Conductive polymers their preparations and catalyses on NADH oxidation at carbon cloth electrodes

ORIGINAL ARTICLE Conductive polymers: Their preparations and catalyses on NADH oxidation at carbon cloth electrodes Zahraa A. Jarjes a , Mohammed Razip Samian b , Sulaiman Ab Ghani a, * a Pusat Pengajian Sains Kimia, Universiti Sains Malaysia, 11800 USM P. Pinang, Malaysia b Pusat Pengajian Sains Kajihayat, Universiti Sains Malaysia, 11800 USM P. Pinang, Malaysia Received 14 April 2012; accepted 23 May 2013 Available online 4 June 2013 KEYWORDS Catalyzes; Cyclic voltammogram; Conductive polymers; NADH oxidation Abstract This study has made comparison to five such polymers viz. poly (methylene green), poly aniline (PANI), poly (ortho-phenylene diamine) (PoPD), poly (4-vinylpyridine) and poly pyrrole in terms of their preparations and capacities to improve the oxidation of NADH. Cyclic voltammogram showed that all the electrode processes involved in the preparation of the polymers on the carbon cloth electrode are diffusion-controlled. The oxidation of NADH was enhanced when mediated by PANI and PoPD. A fast heterogeneous electron transfer was shown through the increase in anodic peak current together with a decrease in the cathodic peak current. ª 2013 Production and hosting by Elsevier B.V. on behalf of King Saud University. 1. Introduction The redox processes of the coenzyme b-nicotinamine adenine dinucleotide (NAD) have been the subject of many studies. Its involvement in various dehydrogenase catalysis reactions in both bioprocesses and analytical applications indicates its significance (Pandey et al., 1998). The electrochemical oxidation of NADH to enzymatically active NAD + would suggest for the development of a wide range of amperometric enzyme sensors and also for the use of dehydrogenase dependent enzymes in biofuel cells (Palmore et al., 1998). However, with the large overvoltage encountered for NADH oxidation at or- dinary electrodes and also surface fouling associated with the accumulation of reaction products (Bartlett and Simon, 2000) this looks like a futile effort after all. Nonetheless, mod- ification of electrode surface with new materials like conductive polymer films (A ´ lvarez-Gonza ´ lez et al., 2000) has shown positive result, in that the polymer displays electrocatalytic activity toward NADH oxidation. Among the conductive polymer, PMG happens to be the mostly used and studied (Yang et al., 1998). PANI is another conductive polymer which is also exten- sively used (Jin et al., 2001). It has a unique conduction ability and high environmental stability which makes it very useful especially in biosensor fabrications (Morrin et al., 2005). An- other conductive polymer of interest is poly ortho-phenylene diamine (PoPD). It has been reported to be used in the preparation of photovoltaic cells, anticorrosion coatings, pH mea- * Corresponding author. Tel.: +60 4 6534030; fax: +60 4 6574854. E-mail address: sag@usm.my (S. Ab Ghani). Peer review under responsibility of King Saud University. Production and hosting by Elsevier Arabian Journal of Chemistry (2015) 8, 726–731 King Saud University Arabian Journal of Chemistry www.ksu.edu.sa www.sciencedirect.com 1878-5352 ª 2013 Production and hosting by Elsevier B.V. on behalf of King Saud University. http://dx.doi.org/10.1016/j.arabjc.2013.05.021 surements, fuel cell and biosensors (Losito et al., 2003). The poly-4-vinyl pyridine (P4VP), on the other hand, has unique balance properties like acidity–basicity, and hydrophilic– hydrophobic (Sahiner, 2009) which has been capitalized mostly in the preparation of biosensors (Wang et al., 2008), ion-ex- change resin, and microfiltration membranes (Lewis et al., 2007). One last conductive polymer that is mostly studied is poly pyrrole (PPy). It is environmentally stable, ease of synthesis, relatively high conductivity as compared to other electronic polymers (Sadki et al., 2000) and is usually, compatible to many enzymes in biosensor fabrications (Ramanavicius et al., 2006). Several reports have been published on the electrocatalytic activity of each of these polymers against the NADH oxidation (Dai et al., 2008; Simon et al., 2002; Lobo et al., 1996; Iyer et al., 2003; Pal et al., 1994), but there has been little effort made to discover the most brilliant polymer in this context. This study aims to fill this gap in research by making a com- parative investigation into the electrochemical synthesis and properties of PMG, PANI, PoPD, P4VP and PPy and their influences on the electrochemical oxidation of NADH. Carbon cloth electrodes modified by glycerol dehydrogenase immobilized on either of these polymers were used to carry out the electrochemical oxidation of glycerol released from the hydro- lysis of refined palm oil in the presence of NAD + . The results could provide the basis for the future development of a novel glycerol bioanode for use in the biofuel cell (Jarjes et al., 2011). 