Electrochemical Oxidation of Paracetamol Mediated by MgB 2 Microparticles Modified Glassy Carbon Electrode

A MgB2 microparticles modified glassy carbon electrode (MgB2/GCE) was fabricated by adhering microparticles of MgB2 onto the electrode surface of GCE. It was used as a working electrode for the detection of paracetamol in 0.1 M KH2PO4 aqueous solution during cyclic voltammetry. Use of the MgB2/GCE the oxidation process of paracetamol with a current enhancement significantly by about 2.1 times. The detection limit of this modified electrode was found to be 30 μM. The sensitivity under conditions of cyclic voltammetry is significantly dependent on pH, supporting electrolyte, temperature and scan rate. The current enhancement observed in different electrolytic media varied in the following order: KH2PO4 > KCl > K2SO4 > KBr. Interestingly, the oxidation of paracetamol using modified GC electrode remain constant even after 15 cycling. It is therefore evident that the MgB2 modified GC electrode possesses some degree of stability. A slope of 0.52 dependent of scan rate on current indicates that the system undergoes diffusion-controlled process.


Introduction
Magnesium diboride (MgB 2 ) is an odourless and brittle compound, exists in the form of powder, crystals and ceramics where the colour varies from one form to the other; black in powder, yellow in crystals and pink in ceramics.It has simple crystal structure with Mg layer alternating with honeycomb B layer 1 .It was not discovered 40 years ago due to difficulty in preparation 1 .However, recently method of growing MgB2 such as single crystal 2 , powder 3 and wires 4 have been developed.Acetaminophenol or paracetamol is one of the most commonly used analgesics in pharmaceutical formulations, for the reduction of fever and also as a painkiller for the relief of mild to moderate pain associated with headache, backache, arthritis and postoperative pain.Acetaminophen is electroactive and voltammetric mechanistic studies for the electrode processes of the acetaminophenol /N-acetyl-pquinoneimine redox system has been presented [5][6][7] .The fabrication of chemically modified electrode (CME) has been widely reported to improve sensitivity and selectivity in determining many amino acids, vitamins, drug, DNA and many more in recent years [8][9][10][11] .The study of electrochemistry for MgB 2 is still rather lacking.Electrochemical characterization and mediation studies using MgB 2 modified solid electrode has not been reported.In this work, an attempt is made to report the work done using MgB 2 as sensing electrode materials which is mechanically attached to the surface of glassy carbon (GC) working electrode via solid phase voltammetry of microparticles (SPVM).

Experimental
Electrochemical workstations of bioanalytical system Inc.USA: Model BAS 50 W with potentiostat driven by electroanalytical measuring softwares was connected to computer to perform cyclic voltammetry (CV).An Ag/AgCl (3 M NaCl) and platinum wire were used as a reference and counter electrodes respectively.The working electrode used in this study was 3 mm diameter glassy carbon (GC).Unless otherwise stated, the voltammetric experiments were carried out at 25±2 o C using 0.1 M KH 2 P0 4 as supporting electrolyte.Solutions were degassed with nitrogen for ten minutes prior to recording of the voltammogram.

Reagents
Magnesium diboride (MgB 2 ) was obtained from A Johnson Mattney Company, with 99.9% purity.Deionized water by reverse osmosis (RO) via Elken's water filteration system (BIO PURE) was used in the preparation of solutions.Unless otherwise specified, the supporting electrolyte was 0.1 M KH 2 PO 4 in aqueous media at room temperature.All solutions were desecrated with oxygen-free nitrogen gas for 15 minutes prior to making the measurement.

Procedures
The solid compound magnesium diboride (MgB 2 ) was transferred to the surface of the glassy carbon (GC) electrode as followed: Sample amount of 1-3 mg of MgB 2 were placed on a coarse grade filter paper.Glassy carbon was pressed onto the substance and rubbed over the material, causing some compound to adhere to the electrode surface.For electrochemical measurements, the electrode was transferred into the solution.The clean glassy carbon surface could be renewed after the measurement by polishing with 0.5 µm alumina slurry, followed by ultrasonic cleaning for about 2-3 minutes, rinsing with Deionized water by reverse osmosis (RO) via Elken's water filtration system (BIO PURE) was used in the preparation of solutions.

