enzyme-catalyzed synthesis of conducting polyaniline nanocomposites with pure and functionalized...

Research Article

Enzyme-Catalyzed Synthesis of ConductingPolyaniline Nanocomposites with Pure andFunctionalized Carbon Nanotubes

The in situ enzymatic polymerization of aniline onto multi-walled carbon nano-tubes (MWCNT) and carboxylated MWCNT (COOH-MWCNT) is reported.Nanostructured composites were prepared by this method. Polymerization wascatalyzed with the enzyme horseradish peroxidase at room temperature in aqu-eous medium of pH 4. Hydrogen peroxide was used in low concentration as theoxidant. The nanocomposites were characterized by Fourier transform infraredspectroscopy, scanning electron microscopy (SEM), transmission electron micro-scopy (TEM), and thermogravimetric analysis (TGA). The TEM studies showedtubular morphology with uniformly distributed MWCNT in the nanocomposites.The SEM and TEM investigations revealed wrapping of the MWCNT with polya-niline (PANI) chains. TGA demonstrated that the PANI component is thermallymore stable in PANI/COOH-MWCNT compared to the PANI/MWCNT compo-sites. The synthesized nanocomposites showed higher conductivity than purePANI, which may be due to the strong interaction between the PANI chains andthe MWCNT.

Keywords: Enzymatic polymerization, Functionalized carbon nanotubes, Horseradishperoxidase, Multi-walled carbon nanotubes

Received: March 16, 2011; revised: February 09, 2012; accepted: May 08, 2012

DOI: 10.1002/ceat.201100149

1 Introduction

Materials at the nanometer scale display novel properties. Theyopen up new possibilities to tailor the physical and chemicalproperties of materials, and there is a strong request for a fun-damental understanding of the new phenomena that materialsmay exhibit at the nanometer scale [1]. Such unique propertiesthat are different from those of the bulk materials are deter-mined by their size and structure [2]. These properties andperformances have drawn much attention to nanomaterials forpotential applications in biosciences, electrochemistry, optics,catalysis, ceramics, and energy storage [3–8].

Horseradish peroxidase (HRP) is an extracellular plant en-zyme known to oxidize a range of substrates by hydrogen per-oxide or by organic peroxides [9]. Because of the importantfunctions of HRP, it has been one of the most studied enzymes

[10]. HRP has been applied in many scientific fields such as di-agnostic assays [11], biosensors [12], and polymer synthesis[13].

Polymer nanocomposites are two-phase systems consistingof polymers loaded with nanostructured fillers. These nano-composites significantly improve both the mechanical strengthand the conductivity of the polymer [14]. However, the insolu-bility and poor dispersibility of carbon nanotubes (CNTs) inwater and organic solvents hindered their application andfurther development. To overcome these obstacles, CNT func-tionalization was developed.

Polyaniline (PANI) is one of the most studied electricallyconducting polymers because of its environmental stability,easy synthesis in aqueous medium, simplicity in doping, andits electrochromic effects [15–18]. The fabrication of CNT/PANI composites has attracted enormous interest in recentyears because of their unique electrical properties as well astheir extensive application in electronic devices [19]. Thesenanocomposites have already found use as amperometric bio-sensors for DNA [20], as sensor for nitrogen oxide [21], asacidity sensor [22], in sensing of bioactive molecules [23], inelectrorheology [24], and in drug delivery systems [25]. Wal-lace et al. [26] have reported that PANI fibers containing CNT

Chem. Eng. Technol. 2012, 35, No. 9, 1707–1712 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com

Mohammad R. Nabid1

Mitra Shamsianpour1

Roya Sedghi1,2

Abdolmajid B. Moghaddam3

1 Department of Chemistry,Faculty of Science,Shahid Beheshti University,Tehran, Iran.

2 Department of Chemistry,Faculty of Science,Alzahra University,Tehran, Iran.

3 Department ofEngineering Science,College of Engineering,University of Tehran,Tehran, Iran.

–Correspondence: Dr. A. B. Moghaddam ([email protected]), Depart-ment of Engineering Science, College of Engineering, University ofTehran, P.O. Box 11155-4563, Tehran, Iran.

