J IRAN CHEM SOC DOI 10.1007/s13738-014-0552-1

ORIGINAL PAPER

Evaluation of antioxidant activity of homo and copolymer of aniline and p‑phenylenediamine electrosynthesized in the presence of calcium chloride Ali Parsa · Maryam Sadeghi · Zahra Parsa · Azadeh Shakeri · Maryam Tehrani · Sulaiman Ab Ghani 

Received: 11 April 2014 / Accepted: 23 October 2014 © Iranian Chemical Society 2014

Abstract  Homo and copolymer of aniline (Ani) and p-phenylenediamine (pPDA) are electrodeposited on the composite graphite (CG) in phosphoric acid (H3PO4) solution which contains calcium chloride (CaCl2) as the supporting electrolyte and different ratios of two monomers. The antioxidant activity is evaluated through the reaction of copolymers with 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical in methanol. The FTIR and UV– visible spectra indicate that the increase of the molar ratio of pPDA in polymer chain has also increased the antioxidant activity of the copolymers. The electrochemical impedance spectroscopy (EIS) is used for further investigation. It was revealed that the increase of the molar ratio of pPDA in polymeric chain decreased the charge transfer resistance (Rct) in the electrochemical content and improved the free radical scavenging activity of copolymers. Keywords  Electrodeposition · Conducting polymer · Antioxidant · DPPH · Free radical · Impedance spectroscopy

A. Parsa (*) · M. Sadeghi · A. Shakeri · M. Tehrani  Department of Chemistry, College of Science, Yadegar -e- Imam Khomeini (RAH) Shahre-rey Branch, Islamic Azad University, Tehran, Iran e-mail: [email protected]; [email protected] Z. Parsa  Iran University of Medical Sciences (IUMS), Tehran, Iran S. Ab Ghani  Pusat Pengajian Sains Kimia, Universiti Sains Malaysia, 11800 USM, Pulau Pinang, Malaysia

Introduction Applications of conducting polymers in electrocatalysis, capacitors, sensors, fuel cells, corrosion protection and biomedical purposes have been studied during the recent years [1–4]. Polyaniline (PAni) and its derivatives have become a great attraction as a conducting polymer in the literature due to their peculiar electrical, optical and antioxidant properties [5]. The aromatic diamines have been employed in the synthesis of new polymers to acquire materials which can exhibit better electrical and antioxidant properties than PAni [6, 7]. It has also been reported [8, 9] that conducting polymers have similar antioxidant properties to polyphenols. Polyphenols are one of the most important compounds in the food diet [10, 11] because of their natural antioxidant ability to scavenge free radicals, thus helping to preserve human health, particularly in relation to heart disease and cancer [12, 13]. Since these compounds contain the hydroxyl groups, they have a strong tendency to react with free radicals, hence acting as an antioxidant [14]. The antioxidant activity of a compound is generally examined by DPPH in methanol [15–17]. In this work, the electrodeposition of homo and copolymer of Ani and pPDA under the novel synthesis condition in phosphoric acid medium and calcium chloride as the supporting electrolyte is innovative. Using this supporting electrolyte facilitates the electrocopolymerization of Ani and pPDA by decreasing the difference in redox peak potentials (Ep) between two monomers [18]. The antioxidant activity of those homo and copolymers is measured via UV–visible absorption with free radicals of DPPH. The effect of the molar ratio of pPDA in the antioxidant capability as well as applying the EIS to emphasize this, in the relation to the previous reports, is unique and considerable

13



[19–21]. In this research, the EIS study is used to investigate the mechanism process of reactions and to determine the impedance elements, e.g., Rct and double-layer capacitance (Cdl) by indicating these values on equivalent circuits and on Nyquist plots, which is done to identify the effective parameters in electrosynthesis, such as the effect of molar ratio of monomers in the structure of copolymer or on the antioxidant activity of them.

