Micellar Nanoreactors for Hematin Catalyzed Synthesis of ElectricallyConducting PolypyrroleSethumadhavan Ravichandran,† Subhalakshmi Nagarajan,† Akshay Kokil,‡ Timothy Ponrathnam,§

Ryan M. Bouldin,∥ Ferdinando F. Bruno,⊥ Lynne Samuelson,⊥ Jayant Kumar,‡

and Ramaswamy Nagarajan*,§

†Department of Chemistry, ‡Department of Physics & Applied Physics, §Department of Plastics Engineering, and ∥Department ofChemical Engineering, University of Massachusetts, Lowell, Massachusetts 01854, United States⊥U.S. Army Natick Soldier Research, Development and Engineering Center, Natick, Massachusetts 01760, United States

*S Supporting Information

ABSTRACT: Enzymatic synthesis of doped polypyrrole (PPy)complexes using oxidoreductases (specifically peroxidases) isvery well established “green” methods for producing conductingpolypyrrole. The importance of this approach is realized by thenumerous potential opportunities of using PPy in biologicalapplications. However, due to very high costs and low acidstability of these enzymes, there is need for more robustalternate biomimetic catalysts. Hematin, a hydroxyferriproto-porphyrin, has a similar iron catalytic active center like theperoxidases and has previously shown to catalyze polymerization of phenol monomers at pH 12. The insolubility of hematin dueto extensive self-aggregation at low pH conditions has prevented its use in the synthesis of conjugated polymers. In this study, wehave demonstrated the use of a micellar environment with sodium dodecylbenzenesulfonate (DBSA) for biomimetic synthesis ofPPy. The micellar environment helps solubilize hematin, generating nanometer size reactors for the polymerization of pyrrole.The resulting PPy is characterized using UV−visible, Fourier transform infrared, and X-ray photoelectron spectroscopy andreveals the formation of an ordered PPy/DBSA complex with conductivities approaching 0.1 S/cm.

■ INTRODUCTION

Since the discovery of π-conjugated polymers more than twodecades ago,1 there has been tremendous interest in thesepolymers due to a variety of potential applications. Acombination of ease of processability and tunable optical andelectronic properties have led to applications in a variety offields such as electrochromic devices,2 photovoltaics,3 batteryapplications,4 light emitting diodes,5 organic transistors,6 andanticorrosion coatings.7

Among conducting polymers, polypyrrole (PPy) is partic-ularly interesting due to its biocompatibility.8 In addition to itsuse in electrical and electronic devices, PPy has potential for usein biomedical applications.9−13 With its unique combination ofproperties, PPy continues to attract attention, particularly in thefield of biotechnology. Traditionally, chemical and electro-chemical methods are most commonly used for the synthesis ofPPy. The synthesis almost often involves harsh organicsolvents, reagents and catalysts to produce electricallyconducting PPy. In order to overcome this problem, severalresearch groups over the past decade have explored the use ofmore environmentally friendly enzymatic approaches for thesynthesis of conjugated polymers such as PPy.14 PPy has beensynthesized using oxidants like H2O2 with naturally occurringoxidoreductase enzymes (peroxidases) as catalysts.15,16 Wehave recently reported the synthesis of water dispersible

conducting PPy and its derivatives using catalysts derived fromsoybeans (soybean peroxidase).17,18 While peroxidases are“greener” oxidation catalysts, their commercial utility insynthesis of electrically conducting polymers has been limiteddue to several factors. Naturally occurring peroxidases usuallyexhibit low stability and activity under acidic conditions.19

Moreover, due to the high cost associated with these enzymes,there is a need to develop cost-effective, robust biomimeticcatalysts as effective surrogates to peroxidases. The hemeprosthetic group (iron porphyrin active center) of theperoxidase catalysts (specifically horseradish and soybeanperoxidase) is the active site that catalyzes oxidative polymer-ization reactions.20,21

Hematin (hydroxyferriprotoporphyrin) is a naturally occur-ring, cost-effective iron-porphyrin extracted from porcineblood. Akkara et al. used hematin as a catalyst for thepolymerization of 4-ethylphenol in a mixed solvent system.22 Inthe same study, it was postulated that hematin forms catalyticintermediates similar to peroxidases. However, hematin has alower oxidation potential and is water-soluble only at a high pH(above pH 8), making it an ineffective catalyst for the

