Electrochemical Investigation of Catechol at Poly ( niacinamide ) Modified Carbon Paste Electrode : A Voltammetric Study

A polymeric thin film modified electrode, that is, poly(niacinamide) modified carbon paste electrode (MCPE), was developed for the electrochemical determination of catechol (CC) by using cyclic voltammetric technique. Compared to bare carbon paste electrode (BCPE), the poly(niacinamide)MCPE shows good electrocatalytic activity towards the oxidation of catechol in phosphate buffer solution (PBS) of physiological pH 7.4. All experimental parameters were optimized. Poly(niacinamide) modified carbon paste electrode gave a linear response between concentration of CC and its anodic peak current in the range within 20.6–229.0 μM. The limit of detection (3S/M) and limit of quantification (10S/M) were 1.497μM and 4.99 μM, respectively. From the study of scan rate variation, the electrode process was found to be adsorption-controlled. The involvement of protons and electrons in the oxidation of CC was found to be equal. The probable electropolymerisation mechanism of niacinamide was proposed. Finally, this method can be used in development of a sensor for sensitive determination of CC.


Introduction
Electrochemically controlled formation of thin polymer films at surface of the electrode is one of the sensible techniques to determine new functional active species.Their uses as a monomer or a polymer for modification of several electrodes have been increasingly used in analytical applications [1].Catechol (CC), that is also known as pyrocatechol or 1,2dihydroxybenzene, is a simple organic compound as shown in Scheme 1.It is a natural polyphenolic compound that extensively exists in higher plants such as tea and vegetables and it can be released to the environment during its manufacture and use.It has a biological significance such as antioxidation and antiviral, flower stimulating effect and affecting the activities of some enzymes [2][3][4].Even in a very low concentration, these isomers are very toxic to animals and human beings and they are difficult to degrade.Because of this fact, they are the major problem and cause for the environment pollution [5].
Cyclic voltammetry (CV) is a powerful analytical technique that provides information about the undergoing characteristic electrochemical processes of an analyte in solution, which is used to diagnose mechanisms of electrochemical reactions, for detection of the species present in solution [15].Several electrodes mainly carbon paste electrodes play an important role in electrochemical investigations because of their low background current, low cost, and renewability with many electron mediators [16].In the literature, poly(niacinamide) modified glassy carbon electrode exhibited several excellent electrochemical properties and high electrochemical stability towards the oxidation of dopamine.These properties enable the poly(niacinamide) glassy carbon electrode to render good reproducibility [17].
Niacinamide is one of the derivatives of pyridine and readily soluble in water.It is also a main constituent of B-vitamin, required for mammalian daily diet.Due to its significant biological activity, it is widely investigated over the last few decades [18].The chemical structure of niacinamide is shown in Scheme 2. The present work describes the electropolymerisation of niacinamide on the surface of the carbon paste electrode for the electrochemical investigation of CC by CV technique.
The CV response of CC at modified electrode shows some major advantages like high sensitivity, stability, reproducibility, and comparatively lower detection limit.Overall, the modification procedure reported in this article was modest, efficient, and sensitive in the electrochemical detection of CC.

Experimental
2.1.Instrumentation.Electrochemical studies were carried out by using an electrochemical work station CHI-660c (CH Instrument-660 electrochemical analyser) coupled with a conventional three-electrode cell comprised of saturated calomel electrode (SCE) as a reference, a platinum wire as a counter electrode, and bare carbon paste electrode (BCPE) or poly(niacinamide) modified carbon paste electrode (MCPE) as working electrode.All the voltammograms were recorded at an ambient temperature of 25 ± 0.2 ∘ C. The corresponding oxidation potential of analytes was recorded versus SCE.

