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Analytical Letters
ISSN: 0003-2719 (Print) 1532-236X (Online) Journal homepage: https://www.tandfonline.com/loi/lanl20
Electrical Energy Generation from a Novel
Polypropylene Grafted Polyethylene Glycol Based
Enzymatic Fuel Cell
Muhammet Samet Kilic , Seyda Korkut & Baki Hazer
To cite this article: Muhammet Samet Kilic , Seyda Korkut & Baki Hazer (2014) Electrical Energy Generation from a Novel Polypropylene Grafted Polyethylene Glycol Based Enzymatic Fuel Cell, Analytical Letters, 47:6, 983-995, DOI: 10.1080/00032719.2013.860536
To link to this article: https://doi.org/10.1080/00032719.2013.860536
Accepted author version posted online: 01 Feb 2014.
Published online: 28 Mar 2014.
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Electrochemistry
ELECTRICAL ENERGY GENERATION FROM A NOVEL
POLYPROPYLENE GRAFTED POLYETHYLENE
GLYCOL BASED ENZYMATIC FUEL CELL
Muhammet Samet Kilic,
1Seyda Korkut,
2and Baki Hazer
11
Department of Chemistry, Bulent Ecevit University, Zonguldak, Turkey
2
Department of Environmental Engineering, Bulent Ecevit University, Zonguldak, Turkey
A recently synthesized polypropylene-g-polyethylene glycol polymer was used for the first time as the working electrode of a fuel cell. Electrodes were prepared for unmediated and mediated enzymatic reactions including ferrocene as the mediator. Glucose oxidase and bilirubin oxidase was used as the anodic and cathodic enzymes for the working electro-des, respectively. The biofuel cell was operated using glucose as the fuel in a single-compartment and membrane-less cell. Electrochemical results demonstrated that the catalytic efficiency of the ferrocene based cathode was approximately 100-fold higher than that of an unmediated cathode. The mediated fuel cell electrodes yielded a power den-sity of 65 nW/cm2at a cell potential ofþ560 mV.
Keywords: Bilirubin oxidase; Enzymatic fuel cell; Ferrocene; Glucose oxidase; Polypropylene
INTRODUCTION
Enzyme based biofuel cells that directly convert chemicals such as glucose into electrical energy have received considerable interest in recent decades (Ammam and Fransaer 2013). The generated output power is sufficiently high to power micro- and mini-scale electronic systems such as small sensor-transmitter systems, microdevices, and pacemakers that require relatively low power (Brunel et al. 2007).
The transfer of electrons between the redox site and the surface of electrode in fuel cells is of paramount importance. Recently, conducting polymers, sol-gels, metal oxides, self-assembled monolayers, and nanocomposites, along with different biomolecule immobilization strategies and polymer modifications, have been used to achieve enhanced electron transfer (Palomera et al. 2011). To create a bridge between the enzyme and the electrode surface, metallic electrode surfaces have been
Received 20 August 2013; accepted 14 October 2013.
Address correspondence to Seyda Korkut, Department of Environmental Engineering, Bulent Ecevit University, 67100, Zonguldak, Turkey. E-mail: s.korkut@karaelmas.edu.tr
Color versions of one or more of the figures in the article can be found online at www.tandfonline. com/lanl.
Copyright # Taylor & Francis Group, LLC ISSN: 0003-2719 print=1532-236X online DOI: 10.1080/00032719.2013.860536
modified by deposition of these polymers. Use of polymeric films possessing ease of fabrication and modification for bio-functionalization has gained a special interest for bioelectrochemical applications. The synthesis of novel polymers and copolymers is still of importance due to the high demand for new materials with defined architectures and improved properties. Up to now, most amphiphilic polymers were comprised from polyethylene glycol and vinyl polymers as hydrophobic segment such as polyethylene, polystyrene, and polymethyl methacrylate (Hazer 1990; Erog˘lu et al. 1996; Hazer et al. 1992; Hazer 2010; Yıldız, Hazer, and Tauer 2012; Ozturk et al. 2013). Polypropylene is one of the most important polyolefines due to its wide industrial production, low cost, good mechanical properties, easy processing, and excellent recyclability (Koike and Cakmak 2009; Lee et al. 2009). Furthermore, it is a versatile, hydrophobic polymer that has medical and industrial applications due to its good film and fiber properties.
