Micellar Nanoreactors for Hematin Catalyzed Synthesis of Electrically

Article pubs.acs.org/Langmuir

Micellar Nanoreactors for Hematin Catalyzed Synthesis of Electrically Conducting Polypyrrole Sethumadhavan 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 of Chemical 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) is very well established “green” methods for producing conducting polypyrrole. The importance of this approach is realized by the numerous potential opportunities of using PPy in biological applications. However, due to very high costs and low acid stability of these enzymes, there is need for more robust alternate biomimetic catalysts. Hematin, a hydroxyferriprotoporphyrin, has a similar iron catalytic active center like the peroxidases and has previously shown to catalyze polymerization of phenol monomers at pH 12. The insolubility of hematin due to extensive self-aggregation at low pH conditions has prevented its use in the synthesis of conjugated polymers. In this study, we have demonstrated the use of a micellar environment with sodium dodecylbenzenesulfonate (DBSA) for biomimetic synthesis of PPy. 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 and reveals the formation of an ordered PPy/DBSA complex with conductivities approaching 0.1 S/cm.

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INTRODUCTION Since the discovery of π-conjugated polymers more than two decades ago,1 there has been tremendous interest in these polymers due to a variety of potential applications. A combination of ease of processability and tunable optical and electronic properties have led to applications in a variety of fields such as electrochromic devices,2 photovoltaics,3 battery applications,4 light emitting diodes,5 organic transistors,6 and anticorrosion coatings.7 Among conducting polymers, polypyrrole (PPy) is particularly interesting due to its biocompatibility.8 In addition to its use in electrical and electronic devices, PPy has potential for use in biomedical applications.9−13 With its unique combination of properties, PPy continues to attract attention, particularly in the field of biotechnology. Traditionally, chemical and electrochemical methods are most commonly used for the synthesis of PPy. The synthesis almost often involves harsh organic solvents, reagents and catalysts to produce electrically conducting PPy. In order to overcome this problem, several research groups over the past decade have explored the use of more environmentally friendly enzymatic approaches for the synthesis of conjugated polymers such as PPy.14 PPy has been synthesized using oxidants like H2O2 with naturally occurring oxidoreductase enzymes (peroxidases) as catalysts.15,16 We have recently reported the synthesis of water dispersible © 2012 American Chemical Society

conducting PPy and its derivatives using catalysts derived from soybeans (soybean peroxidase).17,18 While peroxidases are “greener” oxidation catalysts, their commercial utility in synthesis of electrically conducting polymers has been limited due to several factors. Naturally occurring peroxidases usually exhibit 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 biomimetic catalysts as effective surrogates to peroxidases. The heme prosthetic group (iron porphyrin active center) of the peroxidase catalysts (specifically horseradish and soybean peroxidase) is the active site that catalyzes oxidative polymerization reactions.20,21 Hematin (hydroxyferriprotoporphyrin) is a naturally occurring, cost-effective iron-porphyrin extracted from porcine blood. Akkara et al. used hematin as a catalyst for the polymerization of 4-ethylphenol in a mixed solvent system.22 In the same study, it was postulated that hematin forms catalytic intermediates similar to peroxidases. However, hematin has a lower oxidation potential and is water-soluble only at a high pH (above pH 8), making it an ineffective catalyst for the Received: June 19, 2012 Revised: August 16, 2012 Published: August 20, 2012 13380

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Conductivity. Conductivity measurements were taken in triplicate on pressed pellets of PPy/DBSA (dried overnight) using a four-point probe. Conductivity measurements were performed with a Keithley 2750 DMM and a Keithley 6221 source meter. X-ray Photoelectron Spectroscopy (XPS). Electron spectroscopy for chemical analysis (ESCA) was performed utilizing a VG ESCALAB MKII photoelectron spectrometer with Al Kα X-rays and a base pressure in the 10−10 Torr range. The generated photoelectrons were detected at a takeoff angle of 90°, which was defined as the angle between the surface plane and the entrance of the focusing lens of the concentric hemispherical analyzer. 1 H NMR. The 1H NMR spectra were collected using a Bruker 500 MHz NMR spectrometer. The solvent used was deuterated water (obtained from Cambridge isotope laboratories). A total of 10 μL of pyrrole was dissolved in 600 μL of the deuterated solvent, followed by addition of DBSA in different ratios above and below its critical micelle concentration (CMC). Cyclic Voltammetry (CV). Electrochemical experiments were performed using a Pine Research Instruments Wavenow USB potentiostat using a three-electrode cell in water at room temperature. A platinum wire and Ag/AgCl standard electrode were used as counter and reference electrode, respectively. A thin film was drop casted from micellar dispersion of polypyrrole onto an indium tin oxide coated glass slide and was used as the working electrode. The potential scan range 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.

