (0.76 mmol), and 0.2231 g of Py (0.77 mmol) were added to a 50 mL threenecked flask. Under protection of nitrogen, 30 mL of dry DMF was then added using a syringe. The mixture reacted first at 0 °C for 1 h and then at room temperature for 24 h. At the end of reaction, the urea salt formed from DCC was filtered out, and the product (HPC-Py) was precipitated in n-hexane (100 mL). Then the crude product was redissolved in THF (10 mL) and precipitated again in n-hexane (100 mL). The operation of redissolution-precipitation was repeated three times. The HPCPy product with a color of light yellow was obtained and dried
under vacuum. The yield of HPC-Py was 85%. 1H NMR (360 MHz, CDCl3, δ (ppm)): 8.41-7.79 (9 H, protons of the pyrene
Figure 2. UV-vis absorption spectra of HPC-Py with varied DS in THF (concentration 0.1 mg/5 mL): (a) 0.08, (b) 0.06, (c) 0.05, (d) 0.03.
12936 J. Phys. Chem. B, Vol. 112, No. 41, 2008
Yang et al. of 10 kV. UV-vis absorption spectra were collected from a variable-temperature UV-vis spectrophotometer (Cary 50 Bio, Varian, Inc., Palo Alto, CA). Fluorescence spectra were recorded on an MOS-250 fluorescence spectrometer (Biologic, Claix, France). Raman spectra were obtained on a Raman spectrometer (Aramis CRM, Horiba Jobin Yvon, Edison, NJ) with a 780 nm AlGaAs diode laser source.
Figure 3. UV-vis absorption spectra of HPC-Py aqueous solution at different temperatures (DS ) 0.03).
Figure 4. Emission spectra of HPC-Py with different DSs in THF (λexcitation ) 345 nm, concentration 0.1 mg/5 mL): (a) DS ) 0.08, (b) DS ) 0.03.
SCHEME 2: Illustration of the HPC/MWNT Hybrid
moieties), 4.58-2.78 (5 H, protons of the glucose unit), 2.46 (2 H, -COCH2CH2CH2-), 2.17 (2 H, -COCH2CH2CH2-), 1.79-1.40 (5 H, -COCH2CH2CH2-, -OCH2CHCH3O-), 1.10 (3 H, -OCH2CHCH3O-). 2.3. Preparation of the HPC/MWNT Hybrid. MWNTs (5 mg) were dispersed in a solution of 10 mg of HPC-Py in 2 mL of THF in a 5 mL tube by sonication for 30 min. The dispersion was then centrifuged for 20 min at 14 000 rpm. The black sediment was collected and washed with THF three times to remove free HPC-Py. After the sediment was dried under vacuum, the HPC/MWNT hybrid was obtained. 2.4. Characterization. 1H NMR and 2D HSQC (heteronuclear single-quantum correlation) NMR spectra were recorded on a Bruker DRX-360 NMR spectrometer (360.13 MHz, Bruker, Rheinstetten, Germany) fitted with a 5 mm 1H/broad-band gradient probe with inverse geometry using CDCl3 as the solvent. Scanning electron microscopy (SEM) images were taken using a field-emission microscope (JEOL 6700F, JEOL USA, Inc., Peabody, MA) operated at an acceleration voltage
3. Results and Discussion 3.1. Synthesis of Pyrene-Labeled Hydroxypropyl Cellulose. It was reported that fluorescently labeled hydroxypropyl cellulose was synthesized by a reaction of sodium alkoxidehydroxypropyl cellulose with 4-(1-pyrenyl)butyl tosylate.12,13 However, this synthesis method achieved a relatively low degree of substitution (DS, average number of pyrene moieties introduced to each glucose unit). In the present research, HPCPy was synthesized through a condensation reaction between HPC and Py in the presence of DCC. The reaction is shown in Scheme 1. The effect of the molar feed ratio (R; see the definition in Table 1) on the DS of the HPC-Py products was investigated in detail, and the results are listed in Table 1. The DS was calculated from the 1H NMR spectrum by comparing the pyrene proton signals to the glucose unit proton signals. The results indicated that DS increased nonlinearly with an increase of the R value. Specifically, when R increased from 0.02 to 0.2, DS increased quickly from 0.01 to 0.05. However, a further increase of R from 0.2 to 0.6 only produced a slow increase in DS from 0.05 to 0.08. Bulky Py groups might be the cause. When some of the hydroxyl groups of HPC were substituted by Py, further substitution became difficult and slow due to the steric barrier of the introduced Py groups. The results in Table 1 indicate that the content of pyrene moieties in HPC-Py is controllable by changing the molar feed