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Langmuir 2008, 24, 11076-11081
Controlled Electrochemical Synthesis of Polypyrrole Nanoparticle Thin Film and Its Redox Transition to a Highly Conductive and Stable Polypyrrole Variant Richard West and Xiangqun Zeng* Department of Chemistry, Oakland UniVersity, Rochester, Michigan 48309 ReceiVed March 11, 2008. ReVised Manuscript ReceiVed July 31, 2008 We demonstrated here a unique method to produce a highly stable and conductive polypyrrole (PPY) nanoparticle film. The procedure entails controlling the redox switching and the electrochemical synthesis of PPY. PPY was synthesized at a very low forming potential or reaction rate in nonaqueous CH2Cl2 solvent to promote the PPY nanoparticle formation. Then its property was further optimized by first electrochemically reducing it at a hydrogen evolution potential in a neutral 0.1 M NaClO4, then in a slightly acidic 0.05 M asparagine electrolyte. The PPY nanoparticle thin film was characterized by AFM, UV-vis and EQCM. The procedures described here have proven to be reproducible. The data provided by the EQCM shows a reversible doping and undoping mechanism of asparagine indicating the presence of a highly conductive PPY variant. Both UV-vis and electrochemical characterization suggest that the PPY film made using our approach has excellent redox activity as well as high stability when characterized in asparagine solution. The reversible doping and undoping of asparagine during redox switching shows great potential of these PPY nanoparticle films as biological membranes for a broad range of biological applications.
Pyrrole is a heterocyclic molecule consisting of carbon and nitrogen that is able to polymerize chemically or electrochemically to form polypyrrole (PPY). The primary reactant in pyrrole electropolymerization is pyrrole radical or radical cation. The film deposition does not occur via a true three-dimensional nucleation and growth process, but rather involves solution phase polymerization with subsequent electroprecipitation of oligomer.1,2 Extensive studies show that the conditions such as the nature of the solvent, the supporting electrolyte, temperature, and potential/current in which PPY are electrochemically polymerized have a dramatic effect on its chemical, electrical, and mechanical properties.3,4 PPY must be grown at a potential anodic enough to initiate polymerization, yet when the potential is more anodic than 0.7 V vs SCE, the polymer could be over oxidized resulting in a less conductive film.5,6 Reports show that electrosynthesized PPY is generally composed of several variants.7 The reactivity of the generated oligomeric intermediates depends on their respective charge level. Typically, low oxidation potentials produce “short” chain lengths with lower reactivity. Zhou and coauthors account of several systematic studies described the voltammetric evidence of the structural diversity of PPY (defined as PPY (I), PPY (II) and PPY (III)) prepared at decreasing switching potential (potentiodynamic case) or currents (galvanostatic case).8 Recently, PPY nanoparticles were made by a redox enzyme, glucose oxidase, which initiated * To whom correspondence should be addressed. E-mail: zeng@ oakland.edu. (1) Raymond, D. E.; Harrison, D. J. J. Electroanal. Chem. 1993, 361, 65–76. (2) Scharifker, B. R.; Garcia-Pastoriza, E.; Marino, W. J. Electroanal. Chem. 1991, 300, 85–98. (3) Wallace, G. G.; Spinks, G. M.; Kane-Maguire, L. A. P.; Teasdale, P. R. ConductiVe ElectroactiVe Polymers: Intelligent Materials Systems, 2nd ed.; CRC Press: Boca Raton, FL, 2003; Chapters 2 and 3. (4) (a) Akhtar, P.; Too, C. O.; Wallace, G. G. Anal. Chim. Acta 1997, 339, 201–209. (b) Lee, S.; Sung, H.; Han, S.; Paik, W. J. Phys. Chem. 1994, 98, 1250–1252. (c) Jurevicute, I.; Bruckenstein, S. J. Solid State Electrochem. 2003, 7, 554–560. (5) Li, Y.; Qian, R. Electrochim. Acta 2000, 45, 1727–1731. (6) Deore, B.; Yakabe, H.; Shiigi, H.; Nagaoka, T. Analyst 2002, 127, 935– 939. (7) Zhou, M.; Heinze, J. Electrochim. Acta 1999, 44, 1733–1748. (8) Zhou, M.; Heinze, J. J. Phys. Chem. B 1999, 103, 8451–8457.