J IRAN CHEM SOC

The reactants were continuously stirred for 30 min prior to measurement by the UV–visible spectroscopy. The EIS measurements were obtained by placing the polymer-modified substrates in 1 M H3PO4 containing K3 [Fe(CN)6] and K4[Fe(CN)6] at applied potential of 0.25 V, amplitude 10 mV and in the frequency range of 100 kHz–10 mHz.

Results and discussion Experimental Electrochemical synthesis Materials Aniline of Sigma Chemicals USA was distilled under nitrogenous atmosphere at reduced pressure. The resulting colorless liquid was kept in the dark at 5 °C. p-Phenylenediamine and CaCl2 of Fluka Chemicals Switzerland, H3PO4 and DPPH of Sigma Chemicals USA, and methanol of BDH Chemicals UK, were all of analytical grade and used as received. Potassium ferricyanide (K3 [Fe(CN)6]) of China and potassium ferrocyanide (K4[Fe(CN)6]) of Shanghai Heng Da Chemical China were purchased from Shanghai#1 reagent factory. All aqueous solutions were freshly prepared using pure water (18.2 MΩ cm) of Milli-Q plus USA. Oxygen-free nitrogen (OFN) of Nissan-IOI Malaysia was used as received. Equipment The electrochemical system which consists of compact stat (potentiostat) equipped with the IviumSoft software of Ivium Technologies, Netherlands, was used for the synthesis, characterization and EIS measurements. A custom-made three-neck electrochemical cell was employed throughout. Two 2B pencils CG (1.8 mm o.d.) of Staedtler Lumograph Germany were used as both the (1) working electrode and (2) counter electrode. All measurements were obtained against a pseudo Ag/AgCl reference electrode. The FTIR spectrophotometer System 2000 of Perkin Elmer USA and the UV–visible spectrophotometer V-500 of JASCO Japan were used for polymer characterization and antioxidant activity measurement, respectively. Procedure The electrosynthesis of the polymers was obtained by sequentially sweeping the potential between −0.2 and +1.0 V vs. Ag/AgCl under OFN atmosphere at 25 ± 2 C of a 10-mL mixture of monomers 50 mM Ani and pPDA, solvent 1 M H3PO4 and supporting electrolyte 0.5 M CaCl2. Antioxidant activities were studied by reacting 0.1 mg of polymers with 2 mL 1.0 × 10−4 M DPPH in MeOH.

13

The cyclic voltammograms (CVs) of homo and copolymer of Ani and pPDA in H3PO4 containing CaCl2 on CG are shown in Fig. 1. Figure  1a shows the CVs of PAni. The anodic peak at the first scan in the potential of 0.82 V is due to the formation of radical cation of Ani. In the reverse scan, the cathodic peak is not observed, which indicates that the cation radical is going to content of chemical reaction; hence the oxidation reaction is irreversible. The anodic current is decreased at the subsequent scans, which is related to PAni absorption on the electrode surface and this indicates that PAni-modified electrode is less conductive than CG [22]. The irreversible process continues until the twentieth scan when the oxidative and reductive currents start to appear [23]. Three redox couples are observed at the potential range of −0.2 to +1.0 V. The first redox couple is related to the interconversion of formation of leucoemeraldine base (LE) to emeraldine base (EB) at two potentials of −0.05 and 0.2 V (Fig. 2a) [24]. The second is the oxidation of EB to pernigeraniline (PN) and vice versa at the potential range of 0.61 and 0.75 V (Fig. 2b) [25]. The conducting form of emeraldine salt (ES) can also be formed through the oxidation of LE (Fig. 2c) and protonation of EB (Fig. 2d) [26]. The third is the interconversion of benzenoid (B) and quinoid (Q) units in the PAni polymeric chain at 0.38 and 0.46 V. The CV of Poly (Ani-co-pPDA) in 1 M H3PO4 and 0.5 M CaCl2 at CG electrode is shown in Fig. 1b–e. In the forward scan, the Epa appears at 0.65 V, and it is related to the oxidation of monomers while in the reverse scan, the Epc almost appears at 0.45 V, because of the formation of dimer between pPDA radical cation (pPDA•+), and diimine ion [27]. The disappearance of three redox pairs of PAni is noticeable when the molar ratio of pPDA is increased. Figure 1f shows the electrosynthesis of PpPDA in 1 M H3PO4 and 0.5 M CaCl2. The anodic peak current (Ipa) at Epa 0.65 V decreases with successive scans indicating the dissolution of the PpPDA film on the electrode surface. The pPDA•+ is unstable and might