polymerization of electrically conducting polymers. Hence, hematin was modified using polyethyleneglycol chains. Polyethyleneglycol modified hematin was shown to catalyze the synthesis of conductive Pani23 and PPy/PEDOT,24 but it required a low pH (pH∼1) for the synthesis. However, direct functionalization of hematin is not an efficient process due to aggregation of the porphyrin rings during modification. Recently, it was also shown that there was an improvement in the catalytic activity of hematin in the presence of sodium lauryl sulfonate.25 But its efficacy as an oxidative catalyst was not investigated. This work demonstrates the possibility of using unmodified hematin, as an effective and more economical alternative to peroxidases for oxidative polymerizations. We report for the first time the synthesis of PPy using hematin catalyst in aqueous micellar nanoreactors. This new approach involves dispersing water-insoluble hematin in micellar nanoreactors providing a unique environment that enables a simple one-pot biomimetic, biofriendly synthesis of a conducting, doped PPy complex.

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EXPERIMENTAL SECTION

Materials. Pyrrole (98%), sodium dodecylbenzenesulfonate (Technical grade) (DBSA), sodium laurylsulfonate (SLS), cetyltrimethylammonium bromide (CTAB), Triton X-100, Hematin Porcine, and 30% hydrogen peroxide (H2O2) in water were obtained from Sigma-Aldrich Co. Hydrogen peroxide was diluted with deionized water to a stock solution of 3% (w/v). This diluted H2O2 solution was used for all polymerizations. All other chemicals were of reagent grade or 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 was solubilized in the pH 12 solution at a concentration of 10 mg/mL. At the start of the reaction, 100 μL of this stock solution (1 mg of hematin for a 10 mL reaction) is added to a 10 mM citrate buffer (pH adjusted to 3.5−6). Due to the citrate buffer strength, hematin added to the mixture begins to precipitate out of the reaction solution. This is immediately followed by the addition of 10 mM DBSA to the solution, which on sonication for 15 min renders a clear brown dispersion. The pH of the final stock solution is determined and used in the consecutive steps of the reaction. Alternatively, this stock solution can also be prepared by adding the DBSA to the citrate buffer followed by the 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 by sonication for 10 min. After dispersion of the monomer droplets, 1 mL of 3% H2O2 is added in drops over 2 min. Within 15 min after the addition of the H2O2, there is formation of a gray colored solution which 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 large excess of acetone, which breaks the micelles, precipitating the product. The thermogravimetric yield was in the range of 85−90%. The precipitate is centrifuged and dried under vacuum overnight at 50 °C to obtain biomimetically synthesized PPy in a micellar environment. The biomimetic catalyst, hematin, could not be isolated after the reaction and thus was not recycled. Characterization of Products. UV−Visible Spectroscopy. All products were characterized using an Agilent 8453 photodiode array UV−visible spectrometer. To obtain UV−visible spectra of the polymer, a 100 μL aliquot of the reaction mixture was diluted with 100 mM pH 3.5 citrate buffer to a total volume of 300 μL in a quartz cuvette with optical path length of 1 mm. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR measurements were taken on a Thermo Scientific Nicolet 4700 instrument. Solid powder of the synthesized PPy was directly used with a Smart Orbit attenuated total reflectance (ATR) accessory for all the measurements. All samples were dried under vacuum to ensure the removal of moisture, which can result in free −OH peaks.