polymerization.9 However, most electrosynthesized PPY show fibril or cauliflower structures.10,11 In-situ AFM studies of PPY synthesis and redox switching show that the morphology of PPY depends on the nature of the substrate and PPY films have an extraordinary capacity to alter their morphology over time.12,13 We are interested in investigating conductive polymer redox behavior in amino acid bathing solutions for both fundamental and applied interests.14 PPY formed by electropolymerizing pyrrole in either aqueous or organic solution was often characterized in aqueous inorganic salt solution with various inorganic counterions, e.g., perchlorate, fluoroborate, toluensulfonate, oxalate, sulfate, carbonate, hydrogen phosphate.4 No characterization of PPY dynamic redox behavior in a zwitterionic bathing solution was reported. PPY is biocompatible and can noninvasively control shape and growth of mammalian cells.15 We are interested in the dynamic of acid-base equilibrium of the zwitterions on the doping and undoping of counterions in PPY. Asparagine is selected due to its high solubility in water and its important biological roles. Asparagine is required by the nervous system to maintain equilibrium and is also required for amino acid transformation from one form to the other in the liver. Therefore, understanding PPY redox behavior in amino acids could provide important insight to PPY redox behavior in biological fluids and facilitate its use in life science applications such as tissue engineering or implanted sensors. We intend to prepare PPY nanoparticle thin films that are optically transparent by rationally controlling the redox switching (9) Ramanaviciene, A.; Schuhmann, W.; Ramanavicius, A. Colloids Surf., B 2006, 48, 159–166. (10) Aydin, R.; Ko¨leli, F. Prog. Org. Coat. 2006, 56, 76–80. (11) Mo, X.; Wang, J.; Wang, Z.; Wang, S. Synth. Met. 2004, 142, 217–221. (12) Suarez, M. F.; Compton, R. G. J. Electroanal. Chem. 1999, 462, 211– 221. (13) Cohen, Y. S.; Levi, M. D.; Aurbach, D. Langmuir 2003, 19, 9804–9811. (14) Yu, L.; Sathe, M.; Zeng, X. J. Electrochem. Soc. 2005, 152, E357–E363. (15) (a) George, P. M.; LaVan, D. A.; Burdick, J. A.; Chen, C.-Y.; Liang, E.; Langer, R. AdV. Mater. 2006, 18, 577–581. (b) George, P. M.; Lyckman, A. W.; LaVan, D. A.; Hegde, A.; Leung, Y.; Avasare, R.; Testa, C.; Alexander, P. M.; Langer, R.; Sur, M Biomaterials 2005, 26, 3511–3519. (c) Wang, X.; Gu, X.; Yuan, C.; Chen, S.; Zhang, P.; Zhang, T.; Yao, J.; Chen, F.; Chen, G. J. Biomed. Mater. Res.-A 2004, 68A, 411–422.
10.1021/la800774z CCC: $40.75 2008 American Chemical Society Published on Web 08/26/2008
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Figure 2. CV of freshly made PPY characterized in 0.1 M NaClO4 electrolyte at a scan rate of 5mV/s; electrode area ∼0.23 cm2. Cycle 1 (black); Cycle 2 (red); Cycle 5 (blue).
Figure 1. AFM images of PPY nanoparticles after electrostatic polymerization in 0.1 M TBAP/CH2Cl2; (A) 3D, (B) cross-section.
processes of PPY as well as PPY electrochemical synthesis conditions. PPY was synthesized at a very low forming potential or reaction rate in nonaqueous CH2Cl2 solvent to promote the PPY nanoparticle formation. Then its property was further optimized by first electrochemically reducing it at hydrogen evolution potential in neutral 0.1 M NaClO4, then in a slightly acidic 0.05 M asparagine electrolyte. Our results showed that a highly conductive and stable PPY variant was made when characterized in an aqueous asparagine solution, a step further to broaden PPY’s application in biology.