J IRAN CHEM SOC

Fig. 1  The CVs of 50 mM Ani and 50 mM pPDA with molar ratio of a 1.0:0.0 b 0.9:0.1 c 0.7:0.3 d 0.3:0.7 e 0.1:0.9 f 0.0:1.0 in 1 M H3PO4 and 0.5 M CaCl2. The applied potential, Eapp, −0.2 and +1.0 V (vs. Ag/AgCl), scan rate of 100 mV/s and 20 cycles Pernigeraniline N

N

-2e-

(a) H N

H N

H N

H N

-2en

N

+2e-

N

+2e+

N

Leucoemeraldine Base

Emeraldine Base

N

n

(b) H N

H N

n

+

-2e -

-2H

(c) +2e -

+

+2H

H N

H N

H N

Emeraldine Salt

H N

(d) n

Fig. 2  The chemical structures of oxidation states of PAni and their transitions: a interconversion of LE to EB b interconversion of EB to PN c oxidation of LE to ES and d protonation of EB to ES (non-redox doping process)

destabilize the propagation reaction during the polymeric chain reaction. However, the reaction between pPDA•+ and the monomer of pPDA may form two amino groups in its molecular structure, since it can be oxidized at a positive potential to produce radical cation of pPDA and repeated reaction with pPDA monomers to constitute the PpPDA film. Many studies have been done on the electrochemical oxidation of primary amines [28–30] and aliphatic diamines. Figure 1a–f illustrate that the increase in current with the increase of pPDA molar ratio during the electrosynthesis of Poly (Ani-co-pPDA) is due to the

existence of two amine groups in pPDA molecule. The mechanism of the anodic oxidation of pPDA is shown in Fig. 3. The first step involves the adsorption of pPDA on the surface of the electrode. The second step is the monomer oxidation with the loss of an electron and the formation of a radical cation. The third and fourth steps are the formation of a primary carbonium ion by cleavage of the C–N bond which attacks another pPDA molecule. The fifth step is the expulsion of proton from the protonated amine and additional loss of an electron and C–N bond cleavage.

13



J IRAN CHEM SOC

Fig. 3  The mechanism of reaction of anodic electropolymerization of pPDA

Structural analysis The baseline-corrected FTIR spectra of PAni, PpPDA and Poly(Ani-co-pPDA) at various molar ratios are shown in Fig. 4. The vibration bands at 1574 and 1513 cm−1 are attributed to the quinoid (Q) and benzenoid (B) rings, respectively (Fig. 4a) [31, 32]. The vibration bands at 1018 and 831 cm−1 are due to the in-plane aromatic rings and out of plane C–H bending, respectively (Fig. 4a) [33, 34]. A broad band at 1079 cm−1 corresponds to the phosphate ion [35]. The vibrational bands are located between 1088 and 960 cm−1 at 628 and 471 cm−1 and designated to (PO4)3−groups. The vibrational band at 963 cm−1 is for (HPO4)2− [36]. For PpPDA, the vibration bands at 1648 and 1402 cm−1 are assigned to (Q) and (B) rings, respectively (Fig. 4e). The vibrational band at 1617 and 1511 cm−1 is presumably due to the phenazine molecule in the pPDA polymeric chain [18, 37]. The bands at 1318 and 1227 cm−1 are correlated to C–N stretching vibrations of the Q and B rings. The band at 1084 cm−1 corresponds to the in-plane substituted benzene rings indicating the presence of open rings in the phenazine molecules [38, 39]. Figure 4a–e illustrate that with the increase in molar ratio of pPDA the ratio of B/Q is also increased. This means that the Poly (Ani-co-pPDA) is more in ES formation.