Enzymatic polymerization 1707

exhibit significant improvements in mechanical strength andconductivity. PANI can be synthesized by chemical, electro-chemical and catalytic polymerization. The development ofnew and efficient catalysts plays an important role in polymer-ization [27]. The advantages of enzyme catalysis are superiorcatalytic power and high selectivity under mild reaction condi-tions [28]. HRP has been used to catalyze the polymerizationof aromatic substrates [29]. In enzymatic polymerization, therate of nucleation is controlled and in comparison with chemi-cal polymerization a thinner layer of polymer can be coated onthe nanotubes. In both chemical and electrochemical poly-merization an acidic solution is needed, and in chemicalpolymerization a low temperature is needed. In enzymatic po-lymerization there are no harsh conditions. We have previouslyreported the polymerization of aniline on c-alumina nano-sheets and anatase nano-TiO2 particles [30, 31]. Both single-walled CNTs (SWCNTs) [32, 33] and multi-walled CNTs(MWCNTs) [22, 23, 34–39] have been coated with conductivepolymers by in situ chemical polymerization. To the best ofour knowledge, conducting PANI coating on MWCNT by theenzymatic polymerization method has not yet been studied. Inthis study, we report the in situ enzymatic synthesis of PANIon MWCNT and carboxylated MWCNT (COOH-MWCNT)in the presence of poly(sodium 4-styrenesulfonate) (PSS) as apolyanionic template.

2 Experimental

2.1 Materials and Reagents

The MWCNT were synthesized by the chemical vapor deposi-tion (CVD) method, with an outer wall diameter distributionclose to 10–20 nm. The MWCNT were obtained from the Re-search Institute of Petroleum Industry (Iran). The metallic im-purity of the MWCNT was removed by washing in HCl. ThePSS (molecular weight (MW) of 70 000) used in this study waspurchased from Aldrich Chemical Co. (Milwaukee, WI, USA)and used without further purification. HRP (EC 1.11.1.7)(about 170 U mg–1), hydrogen peroxide (30 wt-%), aniline andall other reagents were obtained from Merck Company.

2.2 Instrumentation Characterization

The Fourier transform infrared (FT-IR) measurements werecarried out using a BOMEM MB-Series FT-IR spectrometer inthe form of KBr pellets. High-resolution transmission electronmicroscopy (HRTEM) was used to characterize the productson grids by using a JEOL 2000FX transmission electron micro-scope (Tokyo, Japan) operating at 200 kV in bright-field modeunder Scherzer defocus conditions. The products were alsostudied by a Philips XL-30 scanning electron microscope. Thethermal stability of the nanocomposites was determined usinga thermogravimetric analyzer (TGAQ50, TA Instruments) un-der air and at a heating rate of 20 °C/min. The conductivitywas measured by a Keithley 213 with a digital multimetersystem.

2.3 Preparation of COOH-MWCNT

Functionalization of CNT with carboxyl groups [22] orp-phenylenediamine [40, 41] is performed to improve the in-teraction of PANI with CNTs in the preparation of CNT com-posites. Functionalized CNTs are also easier to disperse inaqueous and organic solutions [42]. In a typical procedure, theMWCNTs were ultrasonically treated in a mixture of concen-trated H2SO4 and HNO3 (3:1 v/v) at 60 °C for 10 h in order tointroduce carboxylic acid groups at the defect sites and thus toimprove the dispersion of the COOH-MWCNTs in the reac-tion medium. After this acidic treatment, the COOH-MWCNTs were washed with distilled water to remove excessacid and unreacted compounds. The COOH-MWCNTs werethen dried under vacuum conditions at 60 °C.

2.4 Enzymatic Polymerization of PANINanocomposites

The composite of PANI with MWCNT was synthesized via anin situ enzymatic polymerization method. Distilled water(20 mL) at pH 4 (the pH was adjusted with HCl) containing0.026 mg PSS and 0.03 mg MWCNT was sonicated at roomtemperature for 15 min. Of the aniline monomer, 15 lL wasadded to the above MWCNT suspension and stirred for 0.5 hto disperse the aniline in the reaction medium. 2 mg HRP wasadded to the solution. The reaction was initiated by the addi-tion of a stoichiometric amount of H2O2 under vigorous stir-ring. To avoid the inhibition of HRP due to excess H2O2 [40],diluted H2O2 (0.02 M) was added dropwise, incrementally,over 1 h. After the addition of H2O2, the reaction was left tostir for at least 12 h, and then the final mixture was washedthoroughly with acetone and distilled water to remove oligo-mers and unreacted monomers. After drying under vacuum at60 °C for 24 h, the final composites were obtained. The sameprocedure was carried out to synthesize the PANI/COOH-MWCNT composite.

