Electrocatalytic Activity and Electrochemical Impedance Spectroscopy of Poly ( Aniline-Co-Ortho-Phenylenediamine ) Modified Electrode on Ascorbic Acid

The poly(aniline-co-ortho-phenylenediamine) modified composite graphite (poly(Ani– co–oPDA)/CG)) has shown excellent electrocatalytic response towards the oxidation of ascorbic acid (AA). The anodic peak potential (Epa) of AA has shifted from +0.48 V (bare CG) to +0.17 V (poly(Ani–co–oPDA/CG)). The anodic peak currents (Ipa) are linearly dependent upon the square root of scan rate indicating a favourable diffusion controlled process. The electro oxidation of AA on poly(Ani–co–oPDA/CG is more feasible in acidic medium than in either neutral or alkaline medium. This is shown by negative shift of Epa. The charge transfer resistance (Rct) at the poly(Ani–co–oPDA/ CG) shows that the rate of the electro oxidation of AA changes with electrode potential. The Rct and diffusion process are dependant not only on applied potential and electrode material but also on the AA. The poly(aniline-co-ortho-phenylenediamine) modified composite graphite (CG) electrode has shown excellent electrocatalytic response towards the oxidation of ascorbic acid (AA). Charge transfer resistance (Rct) at the poly(Ani–co–oPDA/CG) shows that the rate of the electro oxidation of AA changes with electrode potential. 2052 PARSA & SAlOUT, Orient. J. Chem., Vol. 32(4), 2051-2058 (2016)


INTROdUCTION
The determination of ascorbic acid (AA) has become very important since AA is known to be present in human brain and also involves in several biological processes 1 .Recent studies have demonstrated that the AA content in biological fluids is useful in assessing the amount of oxidative stress in human metabolism 2 .The excessive oxidative stress has been linked to cancer, diabetes mellitus and hepatic disease.
Voltametric studies of AA on bare electrode have not been very successful [3][4][5][6] .The main cause is the high over potential of AA besides the fouling effect of the oxidation products, poor reproducibility, low selectivity and poor sensitivity.However, this is overcome by modifying the electrode with mediators [7][8][9] and polymers [10][11][12] .
The electrochemical copolymerization of aniline (Ani) with its derivatives has been reported [13][14][15][16][17][18][19] These have significantly improved the electrochemical activity of polyaniline (PAni).The electro copolymerization of Ani with other monomers is, however, not feasible and even in some cases impossible.The reason is large peak potentials separation ("Ep) between them which hindered the formation of the copolymer.Hence, the smaller the "Ep the better it is for the copolymerization to occur [20][21][22][23][24] .
T h e e l e c t r o c h e m i c a l i m p e d a n c e spectroscopy (EIS) is a useful technique to investigate the interfacial properties of modified electrodes [25][26][27][28][29][30] .The EIS is, usually, utilized to study parameters such as charge transfer resistance (R ct ), double layer capacitance (C dl ), faradaic capacitance etc. of modified electrodes [31][32][33][34] .EIS on PAni and its derivatives modified electrodes have been reported [34][35][36][37] This work reports on the electrochemical behaviour and catalytic ability of poly(Ani-co-oPDA) modified CG electrode towards electro-oxidation of AA.

MATERIALS ANd METHOdS
Aniline (Sigma Chemicals, USA) was purified by distillation under a nitrogen atmosphere at reduced pressure.The resulting colorless liquid was kept in the dark at 5 °C.The ortho-phenylenediamine (oPDA) and phosphoric acid (Sigma Chemicals, USA) were used as received.Ascorbic acid (AA) (Fluka Chemical, Switzerland) solution was prepared fresh in 0.1 mol l -1 phosphate buffer solution, pH 6.8.All aqueous solutions were prepared using ultra pure water from Milli Q plus system (Millipore Corp., USA).Oxygen-free nitrogen (OFN) was obtained from Nissan-IOI, Malaysia.

Equipment
CV was carried out using an electrochemical Autolab PGSTAT system (Eco.Chemie B.V., Netherlands).The system was run on a PC using general-purpose electrochemical system (GPES 4.9) software.A three-compartment electrochemical cell was employed during synthesis and characterization of copolymer.The 2B pencil composite graphite (CG) lumograph (Staedtler, Germany) was used as working and counter electrodes against pseudo Ag/AgCl reference electrode.EIS measurements were taken using the CompactStatpotentiostat (CompactStat; Ivium Technologies, Netherlands) controlled by a computer equipped with the IviumSoft software package.The structure of copolymer was determined by FTIR spectroscopy System 2000 (Perkin Elmer, USA).