Fig. 4  Baseline-corrected FTIR spectra (2000–400 cm−1 region) of PAni and PpPDA, and Poly(Ani-co-pPDA) at different molar ratios of Ani:pPDA a 1.0:0.0 b 0.7:0.3 c 0.5:0.5 d 0.3:0.7 e 0.0:1.0

The antioxidant activity The antioxidant activity of 0.1 mg of PAni, PpPDA and Poly (Ani-co-pPDA) in methanolic DPPH solution is

13

evaluated using the UV–visible spectrophotometry at maximum wavelength (λmax) of 517 nm. Table 1 shows the UV–visible absorption of DPPH methanolic solution

J IRAN CHEM SOC Table 1  UV–visible absorption of 1.0 × 10−4 M DPPH methanolic solution after 30 min exposure to 0.1 mg homo and copolymers of Ani and pPDA electrosynthesized (absorption of DPPH in absence of copolymer is 0.81) and the relation of B/Q peak area of FTIR spectra of homo and copolymers Molar ratio (Ani:pPDA)

Abs. (DPPH methanolic solution)

B/Q (FTIR spectra)

1:0 0.7:0.3 0.5:0.5 0.3:0.7

0.6628 0.4597 0.3359 0.2431

1.001 1.499 1.998 3.284

0:1

0.2342

7.679

of homo and copolymers of Ani and pPDA electrosynthesized and the relation of B/Q peak area of their FTIR spectra, which have the antioxidant capability and with the increased amount of pPDA in the molar ratio, the antioxidant activity of copolymer is also increased. This means that the absorbance (A) of methanolic DPPH solution decreases presumably due to the existence of the two amine groups in the pPDA monomer; in other words, the ability of the free radical scavenging of copolymers is increased with the increase of the pPDA molar ratio. The reaction mechanism of the DPPH with PAni and PpPDA is then suggested (Fig. 5). The FTIR spectra also indicate that with the increase of the pPDA molar ratio in the Poly (Ani-co-pPDA), the relation of B/Q peak area is increased (Table 1). Thus, this can be another reason for the increase of the antioxidant activity of Poly (Ani-co-pPDA) as compared to APni and PpPDA.

The EIS study The Nyquist plots of homo and copolymer of Ani and pPDA films in 1 mM [Fe(CN)6]3−/4− at an applied DC potential (Eapp) of 0.25 V are shown in Fig. 6. However, it is essential to propose an equivalent circuit to demonstrate the influence of impedance. Figure 6 (insertion) shows the corresponding data that are best fitted with the equivalent circuit model. Hence, this equivalent circuit is considered to represent the electrochemical process occurring in the polymeric films. The impedance in a system is influenced by various factors viz. Rct, Cdl, the electrolyte solution resistance (Rs) and Warburg impedance (Zw) [40, 41]. To take a proper fitting of the data into the Randles’ equivalent circuit, it is necessary to replace Cdl with a constantphase element (CPE) [42, 43]. The values of the Randles’ equivalent circuit elements are listed in Table 2. As shown in Fig. 6, all Nyquist plots have a semicircle in the highfrequency region. This impedance arc is assignable to the polymer film–electrolyte interface, which is expected to be the CPE in parallel with the Rct, due to the ion exchange for charge compensation at the polymer–electrolyte interface [40]. The Rct value of PAni film is 7.8 × 104  Ω (Fig. 6e). This is much larger than the Rct value of PpPDA (1.7 × 104  Ω) (Fig. 6a). The lesser value of Rct in PpPDA indicates that the electrodeposition of pPDA was conducted more conveniently and easily than the one in Ani in terms of the kinetic process. Therefore, it is obvious that whatever molar ratio of pPDA in copolymer is increased, the charge transfer resistance is decreased (due to the lesser diameter of impedance arc in more frequency). This behavior could be the reason for the existence of more pPDA monomers in

Fig. 5  The proposed mechanism of reaction of PAni and PpPDA with DPPH radical. a The plot of absorbance of DPPH (at λmax 517 nm) vs. molar ratio of pPDA

13



J IRAN CHEM SOC Acknowledgments  This research is supported by the Yadegar -e- Imam Khomeini (RAH) Shahre-rey Branch, Islamic Azad University, Tehran, Iran for Research University (RU) Grant 1390/1742:90/07/27 and also by the Ministry of Science, Technology and Innovation (MOSTI), Malaysia for Research University (RU) Grant 1001/PKIMIA/811044.