3 Results and Discussion

3.1 Fourier Transform Infrared Spectroscopy

Fig. 1 shows the FT-IR spectra for COOH-MWCNT, PANI/MWCNT and PANI/COOH-MWCNT composites. ForCOOH-MWCNT (Fig. 1a), the peaks at 1729 and 1093 cm–1

are attributed to the stretching modes of the carboxylic acidgroups [43]. The spectrum of the PANI/MWCNT composite(Fig. 1b) shows absorption peaks at 1579 and 1492 cm–1, whichare assigned to C=C stretching vibrations of the quinoid andbenzenoid rings, respectively. The C=C stretching peaks arered-shifted in the PANI/MWCNT composite compared to purePANI [44, 45], which might be attributed to the favorable inter-action between PANI and the MWCNT [46]. The strong bandat about 1147 cm–1 is considered to be a measure of electron de-localization [44]. The significant intensity of this signal is due tothe interaction between PANI and MWCNT, which can facili-

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1708 A. B. Moghaddam et al.

tate a charge transfer process [47] and increase the degree ofelectron delocalization [46]. As can be observed in Fig. 1c, mostsignals of the COOH-MWCNT peaks are covered by PANIsignals in the PANI/COOH-MWCNT spectrum. The peakcorresponding to -COOH carbonyl groups functionalized onMWCNT at 1720 cm–1 has red-shifted to 1709 cm–1 and has be-come very weak. This may have resulted from the formation ofthe hydrogen bond between the PANI amino groups and thecarboxylic acid groups grafted on the MWCNT. The C=Cstretching peaks are red-shifted to 1560 and 1483 cm–1 respec-tively, due to the same reason as for the PANI/MWCNT com-posite. The peaks at 1124, 1032, and 575 cm–1 are assigned toS–O, S=O, and C–S stretching vibrations in the PSS component,respectively [48]. The red-shift of the PSS peaks may have beenthe result of a doping effect of SO3

– on PANI [46]. The C–Hout-of-plane bending located at 826 cm–1 indicates a head-to-tail coupling of aniline monomers during the polymerization.The band at 1308 cm–1 is characteristic of C–N stretching vibra-tion of linking with the quinoid and benzenoid rings.

3.2 Scanning Electron Microscopy

Scanning electron microscopy (SEM) is an effective techniquefor studying the morphology of nanocomposites as both com-ponents can be identified in the images. SEM images of thepure MWCNT and PANI/MWCNT are presented in Fig. 2a and2b, respectively. As can be observed, the CNT diameters are in-creased compared to the neat MWCNT, which confirms thepolymerization of aniline on the surface of the CNT. In otherwords, the morphological analysis of these nanocompositesreveals that PANI is coated on the CNT.

MWCNTs were carboxylated via thementioned process. Enzymatic poly-merization of aniline in the presence ofCOOH-MWCNTs led to the formationof the PANI/COOH-MWCNT com-posite. SEM images of the COOH-MWCNT and the PANI/COOH-MWCNT composite are shown inFig. 3. In this case, the CNT showed anincrease in diameter, which is due tothe deposition of PANI. A comparisonof the SEM images indicates that themorphology of the MWCNT in thesynthesized composites was not per-turbed during polymerization. Ultra-sonication treatment with concen-trated acid seems to be the mainreason for the change in morphologyof the nanotubes in COOH-MWCNT[49]. Also, comparing the SEM imagesin Figs. 2a and 3a confirms the short-ening of the CNT via functionalization,which is in accordance with the litera-ture [50, 51].

3.3 High-Resolution Transmission ElectronMicroscopy

Fig. 4 presents the HRTEM images of the MWCNT and PANI/MWCNT, respectively, while Fig. 5 shows the HRTEM image


Figure 1. FT-IR spectra of (a) COOH-MWCNT, (b) PANI/MWCNT, and (c) PANI/COOH-MWCNT.

a)

b)

Figure 2. SEM images of (a) MWCNT and (b) PANI/MWCNT.


of PANI/COOH-MWCNTs. The bilayered structure of coatedCNTs demonstrates the uniform deposition of PANI on theCNT. As the internal cavity of the CNT is discernible, it can beconcluded that the polymerization only takes place at the outersurface of the nanotubes. Also the surface structures of theMWCNT were not perturbed by PANI coating.

