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The electrochemical redox mechanism and antioxidant activity of polyphenolic compounds based on inlaid multi-walled carbon nanotubes-modified graphite electrode

  • Xiaofen Li , Yuntao Gao , Huabin Xiong and Zhi Yang EMAIL logo
Published/Copyright: September 24, 2021
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Open Chemistry
From the journal Open Chemistry Volume 19 Issue 1

Abstract

The electrochemical redox mechanism of polyphenolic compounds (gallic acid [GA], caffeic acid [CA], ferulic acid [FA], and vanillic acid [VA]) were investigated by electrochemical methods at the inlaid multi-walled carbon nanotubes-modified graphite electrode (MWCNTs/GCE). The obtained micro-information such as the number of electrons and protons were used to deduce the electrochemical oxidation mechanism of four polyphenolic compounds. The antioxidation activities of these compounds were also studied by two methods. The radical scavenging activity followed the order: GA > CA > FA > VA, which was in agreement with the result from the spectrophotometry method. The result indicated that compounds with lower oxidation peak potential (E pa) showed stronger antioxidation activity. At the same time, compounds with high E pa showed lower antioxidant activity. The greater the number of hydroxyls linked to the aromatic ring, the greater the antioxidation activities of four compounds. Structural analysis of these phenolic-based compounds suggested that multiple OH substitutions and conjugations determine their free radical scavenging activity and electrochemical behavior.

Keywords: antioxidant activity; polyphenolic compounds; cyclic voltammetry; differential pulse voltammetry

1 Introduction

Phenolic compounds attract a lot of attention due to their potent antioxidant activity and, as a result, health hazards [1,2,3,4]. However, these compounds are also used as preventive and therapeutic drugs for some oxidative stress illnesses, such as atherosclerosis, inflammation, cancer, etc. [5,6]. Their main benefits are related to their strong antioxidation activity [7,8]. For example, the antioxidation efficacy of phenolic acids depends on their structures and the OH group presence [9,10,11].

Recently, multi-walled carbon nanotubes (MWCNTs) attracted attention to fabricate MWCNT-modified electrodes because they possess high surface area, excellent mechanical and chemical properties, and superior electrocatalytic and charge transfer features [12,13,14]. However, the MWCNTs/GE was usually prepared by the method of dispensing or doping, and the process was comparatively complicated, which could take about 70–80 min, but stable carbon nanotube films could not be obtained. Therefore, MWCNTs’ applications still demonstrate drawbacks such as poor dispersion and film-forming. For this reason, we inlaid MWCNTs into the electrodes directly by mechanically abrading in order to form a compact film on the electrode surfaces. It is found that the method has the advantage of simple, time-saving, and stable chemical properties.

Electrochemical techniques are versatile and powerful analytical tools, which have the benefit of high sensitivity, low detection limits, low cost, and enable rapid analysis of sample [15,16]. In addition to their fundamental science applications to study oxidation and reduction reactions and their mechanism, they are also used for kinetic and thermodynamic studies as well as to understand electron and ion transfer [17,18]. Phenolic compounds are often called “chain-breaking antioxidants” because of the strong radical scavenging ability of their H from the OH group or electron, both of which donate their charge capacity and delocalize/stabilize the phenoxyl radical [19,20]. This scavenging ability could be predicted by assessing the standard one-electron potentials of the corresponding compounds. For this purpose, voltammetry is frequently applied. Voltammetry is used for analyses of natural and synthetic antioxidants to study their reaction mechanisms. All this knowledge is then applied for the design of new efficient antioxidants [21,22,23]. The structure-property-activity relationship established for these compounds suggested that redox potentials reflect the antioxidant activity, which is useful as guidelines for drug discovery and development [24,25]. Cyclic voltammetry (CV) is also applied for the reduction ability characterization of natural phenols. Similarly, CV demonstrated an excellent correlation between its data on redox potential and phenols’ antioxidation properties [26,27,28,29]. In general, strong scavengers oxidize at lower potential values [30], which was also observed by Firuzi et al. [31] during CV to obtain oxidation potentials and ferric reducing antioxidation power to determine their antioxidation properties. Typically, the higher the antioxidation activity and lower E pa, the higher the OH group contents in the aromatic rings, and vice versa.