References

Fig. 6  Nyquist plots of 1 mM [Fe(CN)6]3−/4− at Eapp 0.25 V with pPDA: Ani molar ratio of a 1.0:0.0 b 0.7:0.3 c 0.5:0.5 d 0.3:0.7 e 0.0:1.0. The ac potential amplitude at 10 mV and frequency range 100 kHz–10 mHz

Table 2  The best fitting values of the Randles’ equivalent circuit elements in Fig. 6 from the simulation of the impedance data for homo and copolymer of Ani and pPDA films Molar Rs Rct (Ω) ratio (Ω) (Ani:pPDA)

W (1/Ω CPE (F) sqrt (Hz))

Na

1.7 × 104 1.6 × 104 6.0 × 103 6.0 × 103

8.6 × 10−1 8.4 × 10−1 8.4 × 10−1 9.4 × 10−1

1:0 0.7:0.3 0.5:0.5 0.3:0.7

10 10 10 10

7.8 × 104 6.4 × 104 4.8 × 104 3.1 × 104

0:1

10

1.5 × 104 3.4 × 103

9.1 × 10−9 1 × 10−8 4 × 10−8 9 × 10−8

8.4 × 10−7 9.8 × 10−1

a

  Degree of electrode surface roughness or cell geometry

the polymer chain which is acted upon with more rates in elimination of the DPPH radical.

Conclusion The PAni, PpPDA and Poly (Ani-co-pPDA) which have been electrodeposited in 1 M H3PO4 and 0.5 M CaCl2 on the CG electrode exhibited an antioxidant activity by the scavenging DPPH radicals. The antioxidant activity increased when the more molar ratio of pPDA was used as a monomer in the electrosynthesis of Poly (Ani-co-pPDA). This was presumably due to the presence of two amines groups in the pPDA and the increase of intensity ratio of B/Q in the backbone of copolymer. Moreover, the increase in the antioxidant activity was related to the decrease of the charge transfer resistance of copolymers in the electrochemical content with the higher presence of pPDA in the polymer chain.