ratio. Hydroxypropyl cellulose is soluble in water and many organic solvents (such as ethanol and THF). However, the solubility of HPC-Py is dependent on the content of pyrene moieties introduced (i.e., DS). The results listed in Table 1 indicate that when the DS approached 0.08, HPC-Py became insoluble in water, but still had good solubility in ethanol and THF. The chemical structure of HPC-Py was confirmed by 2D HSQC NMR. As shown in Figure 1, proton peaks of the glucose unit (1, 2, 3, 4, and 5) and methyl and methylene groups (6, 7, 8, 9, 10, and 11) of HPC were clearly visible. The nine characteristic proton peaks (12) of the pyrene moiety were clearly observed in the 2D HSQC NMR spectrum (also see the magnified inset plot). These NMR data indicate that HPC-Py was successfully synthesized. UV-vis absorption spectroscopy was used to further verify the chemical structure of the pyrene-labeled cellulose. Figure 2 shows the UV-vis absorption spectra of the HPC-Py samples with different DSs in THF. No absorption peak was observed for the hydroxypropyl cellulose backbone. However, six absorption peaks at 244, 267, 277, 314, 328, and 344 nm were clearly observed in the spectra of the HPC-Py samples, which were the characteristic absorption bands of pyrene,3h and the intensities of the characteristic peaks were correlated to the DS of HPCPy. The higher the DS, the greater the intensities of the characteristic peaks. Hydroxypropyl cellulose undergoes a phase transition at the lower critical solution temperature (LCST; about 41 °C) in aqueous solution.15 Above the LSCT, water becomes a poor solvent of hydroxypropyl cellulose, and collapse, association and even precipitation of the HPC molecule chains occur. This
Functionalization of MWNTs by HPC-Py
Figure 5.
1
J. Phys. Chem. B, Vol. 112, No. 41, 2008 12937
H NMR spectra of the HPC-Py/MWNT dispersion (A) and HPC-Py (B) in CDCl3.
Figure 7. Raman spectra of the HPC/MWNT hybrid and MWNTs. Figure 6. UV-vis absorption spectra of HPC-Py and the HPC/MWNT hybrid in THF.
temperature-responsive property of HPC is of great interest and importance for its applications. For example, it can be potentially used as a temperature-sensitive carrier of active reagents.15 HPC may not be suitable for the application at a temperature higher than its LSCT if good solubility of HPC in water is required. In the present research, an HPC derivative (HPC-Py) is used to modify MWNTs to improve their dispersibility in water or other solvents. It is critical to know whether HPC-Py still exhibits the temperature-responsive property as HPC does and the LCST of HPC-Py. Variable UV-vis absorption spectroscopy is a powerful technique to investigate the temperature-responsive property of hydroxypropyl cellulose and its derivatives and to measure the LCST.14 The effect of temperature on the UV-vis
absorption spectra of HPC-Py in water is presented in Figure 3. The spectra were recorded using a cuvette with a lid to prevent the water from evaporation during the measurements. At each temperature, the aqueous solution of the HPC-Py sample was equilibrated for 30 min before measurement, which ensured the molecular chains of HPC-Py to transit from one metastable state to another when the temperature changed. Figure 3 indicates that a high temperature intensified the absorption, but did not affect the wavelength of the absorption bands. The increased absorption intensity was attributable to the formation of polymeric aggregates induced by the temperature increase.14 The formation of metastable polymeric aggregates made the HPCPy solution more scattering and resulted in a higher absorption intensity. The results indicate that after being labeled by pyrene, hydroxypropyl cellulose still exhibited the temperature-respon-
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Figure 8. Emission spectra of HPC-Py and the HPC/MWNT hybrids in THF (λexcitation ) 345 nm).
Figure 9. Dispersion images of HPC/MWNT hybrids in solvents: (A) in water (0.05 mg/mL), (B) in ethanol (0.05 mg/mL), (C) in THF (0.1 mg/mL), (D) in CHCl3 (0.1 mg/mL).