To mediate electron transfer between the enzyme and the electrode surface, low molecular weight redox compounds called mediators have been adsorbed or entrapped into the polymeric films (Ardhaoui et al. 2013). Higher power generation and minimal interferences at lower cell potential was achieved by using mediators in fuel cell systems. A mediator should possess the following characteristics: fast electrochemical kinetics, oxidation states that do not interfere with the enzyme and chemical stability. Also, by choosing a mediator that has a potential close to that of the fuel increases the power density (Pas 2007). A redox mediator of appropriate redox potential is required to shuttle electrons between the enzyme and electrode surface, because direct electron transfer to buried redox sites within these enzymes is generally not possible given the distance of the active site from the electrode sur-face (Barriere et al. 2004). Different types of mediators, such as ferrocene and its derivatives, thionine, quinone, phenazines, Fe(III) ethylenediaminetetraacetic acid (EDTA), methylene blue, and neutral red, have been used in enzymatic fuel cells (Osman, Shah, and Walsh 2010). Among mediators, ferrocene and its derivatives have been popular, as they fulfill most of the requirements of ideal mediators in redox-enzyme catalysis (Palomera et al. 2011; Dursun et al. 2012).
In this work, initial studies of a membrane-less biofuel cell are reported using a newly synthesized polypropylene-g-polyethylene glycol based biofuel cell including anodic enzyme glucose oxidase and cathodic enzyme bilirubin oxidase and glucose as fuel. Polypropylene-g-polyethylene glycol based working electrodes were used in both ferrocene mediated and unmediated fuel cell systems, and performance results were compared.
EXPERIMENTAL Reagents
Chlorinated polypropylene (Mw150,000 Da, three repeating units have 1 Cl in
average), polyethylene glycol with Mn¼ 8,000 Da (PEG8000), tetrahydrofuran
(THF, 99% GC grade), glucose oxidase from Aspergillus niger (10 KU), bilirubin oxidase from Myrothecium verrucaria (25 U), and ferrocene were obtained from Sigma-Aldrich. Sodium hydroxide, hydrochloric acid, potassium dihydrogen phosphate, and di-potassium hydrogen phosphate were purchased from Merck.
Glucose monohydrate and chloroform were supplied from Riedel. Stock solutions of enzymes and glucose were daily prepared in 100 mM pH 7.4 phosphate buffer.
Apparatus
Fuel cell experiments were conducted by using a CHI 1040B Model electro-chemical analyzer. A glassy carbon working electrode (3 mm diameter) for anode and cathode, a platinum wire counter electrode, a Ag=AgCl (3M NaCl) reference electrode, and a conventional electrochemical cell obtained from CH Instruments was used for the experimental setup.
Synthesis of Polypropylene-g-Polyethylene Glycol
The amphiphilic polymer was synthesized by modifying a previously reported procedure (Kalaycı et al. 2010; Balcı et al. 2010; Kalaycı, Duygulu, and Hazer 2013). Typically, chlorinated polypropylene, 1.5 g (10 mmol Cl) was dissolved in 50 mL of freshly distilled tetrahydrofuran. The 20 mL of tetrahydrofuran solution containing polyethylene glycol 8000 (20 g, 10 mmol) and 2 mL of an aqueous NaOH solution (50 wt%) were added dropwise into the chlorinated polypropylene solution within 20 min. The reaction mixture was poured into 500 mL of water containing 1 mL of concentrated HCl after stirring 3 days at room temperature. The polymer was fil-tered, washed with water, and dried under vacuum overnight. For the purification, it was redissolved in tetrahydrofuran and re-precipitated in 200 mL of distilled water and then dried under vacuum overnight at 50C. Routine synthesis of the polymer was presented in Figure 1. Proton nuclear magnetic resonance, infrared, and gel per-meation chromatography characterizations of the final polymer were previously reported by our group (Kalaycı et al. 2010; Balcı et al. 2010; Kalaycı et al. 2013).