Procedure
The electro copolymerization was performed using 25 ml solution of the 50 m mol l -1 Ani and oPDA, 1 mol l -1 H 3 PO 4 and 0.06 mol l -1 Ca 3 (PO 4 ) 2 by sweeping the potential between "0.7 V and +0.8 V (vs.Ag/AgCl), at a scan rate 100 mV s "1 , under OFN atmosphere and at 25 ± 2 ºC.
The electrochemical determination of AA was performed using 25 ml solution containing 2 m mol l -1 AA and 1 mol l -1 H 3 PO 4 by sweeping the potential between "0.45 V to +0.65 V at a scan rate 100 mVs "1 , under OFN atmosphere and at 25 ± 2°C.Copolymer film was repeatedly washed with supporting electrolyte and then placed in monomer free supporting electrolyte containing AA at different applied dc potentials to perform ac impedance measurements.The potential amplitude of ac impedance was kept at 10 mV with its frequency range from 100 kHz to 10 mHz.

Electrosynthesis of copolymer Ani and oPdA
The CVs of copolymerization of Ani and oPDA in 1 mol l -1 H 3 PO 4 containing 0.06 mol l -1 Ca 3 (PO 4 ) 2 is shown in Figure 1a.The details have been reported elsewhere 38 .Figure 1b shows FT-IR spectra bands at 1634-1618 cm -1 indicating the existence of oPDA in the copolymer 39 .The appearance of bands at 1068-1064, 884 and 850 cm -1 suggests the presence of phenazine-like cyclic structures in the copolymer backbone 40 .These could either be due to the presence of PoPDA blocks or of  the adjacent oPDA and Ani unit in the copolymeric chain 38 .

Electrocatalytic oxidation of AA
Figure 2c shows that the anodic peak potential (E pa ) of AA appears at +0.48 V on bare CG with the anodic peak current (I pa ) at 5 mA.It is an irreversible electrode process.In contrast, on poly(Ani-co-oPDA)/CG the I pa has increased to 9 mA and the E pa has shifted negatively to +0.17 V (Fig. 2d) with quasi reversible electrode process.This shows that the electro oxidation of AA is feasible indicating a strong electrocatalytic property of poly(Ani-co-oPDA)/CG towards AA.The shifting of the E pa is due to the kinetic effect in which a substantial increase in the rate of electron transfer is observed 41 .
Rueda et al 42 have studied the oxidation of AA on a gold electrode in a wide pH range.The products of the reaction have been identified by chromatography.From electrochemical experiments, the total number of electrons taking part in the oxidation has been estimated at 1.9.On this basis the following EC mechanism have been proposed: Where H 2 A is AA, A stands for the dehydro ascorbic acid (DHAA) and B is hydrated dehydro ascorbic acid (DHAA.H 2 O).The DHAA is formed from H 2 Avia a radical anion intermediate viz.mono dehydro ascorbic acid.The DHAA undergoes a hydration reaction to form the final product, the electro inactive DHAA.H 2 O.The irreversibility of the electro oxidation of AA is displayed by the absence of the cathodic peak current (I pc ) (Figure 2).

Effect of scan rate
Further investigation is carried out on the transport characteristics of AA on the modified electrodes.It is apparent that with increasing scan rates the E pa is shifted to more positive potentials indicating there is kinetic limitation on the electrode process (Figure 3).But, as I pa is directly proportional to the square root of scan rates the electrode process is, predominantly, diffusion-controlled 43 .This is regarded as normal for a useful electrode process.

Effect of pH
The CVs of 2 m mol l -1 AA in supporting electrolytes at different pH values at poly(Ani-co-oPDA)/CG are shown in Figure 4.It appears that there is a decrease in I pa when pH is raised from 1 to 12.The decrease in the electrode process is due to higher electronic conductivity of poly(Anico-oPDA) in acid than in basic media 44,45 .With an increase in pH the E pa is shifted towards positive indicating the decrease in electro catalysis of poly(Ani-co-oPDA)/CG. Hence, the electro oxidation of AA on the poly(Ani-co-oPDA)/CG is more feasible in acid than in either neutral or alkaline media.
Therefore, the poly(Ani-co-oPDA)/CG is useful for electro oxidation of AA in acidic pH range.

