Layer-by-Layer Assembly of Aqueous Dispersible, Highly Conductive

Apr 30, 2008 - Poly(aniline-co-o-anisidine) (P(An-co-o-As)) ionomers and ... of the P(An-co-o-As) to MWNTs, which effectively delocalize the electrons...
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Langmuir 2008, 24, 5825-5831

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Layer-by-Layer Assembly of Aqueous Dispersible, Highly Conductive Poly(aniline-co-o-anisidine)/Poly(sodium 4-styrenesulfonate)/MWNTs Core–Shell Nanocomposites Fei Wang,† Gengchao Wang,† Shu Yang,‡ and Chunzhong Li*,† Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China UniVersity of Science & Technology, Shanghai 200237, P.R. China, and Department of Materials Science and Engineering, UniVersity of PennsylVania, 3231 Walnut Street, Philadelphia, PennsylVania 19104 ReceiVed August 29, 2007. ReVised Manuscript ReceiVed February 19, 2008 Poly(aniline-co-o-anisidine) (P(An-co-o-As)) ionomers and poly(sodium 4-styrenesulfonate) (PSS) were layerby-layer (LbL) assembled on carboxylic acid-functionalized multiwalled carbon nanotubes (MWNTs). The multilayered polyelectrolyte greatly enhanced the dispersibility and stability of MWNTs in aqueous solutions. More importantly, the nanocomposites showed 3 orders of magnitude of conductivity increase, 4.2 S/cm, compared to that of neat ionomers, 0.004 S/cm. The deposition procedure was monitored with zeta (ζ) potential changes. Fourier transform infrared (FT-IR), ultraviolet-visible (UV–vis), and Raman spectra confirmed charge transfer from the quinoid units of the P(An-co-o-As) to MWNTs, which effectively delocalize the electrons. Further, we explored the pH response of the assembled P(An-co-o-As)/PSS/MWNTs multilayer nanocomposites. The sharp transition of the conductivity in the pH range of 2 to 6 makes the nanocomposites promising candidates for chemical-biological sensing.

Introduction It is well-known that the poor dispersibility of carbon nanotubes (CNTs) in water and most common organic solvents have greatly hindered their broad applications. Many approaches have been studied to functionalize the surface of CNTs and obtain stable dispersions in solvents, specifically in water, mainly including covalent grafting of hydrophilic groups1 and physical wrapping with surfactants,2 water-soluble polymers, such as poly(ethylene oxide) (PEO),3 dye,4 DNA,5 protein,6 polysaccharides7 and polyelectrolytes.8 Compared to the chemical grafting methods and physical stabilizing with surfactants, physical wrapping of polymers on CNTs has been widely studied due to its simplicity and readily available functional polymers. Nevertheless, if the polymers wrapping on CNTs are nonconducting or less conductive, they could impair the electrical conductivity of CNTs.9 It has been shown that wrapping CNTs with conducting polymers such as polyaniline (PANI) may resolve this issue.10–12 However, most PANI-coated CNTs can only be dispersed in organic solvents. Our interest is to develop a novel strategy of physically assembling conducting polymers on CNTs such that the * Author to whom correspondence may be sent. E-mail: [email protected]. † East China University of Science & Technology. ‡ University of Pennsylvania. (1) Peng, H.; Alemany, L. B.; Margrave, J. L.; Khabashesku, V. N. J. Am. Chem. Soc. 2003, 125, 15174. (2) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3, 269. (3) Sano, M.; Kamino, A.; Okamura, J.; Shinkai, S. Langmuir 2001, 17, 5125. (4) Hu, C. G.; Chen, Z. L.; Shen, A. G.; Shen, X. C.; Li, J.; Hu, S. S. Carbon 2006, 44, 428. (5) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; Mclean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338. (6) Karajanagi, S. S.; Yang, H.; Asuri, P.; Sellitto, E.; Dordick, J. S.; Kane, R. S. Langmuir 2006, 22, 1392. (7) Numata, M.; Asai, M.; Kaneko, K.; Bae, A.-H.; Hasegawa, T.; Sakurai, K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 5875. (8) Kim, B.; Park, H.; Sigmund, W. M. Langmuir 2003, 19, 2525. (9) Guo, M.; Chen, J.; Li, J.; Nie, L.; Yao, S. Electroanalysis 2004, 16, 1992. (10) Premamoy, G.; Samir, K. S.; Amit, C. Eur. Polym. J. 1999, 35, 699. (11) Cochet, M.; Maser, W. K.; Benito, A. M.; Callejas, M. A.; Martinez, M. T.; Benoit, J. M.; Schreiber, J.; Chauvet, O. Chem. Commun. 2001, 1, 1450. (12) Wu, T.; Lin, Y.; Liao, C. Carbon 2005, 43, 734.