Results and Discussion A PPY film was electrochemically polymerized on a gold working electrode by applying a potential step from 0.00 to 0.60 V (vs SCE) with 20 mM pyrrole in dichloromethane solvent in 0.1 M tetrabutylammonium perchlorate (TBAP) electrolyte in the absence of light. Dichloromethane was used as the solvent due to its lower toxicity compared to acetonitrile, CH3CN. We used a low pyrrole concentration and electrochemically polymerized the pyrrole using minimum oxidation potential in organic solvent in order to promote a better crystallinity as well as to prevent overoxidation of the monomer unit. Electrochemical quartz crystal microbalance (EQCM) was used to monitor the charge, mass (i.e., QCM frequency) and current during the electrochemical polymerization of the monomer unit (Figure S1 in the Supporting Information). The film thickness was controlled by controlling the oxidation charge to be around 1.92 mC. The thickness of the PPY modified gold electrode was around 0.1 µm or 100 nm. Figure 1 shows a typical AFM image of the PPY nanoparticle thin film made using the above conditions. For AFM experiments, gold was vapor deposited onto mica. The size distribution analysis shows that the PPY nanoparticle sizes are 700 ( 100 nm with a height of 45 ( 1 nm. The PPY nanoparticles do not agglomerate and have a relatively uniform distribution. We further changed the resulting PPY nanoparticle thin film by redox switching it in aqueous 0.1 M NaClO4. First, we
performed a cathodic scan to the hydrogen evolution region (HER) to completely reduce the PPY. Few electrochemical studies of PPY thin films were conducted in the potential region of hydrogen evolution. The common view is that if the potential applied is too negative, hydrogen evolution and subsequent deterioration of the polymer can occur.16 Otero et al. developed an electrochemically simulated conformational relaxation model (ESCR) and used it to characterize the properties of PPY film after polarization at high cathodic potentials.17 Based on their ESCR model, they proposed that polarization of PPY films at high cathodic potential leads to not only the reduction of the PPY, but also the compaction of the PPY polymeric structure, thus promoting further oxidation to be controlled by conformational relaxation processes. Additionally, it was reported that H+ ions play an important role in the electrochemical polymerization of pyrrole and its property.18-20 For example, according to Pei and coauthors, no electrochemical polymerization of pyrrole in aqueous buffer solutions was observed in a buffered solution at pH > 7.18 By electrochemically synthesizing PPY in organic dichloromethane solvent, but redox switching it to the HER in aqueous electrolyte (i.e., ClO4-) bathing solution, we intend to promote the drastic conformational relaxation of PPY chains. Figure 2 is the cyclic voltammograms (CV) of the redox switching of freshly made PPY in 0.1 M NaClO4 electrolyte. As shown in Figure 2, there is a sharp cathodic peak current at -0.29 V which can only be seen at a freshly made PPY film. This initial peak is likely due to the reduction of PPY and ejection of the anionic species, i.e. the ClO4- anion. When the potential approaches -1.20 V, hydrogen evolution reaction takes place, leading to the sharp increase of cathodic current at -1.20 V. In the second and subsequent scans, a small broad cathodic current peak around -0.25 V is observed. This residue peak is also observed when a bare gold electrode is used in either 0.1 M NaClO4 or 0.05 M asparagine bathing solutions. Since we are using a three compartment electrode system; there is trace chloride in our system, this peak is likely due to the trace chloride or oxygen. The true identity of this peak needs further investigation. (16) Jukic, A.; Metikosˆ-Hukovic, M. Electrochim. Acta 2003, 48, 3929–3937. (17) Otero, T. F.; Grande, H.; Rodriguez, J. J. Phys. Chem. B 1997, 101, 8525–8533. (18) (a) Qian, R.; Pei, Q.; Huang, Z. Makromol. Chem. 1991, 192, 1263–1273. (b) Pei, Q.; Qian, R. J. Electroanal. Chem. 1992, 322, 153–166. (19) Zhou, M.; Pagels, M.; Geschke, B.; Heinze, J. J. Phys. Chem. B 2002, 106, 10065–10073. (20) Hepel, J.; Bruckenstein, S.; Hepel, M, Microchem. J. 1997, 55, 179–191.