sive property, but the LSCT moved to a slightly higher temperature of approximately 44 °C (see the inset plot in Figure 3), which is consistent with the literature value.14 Pyrene usually exhibits characteristic monomer emission at low concentration and excimer emission at high concentration.16 Therefore, at the same concentration of HPC-Py, the DS of HPC-Py may theoretically decide its emission type of monomer or excimer. The fluorescence emission spectrum of HPC-Py is shown in Figure 4. The spectra of the two HPC-Py samples with different DSs showed three monomer emissions at 379, 395, and 416 nm, while an excimer emission at 470 nm was only observed in the spectrum of the HPC-Py sample with a higher DS of 0.08.16 3.2. Preparation of the HPC/MWNT Hybrid. It is wellknown that pyrene and its derivatives can interact with CNTs via π-π stacking, which can be verified by 1H NMR at the molecular level.2 In general, proton peaks of the pyrene moiety that is close to the surface of the CNTs become broad and shift downfield because of the π-π stacking interaction. Figure 5 shows the 1H NMR spectra of HPC/MWNTs and HPC-Py in CDCl3. The HPC/MWNTs and HPC-Py had similar 1H NMR
spectra. However, it is clearly seen that the peaks of the pyrene moiety of HPC-Py became broad and shifted downfield in the presence of MWNTs.17 These NMR data verified the existence of a strong π-π stacking interaction between MWNTs and the pyrene-labeled cellulose. Conclusively, the pyrene-labeled cellulose can be used to functionalize MWNTs by π-π stacking interaction. The schematic structure of the HPC/MWNTs hybrid is illustrated in Scheme 2. The composition of the HPC/MWNT hybrid was further verified by UV-vis spectra, as shown in Figure 6. The spectrum of the HPC/MWNT hybrid was similar to that of HPC-Py, indicating the presence of HPC-Py in the HPC/MWNTs hybrid. On the other hand, the absorption intensity of the HPC/MWNT hybrid was lower than that of HPC-Py due to the presence of the MWNTs.18 The HPC/MWNT hybrid was further characterized using Raman spectroscopy. Figure 7 shows the Raman spectra of pristine MWNTs and the HPC/MWNT hybrid. The pristine MWNTs had two characteristic peaks at 1321 and 1565 cm-1, corresponding to the D band (C-C) and G band (CdC), respectively.19 The intensity ratios (ID/IG) of the D-band to the G-band of the MWNTs and HPC/MWNT hybrid were 0.24 and 0.40, respectively. These results indicated that the D-band of the MWNTs was enhanced significantly by the hybridization with HPC-Py, which should be attributed to the π-π stacking interaction. Compared with the pristine MWNTs, the Raman blue-shift phenomenon was observed in the spectrum of the HPC/MWNT hybrid because of an increased extent of disorder and defect.20 Furthermore, a new D′-band appeared at 1611 cm-1 in the spectrum of the HPC/MWNT hybrid, which represented the enhancement of the disorder extent.21 These Raman spectra
Figure 10. SEM images of MWNTs (A) and HPC/MWNT hybrids (B, DS(HPC-Py) ) 0.03; C, DS(HPC-Py) ) 0.08).
Functionalization of MWNTs by HPC-Py provided additional evidence of the interaction between the MWNTs and HPC-Py. The fluorescence property of HPC-Py could be quenched when hybridized with the MWNTs. The fluorescence quenching efficiency of the HPC/MWNT hybrid was quantitatively measured. Figure 8 shows the emission spectra of two HPC-Py samples with varied DS (0.08 and 0.03 in (A) and (B), respectively) and their corresponding HPC/MWNT hybrids. From the spectra, it is clear that the fluorescence intensities of the HPC/MWNT hybrids at 345 nm of excitation were far lower than those of the HPC-Py samples. This phenomenon is called emission quenching.22 This kind of emission quenching is usually observed in the carbon nanotube hybrid.3 The quenching resulted from the π-π stacking between the MWNTs and HPCPy, which was attributed to the disruption of π-conjugation by a conformational change rather than an electronic transfer.23 By calculation, the quenching efficiency was about 98% regardless of the DS of HPC-Py. It is very difficult to disperse pristine MWNTs in most solvents because of their strong hydrophobic graphite structure. However, the HPC/MWNTs were well dispersed in water, ethanol, THF, and CHCl3, as shown in Figure 9. The dispersions were very stable, and no sediment was observed after two weeks of storage at room temperature. It was the HPC layer noncovalently coated on the surface of the MWNTs that prevented the aggregation of MWNTs in the solvents. The dispersibility of the HPC/MWNT hybrids in an aqueous medium was dependent on the solubility of the corresponding HPC-Py in water. In other words, it was dependent on the DS of HPC-Py. When the DS of HPC-Py was below 0.08 (at or above which HPC-Py became insoluble in water, as shown in Table 1), the corresponding HPC/MWNT hybrids were dispersible in water. In addition, despite the fact that HPC-Py had a temperatureresponsive property in water, as discussed above, our results (not shown here) indicated that the HPC/MWNT hybrids had good stability in water at 50 °C. The morphologies of the pristine MWNTs and HPC/MWNT hybrids were observed using SEM. As shown in Figure 10, the pristine MWNTs formed large bundles due to strong hydrophobic interaction, while the HPC/MWNT hybrids existed as individual tubes or small bundles because the thin HPC layer covering the surface of the MWNTs prevented them from aggregation. The SEM images provided additional evidence of the good dispersibility of HPC/MWNTs. 4. Conclusion HPC-Py with varied DS was successfully prepared by a simple condensation reaction. MWNTs were noncovalently functionalized with HPC-Py through π-π stacking interaction. The HPC/MWNT hybrid could be dispersed in water and many organic solvents, which made the hybrid a potential candidate for biosensors. Acknowledgment. We gratefully acknowledge support from the Department of Biological Systems Engineering and the College of Agriculture and Life Science at the University of WisconsinsMadison and USDA McIntire-Stennis Fund.
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