Reagents and Chemicals.
All chemicals were of analytical grade and used without additional purification.Catechol (CC) was purchased from Himedia (molecular weight = 110.1,purity 99%).Niacinamide was obtained from Sigma Ltd., India (molecular weight = 122.12,purity 99.5%).A stock solution of CC (25 × 10 −4 M) was prepared by dissolving in double distilled water (18 MΩ cm).Phosphate buffer solution (PBS) of same ionic strength was prepared (0.2 M) by mixing appropriate ratio of NaH 2 PO 4 ⋅H 2 O and Na 2 HPO 4 .Graphite powder of average particle size 50 M was purchased from Merck and silicon oil from Himedia was used to prepare carbon paste electrode (CPE).All other reagents used were of analytical grade.All solutions were prepared with double distilled water.

Preparation of Bare Carbon Paste Electrode.
The BCPE was prepared by hand mixing of 70% graphite powder and 30% silicone oil in an agate mortar and grinded for about 45 min until a homogeneous paste was formed.The paste was packed into a homemade cavity of Teflon tube of 3 mm internal diameter and the surface was smoothened on a weighing paper.Unless otherwise stated, the paste was carefully removed prior to pressing a new portion into the electrode after every measurement.The electrical contact was provided by a copper wire connected to the end of the tube.

Preparation of Poly(niacinamide) MCPE.
Electrochemical polymerisation of niacinamide at the BCPE was carried out using cyclic voltammetric technique.The BCPE was scanned for 10 multiple cycles in an electrochemical cell containing aqueous solution of 1 mM niacinamide monomer in 0.2 M PBS of pH 7.4.The electropolymerisation was achieved by successive cyclic voltammetric sweep between −0.8 V and +1.8 V with the scan rate of 0.1 Vs −1 .After that, the polymeric thin film modified electrode, that is, poly(niacinamide) MCPE, was rinsed thoroughly with double distilled water and used for the determination of CC.

Electrochemical Polymerisation of Niacinamide on BCPE.
The poly(niacinamide) MCPE was fabricated by CV technique in between the potential window −0.8 V to +1.8 V with scan rate 0.1 Vs −1 for 10 multiple cycles in 0.2 M PBS of pH 7.4 until a stable cyclic voltammogram was obtained.In this process of multiple cycles, the voltammogram was slowly descended with increasing in cyclic times as shown in Figure 1.This indicates that the niacinamide layer was formed and deposited on BCPE.The thickness of the layer also affects the electrocatalytic property of the electrode and it can be maintained by changing the sweep segments.The probable mechanism of electropolymerisation of niacinamide is described in Scheme 3.Such types of mechanisms have been proposed in earlier report [11].

Characterization of Poly(niacinamide) MCPE.
Figure 2 shows the CV responses recorded for the oxidation of 1 mM potassium ferrocyanide in 1 M KCl as a supporting electrolyte with the scan rate 0.05 Vs −1 .The voltammograms obtained at BCPE (dashed line) were with less sensitivity.However, on the other hand, voltammogram obtained at poly(niacinamide) MCPE (solid line) shows remarkable improvement in the electrotransfer process.The improved result obtained for the voltammetric response of potassium ferrocyanide at poly(niacinamide) MCPE confirms that the surface morphology was significantly changed and also result proves electrocatalytic activity of the poly(niacinamide) MCPE.The total active surface area available for reaction of species in solution can be estimated by the Randles-Sevcik equation ( 1) [1,19]: where  p is the peak current in A.  0 is the concentration of the electroactive species (mol cm −3 ),  is the number of electrons exchanged,  is the diffusion-coefficient (9.196 × 10 −4 cm 2 s −1 ),  is the scan rate (Vs −1 ), and  is the electroactive surface area (cm 2 ).For poly(niacinamide) MCPE, the electroactive surface area is maximum (0.0412 cm 2 ) as compared with bare CPE (0.0288 cm 2 ).An approximate surface coverage of the poly(niacinamide) layer formed on the surface of carbon paste electrode (CPE) was calculated by [15,20] where Γ (M/cm 2 ) represents the surface coverage concentration which is proportional to the peak current ( p ),  is the scan rate,  is the geometric surface area of the electrode,  is the number of electrons involved in the reaction, and , ,  have their usual significance.The surface coverage of poly(niacinamide) adhered on the surface of CPE was determined to be 0.0476 × 10 −10 M/cm 2 .