Received: June 19, 2012Revised: August 16, 2012Published: August 20, 2012

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polymerization of electrically conducting polymers. Hence,hematin was modified using polyethyleneglycol chains.Polyethyleneglycol modified hematin was shown to catalyzethe synthesis of conductive Pani23 and PPy/PEDOT,24 but itrequired a low pH (pH∼1) for the synthesis. However, directfunctionalization of hematin is not an efficient process due toaggregation of the porphyrin rings during modification.Recently, it was also shown that there was an improvementin the catalytic activity of hematin in the presence of sodiumlauryl sulfonate.25 But its efficacy as an oxidative catalyst wasnot investigated. This work demonstrates the possibility ofusing unmodified hematin, as an effective and more economicalalternative to peroxidases for oxidative polymerizations. Wereport for the first time the synthesis of PPy using hematincatalyst in aqueous micellar nanoreactors. This new approachinvolves dispersing water-insoluble hematin in micellar nano-reactors providing a unique environment that enables a simpleone-pot biomimetic, biofriendly synthesis of a conducting,doped PPy complex.

■ EXPERIMENTAL SECTIONMaterials. Pyrrole (98%), sodium dodecylbenzenesulfonate

(Technical grade) (DBSA), sodium laurylsulfonate (SLS), cetyltrime-thylammonium bromide (CTAB), Triton X-100, Hematin Porcine,and 30% hydrogen peroxide (H2O2) in water were obtained fromSigma-Aldrich Co. Hydrogen peroxide was diluted with deionizedwater to a stock solution of 3% (w/v). This diluted H2O2 solution wasused for all polymerizations. All other chemicals were of reagent gradeor better and used without further purification.Dispersing Hematin in Low pH. A pH 12 solution was prepared

by adding a few drops of 0.1 M NaOH in distilled water. Hematin wassolubilized in the pH 12 solution at a concentration of 10 mg/mL. Atthe start of the reaction, 100 μL of this stock solution (1 mg ofhematin for a 10 mL reaction) is added to a 10 mM citrate buffer (pHadjusted to 3.5−6). Due to the citrate buffer strength, hematin addedto the mixture begins to precipitate out of the reaction solution. This isimmediately followed by the addition of 10 mM DBSA to the solution,which on sonication for 15 min renders a clear brown dispersion. ThepH of the final stock solution is determined and used in theconsecutive steps of the reaction. Alternatively, this stock solution canalso be prepared by adding the DBSA to the citrate buffer followed bythe addition of the hematin solution.Biomimetic Synthesis of Polypyrrole. Pyrrole (10 mM) is

added to the stock solution of a known pH (3.5−6) followed bysonication for 10 min. After dispersion of the monomer droplets, 1 mLof 3% H2O2 is added in drops over 2 min. Within 15 min after theaddition of the H2O2, there is formation of a gray colored solutionwhich turns black in 1 h. The reaction is carried out at 4 °C for 10 h.After the completion of the reaction, the mixture is poured in a largeexcess of acetone, which breaks the micelles, precipitating the product.The thermogravimetric yield was in the range of 85−90%. Theprecipitate is centrifuged and dried under vacuum overnight at 50 °Cto obtain biomimetically synthesized PPy in a micellar environment.The biomimetic catalyst, hematin, could not be isolated after thereaction and thus was not recycled.Characterization of Products. UV−Visible Spectroscopy. All

products were characterized using an Agilent 8453 photodiode arrayUV−visible spectrometer. To obtain UV−visible spectra of thepolymer, a 100 μL aliquot of the reaction mixture was diluted with100 mM pH 3.5 citrate buffer to a total volume of 300 μL in a quartzcuvette with optical path length of 1 mm.Fourier Transform Infrared (FTIR) Spectroscopy. FTIR measure-

ments were taken on a Thermo Scientific Nicolet 4700 instrument.Solid powder of the synthesized PPy was directly used with a SmartOrbit attenuated total reflectance (ATR) accessory for all themeasurements. All samples were dried under vacuum to ensure theremoval of moisture, which can result in free −OH peaks.

Conductivity. Conductivity measurements were taken in triplicateon pressed pellets of PPy/DBSA (dried overnight) using a four-pointprobe. Conductivity measurements were performed with a Keithley2750 DMM and a Keithley 6221 source meter.