The electrode fabrication step was preceded by cleaning the electrode surface using gamma alumina powder and rinsing with distilled water. For the first time,
mediator-less fuel cell experiments were conducted. For this purpose, 2 mL of polypropylene-g-polyethylene glycol solution, prepared by dissolving the 10 mg of the solid polymer in 1 mL of chloroform, was directly placed on the surface of the cleaned glassy carbon anode and cathode electrode. The electrodes were then allowed to dry for solvent evaporation at room temperature. 30 mL of glucose oxidase (10 mg=mL) and 30 mL of bilirubin oxidase (10 mg=mL) were dropped onto the glassy carbon anode and cathode, respectively. The electrodes were allowed to dry and until enzyme immobilization at room temperature. Working electrodes including the mediator were fabricated by adding 1 mg of ferrocene into the 10 mg=mL of polypropylene-g-polyethylene glycol solution. Subsequent fabrication steps were the same with mediator-less working electrode fabrication as previously mentioned.
All electrochemical measurements were carried out in aerated 100 mM phosphate buffer solution, pH 7.4, with an applied potential of 200 mV for the anode including the ferrocene mediator, 340 mV for mediator-less anode, and þ360 mV for the mediated and unmediated cathode under continuous stirring at 100 rpm by using four-electrode cell. Various concentrations of glucose were added to this reaction medium to produce amperometric current-time curves.
RESULTS AND DISCUSSION
Electrochemical Characterization of Polypropylene-g-Polyethylene Glycol Electrodes
The electrochemical characteristic of the mediator-less polypropylene-g-polyethylene glycol=glucose oxidase and polypropylene-polypropylene-g-polyethylene glycol= bilirubin oxidase film coated electrodes were evaluated by cyclic voltammetry (Figure 2). The cyclic voltammograms were obtained in 10 mL of 100 mM pH 7.4 phosphate buffer at a potential scan between 500 and þ500 mV at a scan rate of 100 mV=s. The voltammograms revealed the absence of a voltammetric peak in the scanning potential range is obtained. The voltammogram shapes suggest that there was no hindrance effect for electron transfer process between the polymeric film and anodic and cathodic enzyme despite the fact that the polymeric film structure was nonconductive.
A cyclic voltammogram study was conducted for mediator-less (Figure 3A) and ferrocene including (Figure 3B) polypropylene-g-polyethylene glycol=bilirubin oxidase film coated cathode of fuel cell system in 100 mM pH 7.4 phosphate buffer with a potential range between 400 and þ600 mV at a scan rate of 100 mV=s. A well-defined cathodic peak observed at þ360 mV was attributed to the reduction of ferrocene present in the polypropylene-g-polyethylene glycol film. The presence of this peak confirms the incorporation of mediator ferrocene molecule into the poly-meric film structure that makes the fuel cell system mediator controlled. The formal reduction potential of ferrocene is betweenþ300 and þ400 mV which is similar with previously reports (Cox, Kittredge, and Ca 2004; Cox et al. 2011; Chen et al. 2013; Saito and Watanabe 1998). The electroactivity of the cathode was significantly increased by the incorporation of ferrocene into the polymeric film in comparison to mediator-less cathode.
