Electrocatlytic oxidation of ascorbic acid mediated by ZNO microcrystalline modified glassy carbon electrode

Introduction

ZnO is material of interest in modified electrode like other transition metal to show particular electrochemical and electrocatalytic behavior via electrooxidation of some substrates¹. Modification of electrode surface with electroactive materials is an important area of research in electrochemistry for one decade^2-4. Zinc oxide has been promising applications in catalytic, electrical, optoelectronic, photochemical fields and sensors^5-7. In recent years, metallic nanoparticles have attracted much attention in electroanalysis because of their unusual physical and chemical properties including electrodeposition attachment of metal oxide particles and nanoparticles such as as Au⁸, Pt^9,10, Ag¹¹, Cu¹², Ni¹³ and ZnO₂ ¹⁴, MnO₂ ¹⁵, SnO₂ ¹⁶, TiO₂ in oxidation of some organic and biological substances¹⁷. The electrode surface can be modified with metal hexacyanoferrate by different ways such as electrodeposition, adsorption, mechanically attaching using gold electrode^18-23. Also the metal hexacyanoferrate modified electrode have been widely used in electrochemistry to catalyze the oxidation of dopamine²⁴, glutathione^25-26, cysteine²⁷. The analysis of ascorbic acid is important because it involves wide area of usage such as in drugs, and as chelating agents for heavy metals in body. Ascorbic acid is essential in human body due to its importance in antioxidant property. Ascorbic acid is a water-soluble type of organic acid, which is also known widely as vitamin C. Many studies have been reported on the catalytic oxidation of ascorbic acid^28-34. In this paper, we are reporting the modification of GC electrode with zinc oxide microparticles using various voltammetric techniques. Finally, the electrocatalytic behavior of ZnO/GC electrode was investigated towards oxidation of ascorbic acid in 0.1 M KH₂P0₄ electrolyte solution.

Experimental

Instrumentation and electroanalytical analysis methods

Electrochemical workstations of Bioanalytical System Inc. USA: Model BAS 50W with potentiostat driven by electroanalytical measuring software were connected to computer to perform cyclic voltammetry (CV), chronoamperometry (CC) and chronoamperometry (CA). Ag/AgCl (3M 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^oC using 0.1 M KH₂P0₄ as supporting electrolyte. Solution was degassed with nitrogen gas for ten minutes prior to recording the voltammogram. Hydrodynamic voltammetry using rotating (carbon) disk electrode (RDE-1) and model: BAS-100A voltammetric workstation was also employed in this study.

Reagents

Zinc oxide was obtained from A Johnson Mattney Company, with 99.9% purity. All solutions were prepared using deionized water from reverse osmosis (RO) water model Elken (BIO PURE). Unless otherwise specified, the supporting electrolyte was 0.1 M KH₂P0₄ in aqueous media at room temperature.

Preparation of zinc oxide modified glassy carbon electrode

The solid compound Zinc oxide (ZnO) was transferred to the surface of glassy carbon (GC) electrode as follows: Sample amounts of 1-3 mg of microcrystalline ZnO were placed on a coarse grade filter paper. The glassy carbon electrode was pressed onto the substance and rubbed over the material, causing some compound to adhere to the electrode surface. 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 distilled water.

Figure 1(a) shows the cyclic voltammograms of 0.5 mM ascorbic acid in 0.1 M KH₂P0₄ supporting electrolyte at pH 4.5 using bare GC electrode. Modified GC electrode with ZnO microparticles shows the effect on the catalytic oxidation of ascorbic acid at the ZnO/GC electrode as shown in Figure 1(b) as it causes the oxidation current of ascorbic acid to increase by 1.5 times as compared to bare GC electrode. The oxidation of ascorbic acid is irreversible during cyclic voltammetry as there is an absence of electroactivity during the reverse scan.

Effect of pH

The pH was varied from pH 2.5 to 8 to determine its effect on the catalytic oxidation of 0.5 mM ascorbic acid at the ZnO/GC modified electrode. Figure 2 shows that the oxidation current of 0.5 mM ascorbic acid increases with an increase in pH between 2.7 to 4.5 with a maximum current response at pH4.5. However, current decreased gradually from pH 5 onward until pH 6.0

Effect of temperature

In general current markedly increased with an increase in temperature. Figure 3 shows the Arrhenium plot of log i_ox versus reciprocal of temperature using ZnO/GC electrode is obeyed for the oxidation of ascorbic acid over the temperature range of 30 to 80 ⁰C. This indicates that the rate of the oxidation process of ascorbic acid species

increases as the temperature increases as expected. The temperature dependence may be associated with a change in conductivity and diffusivity of the solid because temperature has a significant influence on the energy formation of a compound. The effect of temperature on conductivity or diffusivity is given by equations (1) and (2).

s = s⁰ exp (-∆H/RT) …(1)

or

D = D⁰ exp (-∆H/RT) …(2)

where,

s / D = conductivity / diffusivity

s ⁰ / D⁰ = standard conductivity or initial diffusivity of the material

∆H = formation energy

So, conductivity or diffusivity would be predicted to change exponentially with temperature.

Table 1: Effect of amount ZnO attached onto a GC electrode on the oxidation of 0.5mM ascorbic acid in 0.1 M KH₂PO₄ (pH 4.5) electrolyte

Amount of [ZnO]/mg	Ipa/(AA)	Enhancement further
0	3.01	–
0.05mg	3.65	1.2
0.1mg	3.98	1.3
0.2mg	5.43	1.8
0.3mg	5.9	1.9
0.5mg	6.2	2.0

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