composites will maintain their electronic properties while improving the dispersibility of CNTs in aqueous solutions. Here, we synthesized aniline/o-anisidine copolymers (P(Anco-o-As)) and then prepared their water-soluble ionomers. These positively charged ionomers and a negatively charged, watersoluble polyelectrolyte, poly(sodium 4-styrenesulfonate) (PSS) were alternately absorbed on the carboxylic acid-functionalized multiwalled carbon nanotubes (COOH-MWNTs) via layer-bylayer (LbL) electrostatic assembly technique. The resultant multilayer P(An-co-o-As)/PSS/COOH-MWNTs core–shell nanocomposites have high electrical conductivity, good aqueous dispersibility, and pH sensitivity, suggesting that this may serve as a novel method for preparing applicable chemical-biological sensors and antistatic filler in aqueous bases.

Experimental Section Materials. Multiwalled carbon nanotubes (MWNTs, purity g95%) were obtained from Shenzhen Nanotech Port, China. The MWNTs were ultrasonically treated in a mixture of concentrated H2SO4 and HNO3 (3:1 v/v) at 60 °C for 1 h in order to introduce carboxylic acid groups (designated as COOH-MWNTs). The functionalized MWNTs were negatively charged in neutral aqueous solutions. Aniline (An) and o-anisidine (o-As) (analytical grade, Shanghai Chemical Reagent Corp.) were distilled under reduced pressure for copolymerization. Poly(sodium 4-styrene sulfonate) (PSS, Scheme 1a, Mw ) 7000, from Aldrich) was used without further purification. Synthesis of Aqueous Soluble P(An-co-o-As) Ionomers. The emeraldine base form of aniline/o-anisidine copolymer (P(An-coo-As)) (Scheme 1b) were synthesized following the procedure described in the literature.13 1H NMR spectrum showed a molar ratio of 0.63/0.37 aniline/o-anisidine. The number-average molecular weight (Mn) is estimated to be 26800 with a polydispersity of about 4.64 by gel permeation chromatographic analysis (GPC), which was performed using a solution of the copolymer in dimethylformamide (DMF, 1 g/L), and the flow-rate was 1.0 mg/min. (13) Ozdemir, C.; Can, H. K.; Colak, N.; Guner, A. J. Appl. Polym. Sci. 2006, 99, 2182.

10.1021/la8000625 CCC: $40.75  2008 American Chemical Society Published on Web 04/30/2008

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Scheme 1. Chemical Atructures of (a) PSS, and (b) P(An-co-o-As) Ionomers

Scheme 2. Schematic Illustration of the LbL Assembly Procedure

Table 1. The Chemical Composition and Conductivity for Composites samples COOH-MWNTs P(An-co-o-As) ionomer P(An-co-o-As)/MWNTs P(An-co-o-As)/PSS-1/MWNTs P(An-co-o-As)/PSS-2/MWNTs P(An-co-o-As)/PSS-3/MWNTs

layer number of PSS

N content (%)

S content (%)

P(An-co-o-As) content (%)

PSS content (%)

conductivity (S/cm)