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Figure 3. (a) CV of polypyrrole in 0.05 M asparagine at a scan rate of 5 mV/s (electrode area ∼0.23 cm2). Cycle 1 (black); Cycle 2 (red); Cycle 5 (blue). Black Curve ∆E ) 0.325 V; Red Curve ∆E ) 0.307 V; and Blue Curve ∆E ) 0.244 V; (b) frequency vs E curves of polypyrrole doped with 0.05 M asparagine. Cycle 1 (black); Cycle 2 (red); Cycle 3 (green); Cycle 4 (cyan); Cycle 5 (blue). Scheme 1. Asparagine Ionization Equilibria
When the potential is swept back to the anodic region, there are two peaks at E ) 0.25 and 0.38 V. These two peaks are in close proximity to each other during the first cycle. Zhou and Heinze studies conclude that at an extremely low reaction rate, potentiodynamic generated PPY in acetonitrile solvent show several PPY variants (PPY (I), PPY (II) and PPY(III)) corresponding to the various peaks shown in the CVs.7 In the presence of a small amount of acid (∼1 × 10-5 M), PPY (II) is exclusively formed and the PPY(II) represents a very well-defined structure and is highly conductive.19,21 In a solution of high acidity (>10-4 M), a nonconductive PPY(III) forms at the electrode when a low potential is applied to the electrolysis.21 Our experimental conditions are similar to Zhou and Heinze; thus we can assume that several forms of PPY structures exist in the original PPY nanoparticle films. By switching to the HER in 0.1 M NaClO4, it is likely that a more conductive PPY with two forms were made. After PPY film was conditioned in 0.1 M NaClO4 by redox switching to the HER (-1.20 to 0.80 V), it was then studied in 0.05 M asparagine solution (pH 5.4). The slight acidity of 0.05 M asparagine favors PPY(II) formation.21 Figure 3a shows the cyclic voltammograms of PPY in a 0.05 M asparagine bathing solution and their corresponding frequency vs potential changes upon doping and undoping of asparagine. A simple one pair of redox switching peaks was shown. The peak current is increasing with each subsequent scan indicating increasing electroactive centers of PPY during redox cycling. As a result, the mass of the film is also increased with each subsequent scans (Figure 3b). The ∆E (the difference of E cathodic and E anodic) is also getting smaller with each subsequent scan, indicating increasing reversibility. Control experiments of redox features between -1.2 (21) Zhou, M.; Heinze, J. J. Phys. Chem. B 1999, 103, 8443–8450.
and 0.8 V of bare gold electrode in 0.1 M NaClO4 and 0.05 M asparagine electrolytes, respectively, showed that proton is reduced to hydrogen at around -0.9 V in 0.1 M NaClO4 and around -0.8 V in 0.05 M asparagine electrolyte. As expected, the current of the reduction of proton (also called hydrogen evolution process (HER)) is much higher in 0.05 M asparagine electrolyte since the pH of 0.05 M asparagine is lower than that of 0.1 M NaClO4. At PPY modified electrode, the proton reduction current is increasing in the second and subsequent redox cycles compared with that of the first cycle in both 0.1 M NaClO4 and 0.05 M asparagine electrolytes. This result indicates the HER process can effectively move the acid base equilibrium of the solvent or electrolyte (Scheme 1). The HER process allows local increases of anion concentration (i.e., OH- in 0.1 M NaClO4 and asparagine anion in 0.05 M asparagine) which is the dominant dopants during polypyrrole oxidation. We hypothesize the HER process at modified PPY electrode can lead to polypyrrole relaxation and the increasing of local anion concentrations. Both combined effects can result in the increased reversibility and higher peak currents observed during polypyrrole redox switching processes in 0.05 M asparagine solution. The PPY doped with asparagine and glutamine was previously studied for its ionexchange properties using an open circuit potentiometric measurement.22 However, there is currently no report on PPY redox behavior in an amino acid bathing solution. Usually, mechanical and chemical deformations occur in conducting polymers with large dopants like amino acids. The CV peak currents in Figure 3 over time become very well defined with no signs of any (22) (a) Paczosa-Bator, B.; Migdalski, J.; Lewenstam, A. Electrochim. Acta 2006, 51, 2173–2181. (b) Liang, H.-J.; Ling, T.-R.; Rick, J. F.; Chou, T.-C. Anal. Chim. Acta 2005, 542, 83–89.