Comparison of Figs. 4b and 5 indicates that the thickness ofthe PANI coated on the pure MWCNT is significantly largerthan that on the COOH-MWCNT, which is in accordance withthe SEM and thermogravimetric analysis (TGA) results. The

HRTEM image of PANI/COOH-MWCNT demonstrates theexistence of PANI in the internal cavity of some MWCNT, asthe capped ends of the nanotubes may be opened by oxidation,allowing the insertion of PANI inside the nanotubes.

3.4 Thermogravimetric Analysis

TGA was conducted to identify the thermal stability ofMWCNT, COOH-MWCNT, and the PANI/MWCNT andPANI/COOH-MWCNT composites. It was also used to esti-mate the amount of PANI grafted onto the MWCNT in thecomposites. In Fig. 6, the thermograms of the MWCNT andCOOH-MWCNT are presented. The amount of weight de-crease in the temperature range below 20 °C reflects the loss ofhumidity and acid. The weight loss due to organic decomposi-tion for the carboxylic acid is observed in the temperature in-terval of 100–250 °C, which may be due to the thermal degra-dation of carboxylic acid groups formed on the surface of theMWCNT [52]. The thermograms of the MWCNT and COOH-MWCNT indicates that the thermal stability of the functional-ized CNT is improved compared to the pure MWCNT, whichmay have resulted by cutting of the nanotubes to smaller onesvia the carboxylation process.

According to Fig. 7, the thermogram of PANI/MWCNTdemonstrates that PANI is decomposed at about 350 °C,whereas decomposition of the PANI component in the PANI/COOH-MWCNT starts above 400 °C. The increase in thermalstability of the PANI/COOH-MWCNT compared to the PANI/MWCNT composite may be attributed to the interaction be-tween the PANI chains and the carboxylic acid groups of theMWCNT. The amount of PANI on the surface of the nano-


a)

b)

Figure 3. SEM images of (a) COOH-MWCNT and (b) PANI/COOH-MWCNT.

a) b)

Figure 4. HRTEM images of (a) pure MWCNT, scale bar = 5 nm;(b) PANI/MWCNT, scale bar = 5 nm.

Figure 5. HRTEM image of PANI/COOH-MWCNT, scale bar =5 nm.


tubes is approximately 40 % and 20 % in the PANI/MWCNTand PANI/COOH-MWCNT composites, respectively.

3.5 Conductivity

Conductivity measurements were carried out on the nano-composite films using a four-point probe method. The roomtemperature conductivities for protonic acid-doped PANI,PANI/MWCNT and PANI/COOH-MWCNT were 2.7 · 10–3,1.1 · 10–1, and 3.2 · 10–2 S cm–1, respectively. Compared to PANIwithout MWCNTs, the conductivities for PANI with COOH-MWCNT and MWCNT are one and two orders of magnitudehigher, respectively. The enhanced conductivity of the nano-composites may be due to the strong interaction between thePANI chains and the MWCNT, which facilitates electrondelocalization.

4 Conclusions

In this work, both pure and functionalized MWCNTs werecoated with PANI via in situ enzymatic polymerization. HRPwas used as a green catalyst to prepare dispersible nanocom-posites. Carboxylation of MWCNT was conducted throughultrasonication in a mixture of H2SO4 and HNO3. The func-tionalization of the MWCNT was confirmed by FT-IR mea-surements. The morphological studies showed that polymer-ization of aniline took place on the surface of pure MWCNT,while PANI could enter the internal cavity of some COOH-MWCNT. HRTEM images demonstrated that the amount ofPANI coated on the MWCNT was significant. The conductivitymeasurements showed that the conductivity of the nanocom-posites was higher than that of neat PANI. On the basis of theTGA results, we conclude that the PANI component exhibitshigher thermal stability in the PANI/COOH-MWCNT com-pared to PANI coated on pure MWCNT.

Acknowledgment

The authors would like to thank Dr. H. Vali for the perfor-mance of the transmission electron microscopic studies. Thefinancial support provided by the Iran National Science Foun-dation (INSF) is gratefully acknowledged.

The authors have declared no conflict of interest.

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enzyme-catalyzed synthesis of conducting polyaniline nanocomposites with pure and functionalized...

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