X-ray Photoelectron Spectroscopy (XPS). Electron spectroscopyfor chemical analysis (ESCA) was performed utilizing a VG ESCALABMKII photoelectron spectrometer with Al Kα X-rays and a basepressure in the 10−10 Torr range. The generated photoelectrons weredetected at a takeoff angle of 90°, which was defined as the anglebetween the surface plane and the entrance of the focusing lens of theconcentric hemispherical analyzer.

1H NMR. The 1H NMR spectra were collected using a Bruker 500MHz NMR spectrometer. The solvent used was deuterated water(obtained from Cambridge isotope laboratories). A total of 10 μL ofpyrrole was dissolved in 600 μL of the deuterated solvent, followed byaddition of DBSA in different ratios above and below its critical micelleconcentration (CMC).

Cyclic Voltammetry (CV). Electrochemical experiments wereperformed using a Pine Research Instruments Wavenow USBpotentiostat using a three-electrode cell in water at room temperature.A platinum wire and Ag/AgCl standard electrode were used as counterand reference electrode, respectively. A thin film was drop casted frommicellar dispersion of polypyrrole onto an indium tin oxide coatedglass slide and was used as the working electrode. The potential scanrange was set from −0.1 to 1.0 V. The cyclic voltammograms of PPy/DBSA complexes were recorded at scan rates of 20 mV/min.

■ RESULTS AND DISCUSSION

Biomimetic Polymerization of Pyrrole. Biomimeticpolymerization of hematin was performed in a micellar solutionprepared by the addition of DBSA above its CMC in a pH 3.5citrate buffer. The surfactant forms a hydrophobic micellar coreand a hydrophilic shell, enabling dissolution of water insolublesubstances.26 Hematin is normally insoluble in water at low pH.However, hematin can be loaded into these micelles, primarilydriven by hydrophobic interactions. These micelles loaded withthe catalyst serve as nanoreactors for the polymerization of anyhydrophobic monomer. In the present work, pyrrole is addedto these nanoreactors followed by the addition of H2O2. It isexpected that the pyrrole is predominantly dispersed in thehydrophobic core of the DBSA micelles, due to its limitedsolubility in water.Hematin catalyzed polymerization of PPy/DBSA complexes

were monitored using UV−visible spectroscopy, as thecomplexes remain dispersible in water during the reaction.The intensity of bipolaron absorptions (300−450 nm) and thelong wavelength absorptions (1000 nm) is indicative of theformation of PPy/DBSA complex at any time during thepropagation of the reaction. However, the final PPy/DBSAcomplex after isolation is insoluble in common organic solventsand could not be redispersed in an aqueous media. Themolecular weight of the obtained PPy/DBSA complex couldnot be characterized due to the insolubility of the precipitatedPPy/DBSA complex in water or other common organicsolvents. This is not uncommon and has been previouslyreported for PPy/DBSA systems by several groups.27,28

Ionic and Nonionic Surfactants. Several types of surfactantswith different molecular structures and charge were used as asurfactant/dopant for PPy synthesis using this micellarapproach as shown in Figure 1. All three types of surfactants,namely, anionic, cationic, and nonionic forms, were able todisperse hematin and pyrrole droplets in the micelle before theaddition of H2O2. However, CTAB and Triton-X did notproduce conducting PPy (Figure 2) complexes. Therefore, wemay conclude that the presence of a strong anionic charge is

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necessary for the synthesis of electrically conducting poly-pyrrole using hematin. As shown in Figure 2, there is a strongabsorption in the region from 800 to 1100 nm in the presenceof anionic surfactants due to the formation of a doped,conducting form of PPy. PPy doped with DBSA exhibitedhigher conversions as evidenced by stronger bipolaronabsorptions in the 800−1100 nm range, due to a strongerinteraction of the aromatic anion in comparison to SLS.29Theuse of Triton-X as a cosurfactant to DBSA (1:1 molar ratio) didnot result in an increase in the intensity of the charge carrier tail(around 1100 nm) of PPy/DBSA.Local Environment in a Micellar Media. Biomimetic

polymerization of pyrrole does not proceed in normal aqueousreaction media. In the enzymatic synthesis of polyaniline,27 thelocal low-pH environment provided by aqueous micelles hasbeen shown to play an important role. Similarly, during thebiomimetic polymerization of PPy, DBSA micelles served thefollowing important functions.DBSA micelles helped disperse hematin in an aqueous media

at low pH necessary for polymerization. Recently, it was alsoshown that the presence of a micellar media helps improve thecatalytic efficiency of hematin by 20%.16

High local concentration of the monomer and the catalystinside the micelle promotes efficient polymerization. Inaddition, voltammetric studies on pyrrole monomer in DBSAmicelle solutions indicated that with increasing DBSAconcentrations there is a significant reduction in oxidationpotential (measured by monitoring the first irreversibleoxidation peak from cyclic voltagrams) of pyrrole as shownin Figure 3.