0 1 3 5

0 13.46 2.30 2.25 5.82 7.24

0 0 0 0.24 0.7 1.2

0 100 17.1 16.7 43.3 53.8

0 0 0 1.6 4.5 7.8

3.4 0.004 4.2 1.10 0.32 0.18

The aqueous soluble P(An-co-o-As) ionomers were prepared following the procedures by Cheung et al.14 Briefly, P(An-co-oAs)-EB was dissolved in dimethylacetamide (DMAc) solution (40 mg/mL). After filtering, HCl (1 mol/L) was added dropwise until the pH was ∼2.5–4.0. The solution was filtered through a 0.22 µm filter before its use. The P(An-co-o-As) ionomer concentration (0.8-3.2 mg/mL) was varied by adjusting the amount of either P(An-co-o-As) in DMAc or water added. Preparation of P(An-co-o-As)/PSS/MWNTs Multilayer Composites. The LbL assembly of P(An-co-o-As)/PSS/MWNTs nanocomposites is outlined in Scheme 2. COOH-MWNTs aqueous suspension (5 mg/mL) was obtained by sonification. PSS was dissolved in deionized water at a concentration of 1 mg/mL containing 0.04 mol/L NaCl. The COOH-MWNTs suspension (5 mg/mL) and the aqueous solution of P(An-co-o-As) ionomers (0.8 mg/mL) were (14) Cheung, J. H.; Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2712.

first mixed and diluted to 50 mL. After sonification for 0.5 h, the mixture was filtered and washed with deionized water three times. The obtained nanocomposites (P(An-co-o-As)/MWNTs) (100 mg) were dispersed in water (20 mL), and mixed with PSS solution (5 mL), which was subsequently diluted to 50 mL. After filtration and washing, the bilayer composite (P(An-co-o-As)/PSS/MWNTs) was obtained. These steps were repeated until the desired layer numbers were achieved. After filtrating and drying under vacuum at 60 °C for 24 h, the final composites were obtained. The chemical composition for composites was listed in Table 1. Characterization. The chemical states of the MWNTs’ surfaces before and after the carboxylic acid treatment were characterized by X-ray photoelectron spectroscopy on a RBD upgraded PHI 5000C instrument equipped with a monochromatic Mg KR X-ray source (1253.6 eV). The zeta (ζ)-potential change of composites was measured using a Malvern Zetasizer 3000HS analyzer. The ionic strength of the dilute suspension was maintained at 10-3 mol/L using aq KCl. Transmission electron microscopy (TEM) images were taken by a JEOL JEM-2100

Assembly of P(An-co-o-As)/PSS/MWNTs Nanocomposites

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Figure 2. ζ potential of multilayer composite as a function of layer number in water solution at pH ) 5.4. The concentration of P(An-coo-As) was 0.8 mg/mL.

Figure 1. C1s XPS spectra of MWNTs (a) before and (b) after carboxylic acid functionalization (sonification in a mixture of concentrated H2SO4 and HNO3 (3:1 v/v) at 60 °C for 1 h).

electron microscope at 120 kV. The Fourier transform infrared (FT-IR) spectra were recorded from KBr pellets using a Nicolet 5700 spectrometer. Ultraviolet-visible (UV–vis) absorption spectra were obtained using a Shimadzu UV2102PC spectrophotometer in aqueous solutions. Raman spectra were recorded from Reishaw inVia+Reflex using a 50 mW He-Ne laser operating at 514 nm. The conductivity of the sample pellets (compressed under 10 MPa) was measured by the four-probe method using SX 1934 four-probe instrument at room temperature. For pH response study, the aqueous solution of the nanocomposites (1 wt %) was stabilized with a different pH value for 5 min before drying and the following conductivity measurement. The contents of N and S elements were measured from Elementar Vario EL III. The dispersion stability of the samples in water was studied using an analyzer of concentrated liquid dispersion from Formulaction Corporation. The aqueous suspensions of the samples were placed in a cylindrical glass cell to measure the backscatting intensity. The light source is an electroluminescent diode that emits light in the near-infrared (λ ) 880nm).

Results and Discussion -COOH Groups Introduction onto MWNTs Surface. Prior to use, MWNTs were chemically treated by sonification in a mixture of sulfuric acid and nitric acid (3:1) in order to introduce carboxylic acid groups, which made them negatively charged in neutral solutions. The -COOH content was determined from XPS results. Figure 1 shows the C1s XPS spectra of MWNTs before and after acid treatment. The shift of all the binding energies