Synthesis of Polypyrrole Nanoparticle Thin Film
Figure 4. EQCM frequency vs charge curves of PPY in 0.05 M asparagine (a potential trace is included). Inset: multicycle results. The main figure is cycle 1 which can be seen in the inset.
degradation indicating unique properties for our electrochemically synthesized PPY nanoparticle thin film. The damping resistance, R, in the Butterworth-Van Dyke equivalent circuit characterizes the change of damping of the crystal oscillation that is characteristic of a fluid or viscoelastic material. The RQCM instrument can provide data on the change in the frequency and in the damping resistance simultaneously. The damping resistance gets smaller with each subsequent cycle, indicating a more rigid PPY film (Supporting Information, Figure S2). AFM studies (FigureS3) show that the diameter of the PPY nanoparticles is 600 ( 100 nm after redox switching in 0.05 M asparagine solution, which does not vary significantly from its initial value. However, the height increases to 60 ( 1 nm after the doping with asparagine. The EQCM can measure in situ mass changes in the nanogram range during electrochemical measurement. It can demonstrate the existence of adsorbed or doped species on the working electrode and also give information about the change in composition of species on the interface of a working electrode during conductive polymer redox switching processes. Figure 4 shows the change in frequency with respect to the charge consumed in one complete cycle in 0.05 M asparagine bathing solution. During the gradual approach to the HER, the frequency change at the EQCM electrode did not change by any appreciable degree. This would be consistent with the following argument that the film was reconfigured in 0.1 M NaClO4 to a very open and stable structure. In 0.1 M NaClO4, the hydrogen evolution process facilitates the conformational relaxation of PPY synthesized in organic solvent into a highly conductive, stable, and open structure. The production of hydrogen allows the PPYmodified electrode surface to be more basic, which then facilitates the doping of the anion (i.e., OH- and ClO4-) during anodic processes. It also facilitates the dramatic conversion of PPY structures to a more uniformed variant as subsequently scans show increasing hydrogen evolution current and a more reversible CV. Additionally, when cathodic potentials reached the HER (i.e., -1.20 V), the in situ QCM shows a decreasing of the PPY film damping resistance. Once the resistance has decreased, the resistance does not change significantly when scanned back to the anodic region. After the PPY film was conditioned in 0.1 M NaClO4, the PPY film can be thought of as forming a 3-D network, which
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Figure 5. Kinetic study of PPY in 0.05 M asparagine at different scan rates. (Anodic peak currents (9); cathodic peak currents (•)).
would allow for easy incorporation of the anions.17 Figure 4 shows that during the oxidation and reduction of the PPY film in 0.05 M asparagine between 0.00 and 0.80 V, about 0.6 ( moles of asparagine per mole of electrons passed is doped into the PPY film. Redox switching to HER region also allows production of more OH- in the bathing solution; it is likely that a small amount of OH- could also be doped along with asparagine anions.23 In summary, by using a controlled synthesis of PPY in organic solvent and polarizing it to the HER in 0.1 M NaClO4, a uniformed PPY variant was made. The PPY has a very reversible doping and undoping dynamics in 0.05 M asparagine solution (Figure 4) which is very desirable when PPY is used as a biological membrane for various biological applications (e.g., drug delivery). The current peaks of PPY in 0.05 M asparagine solution can be further studied to decide whether they arise from a diffusionor surface-controlled process. If the current peak results from diffusion-controlled process, the peak current should be linear to the square root of scan rate ν1/2. The peak current is proportional to the scan rate ν if it is a surface-controlled process. Figure 5 shows a linear relationship of peak current with the ν1/2 at slower scan rates. Redox conditioning in 0.1 M NaClO4 allows the conformational relaxation to complete and results in a partially open structure of the PPY film. Diffusion of the counterions becomes the rate limiting step of the oxidation and reduction of PPY in 0.05 M asparagine bathing solution at slow scan rates. The deviation from the linearity at high scan rates (>0.320 V s-1) indicates that surface-controlled process is likely to dominate at high scan rates due to the conformational relaxation processes. After subjecting PPY to redox cycling in 0.1 M NaClO4 and 0.05 M asparagine solution, the PPY film was studied once again in 0.1 M NaClO4 using identical potential programming to observe any changes from the initial film characterization. In Figure 6, the original two redox peak currents increased compared to the first exposure to 0.1 M NaClO4 suggesting that the film has apparently increased in conductivity after redox switching in 0.05 M asparagine. Additionally, there is a large mass increase in the first cathodic cycle when the potential approaches the HER (i.e., -1.20 V). This suggests additional conformational relaxation and cation and solvent incorporation. In the anodic scan, assuming all the charge was due to the doping of ClO4-, the apparent molar mass of the dopant was calculated to be 21.7 (23) Sathe, M.; Yu, L.; Mao, Y.; Zeng, X. J. Electrochem. Soc. 2005, 152, E94–E97.