In the present work, the graphite electrode modified with an MWCNTs coating and CV were used to study the electrochemical redox mechanism of four polyphenolic compounds. Their antioxidation properties were assessed based on their redox potentials. The results coincide with that of spectrophotometry, which indicated that redox potentials reflect the antioxidation activity of the phenolic compounds, which, in turn, could be used for pharmaceutical applications.

2 Materials and methods

2.1 Reagents and chemicals

Gallic acid, caffeic acid, ferulic acid, and vanillic acid reference substance (purity >99.0%) were acquired from the National Institute of China for the Control of Pharmaceutical and Biological Products (NICPBP). The 1.0 mg/mL stock solutions of gallic acid and caffeic acid were prepared in CH3OH. The 50 mg/mL solutions of vanillic acid and ferulic acid were also prepared in CH3OH. All the stock solutions were stored at 4°C. 0.10 mol/L of C6H8O7–Na3C6H5O7 buffer solution, 0.10 mol/L of HAc–NaAc buffer solution, and KCl solution were used as supporting electrolytes. All other reagents were of analytical and reagent grades. Twice-distilled water was used throughout all experiments.

The TNMH3 carboxyl-functionalized MWCNTs (>95% pure) were bought from the Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences. Spectrum pure graphite rod (0.6 cm in diameter) was from Qingdao Tengrui Carbon Co., Ltd.

2.2 Experimental setups and instruments

ZAHNER Zennium IM6 Electrochemical Workstation (Germany) was used for all tests. A typical setup contained a three-electrode system with MWCNTs/GE as a working electrode, platinum wire as a counter electrode, and a saturated calomel electrode as a reference electrode. UV-vis spectra were collected using an 8453 UV-Vis spectrophotometer (Agilent Technologies, USA).

2.3 Experimental methods

2.3.1 Preparation of the modified electrode

First, 6 cm graphite rod was taken and sealed in the polyethylene pipe using epoxy resin binder. One of the ends was used as electrode connection and the other was polished to a mirror-like surface by sandpaper. The mirror side was carefully polished for about 15 min using functionalized MWCNTs on parchment paper to embed MWCNTs uniformly on the electrode surface and the inlaid MWCNTs/GE were obtained. Finally, the prepared electrode was electrochemically cleaned in based solution with cycling potentials between −0.4 and +2.0 V at 0.1 V/s until a steady CV curve was observed.

2.3.2 Electrochemical analysis

One milliliter of sample solution at a specific concentration was added to 9.0 mL of appropriate buffer solution. CV and differential pulse voltammetry (DPV) analyses of the mixed solutions were performed using the three-electrode system under the optimization of the pulse amplitude, pulse interval, and pulse width conditions.

2.3.3 Determination of the antioxidant activity of four polyphenolic compounds

The antioxidation activity of four polyphenolic compounds was investigated by the electrochemical analysis method and spectrophotometry [32].

  1. Ethical approval: The conducted research is not related to either human or animal use.

3 Results and discussion

3.1 Electrochemical behavior of the polyphenols under the MWCNTs/GE presence

3.1.1 CV analysis

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).

Figure 1 The CV curve of four polyphenolic compounds on the MWCNTs/GE. (a((a) Na2HPO4–C6H8O7, (b) Na2HPO4–C6H8O7 + 3 mg/L of GA, (c) Na2HPO4–C6H8O7 + 5 mg/L of GA)); (b((a) HAc–NaAc, (b) HAc–NaAc + 0.5 mg/L of CA, (c) HAc–NaAc + 1 mg/L of CA)); (c((a) C6H8O7–Na3C6H5O7, (b) C6H8O7–Na3C6H5O7 + 6 mg/L of FA, (c) C6H8O7–Na3C6H5O7 + 10 mg/L of FA)); and (d((a) KCl, (b) KCl + 10 mg/L of VA, (c) KCl + 15 mg/L of VA)).
Figure 1

The CV curve of four polyphenolic compounds on the MWCNTs/GE. (a((a) Na2HPO4–C6H8O7, (b) Na2HPO4–C6H8O7 + 3 mg/L of GA, (c) Na2HPO4–C6H8O7 + 5 mg/L of GA)); (b((a) HAc–NaAc, (b) HAc–NaAc + 0.5 mg/L of CA, (c) HAc–NaAc + 1 mg/L of CA)); (c((a) C6H8O7–Na3C6H5O7, (b) C6H8O7–Na3C6H5O7 + 6 mg/L of FA, (c) C6H8O7–Na3C6H5O7 + 10 mg/L of FA)); and (d((a) KCl, (b) KCl + 10 mg/L of VA, (c) KCl + 15 mg/L of VA)).

