ORIGINAL ARTICLEConductive polymers: Their preparations and catalyses on NADH oxidation at carbon cloth electrodes a Pusat Pengajian Sains Kimia, Universiti Sains Malaysia, 11800 USM P..
Trang 1ORIGINAL ARTICLE
Conductive polymers: Their preparations
and catalyses on NADH oxidation
at carbon cloth electrodes
a
Pusat Pengajian Sains Kimia, Universiti Sains Malaysia, 11800 USM P Pinang, Malaysia
bPusat 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 voltammo-gram showed that all the electrode processes involved in the preparation of the polymers on the car-bon 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 ano-dic peak current together with a decrease in the cathoano-dic peak current
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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
oxida-tion 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
en-zymes 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 conduc-tive 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 prepa-ration 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
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http://dx.doi.org/10.1016/j.arabjc.2013.05.021
Trang 2surements, 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
synthe-sis, 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
oxida-tion (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
immobi-lized 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
elec-trode All potentials are against Ag/AgCl (3 M KCl) The
working electrode was 1 cm2of 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 s1were used (Parsa
and Ab Ghani, 2009) For 4-vinyl pyridine (4VP) 25 mL
solu-tion of 3 mM 4VP and 0.1 M tetrabutyl ammonium
perchlo-rate in acetonitrile, at pH 3.0 and a scan perchlo-rate of 50 mV s1
was used (Ahmad and Ab Ghani, 2005) For oPD 25 ml
solu-tion containing 50 mM monomer, 1 M H3PO4 and 0.5 M CaCl2 at a scan rate of 100 mV s1was 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 s1 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 KH2PO4, 0.044 M NaOH and 0.15 M NaCl All measurements were per-formed at 25 ± 5C
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 s1 and scanning up to
10 cycles
Trang 33 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) InFig 1a
the CV of PANI shows that the anodic peak potential (Epa)
ap-pears to have shifted toward more positive values indicating
the rapid depletion of the monomer in the vicinity of the
elec-trode by changing to radical cation (Parsa and Ab Ghani,
2008) While the CV of PoPD (result not shown) indicates that
cathodic peak potentials Epc, and, Epa, 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 (Ipa), and cathodic peak current (Ipc) have
in-creased 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
(DEp) 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, Ipa, and Ipcdecreased 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
sig-nificant 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 s1 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 electropolymeriza-tions are diffusion-controlled Additionally, as the scan rates increase the Epashifts toward positive and the Epcmoves to-ward negative, making the DEp 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 Ipaand Ipcare increased, except
in the case of PPy where with increasing of scan rates Ipais in-creased while Ipcis 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 den-sity obtained As the monomer concentration increases Ipa in-creases (Fig 2) However, when the monomer concentration becomes too high exceeding 0.8 mM (MG) 50 mM (ANI), and 50 mM (oPD), Ipastarts 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 t t 1/2 Anodic peak
current Ipa (mA)
Cathodic peak current Ipc (mA)
Linear regression
of Ipa vs t 1/2
Linear regression
of Ipc vs t 1/2
Epa (mV) Epc (mV) DEp
Trang 4lyte 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
electro-chemical response depends on the subsequent reoxidation of
MG (Dai et al., 2008):
NADHþ PMGðOXÞ! NADþþ PMGðredÞþ Hþþ 2e ð1Þ
InFig 3a two redox couples with Epcat0.11 and 0.29 V
and Epa 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 be-come one redox couple (CV 2) A decrease in Ipcwas observed
as the concentration of NADH increases (CV 3) This can be inferred to the mediated oxidation of NADH to NAD+
InFig 3b on the addition of NADH, Ipaincreases in height while Ipc 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
where PANIox and PANIred are the oxidized and reduced forms of polyaniline, respectively (Gerard et al., 1999) The CV of PoPD-modified electrode (Fig 3c) in the ab-sence of NADH (CV1) shows a stable redox couple with Epc and Epa at 283 and 240 mV, respectively With NADH (CV 2 and 3) an enhancement in Ipais observed which is asso-ciated with a decrease in Ipc 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 electrocata-lytic behavior results from a chemical interaction between ac-tive 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 pres-ence 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 of1
to 0.5 V As shown in CV 1, a broad peak was observed in sys-tem without NADH While in CV 2 and after the addition of 0.5 mM NADH, Ipais observed at104 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 diffu-sion-controlled High current densities were obtained at vari-ous 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
2 )
The concentration of monomer (mM)
0
1
2
3
4
5
6
20 30 40 50 60
2 )
The concentration of monomer (mM)
0
1
2
3
4
5
6
30 40 50 60 70
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
Trang 5Figure 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 s1
Trang 6The 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
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