The electroactive proportion of enzyme is essential for electrochemical signal devices rather than total amount of immobilized enzyme. Some of the immobilized enzyme will not provide electroactivity on the electrode surface as the polypropylene structure of the polymer is mainly nonconductive. The enzyme was physically adsorbed onto the long polyethylene glycol side-chain of the polymer. The CVs of the working electrodes were measured in 100 mM phosphate buffer (pH 7.4) at dif-ferent scan rates to investigate the surface concentration of electroactive anodic and cathodic enzymes. Figure 4 presents the CV of the polypropylene-g-polyethylene glycol=ferrocene=bilirubin oxidase electrode for various scan rates ranged between 10 and 600 mV=s at a scanned potential of400 and þ600 mV. The peak currents increased linearly with a scan rate from 10 to 600 mV=s for all working electrodes, Figure 2. Cylic voltammogram of (A) mediatorless polypropylene-g-polyethylene glycol=Glucose oxidase and (B) polypropylene-g-polyethylene glycol=bilirubin oxidase working electrodes at a scan rate of 100 mV=s.
as Ipc¼ 8.109 tþ 106(A, mV=s, r2¼ 0.992) for the polypropylene-g-polyethylene
glycol=ferrocene=bilirubin oxidase electrode and Ipa¼ 1.108 tþ 2.107 (A, mV=s,
r2¼ 0.998) for the polypropylene-g-polyethylene glycol=ferrocene=glucose oxidase electrode. From the integration of the peak currents and using Faraday’s law, the surface concentration of electroactive enzymes for both glucose oxidase and bilirubin oxidase (C) in the working electrode surfaces was estimated according to the
Figure 4. Cylic voltammograms of the polypropylene-g-polyethylene glycol=ferrocene=bilirubin oxidase electrode at different scan rates (from inner to outer): 10, 20, 50, 80, 100, 120, 150, 200, 300, 400, 500, and 600 mV=s.
Figure 3. Cylic voltammogram of (A) mediatorless polypropylene-g-polyethylene glycol=bilirubin oxidase and (B) polypropylene-g-polyethylene glycol=ferrocene=bilirubin oxidase cathodes at a scan rate of 100 mV=s.
following equation (Laviron 1979):
Ip¼
n2F2 ACn
4RT ð1Þ
where Ipis the peak current, A is the electrode surface area, v is the scan rate, n is the
number of electrons, R is the ideal constant, T is the absolute temperature, and F is the Faraday. By considering that at v¼ 300 mV=s, n ¼ 1, A ¼ 0.07068 cm2, F¼ 96,485 C=mol, R ¼ 8.314 J=K mol, and T ¼ 298 K, Ip¼ 3.2 106 A, and
3.4 106 A for polypropylene-g-polyethylene glycol=ferrocene=glucose oxidase
and polypropylene-g-polyethylene glycol=ferrocene=bilirubin oxidase electrode, respectively. The amounts of the electroactive GOX and BOD were found to be
0.28 and 0.19 mg, respectively.
Electrical Power Generation of the Fuel Cell
The basic reaction for a functioning enzymatic fuel cell is a complete circuit comprised of the anodic and cathodic enzyme reactions that release and trap elec-trons. The anodic enzyme glucose oxidase has been commonly used due to its high stability at physiological pH of 7–7.5 and high turnover rates. Bilirubin oxidase has emerged as the best cathodic enzyme primarily because of its high stability at physio-logical pH. Many researchers have chosen to work with another enzyme, laccase. However, laccase performs with outstanding efficiency only at an acidic pH of 5.2 and encounters nearly a 10-fold drop in its performance at physiological pH. The redox reaction of glucose oxidase and bilirubin oxidase is stated as follows (Flexer and Mano 2010): Anode : 2C6H12O6 ! glucose oxidase 2C6H10O6þ 4Hþþ 4e ð2Þ Cathode : O2þ 4Hþþ 4e! bilirubin oxidase 2H2O ð3Þ
These reactions result in the net bioelectrochemical power generation reaction for an enzymatic fuel cell:
2C6H12O6!2C6H10O6þ 2H2O ð4Þ