Table 1

The optimum condition of the CV curve of four polyphenolic compounds

Matter Range of potential (V) Based solution Based solution pH Scan rate (V/s)
GA 0.1–0.8 Na2HPO4–C6H8O7 pH = 4.40 0.07
CA 0–0.8 HAc–NaAc pH = 4.40 0.05
FA −0.4 to 1.0 C6H8O7–Na3C6H5O7 pH = 5.40 0.10
VA –0.2 to 1.3 KCl 100 mmol/L 0.05

3.1.2 The DPV analysis

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.

Figure 2 The DPV curve of four polyphenolic compounds on the MWCNTs/GE.
Figure 2

The DPV curve of four polyphenolic compounds on the MWCNTs/GE.

Table 2

The optimum condition of the DPV curve of four polyphenolic compounds

Matter Range of potential (V) Based solution Pulsewidth (ms) Pulse amplitude (mV) Pulse interval (ms)
GA 0.1–0.8 Na2HPO4–C6H8O7 (pH 4.40) 80 30 130
CA 0–0.8 HAc–NaAc (pH 4.40) 200 30 200
FA −0.4 to 1.0 C6H8O7−Na3C6H5O7 (pH 5.40) 100 30 130
VA −0.2 to 1.3 100 mmol/L KCl 20 30 220
Table 3

The linear relationship between i pa and concentration of four polyphenolic compounds

Matter Range of concentration (g/L) Linear regression equation r RSD (n = 5)
GA 2.0 × 10−6–8.0 × 10−4 i pa (μA) = 0.3187c + 0.3329 0.9996 1.015
CA 6.0 × 10−7–1.0 × 10−4 i pa (μA) = 1.8564c + 0.9736 0.9988 4.038
FA 2.0 × 10−6–1.2 × 10−4 i pa (μA) = 2.0850c − 0.5263 0.9912 1.575
VA 6.0 × 10−6–5.0 × 10−4 i pa (μA) = 0.3411c + 0.8118 0.9996 2.743

3.2 Supporting electrolyte effect

For these tests, we used 0.01 mol/L of citric acid–sodium citrate buffer solution (C6H8O7–Na3C6H5O7), 0.01 mol/L of disodium hydrogen phosphate–potassium dihydrogen phosphate buffer solution (Na2HPO4–KH2PO4), 0.01 mol/L of phosphate buffer solutions (PBS), 0.01 mol/L of citric acid–disodium hydrogen phosphate buffer solution (C6H8O7–Na2HPO4), 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 Na2HPO4–C6H8O7 buffer solution (GA), HAc–NaAc buffer solution (CA), Na2HPO4–C6H8O7 buffer solution (FA), and KCl buffer solution (VA).

Figure 3 The influence of supporting electrolyte on the redox peak current of four polyphenolic compounds. (a((a) Na2HPO4–KH2PO4, (b) PBS, (c) C6H8O7–Na3C6H5O7, (d) Na2HPO4–C6H8O7, (e) HAc–NaAc, c = 0.01 mol/L and ν = 0.05 V/s)); (b((a) C6H8O7–Na3C6H5O7, (b) PBS, (c) HAc–NaAc, (d) B–R e: KCl, c = 0.01 mol/L and ν = 0.05 V/s)); (c((a) Na2HPO4–KH2PO4, (b) C6H8O7–Na3C6H5O7, (c) Na2HPO4–C6H8O7, (d) PBS, (e) HAc–NaAc, c = 0.01 mol/L and ν = 0.10 V/s)); and (d((a) HAc–NaAc, (b) KCl, (c) PBS, (d) B–R, c = 0.01 mol/L and ν = 0.05 V/s)).
Figure 3