was corrected using the C1s level at 284.6 eV as an internal standard. Both spectra have been fitted into an asymmetric peak of graphite (284.5 ( 0.2 eV) and four Gaussian peaks centered at 285.1 ( 0.2, 286.2 ( 0.2, 287.5 ( 0.2, and 288.9 ( 0.2 eV, as shown in Table 2.15–17 The peaks at 284.5 and 285.1 eV are attributed to the graphite carbon (C-C) and the hydrocarbons (C-H), respectively.16 And the peaks with higher binding energies located at 286.2, 287.5, and 288.9 eV can be assigned as -C-O-, >CdO, and -COOH species, respectively. The data show that the spectral contribution of the -COOH peak increases from about 1.51% to 7.65% after acid treatment, which is necessary and advantageous for the following LbL assembly. Fabrication and Characterization of Composites. PANI is a well-known conducting polymer. However, it has limited solubility in water and most organic solvents. In contrast, poly(oanisidine) is soluble in common solvents because of its -OCH3 groups. The incorporation of the anisidine units into the PANI backbone will increase the solubility of the polymer.13 Therefore, we synthesized poly(aniline-co-o-anisidine) (P(An-co-o-As)) as a substitute for PANI, which has a good solubility in DMAc and maintains good electrical conductivity. And we prepared watersoluble ionomers from P(An-co-o-As), which had positive charges to facilitate electrostatic interactions with COOH-MWNTs and PSS. The zeta (ζ) potential changes of P(An-co-o-As)/PSS/ MWNTs composites as a function of layer number are given in Figure 2. The ζ potentials oscillate alternately, confirming that the driving force of assembly is electrostatic attractions. Partially oxidizing MWNTs in H2SO4/HNO3 introduced carboxylic groups on MWNT surfaces, resulting in a net negative charge upon dissociation in water. The positively charged P(An-co-o-As) ionomer and negatively charged PSS were then coated alternately on the surface of the COOH-MWNTs. The periodic change of ζ potential suggests the uniform adsorption of P(An-co-o-As)/ PSS upon each cycle. The fine structure of the nanocomposites assembled at different stages was characterized by TEM (Figure 3). Figure 3a-d shows the TEM images of COOH-MWNTs, P(An-co-o-As)/PSS-1/ MWNTs, P(An-co-o-As)/PSS-2/MWNTs, and P(An-co-o-As)/ (15) Ago, H.; Kugler, T.; Cacialli, F.; Salaneck, W. R.; Shaffer, M. S. P.; Windle, A. H.; Friend, R. H. J. Phys. Chem. B 1999, 103, 8116. (16) Blyth, R. I. R.; Buqa, H.; Netzer, F. P.; Ramsey, M. G.; Besenhard, J. O.; Golob, P.; Winter, M. Appl. Surf. Sci. 2000, 167, 99. (17) Okpalugo, T. I. T.; Papakonstantinou, P.; Murphy, H.; McLaughlin, J.; Brown, N. M. D. Carbon 2005, 43, 153.

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Table 2. Curve Fitting Results of XPS C1s Spectra, Value Given in % of Total Intensity samplesa

C-C 284.5 eVb

C-H 285.1 eV

-C-O- 286.2 eV

>CdO- 287.5 eV

-COOH 288.9 eV

MWNTs COOH-MWNTs

64.63 52.33

15.73 11.21

18.13 28.80

0 0.01

1.51 7.65

a

MWNTs: raw carbon nanotubes; COOH-MWNTs: carboxylic acid-functionalized carbon nanotubes.

b

Binding energy.15–17

Figure 3. TEM images of (a) COOH-MWNTs, (b) P(An-co-o-As)/PSS-1/MWNTs, (c) P(An-co-o-As)/PSS-2/MWNTs, and (d) P(An-co-o-As)/ PSS-3/MWNTs. The red line indicates the thickness of the polymer shells assembled on MWNTs.