Figure 5 shows a schematic configuration of a biofuel cell that employs glucose oxidase and bilirubin oxidase as the anode and cathode, respectively. The anode and cathode were immersed in a working buffer solution which is suitable for conducting the enzymatic reactions. The electrochemical system was worked by applying a proper potential for the anode and cathode. When the biofuel cell system reaches steady-state current conditions, glucose as a fuel source is added to the working buffer. Glucose is then oxidized to gluconolactone by glucose oxidase at the anode side, and electrons are released (Eq. 2). The released electrons are transferred to the polymer from the enzyme and then the anode electrode material. The electrons flow through to the cathode side due to the driving force of the applied anodic potential. The flowing electrons are seized on the cathode side by the applied cathodic potential
at the same time, then on the polymer film. The electrons are received by ferrocene to form a ferrocenium=ferrocene redox couple (Fcþ=Fc) rapidly and the enzyme bilir-ubin oxidase. The cathodic enzyme bilirbilir-ubin oxidase reduces molecular oxygen to water due to these electrons (Eq. 3). The flowing electrons from anode to cathode are observed as an electrical current with an applied cell voltage (Ecell¼ Ecathode
Eanode). The electrons exchanged at each electrode make the cell active. Mediated
electron transfer usually refers to cases where a mediator is used to enhance electron transfer between the redox center of the enzyme and the electrode. The vast majority of enzymes are incapable of unmediated (direct) electron transfer. As a result, most biosensor and biofuel cell systems employ a mediator. In this study, ferrocene, which has high electroactivity, was selected as an anodic and cathodic mediator for the mediated polypropylene-g-polyethylene glycol based fuel cell.
In order to examine the current generation of mediated and unmediated polypropylene-g-polyethylene glycol based fuel cells, amperometric current-time experiments were carried out in 10 mL of 100 mM phosphate buffer at pH 7.4. A magnetic stirrer at 100 rpm provided the convective transport during the study. The current responses of the unmediated anode and cathode were measured at applied potentials of 340 mV and þ360 mV (with a cell potential of þ700 mV) and 200 mV and þ360 mV (with a cell potential of þ560 mV) for the mediated anode and cathode, respectively (vs. Ag=AgCl). The formal redox potential of glucose oxidase is approximately 320 mV in an unmediated system (Mano, Mao, and Heler 2003) and 200 mV in the mediated system (Atanassov et al. 2007). The formal redox potential of bilirubin oxidase is approximatelyþ360 mV for both mediated and unmediated systems (Mano et al. 2003). Figure 6 illustrates typical amperometric responses for the mediated polypropylene-g-polyethylene glycol=
ferrocene=glucose oxidase and unmediated polypropylene-g-polyethylene
glycol=glucose oxidase anode after the addition of successive aliquots of glucose at concentrations of 30, 50, 100, and 300 mM. Surprisingly, compared to the
polypropylene-g-polyethylene glycol=ferrocene=glucose oxidase anode, the polypropylene-g-polyethylene glycol=glucose oxidase anode exhibited higher current changes to the glucose additions. This means that ferrocene used in the anode could not promote electron transfer between the redox center of anodic enzyme glucose oxidase and electrode. In addition, the lower operational cell potential for mediated anode may result in lower anodic current. However, the cathode side of the mediated fuel cell system presented excellent cathodic current (Figure 7B) in comparison to the unmediated cathode (Figure 7A). Electrons flowing toward the cathode designate the electrical generation potential of a fuel cell. Therefore, the cathode performance is the identifier of the overall system performance. Ferrocene is effective for cathodic enzyme bilirubin oxidase to produce higher currents because of the fast electron transport toward to the cathode. In addition, unstable and noisy currents were observed in unmediated cathode to increasing concentrations of glucose additions.