The influence of supporting electrolyte on the redox peak current of four polyphenolic compounds. (a((a) Na2HPO4–KH2PO4, (b) PBS, (c) C6H8O7–Na3C6H5O7, (d) Na2HPO4–C6H8O7, (e) HAc–NaAc, c = 0.01 mol/L and ν = 0.05 V/s)); (b((a) C6H8O7–Na3C6H5O7, (b) PBS, (c) HAc–NaAc, (d) B–R e: KCl, c = 0.01 mol/L and ν = 0.05 V/s)); (c((a) Na2HPO4–KH2PO4, (b) C6H8O7–Na3C6H5O7, (c) Na2HPO4–C6H8O7, (d) PBS, (e) HAc–NaAc, c = 0.01 mol/L and ν = 0.10 V/s)); and (d((a) HAc–NaAc, (b) KCl, (c) PBS, (d) B–R, c = 0.01 mol/L and ν = 0.05 V/s)).

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.

3.3 Influence of pH of supporting electrolyte

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).

Figure 4 pH effect on the redox peak current of four polyphenolic compounds. (a((a) pH = 7.00, (b) pH = 6.00, (c) pH = 5.00, (d) pH = 4.40, (e) pH = 4.00, (f) pH = 3.00, and (g) pH = 2.20)); (b((a) pH = 3.60, (b) pH = 4.00, (c) pH = 4.40, (d) pH = 4.80, (e) pH = 5.40, and (f) pH = 5.80)); (c((a) pH = 3.00, (b) pH = 4.00, (c) pH = 5.00, (d) pH = 5.40, (e) pH = 6.00, (f) pH = 6.60, and (g) pH = 7.0)); and (d((a) 400 mmol/L, (b) 200 mmol/L, (c) 40 mmol/L, (d) 80 mmol/L, and (e) 100 mmol/L)).
Figure 4

pH effect on the redox peak current of four polyphenolic compounds. (a((a) pH = 7.00, (b) pH = 6.00, (c) pH = 5.00, (d) pH = 4.40, (e) pH = 4.00, (f) pH = 3.00, and (g) pH = 2.20)); (b((a) pH = 3.60, (b) pH = 4.00, (c) pH = 4.40, (d) pH = 4.80, (e) pH = 5.40, and (f) pH = 5.80)); (c((a) pH = 3.00, (b) pH = 4.00, (c) pH = 5.00, (d) pH = 5.40, (e) pH = 6.00, (f) pH = 6.60, and (g) pH = 7.0)); and (d((a) 400 mmol/L, (b) 200 mmol/L, (c) 40 mmol/L, (d) 80 mmol/L, and (e) 100 mmol/L)).

Table 4

The linear relationship between pH and peak potential E p of three polyphenolic compounds

Matter Range of pH Linear regression equation r m
GA 2.20–7.00 E pa (V) = −0.0599 pH + 0.8057 0.9984 3.04
CA 4.00–5.80 E pa (V) = −0.6190 pH + 0.9786 0.9879 2.10
3.60–5.80 E pc (V) = −0.0586 pH + 0.5824 0.9983 1.99
FA 3.00–7.00 E pa (V) = −0.0848 pH + 0.9680 0.9908 1.44
3.00–7.00 E pc (V) = −0.0731 pH + 0.6791 0.9902 1.24

3.4 Scan rate effect on the electrodes

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).

Figure 5 Electrochemical behavior of the polyphenols as a function of the CV scan rate (υ).
Figure 5

Electrochemical behavior of the polyphenols as a function of the CV scan rate (υ).