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RESULTS AND DISCUSSION Biomimetic Polymerization of Pyrrole. Biomimetic polymerization of hematin was performed in a micellar solution prepared by the addition of DBSA above its CMC in a pH 3.5 citrate buffer. The surfactant forms a hydrophobic micellar core and a hydrophilic shell, enabling dissolution of water insoluble substances.26 Hematin is normally insoluble in water at low pH. However, hematin can be loaded into these micelles, primarily driven by hydrophobic interactions. These micelles loaded with the catalyst serve as nanoreactors for the polymerization of any hydrophobic monomer. In the present work, pyrrole is added to these nanoreactors followed by the addition of H2O2. It is expected that the pyrrole is predominantly dispersed in the hydrophobic core of the DBSA micelles, due to its limited solubility in water. Hematin catalyzed polymerization of PPy/DBSA complexes were monitored using UV−visible spectroscopy, as the complexes remain dispersible in water during the reaction. The intensity of bipolaron absorptions (300−450 nm) and the long wavelength absorptions (1000 nm) is indicative of the formation of PPy/DBSA complex at any time during the propagation of the reaction. However, the final PPy/DBSA complex after isolation is insoluble in common organic solvents and could not be redispersed in an aqueous media. The molecular weight of the obtained PPy/DBSA complex could not be characterized due to the insolubility of the precipitated PPy/DBSA complex in water or other common organic solvents. This is not uncommon and has been previously reported for PPy/DBSA systems by several groups.27,28 Ionic and Nonionic Surfactants. Several types of surfactants with different molecular structures and charge were used as a surfactant/dopant for PPy synthesis using this micellar approach as shown in Figure 1. All three types of surfactants, namely, anionic, cationic, and nonionic forms, were able to disperse hematin and pyrrole droplets in the micelle before the addition of H2O2. However, CTAB and Triton-X did not produce conducting PPy (Figure 2) complexes. Therefore, we may conclude that the presence of a strong anionic charge is 13381

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

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

The presence of a micellar media assisted the interaction of DBSA dopant with the pyrrole monomer prior to polymerization. 1H NMR studies were used to observe any interactions between these two compounds in D2O with varying ratios of pyrrole (data provided in the Supporting Information). The alpha and beta protons of pyrrole appear at 6.84 and 6.18 ppm, respectively, and shift continually with an increase in molar concentration of DBSA. Unlike the previously reported aniline system wherein the interactions are purely electrostatic,27 the pyrrole monomer is unlikely to be protonated neither in neutral D2O nor at the reaction pH. Hence, this shift is more likely due to the interactions between the pyrrole protons and the aromatic sulfonate group of the DBSA dopant, increasing the probability of radical stabilization. Our experimental results seem to indicate that these interactions play a very important role in the biomimetic polymerization of pyrrole. Optimization of Reactions: Influence of Reaction Conditions. Effect of pH. UV−visible spectra of PPy/DBSA reaction after 24 h carried out at various reaction pHs revealed that lower reaction pH results in higher product formation. In spite of stabilizing hydrophobic interactions due to DBSA micelles, hematin could not form stable dispersions (without precipitation) at pH lower than 3. Interestingly, the use of hematin enabled the formation of PPy/DBSA complex even at pH approaching physiological conditions, whereas no polymerization 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 with conductivities ranging from 10−6 to 10−7 S/cm. XPS analysis of PPy/DBSA complexes indicated that, with increase in reaction pH, there is a large decrease in pyrrole carbons with respect to the dopant. At pH 3.5, the percentage of pyrrole carbons in the system is 70 and decreases to 50 at pH 6. These results are consistent with the conductivities of the synthesized PPy/DBSA complexes. Effect of Time. The time taken for the hematin catalyzed polymerization reactions is significantly longer than that of enzyme catalyzed systems, due to lower catalytic activity and redox potentials of these native porphyrins. However, the presence of a micellar environment has enabled faster formation of PPy/DBSA complex, with absorption maxima at 460, 1000, and 370 nm attributed to bipolaron peaks and porphyrin Soret peak, respectively, as shown in Figure 4. The bipolaron absorptions at around 900−1000 nm reach a