The electrical power density, P, of a biofuel cell is defined as the product of the cell potential and the generated current density: P¼ I Vcell. The power density can
be calculated by normalizing the power with respect to the electrode cross-sectional area or the electrode volume (Osman, Shah, and Walsh 2011). Figure 8 shows the power generation of mediated fuel cell system for various concentrations of glucose with a cell potential ofþ560 mV (vs. Ag=AgCl). Generally, an enzymatic fuel cell is operated around at a cell potential of 1 V (Atanassov et al. 2007). When a mediator is used in such an electrochemical system, the cell potential decreases and hence reduced power generation can be observed. In this study, approximately 100-fold higher power generation was obtained from the ferrocene including fuel cell system Figure 6. Amperometric responses of polypropylene-g-polyethylene glycol=glucose oxidase (with a cell potential of þ700 mV) and polypropylene-g-polyethylene glycol=ferrocene=glucose oxidase (with a cell potential of þ560 mV) electrodes to increasing glucose concentrations ranging between 30 and 300 mM in the 100 mM pH 7.4 phosphate buffer vs. Ag=AgCl in 3 M NaCl.
in comparison with the unmediated one. While electrons flow to the cathode, ferro-cene catches them from the electrode material then a ferroferro-cene=ferrocenium redox couple is formed. The electrons are more efficiently transferred from this redox cou-ple to the anodic enzyme bilirubin oxidase. The obtained power density was 65 nW= cm2for the mediated fuel cell system. A number of membrane-less systems have been developed, exhibiting varying degrees of electrical performance. Membrane-less sys-tems employing mediators, and either glucose dehydrogenase=bilirubin oxidase (Gao et al. 2007), alcohol dehydrogenase=bilirubin oxidase (Topcagic and Minteer 2006), glucose dehydrogenase=laccase (Deng et al. 2008), or fructose dehydrogenase= Figure 7. Amperometric responses of (A) polypropylene-g-polyethylene glycol=bilirubin oxidase (with a cell potential of þ700 mV) and (B) polypropylene-g-polyethylene glycol=ferrocene=bilirubin oxidase (with a cell potential of þ560 mV) electrodes to glucose concentrations between 30 and 300 mM in 100 mM pH 7.4 phosphate buffer vs. Ag=AgCl in 3 M NaCl.
laccase (Kamitaka et al. 2007), can have power densities 1–2 orders of magnitude higher than those yielded by direct electron transferred system.
Anodic and cathodic enzymes were physically adsorbed onto the side-chain polyethylene group of the polypropylene-g-polyethylene glycol polymer at nanogram level. The obtained power density from the mediated fuel cell system was sufficient at this electroactive enzyme amount to develop and operate this type of membrane-less fuel cell system. The polypropylene-g-polyethylene glycol polymer is suitable as a fuel cell electrode material due to its structure which keeps enzymes and the mediator ferrocene inside the polymeric film.
CONCLUSION
In this study, a novel polypropylene-g-polyethylene glycol film based membrane-less biofuel cell using glucose as a fuel was prepared. Anodic and catho-dic enzymes were adsorbed on the polyethylene side-chain groups of the polymer. Ferrocene was successfully incorporated into the polymer and acted as a mediator to promote electron transfer for the fuel cell system. The electrochemical results demonstrated that the catalytic efficiency of the ferrocene-based cathode was approximately 100-fold higher than that of unmediated cathode. As a result, the mediated fuel cell electrodes yielded a power density of 65 nW=cm2at a cell potential ofþ560 mV employing minimal electroactive enzyme amounts.
FUNDING
This work was financially supported by; both ‘‘The Scientific & Technological Research Council of Turkey’’ (TUBITAK) (Grant no. 112Y100 and 211T016) and the Bulent Ecevit University Research Fund (Grant no. BEU-2012-10-03-13). Figure 8. Power density of the polypropylene-g-polyethylene glycol based fuel cell including ferrocene at a cell potential ofþ560 mV obtained from glucose concentrations between 30 and 300 mM in 100 mM pH 7.4 phosphate buffer vs. Ag=AgCl and 3 M NaCl.
REFERENCES
Ammam, M., and J. Fransaer. 2013. Combination of laccase and catalase in construction of H2O2–O2based biocathode for applications in glucose biofuel cells. Biosens. Bioelectron. 39:
274–281.