13

1. U. Lange, N.V. Roznyatovskaya, V.M. Mirsky, Anal. Chim. Acta 614, 1 (2008) 2. Y. Yang, S. Mu, Electrochim. Acta 55, 4706 (2010) 3. T.K. Das, S. Prusty, Polym. Plast. Technol. Eng. 51, 1487 (2012) 4. M.R. Majidi, T. Khoshkdaman, M. Nazarpur, Int. J. Polym. Anal. Charact. 15, 351 (2010) 5. A.G. MacDiarmid, Angew. Chem. Int. Ed. 40, 2581 (2001) 6. X.-G. Li, M.-R. Huang, W. Duan, Y.-L. Yang, Chem. Rev. 102, 2925 (2002) 7. M.A. Abd El-Ghaffar, D.E. El-Nashar, E.A.M. Youssef, Polym. Degrad. Stab. 82, 47 (2003) 8. M. Gizdavic-Nikolaidis, J. Travas-Sejdic, G.A. Bowmaker, R.P. Cooney, C. Thompson, P.A. Kilmartin, Curr. Appl. Phys. 4, 347 (2004) 9. P.D. MacLean, E.E. Chapman, S.L. Dobrowolski, A. Thompson, L.R.C. Barclay, J. Org. Chem. 73, 6623 (2008) 10. M.I. Orozco-Solano, C. Ferreiro-Vera, F. Priego-Capote, M.D. Luque de Castro, J. Chromatogr. A 1258, 108 (2012) 11. J.S. Perona, M. Fito, M.-I. Covas, M. Garcia, V. Ruiz-Gutierrez, Metabolism 60, 893 (2011) 12. L. Zhang, J. Solid State Electrochem. 11, 365 (2007) 13. L. Zhang, J. Lian, J. Electroanal. Chem. 611, 51 (2007) 14. G.-L. Xi, Z.-Q. Liu, Eur. J. Med. Chem. 68, 385 (2013) 15. W. Brand-Williams, M.E. Cuvelier, C. Berset, LWT–Food Sci. Technol. 28, 25 (1995) 16. A. Parsa, S. AbGhani, Electrochim. Acta 54, 2856 (2009) 17. F. Abderrahim, S.M. Arribas, M.C. Gonzalez, L. Condezo-Hoyos, Food Chem. 141, 788 (2013) 18. A. Parsa, S. AbGhani, Polymer 49, 3702 (2008) 19. J. Prokeš, J. Stejskal, I. Krˇivka, E. Tobolková, Synth. Met. 102, 1205 (1999) 20. A. Cot, S. Lakard, J. Dejeu, P. Rougeot, C. Magnenet, B. Lakard, M. Gauthier, Synth. Met. 162, 2370 (2012) 21. T. Li, C. Yuan, Y. Zhao, Q. Chen, M. Wei, Y. Wang, High Perform. Polym. 25, 348 (2013) 22. S.J. Choi, S.M. Park, J. Electrochem. Soc. 149, E26 (2002) 23. W.-S. Huang, B.D. Humphrey, A.G. MacDiarmid, J. Chem. Soc. Faraday Trans. 1(82), 2385 (1986) 24. A.J. Motheo, M.F. Pantoja, E.C. Venancio, Solid State Ionics 171, 91 (2004) 25. A. Ray, A.F. Richter, A.G. MacDiarmid, A.J. Epstein, Synth. Met. 29, 151 (1989) 26. D.E. Stilwell, S.-M. Park, J. Electrochem. Soc. 135, 2497 (1988) 27. M. Hebert, D. Rochefort, Electrochim. Acta 53, 5272 (2008) 28. K.K. Barnes, C.K. Mann, J. Org. Chem. 32, 1474 (1967) 29. J. Ghilane, P. Martin, H. Randriamahazaka, J.-C. Lacroix, Electrochem. Commun. 12, 246 (2010) 30. M. Govindan, I.-S. Moon, J. Hazard. Mater. 260, 1064 (2013) 31. X. Lei, X. Guo, L. Zhang, Y. Wang, Z. Su, J. Appl. Polym. Sci. 103, 140 (2007) 32. Y. Furukawa, F. Ueda, Y. Hyodo, I. Harada, T. Nakajima, T. Kawagoe, Macromolecules 21, 1297 (1988) 33. Y. Furukawa, J. Phys. Chem. 100, 15644 (1996) 34. P. Colomban, S. Folch, A. Gruger, Macromolecules 32, 3080 (1999)

J IRAN CHEM SOC 3 5. M. Weber, F.C. Nart, Electrochim. Acta 41, 653 (1996) 36. M. Weber, I.R. de Moraes, A.J. Motheo, F.C. Nart, Colloids Surf A 134, 103 (1998) 37. R.H. Sestrem, D.C. Ferreira, R. Landers, M.L.A. Temperini, G.M. do Nascimento, Polymer 50, 6043 (2009) 38. I.M. Khan, A. Ahmad, J. Mol. Struct. 1050, 122 (2013) 39. B. Duran, G. Bereket, M. Duran, Prog. Org. Coat. 73, 162 (2012)

40. V. Horvat-Radosevic, K. Kvastek, J. Electroanal. Chem. 631, 10 (2009) 41. P. Cordoba-Torres, T.J. Mesquita, O. Devos, B. Tribollet, V. Roche, R.P. Nogueira, Electrochim. Acta 72, 172 (2012) 42. J. Tymoczko, W. Schuhmann, A.S. Bandarenka, Electrochem. Commun. 27, 42 (2013) 43. C.-C. Hu, C.-H. Chu, J. Electroanal. Chem. 503, 105 (2001)

13