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

necessary for the synthesis of electrically conducting polypyrrole using hematin. As shown in Figure 2, there is a strong absorption in the region from 800 to 1100 nm in the presence of anionic surfactants due to the formation of a doped, conducting form of PPy. PPy doped with DBSA exhibited higher conversions as evidenced by stronger bipolaron absorptions in the 800−1100 nm range, due to a stronger interaction of the aromatic anion in comparison to SLS.29The use of Triton-X as a cosurfactant to DBSA (1:1 molar ratio) did not 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 aqueous reaction media. In the enzymatic synthesis of polyaniline,27 the local low-pH environment provided by aqueous micelles has been shown to play an important role. Similarly, during the biomimetic polymerization of PPy, DBSA micelles served the following important functions. DBSA micelles helped disperse hematin in an aqueous media at low pH necessary for polymerization. Recently, it was also shown that the presence of a micellar media helps improve the catalytic efficiency of hematin by 20%.16 High local concentration of the monomer and the catalyst inside the micelle promotes efficient polymerization. In addition, voltammetric studies on pyrrole monomer in DBSA micelle solutions indicated that with increasing DBSA concentrations there is a significant reduction in oxidation potential (measured by monitoring the first irreversible oxidation peak from cyclic voltagrams) of pyrrole as shown in Figure 3. 13382

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

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

set of reactions producing maximum yields at 10 and 2 h, respectively. Reactions at 40 °C did not produce bipolaron absorptions 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 early stages of the reaction. However these reactions undergo drastic retardation beyond this time, whereas the low temperature reactions continue to progress until a time period of 10 h. To understand this unique behavior, products from all reactions were isolated when their bipolaron absorptions reached a maxima (2 h for 20/30 °C and 10 h for 4/10 °C respectively) and their conductivities were plotted on the same graph as shown in Figure 7. The conductivities of the PPy/DBSA

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 of the propagating species, indicating the possibility of overoxidation or defect-site generation.30 The obtained values of conductivities are in good agreement with UV−visible measurements with PPy/DBSA isolated at 10 h showing a maximum conductivity of ∼10−1 S/cm (Figure 5). These conductivity values are an order of magnitude higher than the values for conductivities of enzymatically synthesized PPy/ DBSA complex.

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

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

Effect of Temperature. Previous reports on enzymatic, chemical,31 and electrochemical32 polymerization identified temperature control as an important variable to obtain conducting PPy. To study the effects of temperature on the synthesis of PPy/DBSA complexes using hematin, the synthesis was carried out at 4, 10, 20, 30, and 40 °C, and the rate of formation of PPy/DBSA complexes was monitored with time by UV−visible absorption spectroscopy. The bipolaron peak at 950 nm is taken as a measure of the conversion at any time in the reaction mixture as shown in Figure 6. Two distinct trends were observed for polymers synthesized at 4 and 10 °C in comparison to the ones synthesized at 20 and 30 °C, with each

complexes synthesized at 4 and 10 °C are roughly 3−4 orders of magnitude higher than that of the ones synthesized at 20 and 30 °C respectively. This is corroborated by XPS analysis, which indicates that increase in temperature results in a drastic reduction in the ratio of α/β carbon atoms from 1.5 (4 °C) to 0.7 (30 °C). A decrease in α/β protons increases the possibility of 2,3-coupling modes providing additional doping centers (Dopant/Monomer ratio increases from 0.3 to 0.38 with increasing temperature) resulting in poorly conducting samples.33 In addition, recent work on enzymatic synthesis of PPy also follows a similar trend, producing PPy/PSS complexes 13383

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PSS complex with very low degree of disorder of 13%.17 PPy/ DBSA synthesized using hematin has a slightly higher disorder percentage at 19.5, possibly due to the use of small molecule dopant DBSA instead of PSS. But this value is still comparable to chemical methods of PPy/DBSA synthesis yielding complexes with good conductivities. Cyclic Voltammetry. The cyclic voltammetry of a drop casted film of PPy/DBSA complex (shown in the Supporting Information) synthesized at pH 3.5 and temperature 4 °C is characterized by an oxidation peak at around 0.12 V and a reduction peak at −0.42 V in the first cycle which is very similar to reported values for chemically synthesized PPy/DBSA complexes.35 Fourier Transform Infrared Spectroscopy. The FTIR spectra of PPy/DBSA complex synthesized using hematin at 4 °C is shown in Figure 9. The bands at 890, 1034, and 961