Ardhaoui, M., M. Zheng, J. Pulpytel, D. Dowling, C. Jolivalt, and F. A. Khonsari. 2013. Plasma functionalized carbon electrode for laccase-catalyzed oxygen reduction by direct electron transfer. Bioelectrochemistry 91: 52–61.
Atanassov, P., C. Apblett, S. Banta, S. Brozik, S. C. Barton, M. Cooney, B. N. Liaw, S. Mukerjee, and S. D. Minteer. 2007. Enzymatic biofuel cells. Interface-Electrochem. Soc. 16: 28–31. Balcı, M., A. Allı, B. Hazer, O. Gu¨ven, K. Cavicchi, and M. Cakmak. 2010. Synthesis and
characterization of novel comb-type amphiphilic graft copolymers containing polypropylene and polyethylene glycol. Polym. Bull. 64: 691–705.
Barriere, F., Y. Ferry, D. Rochefort, and D. Leech. 2004. Targetting redox polymers as mediators for laccase oxygen reduction in a membrane-less biofuel cell. Electrochem. Commun. 6: 237–241.
Brunel, L., J. Denele, K. Servat, K. B. Kokoh, C. Jolivalt, C. Innocent, M. Cretin, M. Rolland, and S. Tingry. 2007. Oxygen transport through laccase biocathodes for a membrane-less glucose=O2 biofuel cell. Electrochem. Commun. 9: 331–336.
Chen, Q., L. Zhang, S. Ebrahim, M. Soliman, C. Zhang, and Q. Qiao. 2013. Synthesis and structure study of copolymers from thiadiazole fused indolocarbazole and dithienosilole. Polymer 54: 223–229.
Cox, J. A., K. W. Kittredge, and D. V. Ca. 2004. Measurement platforms fabricated by layer-by-layer assembly of crown ether functionalized gold nanoclusters. J. Solid State Electrochem. 8: 722–726.
Cox, J. A., K. M. Wiaderek, B. Layla Mehdi, B. P. Gudorf, D. Ranganathan, S. Zamponi, and M. Berrettoni. 2011. Influence of silanization on voltammetry at electrodes modified with silica films of controlled porosity formed by electrochemically initiated sol-gel processing. J. Solid State Electrochem. 15: 2409–2417.
Deng, L., F. Wang, H. Chen, L. Shang, L. Wang, T. Wang, and S. Dong. 2008. A biofuel cell with enhanced performance by multilayer biocatalyst immobilized on highly ordered macroporous electrode. Biosens. Bioelectron. 24: 329–333.
Dursun, F., S. Korkut Ozoner, A. Demirci, M. Gorur, F. Yilmaz, and E. Erhan. 2012. Vinylferrocene copolymers based biosensors for phenol derivatives. J. Chem. Technol. Biotechnol. 87: 95–104.
Erog˘lu, M. S., B. Hazer, O. Gu¨ven, and B. M. Baysal. 1996. Preparation and thermal characterization of block copolymers by macroazonitriles having glycidyl azide and epichlorohydrin moities. J. Appl. Polm. Sci. 60: 2141–2147.
Flexer, V., and N. Mano. 2010. From dynamic measurements of photosynthesis in a living plant to sunlight transformation into electricity. Anal. Chem. 82: 1444–1449.
Gao, F., Y. Yan, L. Su, L. Wang, and L. Mao. 2007. An enzymatic glucose=O2 biofuel
cell: Preparation, characterization and performance in serum. Electrochem. Commun. 9: 989–996. Hazer, B. 1990. Cationic polymerization of tetrahydrofuran initiated by difunctional
initiators. Synthesis of block copolymers. Eur. Polm. J. 26: 1167–1170.