Table 5

Linear relationship of four polyphenolic compounds between oxidation peak current and scan rate

Matter Range of scan rate (V/s) Linear regression equation r Control process
GA 0.02–0.07 i pa (μA) = 240.0ν + 21.03 0.9987 Adsorption control
0.07–0.12 i pa (μA) = −182.9ν + 51.22 0.9984
CA 0.01–0.12 i pa (μA) = 255.7ν + 27.49 0.9979 Adsorption control
0.01–0.12 i pa (μA) = 246.3ν + 27.04 0.9980
FA 0.02–0.10 i pa (μA) = 326.48ν + 10.60 0.9924 Adsorption control
0.02–0.10 i pa (μA) = 481.80ν + 1.526 0.9972
VA 0.02–0.08 i pa (μA) = 283.2ν + 29.97 0.9958 Adsorption control
0.02–0.08 i pa (μA) = 385.4ν + 14.54 0.9942

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 = 8.1345 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 0.2826 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 ) ,

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) = 0.0477 lg ν + 0.5054 0.9900 3 2.27 × 10−10 3 0.41 0.27
CA E pa (V) = 0.0843 lg ν + 0.6150 0.9967 2 6.18 × 10−10 2 0.35 2.32
FA E pa (V) = 0.0683 lg ν + 0.4334 0.9992 1 1.41 × 10−10 1 0.87 0.86
VA E pa (V) = 0.0619 lg ν + 0.5512 0.9983 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 The electrochemical oxidation mechanism of four polyphenolic compounds.
Figure 6

The electrochemical oxidation mechanism of four polyphenolic compounds.

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.

3.6 Comparison of antioxidant activity of four polyphenolic compounds

3.6.1 The structure of four polyphenolic compounds (Figure 7)

3.6.2 Comparison of antioxidation activity of the polyphenol

The electrochemical response of four polyphenolic compounds was investigated in 0.01 mol/L of HAc–NaAc buffer solution (pH = 4.00) on the MWCNTs/GE at 50 mV/s scan rate. The peak potential of four polyphenolic compounds (GA, CA, VA, and FA) is 0.48, 0.51, 0.60, and 0.65 V, respectively. And the CV of four polyphenolic compounds is showed in Figure 8.

Figure 7 The structure of four polyphenolic compounds.
Figure 7

The structure of four polyphenolic compounds.

Figure 8 The cyclic voltammogram of four polyphenolic compounds.
Figure 8

The cyclic voltammogram of four polyphenolic compounds.

Wellington et al. investigated the electrochemical oxidation behavior of caffeic acid, ferulic acid, chlorogenic acid, and p-coumaric acid in HAc–NaAc (pH = 5.6) by CV. They concluded that the antioxidation activity of these polyphenols is related to the oxidation potential (E pa), and caffeic acid (E pa = 0.31 V) > chlorogenic acid (E pa = 0.38 V) > ferulic acids (E pa = O.43 V) > p-coumaric acid (E pa = 0.73 V). The result indicated that compounds with lower oxidation potentials show stronger antioxidation activity, whereas compounds with high E pa values show lower antioxidant activity. So, we can infer that the antioxidant activity of four polyphenolic compound: GA (E pa = 0.48 V) > CA (E pa = 0.51 V) > FA (E pa = 0.60 V) > VA (E pa = 0.65 V).

Based on the theoretical analysis, polyphenolic acids are suitable hydrogen donors, whose ability to remove free radicals is mainly associated with phenolic OH groups, and –CHCHCOOH groups are not the necessary groups for phenolic acids scavenging free radicals [45]. Their antioxidation activities are related to the number and position of phenolic OH groups and the other factors such as dehydrogenation reaction and the stability of the generated free radicals. The more hydroxyl groups on the benzene ring, the stronger the antioxidant activity is. The antioxidant activity decreases when the hydroxyl groups on the benzene ring are replaced by the electron-withdrawing groups, whereas the antioxidant activity will be increased.

From Figure 8, GA with three ortho-hydroxyls showed the strongest antioxidant capacity compared with the other four polyphenolic compounds. CA has less antioxidant capacity than GA, and CA turns into FA when the ortho-hydroxyl of CA is replaced by electron-withdrawing group (methoxyl group), and the antioxidant capacity decreased, which indicated the antioxidant capacity of CA stronger than FA. The results further confirmed the sequence of antioxidant activities of the four polyphenols.

3.6.3 Comparison of antioxidant activity of four polyphenolic compounds by spectrophotometry

DPPH free radical scavenging of our polyphenols possessed an excellent linear relationship between concentration and clearance E before reaching the maximum clearance (Figure 9).