Enhancement study
The cyclic voltammograms of 0.1 mM paracetamol in 0.1 M KH 2 PO 4 showed two folds increase in the oxidation current enhancement of modified GC electrode with microparticles of MgB 2 as compared to a bare GC electrode as shown in Figure 1.The oxidation current enhancement of paracetamol at the MgB 2 /GC electrode was caused by the catalytic effect.The oxidation of paracetamol is reversible during CV.

Effect of pH
The pH was varied from pH 3.0 to 11.0 to determine its effect on the catalytic oxidation of 0.1 mM paracetamol at the MgB 2 /GC modified electrode.Figure 2 shows that the oxidation current of 0.1 mM paracetamol increases with an increase in pH between 3.0 to 6.0 with maximum current enhancement at pH 6.The current slowly decreased from pH 6 to pH 10.0.As can be seen, the peak potential for paracetamol oxidation varies linearly with pH and is shifted to more negative potentials with increase in pH.expectation of Equations 1 and 2. The conductivity of the MgB 2 microparticles with the increase in temperature also played a significant influence on the activation energy for diffusion of the substrate of interest, Ea as described by the Arrhenius equation (Eqs.1 and 2) given below.
Where σ /D are conductivity/diffusibility and σ 0 / D° are standard conductivity the initial diffusibility.However while use at elevated temperature, it can result to an increase in current, in microparactical sense application needs to be continued close to ambient temperature.

Effect of varying paracetamol concentrations
Voltammograms in Figure 3 show peak current of paracetamol increases rapidly and linearly with increasing concentration from (5.0x10 -7 to 2.0x10 -3 mol/L), as described by Equations y = 21.28x+0.65,which showed excellent correlation of 0.995 R 2 value.Based on the slope, m of the linear graph, a good sensitivity response of 21.22 µA/mM was obtained.

Effect of varying scan rate
The effect of varying scan rates (v) on the cyclic voltammograms of 0.1 mM paracetamol using modified MgB 2 /GC as working electrode in 0.1 M KH 2 PO 4 supporting electrolyte was studied over 5-1000 mV/s.Oxidation currents of ascorbic acid was observed to increase with scan rate due to heterogeneous kinetics.Figure 4 shows the plot of log (peak current) versus log (scan rate, v) for oxidation current of the first cycle, a straight line was obtained fulfilling the equation y = 0.516x-5.687with R 2 =0.969.A slope of 0.5, which is comparable with theoretical slope of 0.5 for diffusion-controlled process, was obtained.From this study, approximately two protons were transferred in the reaction.Paracetamol oxidation is a twoelectron two-proton process in Equations 3 given below.A conclusion that is consistent with that reported in the literature 11 .

Effect of potential cycling
The potential cycling of the oxidation of paracetamol solution, 0.1 mM paracetamol was carried out at the electrode surface of a glassy carbon electrode (3 mm diameter) modified with MgB 2 microparticles during cyclic voltammetry.Continuous potential cycling to Continuous potential cycling did seem to affect the oxidation peak of paracetamol at magnesium diboride modified GC electrode since the faradaic activity appears reproducible even after 15 cycles reflecting the the oxidation peak of paracetamol remained high; consequently , its can stability of the MgB 2 coat/film at the surface of GC electrode.

Effect of varying supporting electrolyte
Based on the pH study, different types of 0.1 M aqueous supporting electrolytes with neutral conditions were studied.In the presence of H 2 PO 4 -in electrolyte, paracetamol peak currents were increase the peak of oxidation as compared to others in electrolyte solution (Figure 6).In noncomplexing solutions (Table 1), most of the aqueous solutions observed showed no distinct changes.Similar potential ranges were obtained using electrolytes solutions of KCl>K 2 SO 4 >KBr However, the oxidation peak of paracetamol show that the greatest enhancement effect is obtained when potassium dihydrogen phosphate is used as electrolyte.In general, degree of current enhancement in varying electrolyte varies in the following order KH 2 PO 4 > KCl > K 2 SO 4 > KBr (Table 1).