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Figure 6. (a) Current vs potential at a scan rate of 5 mV/s (electrode area ∼0.23 cm2) (b) frequency vs potential curves of polypyrrole characterized in 0.1 M NaClO4 following redox cycling first in 0.1 M NaClO4 and then 0.05 M asparagine. Cycle 1 (black); Cycle 2 (red); Cycle 3 (green); Cycle 4 (cyan); Cycle 5 (blue).
Figure 7. UV-vis of PPY (a) freshly made PPY film (black); (b) cycled five times in 0.1 M NaClO4 (red) and (c) then followed by five cycles in 0.05 M asparagine (blue).
g/mol using cycle 4 data. This is much smaller than the molecular weight of ClO4- (99.5 g/mol). Thus, the mass change is not only caused by the doping of ClO4- but also there is simultaneous ejection of some of the species doped in the first cathodic cycle from the PPY film. The subsequent cycles show very reproducible mass- and current-potential curves. As a result, the conduction of the electron transfer along the conjugated backbone of the PPY has been enhanced after it has been cycled in asparagine solutions. UV-vis was used to characterize the absorbance bands of the PPY films made. A transparent gold thin film was made by evaporating gold onto a mica sheet until a thickness of 150 nm was obtained. A PPY film was formed using identical synthesis conditions and redox cycling procedures described above. As shown in Figure 7, the freshly made PPY film shows a broad absorbance band around 500 nm and a well-defined absorbance band at 740 nm. The absorbance at 740 nm is due to the electronic transition between the valance band and the bipolaron band.24 (24) Kim, D. Y.; Cho, H. N.; Kim, C. Y. J. Intell. Mater. Syst. Struct. 1994, 5, 626–630.
The absorbance at 500 nm is due to the electronic transition between the valance band and the antibipolaron band.24 Both bands occur at higher wavelength compared to the literature studies indicating a higher conductivity of our PPY film.25,26 Conditioning of PPY in 0.1 M NaClO4 leads to an increasing of absorbance of both bands. The broadband at 500 nm indicates the presence of several PPY forms in freshly made PPY and in the 0.1 M NaClO4 conditioned PPY. However, the broad absorbance band of PPY at 500 nm becomes much sharper after conditioning the PPY film in 0.05 M asparagine which suggests a transition of several PPY forms to one variant.27 There is also a slightly positive shift of the absorption band at lower energy (i.e., 740 nm) after each redox conditioning in 0.1 M NaClO4 and 0.05 M asparagine suggesting increasing conductivity of the PPY film. The PPY nanoparticle films were studied further in a 10 month period for their stability. A new film was made and stored in the dark for this study. The two variables that could not be eliminated were moisture from the air and the oxygen atmosphere. As shown in Figure 8, after 3 weeks of storage, the redox peak of PPY was sharper and the cathodic and anodic peak separation was reduced. After 39 weeks of storage, the peak current reduced to about 60%. However, within the 39-week study, the major features of the PPY redox doping in 0.05 M asparagine bathing solutions remain, indicating the PPY variant made can be maintained.
Conclusions In summary, we demonstrated here a unique method to produce a highly stable and conductive PPY nanoparticle film via slow electrochemical polymerization in organic solvent and redox switching in HER in aqueous 0.1 M NaClO4 and 0.05 M asparagine electrolytes respectively. The nanoparticle PPY thin film was characterized by AFM, UV-vis and EQCM. The procedures described here have proven to be reproducible. The data provided by EQCM shows a reversible doping and undoping mechanism of asparagine indicating the presence of a highly conductive PPY variant. Both UV-vis and electrochemical characterizations in asparagine solution suggest that the PPY (25) Lee, E. S.; Park, J. H.; Wallace, G. G.; Bae, Y. H. Polym. Int. 2004, 53, 400–405. (26) Kim, D. Y.; Lee, J. Y.; Moon, D. K.; Kim, C. Y. Synth. Met. 1995, 471–474. (27) Bufon, C. C. B.; Vollmer, J.; Heinzel, T.; Espindola, P.; John, H.; Heinze, J. J. Phys. Chem. B 2005, 109, 19191–19199.
