The electrochemical redox mechanism and antioxidant ...

The DPV curve of four polyphenolic compounds under optimum conditions are shown in Figure 2 and Table 2 . The DPV oxidation peak currents (i pa ) were detected. The linear relationships between i pa and concentrations are shown in Table 3 .

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The CV curves of four polyphenolic compounds under the optimum condition is shown in Figure 1 . It is shown that four polyphenolic compounds have the obvious electrochemical response signal on the electrode, GA appeared as an irreversible oxidation peak at E pa = 0.46 V; CA appeared as a pair of prominent redox peak at E pa = 0.42 V, E pc = 0.27 V, and ΔE = 0.15 V; FA appeared as a pair of obvious redox peak at E pa = 0.37 V, E pc = 0.26 V, and ΔE = 0.11 V; VA appeared as a pair of prominent redox peak at E pa = 0.63 V, and E pc = 0.55 V, and ΔE = 0.08 V, which indicated that redox reaction of four polyphenols occurred on the electrode surface ( Table 1 ).

The supporting electrolytes defined the electrochemical reactions discussed above; their chemistry affected the reaction mechanism, similar to how a solvent affects a chemical reaction [ 33 ]. A strong role of a supporting electrolyte is also related to its ability to tune the ionic conductivity of the system.

For these tests, we used 0.01 mol/L of citric acid&#;sodium citrate buffer solution (C 6 H 8 O 7 &#;Na 3 C 6 H 5 O 7 ), 0.01 mol/L of disodium hydrogen phosphate&#;potassium dihydrogen phosphate buffer solution (Na 2 HPO 4 &#;KH 2 PO 4 ), 0.01 mol/L of phosphate buffer solutions (PBS), 0.01 mol/L of citric acid&#;disodium hydrogen phosphate buffer solution (C 6 H 8 O 7 &#;Na 2 HPO 4 ), 0.01 mol/L of acetic acid&#;sodium acetate buffer (HAc&#;NaAc), 0.01 mol/L of Britton&#;Robinson buffer (B&#;R), and 0.01 mol/L of potassium chloride buffer solutions (KCl) as supporting electrolytes for the MWNTs/GE system. Two CV redox peaks were observed for only five electrolytes ( Figure 3 ). At the same time, well-defined CV responses with high redox peak currents for all four polyphenols were obtained in Na 2 HPO 4 &#;C 6 H 8 O 7 buffer solution (GA), HAc&#;NaAc buffer solution (CA), Na 2 HPO 4 &#;C 6 H 8 O 7 buffer solution (FA), and KCl buffer solution (VA).

The influence of pH on the CV behavior of four polyphenolic compounds was investigated using the optimized buffer solutions. The oxidation peak potentials (E pa ) shifted negatively as pH increased ( Figure 4 ). This dependency was linear ( Table 4 ).

The influence of scan rate (υ) on the CV response of four polyphenolic compounds is shown in Figure 5 . The oxidation peak potential (E pa ) shifted positively, while the reduction potential (E pc ) shifted towards more negative values as the scan rate increased. The oxidation and the reduction peak currents (i pa and i pc , respectively) increased linearly with the scan rate for all four polyphenols tested in this work ( Table 5 ). Thus, the electrode reactions of these polyphenols at the MWCNTs/GE were controlled by the adsorption [ 34 ]. A high and stable peak current for four polyphenolic compounds was obtained at the scan rate of 70 mV/s (GA), 50 mV/s (CA), 100 mV/s (FA), and 50 mV/s (VA).

3.5 Analysis of the kinetic parameters

Electron transfer number, n [35] is a key parameter of an electrochemical reaction at the electrode. Because the reactions of four polyphenols on the electrode were adsorption-controlled [36], they could be described by the Langmuir isotherm described as follows [37]:

(1)

i p = n 2 F 2 A Γ T ν 4 R T = n F Q ν 4 R T ,

(2)

Q = n F A Γ T ,

where Q is the charge value calculated from the oxidation or reduction CV peak areas, i p is peak current (in μA), F = 96,485&#;C/mol, R = 8.&#;J/K/mol, T = 298.15&#;K, Γ T is the surface coverage of the electrode (in mol/cm2); and A is the electrode surface area (equal to 0.&#;cm2). Our calculations showed that the electron transfer numbers (n) of four polyphenolic compounds calculated from the data obtained at 0.05&#;V/s of scan rate were equal to 3.01 (GA), 1.98 (CA), 1.31 (VA), and 1.29 (FA).