Figure 9 DPPH free radical scavenging of polyphenols studied in this work.
Figure 9

DPPH free radical scavenging of polyphenols studied in this work.

In addition, the 50% radical scavenging activity (IC50) of our polyphenols was obtained using linear equations shown in Table 7. Lower IC50 values imply higher radical scavenging. Thus, the radical scavenging ability was in the following order: GA > CA > FA > VA, which agreed with the electrochemical method’s result. Thus, our electrochemical method to evaluate antioxidation activities of polyphenols is reliable, and the micro-information of the reaction mechanism is also obtained.

Table 7

The linear relationship of four polyphenolic compounds on DPPH free radical clearance

Matter Linear equation R 2 IC50 Linear range
GA y = 49.34x + 4.5849 0.9755 0.92 μg/mL 0–1.6 μg/mL
CA y = 14.518x + 0.1982 0.9913 3.43 μg/mL 0–6.0 μg/mL
FA y = 74.736x + 9.116 0.9282 0.55 mg/mL 0–1.0 mg/mL
VA y = 17.457x + 10.388 0.9565 2.27 mg/mL 0–4.75 mg/mL

4 Conclusion

Electrochemical methods are powerful analytical tools capable of providing insights into the reaction mechanisms and electron transfer. The electrochemical behavior of four polyphenolic compounds was investigated by CV using the inlaid MWCNTs/GE. The obtained micro-information such as the number of electrons and protons were used to deduce the electrochemical oxidation mechanism of four polyphenolic compounds. The antioxidation properties of these polyphenols were analyzed electrochemically and by UV-vis spectrophotometry. The compounds with lower oxidation potentials demonstrated stronger antioxidation abilities. The compounds with high E pa values showed lower antioxidant activity. The scavenging ability of our polyphenols could be described in the following order: GA > CA > FA > VA, which was corroborated by the spectrophotometry data. Thus, experimentally obtained oxidation potentials strongly correlate with the structures of the polyphenolic compounds. The polyphenol structures also influence their corresponding antioxidation activities. The compounds with several electron-donating groups typically possess lower anodic peak potentials and higher antioxidant abilities than monosubstituted phenols. However, phenolic OH groups have stronger effects than OCH3 ones.

  1. Funding information: This work was supported by the National Natural Science Foundation of China (No. 201367025), program for the National Natural Science Foundation of China (No. 21665027), program for the Applied Basic Research Project of Yunnan Province Youth Program (No. 2017FD118), and Scientific research fund project of Yunnan Education Department (No. 2021J0656).

  2. Author contributions: Investigation: L.X.F., X.H.B.; resources: G.Y.T., Y. Z.; funding acquisition: Y.Z.; methodology: G.Y.T., Y. Z.; data curation: L.X.F., X.H.B.

  3. Conflict of interest: We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service, and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “The Electrochemical Redox Mechanism and Antioxidant Activity of polyphenolic compounds Based on inlaid MWCNTs modified graphite electrode.”

  4. Data availability statement: Raw data were generated at the facility: ZAHNER Zennium IM6 Electrochemical Workstation using UV-Vis spectrophotometer, etc. Derived data supporting the findings of this study are available from the corresponding author on request.

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Received: 2021-05-08
Revised: 2021-08-17
Accepted: 2021-08-30
Published Online: 2021-09-24