PSS-3/MWNTs, respectively. It can be clearly seen that the MWNTs used in our experiments are of herringbone-type. Also, the tubular morphology of the composite, which has a core–shell structure where the CNTs are core encapsulated by the P(Anco-o-As)/PSS shell, is present in all products with different diameters, which are proportional to LbL cycle number. The P(An-co-o-As)/PSS shell of P(An-co-o-As)/PSS-1/MWNTs is smooth (Figure 3b); however, it becomes rougher in P(An-coo-As)/PSS-2/MWNTs and P(An-co-o-As)/PSS-3/MWNTs (Figure 3c,d). The positively charged P(An-co-o-As) ionomers were initially mainly absorbed to the sites existing of -COOH groups on the surface of MWNTs. The interaction between polymers and MWNTs might hinder the crystallization of P(An-co-o-As) and thus form the smooth amorphous layer on the interface (Figure 3b). With the interaction weakening, the outer-layer polymers tend to form a regular structure on the surface of the amorphous layer.12 The same thing happened at the interface between P(Anco-o-As) and PSS. So the P(An-co-o-As)/PSS shell has amorphous structures at the P(An-co-o-As)/MWNTs and P(An-co-o-As)/ PSS interfaces and polycrystalline structure between the two interfaces. But interpenetration between the P(An-co-o-As) layers and the PSS layers would happen, and thus the structures of shells around MWNTs would not arrange in regular layers in some places (Figure 3c,d). From the thickness of 1, 3, and 5 LbL cycle shells (4, 12, and 22 nm, respectively), it can be estimated that the shell thickness of each LbL cycle is about 4 nm. This

Figure 4. FT-IR spectra of (a) COOH-MWNTs, (b) P(An-co-o-As) ionomer, (c) P(An-co-o-As)/MWNTs, (d) (P(An-co-o-As)/PSS-3/ MWNTs, and (e) PSS.

demonstrates that the thickness of the polymer shell can be easily controlled by changing the LbL cycle number. Figure 4 shows the FT-IR spectra for the COOH-MWNTs, P(An-co-o-As) ionomers, P(An-co-o-As)/MWNTs, P(An-co-oAs)/PSS-3/MWNTs composites, and PSS, respectively. For

Assembly of P(An-co-o-As)/PSS/MWNTs Nanocomposites

COOH-MWNTs, the peaks at 1720 cm-1, 1195, and 1093 cm-1 correspond to the stretching modes of the carboxylic acid groups18 (Figure 4a). The spectrum of P(An-co-o-As) ionomer (Figure 4b) is similar to that of polyaniline emeraldine salt (PANI-ES), where the absorption peaks at 1598 cm-1 and 1500 cm-1 are assigned to CdC stretching vibrations of the quinoid and benzenoid rings, respectively.19,20 As commonly observed for ES PANI, the quinoid band at about 1600 cm-1 is less intense than that of the benzenoid band at 1500 cm-1. The strong characteristic band at about 1147 cm-1 was described by Quillard et al. as the “electron-like band” and is considered to be a measure of delocalization of electrons.19 Most signals of the COOH-MWNTs peaks have been covered by those of P(An-co-o-As) in the P(An-co-o-As)/MWNTs curve (Figure 4c). The peak corresponding to COOH groups on MWNTs at 1720 cm-1, however, can be observed to red-shift to 1708 cm-1 and become very weak. This may be attributed to the formation of the hydrogen bonding between the amino groups of P(An-co-o-As) and the carboxylic acid groups of MWNTs. The CdC stretching peaks are red-shifted to 1576 cm-1 and 1491 cm-1, respectively, in the P(An-co-o-As)/MWNTs composite, which might be attributed to the favorable interaction between MWNTs and P(An-co-o-As). In addition, the P(Anco-o-As)/MWNTs composite exhibits an inverse intensity ratio of 1600/1500 cm-1 compared to that of the ionomers, indicating that the P(An-co-o-As) in the composite is richer in quinoid units than the P(An-co-o-As) ionomers. That is because the π-bonded surface of MWNTs could interact strongly with the conjugated structure of P(An-co-o-As), especially through the quinoid ring, and therefore, stabilize the quinoid ring structure. The increasing intensity of the signal at 1147 cm-1 and its redshift to 1134 cm-1 may indicate that the interaction between P(An-co-o-As) and MWNTs facilitates a charge transfer process21 and enhances the degree of electron delocalization, thus increasing the conductivity of the composites. This agrees with our increased conductivity measurements. The characteristic peaks of COOH-MWNTs, P(An-co-o-As), and PSS are all observed in the spectrum of the P(An-co-oAs)/PSS-3/MWNTs composite and shifted to lower wave numbers (Figure 4d). Besides the signals similar to those in P(An-co-oAs)/MWNTs, the peaks at 1124, 1031, and 575 cm-1 are assigned to S-O, SdO, and C-S stretching vibrations in the PSS component, respectively.22 The red-shift of PSS characteristic peaks may have resulted from the doping effect of SO3- on P(An-co-o-As). Moreover, the intensity ratio of 1600/1500 cm-1 in P(An-co-o-As)/PSS-3/MWNTs decreases relative to that of P(An-co-o-As)/ MWNTs and becomes similar to that of the neat P(An-co-o-As) ionomers. It is presumed that the introduction of the polyelectrolyte PSS could cut off the interaction between MWNTs and P(An-co-o-As) ionomers, hence, the P(An-co-oAs) in external layers (adsorbed on PSS layers) acted as neat P(An-co-o-As) ionomers with no “charge transfer” effect. As seen in the UV–vis spectra (Figure 5), the PSS and the COOH-MWNTs show no absorption band over 300-1000 nm (Figure 5a,d). The P(An-co-o-As) ionomers, however, exhibit three absorption bands (Figure 5b): 311 nm corresponding to a π-π* transition of the benzenoid ring of P(An-co-o-As), and (18) Porro, S.; Musso, S.; Vinante, M.; Vanzetti, L.; Anderle, M.; Trotta, F.; Tagliaferro, A. Physica E 2007, 37, 58. (19) Ouillard, S.; Louarn, G.; Lefrant, S.; MacDiarmid, A. G. Phys. ReV. B 1994, 50, 12496. (20) Furnkawa, Y.; Ueda, F.; Hyodo, Y.; Harada, I.; Nakajima, T.; Kawagoe, T. Macromolecules 1988, 21, 1297. (21) Zengin, H.; Zhou, W.; Jin, J.; Czera, R.; Smith, D. W.; Echegoyen, L.; Carroll, D. L.; Foulger, S. H.; Ballato, J. AdV. Mater. 2002, 14, 1480. (22) Jamróz, D.; Maréchal, Y. J. Phys. Chem. B 2005, 109, 19664.