The presence of a micellar media assisted the interaction ofDBSA dopant with the pyrrole monomer prior to polymer-ization. 1H NMR studies were used to observe any interactionsbetween these two compounds in D2O with varying ratios ofpyrrole (data provided in the Supporting Information). Thealpha and beta protons of pyrrole appear at 6.84 and 6.18 ppm,respectively, and shift continually with an increase in molarconcentration of DBSA. Unlike the previously reported anilinesystem wherein the interactions are purely electrostatic,27 thepyrrole monomer is unlikely to be protonated neither in neutralD2O nor at the reaction pH. Hence, this shift is more likely dueto the interactions between the pyrrole protons and thearomatic sulfonate group of the DBSA dopant, increasing theprobability of radical stabilization. Our experimental resultsseem to indicate that these interactions play a very importantrole in the biomimetic polymerization of pyrrole.

Optimization of Reactions: Influence of ReactionConditions. Effect of pH. UV−visible spectra of PPy/DBSAreaction after 24 h carried out at various reaction pHs revealedthat lower reaction pH results in higher product formation. Inspite of stabilizing hydrophobic interactions due to DBSAmicelles, hematin could not form stable dispersions (withoutprecipitation) at pH lower than 3. Interestingly, the use ofhematin enabled the formation of PPy/DBSA complex even atpH approaching physiological conditions, whereas no polymer-ization was observed when using peroxidases at pH above 5.17

However, the PPy/DBSA complexes synthesized at higher pH,namely, 5 and 6, only produced PPy complexes withconductivities ranging from 10−6 to 10−7 S/cm. XPS analysisof PPy/DBSA complexes indicated that, with increase inreaction pH, there is a large decrease in pyrrole carbons withrespect to the dopant. At pH 3.5, the percentage of pyrrolecarbons in the system is 70 and decreases to 50 at pH 6. Theseresults are consistent with the conductivities of the synthesizedPPy/DBSA complexes.

Effect of Time. The time taken for the hematin catalyzedpolymerization reactions is significantly longer than that ofenzyme catalyzed systems, due to lower catalytic activity andredox potentials of these native porphyrins. However, thepresence of a micellar environment has enabled fasterformation of PPy/DBSA complex, with absorption maxima at460, 1000, and 370 nm attributed to bipolaron peaks andporphyrin Soret peak, respectively, as shown in Figure 4. Thebipolaron absorptions at around 900−1000 nm reach a

Figure 1. Structures of surfactants used in biomimetic polymerizationof pyrrole.

Figure 2. UV−visible spectra of PPy/DBSA complexes synthesizedusing DBSA and SLS.

Figure 3. Cyclic voltammetry of pyrrole in pH 3.5 citrate buffer withvarying DBSA concentrations. Scan range: −0.8 to 0.9 V.

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maximum at around 10 h after initiation of the polymerization,indicating strong charge interaction of PPy with DBSA dopant.Beyond 10 h, there is a decline in the bipolaron absorptions ofthe propagating species, indicating the possibility of over-oxidation or defect-site generation.30 The obtained values ofconductivities are in good agreement with UV−visiblemeasurements with PPy/DBSA isolated at 10 h showing amaximum conductivity of ∼10−1 S/cm (Figure 5). Theseconductivity values are an order of magnitude higher than thevalues for conductivities of enzymatically synthesized PPy/DBSA complex.

Effect of Temperature. Previous reports on enzymatic,chemical,31 and electrochemical32 polymerization identifiedtemperature control as an important variable to obtainconducting PPy. To study the effects of temperature on thesynthesis of PPy/DBSA complexes using hematin, the synthesiswas carried out at 4, 10, 20, 30, and 40 °C, and the rate offormation of PPy/DBSA complexes was monitored with timeby UV−visible absorption spectroscopy. The bipolaron peak at950 nm is taken as a measure of the conversion at any time inthe reaction mixture as shown in Figure 6. Two distinct trendswere observed for polymers synthesized at 4 and 10 °C incomparison to the ones synthesized at 20 and 30 °C, with each