Electrochemical Response of CC at Poly(niacinamide) MCPE.
Figure 3 shows the cyclic voltammograms of 0.2 mM  CC at BCPE and poly(niacinamide) MCPE in 0.2 M PBS of pH 7.4 with the scan rate 0.05 Vs −1 .At BCPE, catechol shows a poor voltammetric response due to slow electron transport phenomenon, and the oxidation peak potential occurred at 0.322 V versus SCE.On the other hand, poly(niacinamide) MCPE showed great increment in current signals and the oxidation peak potential was observed at 0.204 V.This minimisation of overpotential and enhancement of current response confirms electrocatalytic activity of poly(niacinamide) MCPE towards the electrochemical oxidation of CC [10].
In this proposed method, the oxidation steps of CC were located on benzene ring containing two hydroxyl groups, which represents a typical redox system with twoelectron oxidation process taking place.In this mechanism, CC (4a) loses two electrons and also loses two protons, forming a cation radical (4b), which further takes place in delocalization of electron (4c).Thus, the resulting 1,2benzoquinone or cyclohexa-3,5-diene-1,2-dione compound was formed (4d).The mechanism is shown in Scheme 4. Such types of mechanism have been proposed in earlier reports [21,22].

Effect of Scan Rate on the Peak Current of CC.
The effect of scan rate for the oxidation of 0.2 mM CC in 0.2 M PBS of pH 7.4 was studied by CV technique at poly(niacinamide) MCPE.According to Randles-Sevcik equation, the peak current is directly proportional to scan rate.Figure 4(a) shows the increase of anodic peak currents with increase in scan rate with good linearity in the range from 0.025 Vs −1 to 0.2 Vs −1 .The plot of peak current ( p ) versus scan rate () and  p versus square root scan rate ( 1/2 ) was plotted, which gives a straight line with good linearity.A good correlation coefficient ( 2 ) was observed between  p and  1/2 with the values 0.9974 and 0.9966 as shown in Figures 4(b) and 4(c).This result suggests that the electrode phenomenon was diffusion controlled process [23,24].

Effect of pH Value on the Determination of CC at
Poly(niacinamide) MCPE.The effect of solution pH has a significant contribution on the electrochemical oxidation and was carefully investigated by CV technique.Figure 5(a) shows the dependence of the CV responses of 0.2 mM CC at poly(niacinamide) MCPE in the pH range of 6.0-8.0.
The result shows that, by increasing the pH of 0.2 mM PBS, the oxidation potential was shifted to more negative potential side.The anodic peak potential ( pa ) versus pH graph clearly indicates that the catalytic oxidation potential depends linearly on the pH with a slope of 0.0558 V/pH ( 2 = 0.9995) as shown in Figure 5(b).This signifies that there is an involvement of equal number of protons and electrons in the redox mechanism according to Nernst equation.This was consistent with the earlier report [10,22,24,25].

Conclusion
In the present work the poly(niacinamide) MCPE was fabricated for the electrooxidation of CC in PBS of pH 7.4 by CV technique.The modified electrode shows a detection and quantification limit of 1.497 M and 4.99 M, respectively.The sensitivity, stability, and reproducibility were shown by the modified electrode.Overall, a simple modification procedure was reported for the determination of catechol by cyclic voltammetric technique.

Scheme 3 :Figure 1 :
Scheme 3: Mechanism of electropolymerisation of niacinamide on the surface of BCPE.

Figure 5 :Figure 6 :
Figure 5: (a) Cyclic voltammograms of 0.2 mM CC at poly(niacinamide) MCPE in 0.2 M PBS solution of different pH values (-: 6.0 to 8.0) at scan rate of 0.05 Vs −1 .(b) The effect of pH on the peak potential response of CC.

Table 1 :
Comparison of linear range and detection limits for catechol with different classical methods and electrodes.