The proton number m is also one of the most essential electrochemical parameters. We used the Nernst equation and data from Table 4 to calculate m as shown below:

(3)

E = E Θ + R T n F ln [ O ] [ H + ] R = E Θ + R T n F ln [ O ] R + m R T n F ln [ H + ] ,

(4)

E = k &#; m R T n F pH = k &#; m n × 0.059 ( 25 ° C ) ,

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where the slope k is equal to 0.059&#;m/n. Thus, m of the oxidation peak can be obtained from the corresponding n values.

The electron transfer coefficient α was calculated from the equation below:

(5)

E p = k + R T α n F lg ν ,

According to the equation, the slope (k) = k = 2.303RT/(αnF), and α could be calculated using corresponding n values.

The apparent constant of electron transfer rate (k s) was calculated by the following equation:

(6)

lg k s = α lg ( 1 &#; α ) + ( 1 &#; α ) lg α &#; lg R T n F ν &#; ( 1 &#; α ) α n F Δ E p 2.303 R T .

All kinetic parameters of our polyphenolic compounds were calculated using the formulas shown in Table 6.

Table 6

The determination of kinetic parameters of four polyphenolic compounds

Sample The linear relationship r n Γ T (mol/cm2) m α k s (/s) GA E pa (V) = 0. lg&#;ν + 0. 0. 3 2.27 × 10&#;10 3 0.41 0.27 CA E pa (V) = 0. lg&#;ν + 0. 0. 2 6.18 × 10&#;10 2 0.35 2.32 FA E pa (V) = 0. lg&#;ν + 0. 0. 1 1.41 × 10&#;10 1 0.87 0.86 VA E pa (V) = 0. lg&#;ν + 0. 0. 1 2.08 × 10&#;9 0.96 1.08

Polyphenolic compounds are antioxidants with the ability to convey electrons and protons. Their oxidation reaction removes free radicals, when phenolic hydroxyl was oxidized to the ketone group [38]. That is, the mechanism of phenolic acids scavenging free radicals is by supplying the hydrogen on phenolic hydroxyl groups to the free radicals and forming relatively stable phenolic hydroxyl radicals to prevent a free radical chain reaction. Due to the reaction mechanism of natural antioxidants scavenging free radicals is consistent with the electrochemical oxidation mechanism, which are both achieved through the electron transfer, the oxidation reaction of antioxidants on the electrode is similar to its antioxidative reactions in vivo [39]. The antioxidant reaction process in vivo of polyphenolic compounds can be learned by the determination of electrochemical behavior.

From the results of the research, for GA, the electrochemical oxidation reaction takes place involving three electrons and protons; For CA, quasi reversible adsorption reaction takes place involving two electrons and protons; For FA and VA, single electron adsorption reaction takes place, and a proton is involved in the reaction of FA. So, we concluded that electrochemical oxidation reaction involving electrons and protons take place in polyphenolic compounds. We can infer the possible reaction mechanism as follow:

Enhanced selectivity of the sensors could be achieved by the electrochemical formation of conducting polymeric layers. Phenolic polymerizable monomers are widely used to obtain electrochemically deposited coatings [40]. The essential feature of all polyphenols is the presence of one or more hydroxylated benzene rings. For example, GA contains both three hydroxyls (&#;OH) and carboxyl (&#;COOH) groups. Thus, it can be electrochemically polymerized because of its two electrochemically active groups, which could be oxidized differently by the benzoic acid and its derivatives. These hydroxyl and carboxyl groups could also act as reactive sites to selectively bond to metals or as biomolecules active as electrocatalysts and chemical and biological sensors [41].

Polyphenol oxidation occurs by an electron transfer through the phenoxyl radical (semiquinone) formation [42]. This radical is unstable and decays by dimerization or polycondensation reactions. Thus, these four compounds could also undergo dimerization or polycondensation. Therefore, the oxidation mechanism of GA is described from this point of view (Figure 6). Abdel-Hamid and Newair have suggested an irreversible transformation of GA to its semiquinone radical cation (GA˙+) by transferring one electron. After this radical releases a proton, it forms a semiquinone radical (GA˙) [43], which, in turn, undergoes a second irreversible electron transfer forming quinone cation (GA+) [44]. GA + deprotonation ends the two-electron process yielding quinone, which manifested as an irreversible peak.

Figure 6

Summarizing all comments and discussions presented above, it was proposed that the electrochemical oxidation of polyphenols induced electrochemical polymerization. Potential GA cycling in acidic buffer solutions could form polymeric layers, which will further grow during anodic electropolymerization, which involves deprotonation of the phenols. These protons first become chemisorbed at the electrode surface, which then gradually oxidizes at anodic potentials.

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