© 2021 Xiaofen Li et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/chem-2021-0087
Keywords for this article
antioxidant activity; polyphenolic compounds; cyclic voltammetry; differential pulse voltammetry
Creative Commons
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  8. Induction of apoptosis and cell cycle arrest by chloroform fraction of Juniperus phoenicea and chemical constituents analysis
  9. Recovery of γ-Fe2O3 from copper ore tailings by magnetization roasting and magnetic separation
  10. Effects of different extraction methods on antioxidant properties of blueberry anthocyanins
  11. Modeling the removal of methylene blue dye using a graphene oxide/TiO2/SiO2 nanocomposite under sunlight irradiation by intelligent system
  12. Antimicrobial and antioxidant activities of Cinnamomum cassia essential oil and its application in food preservation
  13. Full spectrum and genetic algorithm-selected spectrum-based chemometric methods for simultaneous determination of azilsartan medoxomil, chlorthalidone, and azilsartan: Development, validation, and application on commercial dosage form
  14. Evaluation of the performance of immunoblot and immunodot techniques used to identify autoantibodies in patients with autoimmune diseases
  15. Computational studies by molecular docking of some antiviral drugs with COVID-19 receptors are an approach to medication for COVID-19
  16. Synthesis of amides and esters containing furan rings under microwave-assisted conditions
  17. Simultaneous removal efficiency of H2S and CO2 by high-gravity rotating packed bed: Experiments and simulation
  18. Design, synthesis, and biological activities of novel thiophene, pyrimidine, pyrazole, pyridine, coumarin and isoxazole: Dydrogesterone derivatives as antitumor agents
  19. Content and composition analysis of polysaccharides from Blaps rynchopetera and its macrophage phagocytic activity
  20. A new series of 2,4-thiazolidinediones endowed with potent aldose reductase inhibitory activity
  21. Assessing encapsulation of curcumin in cocoliposome: In vitro study
  22. Rare norisodinosterol derivatives from Xenia umbellata: Isolation and anti-proliferative activity
  23. Comparative study of antioxidant and anticancer activities and HPTLC quantification of rutin in white radish (Raphanus sativus L.) leaves and root extracts grown in Saudi Arabia
  24. Comparison of adsorption properties of commercial silica and rice husk ash (RHA) silica: A study by NIR spectroscopy
  25. Sodium borohydride (NaBH4) as a high-capacity material for next-generation sodium-ion capacitors
  26. Aroma components of tobacco powder from different producing areas based on gas chromatography ion mobility spectrometry
  27. The effects of salinity on changes in characteristics of soils collected in a saline region of the Mekong Delta, Vietnam
  28. Synthesis, properties, and activity of MoVTeNbO catalysts modified by zirconia-pillared clays in oxidative dehydrogenation of ethane
  29. Synthesis and crystal structure of N,N′-bis(4-chlorophenyl)thiourea N,N-dimethylformamide
  30. Quantitative analysis of volatile compounds of four Chinese traditional liquors by SPME-GC-MS and determination of total phenolic contents and antioxidant activities
  31. A novel separation method of the valuable components for activated clay production wastewater
  32. On ve-degree- and ev-degree-based topological properties of crystallographic structure of cuprite Cu2O
  33. Antihyperglycemic effect and phytochemical investigation of Rubia cordifolia (Indian Madder) leaves extract
  34. Microsphere molecularly imprinted solid-phase extraction for diazepam analysis using itaconic acid as a monomer in propanol
  35. A nitric oxide-releasing prodrug promotes apoptosis in human renal carcinoma cells: Involvement of reactive oxygen species
  36. Machine vision-based driving and feedback scheme for digital microfluidics system
  37. Study on the application of a steam-foam drive profile modification technology for heavy oil reservoir development
  38. Ni–Ru-containing mixed oxide-based composites as precursors for ethanol steam reforming catalysts: Effect of the synthesis methods on the structural and catalytic properties
  39. Preparation of composite soybean straw-based materials by LDHs modifying as a solid sorbent for removal of Pb(ii) from water samples
  40. Synthesis and spectral characterizations of vanadyl(ii) and chromium(iii) mixed ligand complexes containing metformin drug and glycine amino acid
  41. In vitro evaluation of lactic acid bacteria with probiotic activity isolated from local pickled leaf mustard from Wuwei in Anhui as substitutes for chemical synthetic additives
  42. Utilization and simulation of innovative new binuclear Co(ii), Ni(ii), Cu(ii), and Zn(ii) diimine Schiff base complexes in sterilization and coronavirus resistance (Covid-19)