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Figure 5. UV–vis spectra of (a) PSS, (b) P(An-co-o-As) ionomer, (c) (P(An-co-o-As)/PSS-3/MWNTs, and (d) COOH-MWNTs in neutral aqueous solutions. Inset: intensity of the absorption band at 958 nm vs the number of LbL cycles.

two characteristic bands at 446 and 847 nm assigned to the localized polarons.23 For the P(An-co-o-As)/PSS-3/MWNTs, three characteristic bands are observed. The bands at 311 and 446 nm were both shifted to smaller wavelength (292 nm and 442nm, respectively). This may be attributed to the interaction between the nanotubes and P(An-co-o-As) chains as well as the doping effect of SO3- in PSS, which effectively improve the degree of electron delocalization of the P(An-co-o-As) chains.24 Moreover, the characteristic band at 847 nm red-shifted to 958 nm. This longer localization length may reflect a more extended conformation of P(An-co-o-As) chains in P(An-co-o-As)/PSS3/MWNTs.25 The absorbance at 958 nm increases linearly with the number of LbL cycles (Figure 5, inset), confirming a reproducible adsorption of P(An-co-o-As) on the MWNT surfaces from cycle to cycle. To further study the interactions between the nanotubes and P(An-co-o-As) chains, the Raman spectra of the COOH-MWNTs, P(An-co-o-As) ionomers, P(An-co-o-As)/MWNTs, and P(Anco-o-As)/PSS-3/MWNTs composites were recorded (Figure 6). For the COOH-MWNTs, two strong peaks were observed around 1581 and 1352 cm-1, which could be assigned to the D and G modes of the MWNTs, respectively. The Raman spectrum of P(An-co-o-As) ionomers was found to be similar to that of typical PANI,26 including out-plane bending of the C-H band of the quinoid ring at 750 and 780 cm-1, in-plane C-H bending of the quinoid/benzenoid ring at 1163 and 1217 cm-1, C-N•+ stretching at 1340 cm-1, CdC stretching of the quinoid ring at 1486 cm-1, and C-C stretching of the benzenoid ring at 1590 cm-1 due to the presence of benzene diamine and quinine diimine structures of polymer.27,28 In the case of the P(An-co-o-As)/MWNTs, a new C-N stretching at 1411 cm-1 was observed, demonstrating a site-selective interaction between the quinoid ring of the polymer and the MWNTs, which facilitated the charge-transfer processes. (23) Wei, Z.; Wan, M.; Lin, T.; Dai, L. AdV. Mater. 2003, 15, 136. (24) Zhang, X.; Zhang, J.; Liu, Z. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 1813. (25) Dominis, A. J.; Spinks, G. M.; Kane-Maguire, L. A. P.; Wallace, G. G. Synth. Met. 2002, 129, 165. (26) Cochet, M.; Louam, G.; Quillard, S.; Buisson, J. P.; Lefrant, S. J. Raman Spectrosc. 2000, 31, 1041. (27) Zhou, Y.; Lei, L.; Jing, X.; Tang, J.; Wang, F. Acta Polym. Sin. 1992, (4), 438. (28) Lei, L.; Zhou, Y.; Jing, X.; Tang, J.; Wang, F. Spectrosc. Spectral Anal. 1991, 11, 12.