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Figure 8. CV of redox recycling of a PPY film in 0.05 M asparagine at various PPY storage times: black (day 1), red (1 week old), green (2 weeks old), cyan (3 weeks old), blue (4 weeks old), pink (39 weeks old). In week 39, the first cycle has no oxidation peak, but it reappears in cycle 2 and increases in cycle 3 (third cycle is depicted in Figure 8 alone). The anodic peaks have shifted toward more negative potentials compared to the initial study of the PPY film. All scans were done at 5 mV/s (electrode area ∼0.23 cm2).
film made using our approach has excellent redox activity with high stability. The reversible dynamics of doping and undoping asparagine during redox switching shows great potential of so formed PPY nanoparticle films as a drug delivery substrate or biological membranes for cell attachment and growth.
Methods and Materials Tetrabutylammonium perchlorate (TBAP) (GFS Chemicals), sodium perchlorate (98%, Sigma), asparagine (99%, Acros) and dichloromethane (HPLC grade (99.9%), Acros) were used as received. Pyrrole was purchased from Alfa Aesar and further purified by distillation. Deionized water was purified using a Milli-Q water system (18 MΩ). Electrochemical Polymerization of Pyrrole. A PPY film was electrochemically polymerized onto a gold working electrode with 0.1 M TBAP in dichloromethane solvent. A Pt wire was used as a counter electrode and a SCE was used as a reference electrode. A concentration of 20 mM pyrrole was used and the potential was stepped from 0.00 to 0.60 V until a change of about 2300 Hz and/or 1.92 mC charge was reached. Two types of gold working electrodes were employed as the substrate for PPY electrochemical polymerization. The Au electrode vapor deposited on a quartz crystal
Langmuir, Vol. 24, No. 19, 2008 11081 substrate was used for all EQCM studies. Since UV-vis and AFM requires smooth substrate, a gold electrode vapor deposited on a mica sheet was used for these studies. The pyrrole was purified by distillation under N2 atmosphere. Great care was taken to prevent the monomer from the exposure to any stray light. It was observed that the pyrrole solution will turn from a crystal clear solution, to a yellow color, and finally to a dark amber color when it was exposed to light. As a result, aluminum foil was used to cover the glassware during the pyrrole distillation. The distilled pyrrole was then purged with N2 and wrapped with aluminum and placed in the refrigerator. Electrochemical polymerization of pyrrole was carried out immediately after pyrrole purification. Electrochemical Quartz Crystal Microbalance (EQCM). The gold quartz crystal electrode used in the EQCM study was a 10 MHz AT cut quartz, coated with 1000 Å gold with a ∼0.23 cm2 geometric area (International Crystal Company, Oklahoma). A platinum wire and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. Before each experiment was carried out, the electrochemical cell was purged with N2 for 15 min to remove oxygen. Then, the electrochemical cell was sealed with parafilm to prevent oxygen from dissolving back into the electrolyte solutions. Two small holes were made in the parafilm cover to allow for a stream of N2 blanketing the surface of the electrolyte solution during the course of the experiments. An AFRDE 5 Bipotentiostat (Pine Instrument Company) and the Research Quartz Crystal Microbalance (RQCM) from Maxtek were used for EQCM experiments. The RQCM Data Logging Software (v2.0) collected the current, potential, charge, frequency, and damping resistance during the course of the experiments. All data obtained with the RQCM was analyzed with the Microcal Origin software (v6.0). All experiments were carried out at room temperature. Spectroscopy. Spectroelectrochemistry studies were carried out using a Cary 100 Bio UV-vis Spectrophotometer. The sample and background scans were acquired with a scan rate of 100 nm/min, with a data interval of 0.167 nm. Atomic Force Microscopy. Atomic Force Microscopy (AFM) studies were done using a Molecular Imaging PicoPlus. The AFM images were acquired in air and the cantilever used in all experiments was a silicon nitride tip (Si3N4) with a spring constant of k ) 0.12 N m-1. The images were scanned in contact mode with a scan rate of 1.0 lines/s. The resolution of each image is acquired at 512 × 512. The only processing of the AFM images for each experiment was the flattening.
Acknowledgment. This work was supported by the Oakland University (Bennett tuition award and Research Excellent Funds). Supporting Information Available: Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. LA800774Z
