enzyme-catalyzed synthesis of conducting polyaniline nanocomposites with pure and functionalized...
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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 MWCNTwith 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
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 . Such unique propertiesthat are different from those of the bulk materials are deter-mined by their size and structure . These properties andperformances have drawn much attention to nanomaterials forpotential applications in biosciences, electrochemistry, optics,catalysis, ceramics, and energy storage .Horseradish peroxidase (HRP) is an extracellular plant en-
zyme known to oxidize a range of substrates by hydrogen per-oxide or by organic peroxides . Because of the importantfunctions of HRP, it has been one of the most studied enzymes
. HRP has been applied in many scientific fields such as di-agnostic assays , biosensors , and polymer synthesis.Polymer nanocomposites are two-phase systems consisting
of polymers loaded with nanostructured fillers. These nano-composites significantly improve both the mechanical strengthand the conductivity of the polymer . 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 electrically
conducting polymers because of its environmental stability,easy synthesis in aqueous medium, simplicity in doping, andits electrochromic effects . 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 . Thesenanocomposites have already found use as amperometric bio-sensors for DNA , as sensor for nitrogen oxide , asacidity sensor , in sensing of bioactive molecules , inelectrorheology , and in drug delivery systems . Wal-lace et al.  have reported that PANI fibers containing CNT
Chem. Eng. Technol. 2012, 35, No. 9, 17071712 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com
Mohammad R. Nabid1
Abdolmajid B. Moghaddam3
1Department of Chemistry,Faculty of Science,Shahid Beheshti University,Tehran, Iran.
2Department of Chemistry,Faculty of Science,Alzahra University,Tehran, Iran.
3Department ofEngineering Science,College of Engineering,University of Tehran,Tehran, Iran.
Correspondence: Dr. A. B. Moghaddam (firstname.lastname@example.org), 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 . The advantages of enzyme catalysis are superiorcatalytic power and high selectivity under mild reaction condi-tions . HRP has been used to catalyze the polymerizationof aromatic substrates . 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, 3439] 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.
2.1 Materials and Reagents
The MWCNT were synthesized by the chemical vapor deposi-tion (CVD) method, with an outer wall diameter distributionclose to 1020 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 188.8.131.52)(about 170Umg1), hydrogen peroxide (30wt-%), 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  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 . 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(20mL) at pH 4 (the pH was adjusted with HCl) containing0.026mg PSS and 0.03mg MWCNT was sonicated at roomtemperature for 15min. 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. 2mg 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 ,diluted H2O2 (0.02M) 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 cm1
are attributed to the stretching modes of the carboxylic acidgroups . The spectrum of the PANI/MWCNT composite(Fig. 1b) shows absorption peaks at 1579 and 1492 cm1, 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 . The strong bandat about 1147 cm1 is considered to be a measure of electron de-localization . The significant intensity of this signal is due tothe interaction between PANI and MWCNT, which can facili-
www.cet-journal.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2012, 35, No. 9