EIS studies
It is well known that EIS is a useful method for studying the interfacial properties of the modified electrodes 46 .The impedance is the summation of real, Z re , and imaginary, Z im , components contributed by the resistance and capacitance of the cell 47 .In this study EIS was employed to investigate electro oxidation of AA at poly(Ani-co-oPDA)/CG and bare CG electrodes.EIS measurements of AA in a monomer-free supporting electrolyte solution were performed at two different potentials.
From Figure 5a it is obvious that the magnitude of charge transfer resistance (R ct ), the diameter of the semi circle, of poly(Ani-co-oPDA)/ CG is smaller (2.900×10 3 ohm) than bare CG (2.750×10 4 ohm) indicating improved interfacial capacitance.In short, double-layer capacitance (C dl ) of constant phase element (CPE or Q) has encouraged charge transfer.The presence of straight line on poly(Ani-co-oPDA)/CG with a slope of 45 o C at the lower frequency indicates a favourable diffusion-controlled mass transport process 48 .
The total impedance is measured by several parameters: the bulk electrolyte solution resistance (R s ), charge transfer resistance (R ct ) that corresponds to the kinetic control of the chargetransfer process, double layer capacitance (C dl ) and Warburg impedance (Z w ).To take a proper fitting of the Nyquist plots, it was needed to replace C dl with a CPE in the Randles' equivalent circuit.It's widely believed explanation for the appearance of depressed semi circles and the presence of CPE in the Nyquist plots is due to microscopic roughness, which causes an inhomogeneous distribution in both C dl and R s 49-51   .The circuit element that is of most interest to this work is R ct .This is because it often relates directly to the accessibility of the modified electrode and also reflects the flow of charge across the modified interface into the substrate electrode.
The decrease in diameter of the semicircle confirms the electrocatalytic activity of poly(Ani-co-oPDA)/CG in the oxidation of AA at E app 0.2 V.The electro oxidation of AA that occurred via phenazinelike cyclic species virtually led to an increase in the surface concentration of low-valence species of the electrocatalyst and the decline of the R ct depending on the concentration of AA in the solution as shown in Figure 5c, R ct is high enough at 0.2 V, where the mass transfer process is not performed, so in the Nyquist plots no Z w is observed.
At E app 0.5 V the result shows the inverse as magnitude of R ct of poly(Ani-co-oPDA)/CG (15000 ohm) is much higher than the bare CG (3400 ohm) and with Z w is not observed (Figure 6b).This suggests on the dependancy of diffusion process of AA on the E app .Values of the Randles equivalent circuit elements obtained by fitting the experimental results for the Nyquist plots in monomer free background electrolyte containing AA at the poly(Ani-co-oPDA)/CG and bare CG at E app of 0.2 and 0.50 V are listed in Table 1.
Further investigation shows that the poly(Ani-co-oPDA)/CG electrode has minimum Z re at E app 0.2 V, whereas the CG electrode shows minimum Z re at E app 0.5 V at frequency 100 kHz to 10 mHz (Figures 7a and 7b). Figure 7c shows real impedance spectroscopy (Z re or Z×) of poly(Ani-co-oPDA)/CG electrode in monomer free background electrolyte containing uric acid (UA) obtained by sweeping the potential between "0.1 and 0.8 V (vs.Ag/AgCl).The poly(Ani-co-oPDA)/CG electrode has its minimum Z re at E app 0.3 V. Thus, the R ct and diffusion process depend not only on E app and electrode material but also on type of analyte available.

CONCLUSION
The poly(Ani-co-oPDA)/CG electrode is shown to possess catalytic activity towards the oxidation reaction of AA.As a result, the electro oxidation of AA on the modified electrode is more feasible in acidic than in neutral and alkaline medium.This is shown by negative shift of E pa of AA.The extension of catalytic reaction depends on charge transfer.The electrode process is diffusion-controlled.
The study indicates the poly(Ani-co-oPDA)/CG electrode is useful for oxidation of AA in an acidic range of pH.
T h e e l e c t r o c h e m i c a l i m p e d a n c e spectroscopy was successfully used to interpret the electrocatalytic effect of modified electrode on AA.The equivalent circuit elements obtained by fitting the experimental results confirmed that proposed equivalent circuits were matched and able to explain phenomenon in the modified and unmodified electrode.The R ct and diffusion process are not only dependent on applied potential and electrode material but also on type of analyte used.The R ct is the only circuit element that is meaningful.It describes on the kinetic of charge transfer during the electro oxidation of AA at different applied potential.

Fig. 5 :
Fig. 5: (a) Nyquist plots of AA at the poly(Anico-oPdA)/CG and bare CG electrodes at E app 0.20 V. (b) and (c) are the respective electrodes with Randle's equivalent circuit and circuit description code (CdC) (inset).The ac potential amplitude was kept at 10 mV and frequency range used was from 100 kHz to 10 mHz

Fig. 6 :Fig. 7 :
Fig. 6: (a) Nyquist plots of AA at the poly(Anico-oPdA)/CG and bare CG electrodesat E app 0.50 V. (b) and (c) are the respective electrodes with Randle's equivalent circuit and circuit description code (CdC) (inset).The ac potential amplitude was kept at 10 mV and frequency range used was from 100 kHz to 10 mHz