set of reactions producing maximum yields at 10 and 2 h,respectively. Reactions at 40 °C did not produce bipolaronabsorptions characteristic of a PPy/DBSA complex.From this plot, it is evident that the reactions carried out at

higher temperatures proceed at a faster rate during the earlystages of the reaction. However these reactions undergo drasticretardation beyond this time, whereas the low temperaturereactions continue to progress until a time period of 10 h. Tounderstand this unique behavior, products from all reactionswere isolated when their bipolaron absorptions reached amaxima (2 h for 20/30 °C and 10 h for 4/10 °C respectively)and their conductivities were plotted on the same graph asshown in Figure 7. The conductivities of the PPy/DBSA

complexes synthesized at 4 and 10 °C are roughly 3−4 ordersof magnitude higher than that of the ones synthesized at 20 and30 °C respectively. This is corroborated by XPS analysis, whichindicates that increase in temperature results in a drasticreduction in the ratio of α/β carbon atoms from 1.5 (4 °C) to0.7 (30 °C). A decrease in α/β protons increases the possibilityof 2,3-coupling modes providing additional doping centers(Dopant/Monomer ratio increases from 0.3 to 0.38 withincreasing temperature) resulting in poorly conductingsamples.33 In addition, recent work on enzymatic synthesis ofPPy also follows a similar trend, producing PPy/PSS complexes

Figure 4. UV−visible spectra of PPy/DBSA complexes with increasingreaction time.

Figure 5. Conductivity of PPy/DBSA samples synthesized at varioustime intervals.

Figure 6. Plot of reaction time vs absorbance at 950 nm (bipolaronpeak) for PPy/DBSA complexes synthesized at various temperatures.

Figure 7. Conductivity of PPy/DBSA samples isolated at maximumconversions at various temperatures.

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with higher conductivities at lower temperatures. Thecosurfactant system containing Triton-X and DBSA resultedin conductivities of 10−3 S/cm, roughly an order of magnitudelower than PPy/DBSA complexes.X-ray Photoelectron Spectroscopy. XPS analysis of the

asymmetric carbon peak in PPy/dopant complexes can bedeconvoluted into four prominent peaks, namely, alpha andbeta carbons of pyrrole, carbons of the dopant, and the disordercarbon peak (interchain cross-links, side chains, and chainends) using well-established line shape analysis and Gaussianfitting methods.34 Additionally, it is also possible to determineany residual heme catalytic impurity present in these complexeswhich could interfere with the final conductivities of theproducts. Figure 8 displays the carbon 1s core level scan ofhematin catalyzed PPy/DBSA complex synthesized at 10 °C.

Table 1 shows the deconvoluted pyrrole carbon peaks forbiomimetically synthesized PPy/DBSA complexes in compar-

ison to enzymatic, chemical and electrochemical methods. Theabsolute binding energies of the pyrrole carbon peaks fromvarious methods are slightly different due to varied chargingeffects in the samples. However, a constant energy gap 0.9 eV isobserved between α and β pyrrole carbons in all samples.Degree of disorder was calculated from the ratio of total areaunder the asymmetric carbon to the area under the disorderpeak. Pfluger and Street identified a large concentration ofinterchain cross-links in electrochemically synthesized PPyleading to 33% disorder.34 Joo and Epstein reported chemicalsynthesis, unlike the electrochemical approach, yielded aproduct with a lower degree of disorder close to 22%.33

Recently, our group reported the enzymatic synthesis of a PPy-

PSS complex with very low degree of disorder of 13%.17 PPy/DBSA synthesized using hematin has a slightly higher disorderpercentage at 19.5, possibly due to the use of small moleculedopant DBSA instead of PSS. But this value is still comparableto chemical methods of PPy/DBSA synthesis yieldingcomplexes with good conductivities.