2. Experimental 2.1. Chemicals and reagents All chemicals used were of analytical grade, obtained from various sources and used as received without any further puri- fication. NADH disodium salt hydrate (98%) was obtained from Sigma Aldrich, USA. All solutions were freshly prepared prior to each experiment. Aqueous solutions were prepared using water (conductivity 18.2 Mcm) from Milli-Q plus of Mil- lipore, USA. 2.2. Apparatus Cyclic voltammetric experiments were carried out with BAS Epsilon 2 workstation of Bioanalytical System, USA. A three electrode system was employed together with a platinum wire as counter electrode and Ag/AgCl (3 M KCl) as reference electrode. All potentials are against Ag/AgCl (3 M KCl). The working electrode was 1 cm 2 of carbon cloth B-1 of Clean Fuel Cell Energy LLC, USA. 2.3. Preparation of the modified electrodes The electropolymerizations of monomers were carried out as in the following, for aniline and pyrrole (py) 25 mL solutions containing 50 mM monomer in 0.2 M p-toluene sulfonic acid and 0.5 M KCl at a scan rate of 50 mV s À1 were used (Parsa and Ab Ghani, 2009). For 4-vinyl pyridine (4VP) 25 mL solution of 3 mM 4VP and 0.1 M tetrabutyl ammonium perchlorate in acetonitrile, at pH 3.0 and a scan rate of 50 mV s À1 was used (Ahmad and Ab Ghani, 2005). For oPD 25 ml solution containing 50 mM monomer, 1 M H 3 PO 4 and 0.5 M CaCl 2 at a scan rate of 100 mV s À1 was used (Parsa and Ab Ghani, 2008). For methylene green 50 mL solution containing 0.4 mM MG and 0.1 M sodium nitrate in 10 mM sodium tet- raborate at a scan rate of 50 mV s À1 was used (Akers et al., 2005). The electrodes were rinsed and then allowed to dry overnight prior to the investigation. 2.4. Cyclic voltammetry procedure Cyclic voltammograms (CV) of NADH were recorded at dif- ferent concentrations at modified carbon electrodes. Measure- ments were carried out in an aqueous phosphate buffer solution which was prepared from 0.044 M KH 2 PO 4 , 0.044 M NaOH and 0.15 M NaCl. All measurements were per- formed at 25 ± 5 °C. Figure 1 Cyclic voltammogram of (a) 50 mM ANI in 0.2 M p- toluene sulfonic acid and 0.5 M KCl, (b) 3 mM 4VP in 0.1 M tetrabutyl ammonium perchlorate in acetonitrile, pH 3.0. All experiment are at a scan rate of 50 mV s À1 and scanning up to 10 cycles. Conductive polymers: Their preparations and catalyses on NADH oxidation at carbon cloth electrodes 727 3. Results and discussion 3.1. Electropolymerization of the monomers In the CV of PMG (result not shown) a significant increase in current density from the 1st to the 10th cycle is observed, which indicates the progress of polymerization as well as the increase of accessible surface area of deposition of a PMG film on the surface of the electrode (Rinco ´ n et al., 2010). In Fig. 1a the CV of PANI shows that the anodic peak potential (E pa ) ap- pears to have shifted toward more positive values indicating the rapid depletion of the monomer in the vicinity of the electrode by changing to radical cation (Parsa and Ab Ghani, 2008). While the CV of PoPD (result not shown) indicates that cathodic peak potentials E pc , and, E pa ,atÀ0.34 and 0.08 V, respectively are its redox couples (Jang et al., 1995). Another cathodic peak was observed at 0.15 V which has decreased proportionally with the number of scans due to the growth of polymer film on the electrode surface. However, the anodic peak current (I pa ), and cathodic peak current (I pc ) have increased indicating the growth of polymer deposit layer on the electrode surface. The CV of 4VP (Fig. 1b) shows a typical redox process of quasi-reversible type as its peak separation (DE p ) value is 200 mV. The formal standard potential (E ° ) for this redox couple is 345 mV. Along with the increase in the number of cycles, I pa , and I pc decreased significantly. This is due to an inherent increase in the thickness of the surface of electrode by newly formed polymer film, hence, the electrical double layer. The CV of PPy (result not shown) shows the significant increase in oxidation currents along with the decrease in reduction currents during the successive cycles indicating that electropolymerization is via the oxidation of Py anodic polymerization (Uang and Chou, 2003). 