Hazer, B. 2010. Amphiphilic Poly (3-Hydroxy Alkanoate)s: Potential candidates for medical applications. Int. J. Polym. Sci., Article ID, 8 pages, 423460 doi: 10.1155=2010=423460 Hazer, B., I˙. C¸ akmak, S. Denizligil, and Y. Yag˘cı. 1992. Preparation of multiphase block
copolymers by redox polymerization. Angew. Macromol. Chem. 195: 121–127.
Kalaycı, O¨ ., F. Co¨mert, B. Hazer, T. Atalay, K. Cavicchi, and M. Cakmak. 2010. Synthesis, characterization, and antibacterial activity of metal nanoparticles embedded into amphiphi-lic comb-type graft copolymers. Polym. Bull. 65: 215–226.
Kalaycı, O¨ ., O¨. Duygulu, and B. Hazer. 2013. Optical characterization of CdS nanoparticles embedded into the comb-type amphiphilic graft copolymer. Nanopart. Res. 15: 1355–1366. Kamitaka, Y., S. Tsujimura, N. Setoyama, T. Kajino, and K. Kano. 2007. Fructose=dioxygen biofuel cell based on direct electron transfer-type bioelectrocatalysis. Phys. Chem. Chem. Phys. 9: 1793–1801.
Koike, Y., and M. Cakmak. 2009. Atomic force microscopy observations on the structure development during uniaxial stretching of pp from partially molten state: effect of isotacti-city. Macromolecules 37: 2171–2181.
Laviron, E. 1979. The use of linear potential sweep voltammetry and of a.c. voltammetry for the study of the surface electrochemical reaction of strongly adsorbed systems and of redox modified electrodes. J. Electroanal. Chem. 100: 263–270.
Lee, K. H., O. Ohsawa, K. Watanabe, I. S. Kim, S. R. Givens, B. Chase, and J. F. Rabolt. 2009. Electrospinning of syndiotactic polypropylene from a polymer solution at ambient temperatures. Macromolecules 42: 5215–5218.
Mano, N., F. Mao, and A. Heler. 2003. Characteristics of a miniature compartment-less glucose-O2biofuel cell and its operation in a living plant. J. Am. Chem. Soc. 125: 6588–6594.
Osman, M. H., A. A. Shah, and F. C. Walsh. 2010. Recent progress and continuing challenges in bio-fuel cells. Part II: Microbial. Biosensor. Bioelectron. 26: 953–963.
Osman, M. H., A. A. Shah, and F. C. Walsh. 2011. Recent progress and continuing challenges in bio-fuel cells. Part I: Enzymatic cells. Biosens. Bioelectron. 26: 3087–3102.
Ozturk, T., M. N. Atalar, M. Goktas, and B. Hazer. 2013. One-step synthesis of block-graft copolymers via simultaneous reversible-addition fragmentation chain transfer and ring-opening polymerization using a novel macroinitiator. J. Polym. Sci.: Part A 51: 2651–2659.
Palomera, N., L. Jose´, C. Vera, E. Mele´ndez, E. Jaime, Ramirez-Vick, M. S. Tomar, S. K. Arya, and P. S. Surinder. 2011. Redox active poly(pyrrole-N-ferrocene-pyrrole) copolymer based mediator-less biosensors. J. Electroanal. Chem. 658: 33–37.
Pas, L. 2007. Alternative energy: Enzyme-based biofuel cells. Basic Biotechnol. eJournal 3: 93–97. Saito, T., and M. Watanabe. 1998. Characterization of poly(vinylferrocene-co-2-hydroxyethyl methacrylate) for use as electron mediator in enzymatic glucose sensor. Reactive Function. Polym. 37: 263–269.
Topcagic, S., and S. D. Minteer. 2006. Development of a membraneless ethanol=oxygen biofuel cell. Electrochim. Acta 51: 2168–2172.
Yıldız, U., B. Hazer, and K. Tauer. 2012. Tailoring polymer architectures with macromonomer azoinitiators. Polym. Chem. 3: 1107–1118. doi: 10.1039=C2PY00513A.