Cyclic Voltammetry. The cyclic voltammetry of a dropcasted film of PPy/DBSA complex (shown in the SupportingInformation) synthesized at pH 3.5 and temperature 4 °C ischaracterized by an oxidation peak at around 0.12 V and areduction peak at −0.42 V in the first cycle which is very similarto reported values for chemically synthesized PPy/DBSAcomplexes.35

Fourier Transform Infrared Spectroscopy. The FTIRspectra of PPy/DBSA complex synthesized using hematin at4 °C is shown in Figure 9. The bands at 890, 1034, and 961

cm−1 are due to C−H in and out of plane and C−C in planevibrations respectively. The band at 1167 cm−1 can beattributed to the vibration of the aromatic pyrrole ring. Thepresence of a broad band at 1285 cm−1 is ascribed to −SOgroups and −S-phenyl vibrations from the dopant in the PPycomplex. The PPy/DBSA complexes also had characteristic C−C and C−N vibrations at 1545 and 1455 cm−1, whose ratio isrepresentative of the conjugation length of the conductingpolymer.36 The small band at 1730 cm−1 is due to the very lowconcentration of carbonyl species from pyrrolidone end groupspresent due to slow rates of oxidations of pyrrole in biomimeticsystems in comparison to chemical or enzymatic methods.FTIR-ATR spectroscopy is also used for estimation of

conjugation lengths of PPy. Tian and Zerbi have shown that themain infrared bands of PPy is strongly influenced by theconjugation length. It was theoretically predicted that, as theconjugation length increases, the ratio of IR intensity of theantisymmetric (C−N) ring stretching mode at 1550 cm−1 tothe symmetric mode (C−C) stretching mode at 1460 cm−1

should decrease.37 This has also been experimentally validatedby Menon et al.38 As shown in Table 2, the ratio of theintensities of the 1550 and 1460 cm−1 bands in the IR spectrumhas been used to estimate the conjugation length. Clearly, thereis a correlation of the T1550/T1460 ratio (which is related toconjugation length) with the final product conductivities.Increase in reaction pH, increases T1550/T1460 ratio. This

Figure 8. Carbon 1s core level scan for PPy/DBSA complexsynthesized using hematin at 10 °C.

Table 1. Carbon 1s Core Level Data for PPy SynthesizedUsing Different Methodologies

pyrrole carbons (eV)

synthesis method β α disorder

energy gap(α − β)(eV)

disorder(%)

biomimetic 285.17 286.09 288.26 0.92 19.6enzymatic 284.1 284.9 289.0 0.82 13.1chemical 285.47 286.37 0.90 22.1electrochemical 283.6 284.5 285.4 0.90 33.3

Figure 9. FTIR-ATR spectra of PPy/DBSA complex synthesized at 2°C.

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indicates that the conjugation length is lower when the T1550/T1460 ratio is higher. Consequently product conductivitiesdecrease at high T1550/T1460 ratio. Higher reaction temperaturesproduce PPY polymers with lower conjugation lengths andelectrical conductivities.

■ CONCLUSIONS

Biomimetic hematin was utilized for the synthesis ofconducting PPy/DBSA complexes in a aqueous micellarmedia. The micelles formed by strong acidic surfactants suchas DBSA and SLS are suitable templates for the synthesis ofconducting PPy. These micelles act as small nanoreactorsenabling the solubility of hematin at low pH and also providingthe necessary local environment for effective catalysis. Thislocal environment also caused a slight reduction in theoxidation potential of pyrrole monomer prior to polymerizationenabling ease of catalysis using hematin (with a very lowoxidation potential). Biomimetic synthesis yielded complexeswith higher conductivities than enzymatically synthesized PPyand also helped synthesis of PPy at a near-neutral pHconditions. Lower temperatures yielded more conductivesamples, due to higher concentration of 2,5-coupled products.In conclusion, this research effort has demonstrated the use

of hematin as an effective low cost alternative to peroxidaseenzymes for the polymerization of hydrophobic monomers inan aqueous micellar media. This opens new possibilities for thepolymerization of several substituted hydrophobic pyrrole andthiophene monomers using this micellar technique tosynthesize a range of conjugated polymers with novel andinteresting properties.

■ ASSOCIATED CONTENT

*S Supporting InformationAdditional figures. This material is available free of charge viathe Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Mailing address: Ball 209A, One University Ave., Lowell, MA01854, USA.

Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors would like acknowledge Dr. Daniel Sandman forhis insightful suggestions on the effective use of biomimeticcatalysts. Dr. Earl Ada of the center for high rate nano-manufacturing is gratefully acknowledged for his help withTEM imaging. Financial support from University of Massachu-setts Lowell new faculty start-up fund is also gratefullyacknowledged.