3.2. Effect of scan rate CV at scan rates of 50–250 mV s À1 are recorded for PMG, PANI, PoPD, P4VP and PPy. Both anodic and cathodic peak currents depend linearly on the square root of the scan rate over the whole scan rate range examined (Table 1). The slope values approaching unity imply that the electropolymerizations are diffusion-controlled. Additionally, as the scan rates increase the E pa shifts toward positive and the E pc moves toward negative, making the DE p values become larger which indicates that the electropolymerization is of quasi-reversible and dependant on charge transfer (Kumar and Chen, 2007). With increasing of scan rates I pa and I pc are increased, except in the case of PPy where with increasing of scan rates I pa is increased while I pc is decreased. This means that the oxidation process of the electropolymerization occurs at higher charge electron transfer rates. 3.3. Effect of monomer concentration The monomer concentration in the electropolymerization determines the amount and thickness of the polymers on the surface of the electrode. This will then affect the current density obtained. As the monomer concentration increases I pa increases (Fig. 2). However, when the monomer concentration becomes too high exceeding 0.8 mM (MG) 50 mM (ANI), and 50 mM (oPD), I pa starts to decrease which may be due to the difficulty to dissolve all of the monomers in the electro- Table 1 The relationships between cathodic and anodic peak currents and the square root of scan rates. Polymer tt 1/2 Anodic peak current I pa (mA) Cathodic peak current I pc (mA) Linear regression of I pa vs t 1/2 Linear regression of I pc vs t 1/2 E pa (mV) E pc (mV) DE p PMG 50 7.01 0.88 À0.8 0.967 0.987 À38 À161 123 100 10 1.3 –1.6 –22 –206 184 150 12.2 2 –2.06 4 À239 243 200 14.1 2.4 –2.3 28 –254 282 250 15.8 2.5 –2.8 49 À271 320 PANI 50 7.01 5.5 À3.3 0.919 0.958 527 341 186 100 10 12 –14.1 573 315 258 150 12.2 17.8 –17 608 286 322 200 14.1 25 À20 626 265 361 250 15.81 39 –28 637 241 396 PoPD 50 7.01 16 À12.7 0.980 0.951 107 À261 368 100 10 38 –45 153 –331 484 150 12.2 61 À66 215 À372 587 200 14.1 70 –76 320 –375 695 250 15.81 78 À80 413 À413 826 P4VP 50 7.01 1.21 À0.69 0.998 0.996 430 302 128 100 10 1.7 –1.35 462 224 238 150 12.2 2.15 –1.82 494 175 319 200 14.1 2.5 –2.1 520 144 376 250 15.81 2.78 –2.48 525 132 393 PPy 50 7.01 1.5 À1.35 0.955 0.973 À509 À509 0 100 10 1.8 –1.05 –501 –523 22 150 12.2 2.3 –0.6 À490 À532 42 200 14.1 2.5 –0.4 –482 –544 62 250 15.81 3.1 –0.3 À480 À550 70 728 Z.A. Jarjes et al. lyte. Also, a high monomer concentration may lead to a high rate of initiation relative to that of propagation resulting in a polymer with higher solubility in the electrolyte (Ling et al., 2000). In case of P4VP and PPy the best CV is obtained in the concentrations of 3 and 50 mM, respectively (results not shown). 3.4. Electrocatalysis of NADH oxidation This investigation is to reveal the role of these polymers as an electrocatalyst for NADH. The PMG is a two-electron media- tor which reacts with NADH, followed by the regeneration of reduced PMG and biologically active NAD + . The electrochemical response depends on the subsequent reoxidation of MG (Dai et al., 2008): NADH þ PMG ðOXÞ ! NAD þ þ PMG ðredÞ þ H þ þ 2e À ð1Þ In Fig. 3a two redox couples with E pc at À0.11 and À0.29 V and E pa at À0.05 and À0.23 V (CV 1) were obtained from PMG-modified electrode without the addition of NADH. However, upon addition of NADH the two redox couples become one redox couple (CV 2). A decrease in I pc was observed as the concentration of NADH increases (CV 3). This can be inferred to the mediated oxidation of NADH to NAD + . In Fig. 3b on the addition of NADH, I pa increases in height while I pc is significantly reduced. Shifting