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Figure 6. Raman spectra of (a) COOH-MWNTs, (b) P(An-co-o-As) ionomers, (c) P(An-co-o-As)/MWNTs, and (d) P(An-co-o-As)/PSS-3/ MWNTs.

Figure 7. Backscattering percentage vs sediment time: (a) P(An-coo-As)/MWNTs, (b) P(An-co-o-As)/PSS-1/MWNTs, (c) P(An-co-o-As)/ PSS-2/MWNTs, and (d) P(An-co-o-As)/PSS-3/MWNTs.

With the introduction of PSS, the characteristic bands of P(Anco-o-As) decreased remarkably with the appearance of two new bands at 1185 and 822 cm-1, indicating that PSS doped the polymer and destroyed the quinine diimine structures. As we will discuss later, charge transfer between COOH-MWNTs and P(An-co-o-As) plays an important role to the conductivity of the nanocomposites. Aqueous Dispersing Stability of the P(An-co-o-As)/PSS/ MWNTs Composites. The aqueous dispersion stability of MWNTs and their composites will be important to their application to biotechnology, such as DNA and protein biosensors,29 ion channel blockers,30 and as bioseparators and biocatalysts.31 To study their dispersiblity and stability in water, the change of the backscattering intensity of the suspension over time was collected, which increases with the size of suspended particles. As seen in Figure 7a, the aggregation of P(An-co-oAs)/MWNTs was quite fast, and, in an hour, the backscattering percentage changed from 2.1% to 6.0%, indicating aggregation of the suspended particles of the P(An-co-o-As)/MWNTs and (29) Wang, J.; Liu, G.; Jan, M. R. J. Am. Chem. Soc. 2004, 126, 3010. (30) Park, K. H.; Chhowalla, M.; Iqbal, Z.; Sesti, F. J. Biol. Chem. 2003, 278, 50212. (31) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N.; Nevanen, T. K.; Söderlund, H.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11864.

Wang et al.