■ ABBREVIATIONSFTIR, Fourier transform infrared spectroscopy; UV, ultraviolet;TGA, therogravimetric analysis; PPy, polypyrrole; DBSA,sodium dodecylbenzenesulfonate; XPS, X-ray photoelectronspectroscopy

■ REFERENCES(1) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.;Heeger, A. J. Synthesis of electrically conducting organic polymers:halogen derivatives of polyacetylene, (CH)x. J. Chem. Soc., Chem.Commun. 1997, 578−580.(2) Nicoletta, F. P.; Chidichimo, G.; Cupelli, D.; DeFilpo, G.;DeBenedittis, M.; Gabriele, B.; Salerno, G.; Fazio, A. ElectrochromicPolymer-Dispersed Liquid-Crystal Film: A New Bifunctional Device.Adv. Funct. Mater. 2005, 6, 995−999.(3) Coakley, K.; Mcgehee, M. D. Conjugated Polymer PhotovoltaicCells. Chem. Mater. 2004, 16, 4533−4542.(4) Nystrom, G.; Razaq, A.; Strømme, M.; Nyholm, L.; Mihranyan,A. Ultrafast All-Polymer Paper-Based Batteries. Nano Lett. 2009, 9,3635−3639.(5) Alsalhi, M. S.; Alam, J.; Dass, L. A.; Raja, M. Recent Advances inConjugated Polymers for Light Emitting Devices. Int. J. Mol. Sci. 2011,12, 2036−2054.(6) Kola, S.; Sinha, J.; Katz, H. E. Organic transistors in the newdecade: Toward n-channel, printed, and stabilized devices. J. Polym.Phys. 2012, 50, 1090−1120.(7) Lucio Garcia, M. A.; Smith, M. A. Study of electrodepositedpolypyrrole coatings for the corrosion protection of stainless steelbipolar plates for the PEM fuel cell. J. Power Sources 2006, 158, 397−402.(8) Kim, S.; Oh, W. K.; Jeong, Y. S.; Hong, J. Y.; Cho, B. R.; Hahn, J.S.; Jang, J. Cytotoxicity of, and innate immune response to, size-controlled polypyrrole nanoparticles in mammalian cells. Biomaterials2011, 32, 2342−2350.(9) Ateh, D. D.; Navsaria, H. A.; Vadgama, P. Polypyrrole-basedconducting polymers and interactions with biological tissues. J. R. Soc.Interface 2006, 3, 741−752.(10) Couves, L. D. Polypyrrole as a potentiometric glucose sensor.Synth. Met. 1989, 28, C761−C768.(11) Osaka, T.; Komaba, S.; Amano, A.; Fujino., Y.; Mori., H.Electrochemical molecular sieving of the polyion complex film fordesigning highly sensitive biosensor for creatinine. J. Elect. Soc. 2000,65, 58−63.(12) Shi, J.; Ci, P.; Wang, F.; Peng, H.; Yang, P.; Wang, L.; Ge, S.;Wang, Q.; Chu, P. K. Nonenzymatic glucose sensor based on over-oxidized polypyrrole modified Pd/Si microchannel plate electrode.Biosens. Bioelectron. 2011, 26, 2579−2584.(13) Wong, J. Y.; Langer, R.; Ingber, D. E. Electrically conductingpolymers can noninvasively control the shape and growth ofmammalian cells. Proc. Natl. Acad. Sci.. U.S.A. 1994, 91, 3201−3204.(14) Bouldin, R.; Kokil, A.; Ravichandran, S.; Nagarajan, S.; Kumar,J.; Samuelson, L. A.; Bruno, F. F.; Nagarajan, R. Enzymatic Synthesisof Electrically Conducting Polymers. ACS Symp. Ser. 2010, 1043, 315−341.(15) Tewari, A.; Kokil, A.; Ravichandran, S.; Nagarajan, S.; Bouldin,R.; Samuelson, L. A.; Nagarajan, R.; Kumar, J. Soybean PeroxidaseCatalyzed Enzymatic Synthesis of Pyrrole/EDOT Copolymers. Macro.Chem. Phys. 2010, 211, 1610−1617.

Table 2. Conjugation Length Calculation (T1550/T1460 ratio)for PPy/DBSA Complexes Synthesized at Various ReactionConditions Using FTIR and Correlation with Final ProductConductivities

reactionconditions

pH T (°C) conjugation length T1550/T1460 conductivity (S/cm)

6 4 14.9 <10−7

5 4 10.9 ∼10−6

3.5 4 2.2 ∼10−2

3.5 20 5.8 10−4−10−5

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Langmuir Article

dx.doi.org/10.1021/la302494a | Langmuir 2012, 28, 13380−1338613386