of 50 mV toward more positive potential values (CV) is also observed. This behavior indicates the electrocatalytic oxidation of NADH by the PANI film (Bartlett et al., 1997). These results can be understood by noting that NADH is oxidized to NAD + at the working electrode releasing two electrons and PANI being electron acceptor gets reduced (intermediate state) and finally becoming oxidized (stable state) as indicated below. NADH ! NAD þ þ H þ þ 2e À ð2Þ PANI OX þ 2e À ! PANI red ð3Þ PANI red ! PANI OX þ 2e À ð4Þ where PANI ox and PANI red are the oxidized and reduced forms of polyaniline, respectively (Gerard et al., 1999). The CV of PoPD-modified electrode (Fig. 3c) in the absence of NADH (CV1) shows a stable redox couple with E pc and E pa at À283 and À240 mV, respectively. With NADH (CV 2 and 3) an enhancement in I pa is observed which is associated with a decrease in I pc indicating a favorable charge transfer on the oxidation of NADH (Golabi and Nozad, 2002). This sacrifices the reversibility of the electrode process. The voltammetric observations suggest that the electrocatalytic behavior results from a chemical interaction between active sites of the polymeric film and adsorbed molecules of NADH (Lobo et al., 1996). Fig. 3d shows the apparent increase in anodic peak current along with diminishment of cathodic peak current, whereas shifting toward a positive direction for anodic peak in the presence of NADH (CV 2) indicates a quasi-reversible electrode process when compared to CV in the absence of NADH (CV 1) which clearly shows that the P4VP film is active for NADH oxidation. Fig. 3e shows the electrochemical behaviors of PPy- modified electrode toward NADH in potential window of À1 to 0.5 V. As shown in CV 1, a broad peak was observed in system without NADH .While in CV 2 and after the addition of 0.5 mM NADH, I pa is observed at À104 mV. This result sug- gests that the oxidation of NADH is catalyzed by PPy on the surface of the electrode. 4. Conclusions Electropolymerizations of five monomers i.e. MG, ANI, oPD, P4VP and Py at carbon cloth electrodes and its catalytic effect on NADH were carried out by cyclic voltammetry. The CV of polymerization showed that the electrode processes are diffusion-controlled. High current densities were obtained at various monomer concentrations depending on the type of monomer used. All of these polymers have exhibited to some degree of electrocatalytic activity toward the oxidation of NADH. But only PANI and PoPD have displayed relatively superior performances. The electrodes modified with PANI and PoPD are being studied for future application in bioanode in fuel cell fabrications. 0 0.5 1 1.5 2 2.5 3 3.5 0.4 0.6 0.8 1 1.2 The oxidation current density (mA/cm 2 ) The concentration of monomer (mM) 0 1 2 3 4 5 6 20 30 40 50 60 The concentration current density (mA/cm 2 ) The concentration of monomer (mM) 0 1 2 3 4 5 6 30 40 50 60 70 The oxidation of current density (mA/cm 2 ) The concentration of monomer (mM) (a) (b) (c) Figure 2 The effect of monomer concentration on the oxidation current density of the electropolymerization of polymers (a) PMG, (b) PANI and (c) PoPD. Conductive polymers: Their preparations and catalyses on NADH oxidation at carbon cloth electrodes 729 Figure 3 Cyclic voltammograms for the (a) PMG, (b) PANI, (c) PoPD, (d) P4VP and (e) PPy recorded in phosphate buffer solutions (pH7) in the absence (1) and presence of: 0.1 mM CV (2) and 0.5 mM CV (3) (except for the P4VP and PPy the CV 2) represent the presence of 0.5 mM of NADH. Scan rate: 50 mV s À1 . 730 Z.A. Jarjes et al. Acknowledgements The authors are indebted to the Ministry of Higher Education, Malaysia and the University for (i) Research University Grant # 1001/PKIMIA/811044 and (ii) Postgraduate Incentive Grant # 1001/PKIMIA/842038. One of us (Z.A. Jarjes) is thankful to the Universiti Sains Malaysia for the Fellowship awarded. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.arabjc. 2013.05.021. References Ahmad, F., Ab Ghani, S., 2005. 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Conductive polymers: Their preparations and catalyses on NADH oxidation at carbon cloth electrodes 729 Figure 3 Cyclic voltammograms for the (a) PMG, (b) PANI, (c) PoPD, (d) P4VP and (e)

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