Chronocoulometry study
Chronocoulometry (CC) is the integral analogs of the corresponding chronoamperometry approaches.The integral of the cottrell equation gives the cumulative charge passed in reducing or oxidizing the diffused component 12: Q = 2nFAD1/2Co2π1/2t1/2 (4) Where Q = Charge and the other parameters have its usual meanings.The analysis of chronocoulometry (CC) data is based on the Anson equation above (Eq.4), which defines the charge-time dependence for linear diffusion control.The Anson plot shows linear dependency of Q upon t1/2 (Figure 7) indicating that the process involved is diffusion controlled.
Based on Anson's plot of the above, surface charge can be calculated by taking the difference between two intercepts of forward and reverse steps at t=0.It was found that the MgB 2 /GC has a total charge transferred of 3.0 µC when 1 mM paracetamol was reduced.

Conclusion
Among the various solid electrodes used GC electrode by far a better choice of solid state electrode to be modified with MgB 2 microparticle mediates the oxidation process of paracetamol during cyclic voltammetry.Resulted in the optimization of the oxidation current, which appeared to be 2 times more enhanced compared to bare GC electrode.The sensitivity under conditions of cyclic voltammetry is significantly dependent on pH, electrolyte and scan rate.Some physical constants such as diffusion coefficient, MgB 2 film appearances of well defined edges remain intact, even after 20 potential cycling.

Figure 1 .
Figure 1.Cyclic voltammograms for oxidation of 0.1 mM paracetamol obtained in 0.1 M KH 2 P0 4 (pH 6.0 ), with potential scanning in the positive direction from -1000 to +1000 mV vs. Ag/AgCl at a scan rate of 100 mV/s at 25 °C at (a) bare GC electrode and (b) modified MgB 2 /GC

Figure 2 .
Figure 2. Cyclic voltammograms with varying pH levels for GC electrode modified with modified MgB 2 /GC in 0.1 M KH 2 P0 4 .potential scanning in the positive direction from -1000 to +1000 mV vs. Ag/AgCl at a scan rate of 100 mV/s .Effect of temperatureEffect of temperature on the oxidation process of paracetamol was studied.The current increases gradually from 20 to 80 0 C. The plot of log oxidation current of paracetamol versus reciprocal of temperature was found to be fairly linear in agreement with thermodynamic

Figure 3 .
Figure 3.A plot showing the dependence of the oxidation current on different concentration of paracetamol MgB 2 /GC in 0.1 M KH 2 PO 4 solution with a scan rate of 100 mV/s.

Figure 4 .
Figure 4. Plot of log Ip a versus log v. Effect of varying scan rates of 0.1 mM paracetamol using MgB 2 /GC electrode in 0.1 M KH 2 PO 4 (pH 6.0) at a scan rate rang from 5-1000 mV/s

Figure 5 .
Figure 5. Cyclic voltammogram of potential cycling for the oxidation of paracetamol at the MgB 2 /GCE in 0.1 M KH 2 PO 4 (pH 6.0).potential scanning in the positive direction from -1000 to +1000 mV vs. Ag/AgCl at a scan rate of 100 mV/s for 10 cycles

Figure 6 .
Figure 6.Tile graph (background subtracted) of the oxidation current of paracetmol at the in different supporting electrolytes at (a) bare GC electrode and (b) modified MgB 2 /GC with a scan rate of 100 mV/ sIn noncomplexing solutions (Table1), most of the aqueous solutions observed showed no distinct changes.Similar potential ranges were obtained using electrolytes solutions of KCl>K 2 SO 4 >KBr However, the oxidation peak of paracetamol show that the greatest enhancement effect is obtained when potassium dihydrogen phosphate is used as electrolyte.In general, degree of current enhancement in varying electrolyte varies in the following order KH 2 PO 4 > KCl > K 2 SO 4 > KBr (Table1).

Table 1 .
Effect of varying supporting electrolytes of 1mM paracetamol, during cyclic voltammetry using at MgB 2 /GCE with a scan rate of 100 mV/s