with higher conductivities at lower temperatures. The cosurfactant system containing Triton-X and DBSA resulted in conductivities of 10−3 S/cm, roughly an order of magnitude lower than PPy/DBSA complexes. X-ray Photoelectron Spectroscopy. XPS analysis of the asymmetric carbon peak in PPy/dopant complexes can be deconvoluted into four prominent peaks, namely, alpha and beta carbons of pyrrole, carbons of the dopant, and the disorder carbon peak (interchain cross-links, side chains, and chain ends) using well-established line shape analysis and Gaussian fitting methods.34 Additionally, it is also possible to determine any residual heme catalytic impurity present in these complexes which could interfere with the final conductivities of the products. Figure 8 displays the carbon 1s core level scan of hematin catalyzed PPy/DBSA complex synthesized at 10 °C.

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

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

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

cm−1 are due to C−H in and out of plane and C−C in plane vibrations respectively. The band at 1167 cm−1 can be attributed to the vibration of the aromatic pyrrole ring. The presence of a broad band at 1285 cm−1 is ascribed to −SO groups and −S-phenyl vibrations from the dopant in the PPy complex. The PPy/DBSA complexes also had characteristic C− C and C−N vibrations at 1545 and 1455 cm−1, whose ratio is representative of the conjugation length of the conducting polymer.36 The small band at 1730 cm−1 is due to the very low concentration of carbonyl species from pyrrolidone end groups present due to slow rates of oxidations of pyrrole in biomimetic systems 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 the main infrared bands of PPy is strongly influenced by the conjugation length. It was theoretically predicted that, as the conjugation length increases, the ratio of IR intensity of the antisymmetric (C−N) ring stretching mode at 1550 cm−1 to the symmetric mode (C−C) stretching mode at 1460 cm−1 should decrease.37 This has also been experimentally validated by Menon et al.38 As shown in Table 2, the ratio of the intensities of the 1550 and 1460 cm−1 bands in the IR spectrum has been used to estimate the conjugation length. Clearly, there is a correlation of the T1550/T1460 ratio (which is related to conjugation length) with the final product conductivities. Increase in reaction pH, increases T1550/T1460 ratio. This

Table 1. Carbon 1s Core Level Data for PPy Synthesized Using Different Methodologies pyrrole carbons (eV)

synthesis method

β

α

disorder

biomimetic enzymatic chemical electrochemical

285.17 284.1 285.47 283.6

286.09 284.9 286.37 284.5

288.26 289.0 285.4

energy gap (α − β) (eV)

disorder (%)

0.92 0.82 0.90 0.90

19.6 13.1 22.1 33.3

ison to enzymatic, chemical and electrochemical methods. The absolute binding energies of the pyrrole carbon peaks from various methods are slightly different due to varied charging effects in the samples. However, a constant energy gap 0.9 eV is observed between α and β pyrrole carbons in all samples. Degree of disorder was calculated from the ratio of total area under the asymmetric carbon to the area under the disorder peak. Pfluger and Street identified a large concentration of interchain cross-links in electrochemically synthesized PPy leading to 33% disorder.34 Joo and Epstein reported chemical synthesis, unlike the electrochemical approach, yielded a product with a lower degree of disorder close to 22%.33 Recently, our group reported the enzymatic synthesis of a PPy13384

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ACKNOWLEDGMENTS The authors would like acknowledge Dr. Daniel Sandman for his insightful suggestions on the effective use of biomimetic catalysts. Dr. Earl Ada of the center for high rate nanomanufacturing is gratefully acknowledged for his help with TEM imaging. Financial support from University of Massachusetts Lowell new faculty start-up fund is also gratefully acknowledged.

Table 2. Conjugation Length Calculation (T1550/T1460 ratio) for PPy/DBSA Complexes Synthesized at Various Reaction Conditions Using FTIR and Correlation with Final Product Conductivities reaction conditions pH

T (°C)

conjugation length T1550/T1460

conductivity (S/cm)

6 5 3.5 3.5

4 4 4 20

14.9 10.9 2.2 5.8