poor dispersion stability. The positive charges of P(An-co-o-As) decreased during the charge neutralization between P(An-coo-As) and MWNTs. The resulting static repulsion force among P(An-co-o-As) also became weak, and thus the P(An-co-o-As) molecules could easily aggregate owing to their inherently strong interattraction. In contrast, when the polyelectrolyte PSS was introduced into the nanocomposites, little change of backscattering percentage was observed, even after sedimentation for more than 200 h (Figure 7b-d). Well aqueous dispersibility was obtained only after one coating cycle (Figure 6b, P(An-co-o-As)/PSS1/MWNTs). With the increase of LbL cycles, the dispersing stability did not decrease in spite of the size increment of P(Anco-o-As)/PSS/MWNTs nanotubular composites. Electrical Conductivity of P(An-co-o-As)/PSS/MWNTs Composites. It is clear that the aqueous dispersion stability of the MWNTs is improved after the LbL assembly of P(An-coo-As) and PSS. The remaining question is whether LbL assembly will affect the electrical conductivity of MWNTs. As seen from Table 1, the conductivity of P(An-co-o-As) ionomers is rather low (0.004 S/cm). However, the introduction of COOH-MWNTs dramatically increases the conductivity of the P(An-co-o-As)/ MWNTs by 3 orders of magnitude to 4.2 S/cm, which is slightly higher than that of the COOH-MWNTs (3.4 S/cm). Because MWNTs are excellent electron acceptors,32 while P(An-co-oAs) can be considered a good electron donor, as shown earlier in the FT-IR study, charge transfer from the quinoid units of the P(An-co-o-As) to the MWNTs occurs, thus, greatly increasing the conductivity of the composites. When introducing the insulating PSS layers on the composite surface, charge transfer between the P(An-co-o-As) and the MWNTs is blocked, and the electrical conductivity decreases as the layer of PSS increases. Nevertheless, all composites show higher electrical conductivity than that of neat P(An-co-o-As). Overall, the LbL assembly approach is quite effective to improve the aqueous dispersibility of CNTs without sacrificing their electrical conductivity. pH Response on Electrical Conductivity of the P(An-coo-As)/PSS/MWNTs Nanocomposites. It is well-known that the electrical properties and structure of PANI are dependent on doping extent, and the doping/dedoping process is reversible.10 For example, when the pH surrounding increases, the PANI is dedoped and its doping extent decreases, thus the conductivity of PANI will decrease. This process will be reversed when pH decreases. This characteristic may be enhanced by charge transfer between P(An-co-o-As) and MWNTs when P(An-co-o-As)/PSS/ MWNTs nanocomposites are synthesized. The conductivity changes of P(An-co-o-As)/PSS-1/MWNTs and P(An-co-o-As)/PSS-3/MWNTs as a function of pH value are shown in Figure 8. The curve of P(An-co-o-As)/PSS-1/ MWNTs (Figure 8a) is quite even because the P(An-co-o-As) content in it is low and the characteristic discussed above is not clear. This also demonstrates the necessity of multilayer coating by LbL assembly. As seen in the curve of P(An-co-o-As)/PSS3/MWNTs (Figure 8b), however, when the pH value is less than 2, the conductivity of the composites is high (7.6 × 10-5 S/cm at pH ) 0) and decreases slowly with the increase of the pH value. In the pH range of 2 to 6, the conductivity of composite drops 3 orders of magnitude. Above pH ) 6, the conductivity decreases slowly again. That means the main doping/dedoping process of P(An-co-o-As) occurs in the pH range of 2-6. The sharp transition of the nanocomposite conductivity may provide promising candidates as low-cost, highly sensitive chemicalbiological sensors. (32) Sun, Y.; Wilson, S. R.; Schuster, D. I. J. Am. Chem. Soc. 2001, 123, 5348.

Assembly of P(An-co-o-As)/PSS/MWNTs Nanocomposites

Langmuir, Vol. 24, No. 11, 2008 5831

P(An-co-o-As) ionomers and polyelectrolyte PSS on carboxylic acid-functionalized MWNTs surface. According to zeta (ζ) potential, FT-IR, UV–vis, and Raman data, the multilayer nanocomposite formation is based on the electrostatic attraction, the interaction between the quinoid units of the P(An-co-o-As) and the MWNTs, as well as the doping effect of PSS on P(Anco-o-As). TEM images demonstrate that the polymer shell thickness can be controlled through changing the LbL layer number. The resultant multilayer composites commonly have good electronic properties and aqueous dispersibility. Further, their pH-sensitive character makes them useful in the application of chemical-biological sensors.

Figure 8. Conductivity of (a) P(An-co-o-As)/PSS-1/MWNTs and (b) P(An-co-o-As)/PSS-3/MWNTs composites as a function of pH value.

Conclusions In this work, stable MWNTs nanocomposite dispersion in aqueous solution were prepared by LbL assembly of conducting

Acknowledgment. This work was supported by the National High Technology Research and Development Program of China (2006AA03Z358), the National Natural Science Foundation of China (20236020), the Special Projects for Key Laboratories in Shanghai (06DZ22008), and the Special Projects for Nanotechnology of Shanghai (0552nm001, 0652nm034). LA8000625