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

2.1 Chemicals and reagents

All chemicals used were of analytical grade, obtained from various sources and used as received without any further purification. 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 Millipore, 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 cm² 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⁻¹ 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⁻¹ was used (Ahmad and Ab Ghani, 2005). For oPD 25 ml solution containing 50 mM monomer, 1 M H₃PO₄ and 0.5 M CaCl₂ at a scan rate of 100 mV s⁻¹ 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 tetraborate at a scan rate of 50 mV s⁻¹ 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 different concentrations at modified carbon electrodes. Measurements were carried out in an aqueous phosphate buffer solution which was prepared from 0.044 M KH₂PO₄, 0.044 M NaOH and 0.15 M NaCl. All measurements were performed at 25 ± 5 °C.

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 (Rincón et al., 2010). In Fig. 1a the CV of PANI shows that the anodic peak potential (E_pa) appears 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, at −0.34 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 (ΔE_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).

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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⁻¹ and scanning up to 10 cycles.

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3.2 Effect of scan rate

CV at scan rates of 50–250 mV s⁻¹ 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 ΔE_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 electrolyte. 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).

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Figure 2 The effect of monomer concentration on the oxidation current density of the electropolymerization of polymers (a) PMG, (b) PANI and (c) PoPD.

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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 mediator 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):

In Fig. 3a two redox couples with E_pc at −0.11 and −0.29 V and E_pa at −0.05 and −0.23 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⁺.

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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⁻¹.

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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.

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 −283 and −240 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 −1 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 −104 mV. This result suggests that the oxidation of NADH is catalyzed by PPy on the surface of the electrode.