  43. Phosphorylation of Pit-1 by cyclin-dependent kinase 5 at serine 126 is associated with cell proliferation and poor prognosis in prolactinomas
  44. Molecularly imprinted membrane for transport of urea, creatinine, and vitamin B12 as a hemodialysis candidate membrane
  45. Optimization of Murrayafoline A ethanol extraction process from the roots of Glycosmis stenocarpa, and evaluation of its Tumorigenesis inhibition activity on Hep-G2 cells
  46. Highly sensitive determination of α-lipoic acid in pharmaceuticals on a boron-doped diamond electrode
  47. Synthesis, chemo-informatics, and anticancer evaluation of fluorophenyl-isoxazole derivatives
  48. In vitro and in vivo investigation of polypharmacology of propolis extract as anticancer, antibacterial, anti-inflammatory, and chemical properties
  49. Topological indices of bipolar fuzzy incidence graph
  50. Preparation of Fe3O4@SiO2–ZnO catalyst and its catalytic synthesis of rosin glycol ester
  51. Construction of a new luminescent Cd(ii) compound for the detection of Fe3+ and treatment of Hepatitis B
  52. Investigation of bovine serum albumin aggregation upon exposure to silver(i) and copper(ii) metal ions using Zetasizer
  53. Discoloration of methylene blue at neutral pH by heterogeneous photo-Fenton-like reactions using crystalline and amorphous iron oxides
  54. Optimized extraction of polyphenols from leaves of Rosemary (Rosmarinus officinalis L.) grown in Lam Dong province, Vietnam, and evaluation of their antioxidant capacity
  55. Synthesis of novel thiourea-/urea-benzimidazole derivatives as anticancer agents
  56. Potency and selectivity indices of Myristica fragrans Houtt. mace chloroform extract against non-clinical and clinical human pathogens
  57. Simple modifications of nicotinic, isonicotinic, and 2,6-dichloroisonicotinic acids toward new weapons against plant diseases
  58. Synthesis, optical and structural characterisation of ZnS nanoparticles derived from Zn(ii) dithiocarbamate complexes
  59. Presence of short and cyclic peptides in Acacia and Ziziphus honeys may potentiate their medicinal values
  60. The role of vitamin D deficiency and elevated inflammatory biomarkers as risk factors for the progression of diabetic nephropathy in patients with type 2 diabetes mellitus
  61. Quantitative structure–activity relationship study on prolonged anticonvulsant activity of terpene derivatives in pentylenetetrazole test
  62. GADD45B induced the enhancing of cell viability and proliferation in radiotherapy and increased the radioresistance of HONE1 cells
  63. Cannabis sativa L. chemical compositions as potential plasmodium falciparum dihydrofolate reductase-thymidinesynthase enzyme inhibitors: An in silico study for drug development
  64. Dynamics of λ-cyhalothrin disappearance and expression of selected P450 genes in bees depending on the ambient temperature
  65. Identification of synthetic cannabinoid methyl 2-{[1-(cyclohexylmethyl)-1H-indol-3-yl] formamido}-3-methylbutanoate using modern mass spectrometry and nuclear magnetic resonance techniques
  66. Study on the speciation of arsenic in the genuine medicinal material honeysuckle
  67. Two Cu(ii)-based coordination polymers: Crystal structures and treatment activity on periodontitis
  68. Conversion of furfuryl alcohol to ethyl levulinate in the presence of mesoporous aluminosilicate catalyst
  69. Review Articles
  70. Hsien Wu and his major contributions to the chemical era of immunology
  71. Overview of the major classes of new psychoactive substances, psychoactive effects, analytical determination and conformational analysis of selected illegal drugs
  72. An overview of persistent organic pollutants along the coastal environment of Kuwait
  73. Mechanism underlying sevoflurane-induced protection in cerebral ischemia–reperfusion injury
  74. COVID-19 and SARS-CoV-2: Everything we know so far – A comprehensive review
  75. Challenge of diabetes mellitus and researchers’ contributions to its control
  76. Advances in the design and application of transition metal oxide-based supercapacitors
  77. Color and composition of beauty products formulated with lemongrass essential oil: Cosmetics formulation with lemongrass essential oil
  78. The structural chemistry of zinc(ii) and nickel(ii) dithiocarbamate complexes
  79. Bioprospecting for antituberculosis natural products – A review
  80. Recent progress in direct urea fuel cell
  81. Rapid Communications
  82. A comparative morphological study of titanium dioxide surface layer dental implants
  83. Changes in the antioxidative properties of honeys during their fermentation
  84. Erratum
  85. Erratum to “Corrosion study of copper in aqueous sulfuric acid solution in the presence of (2E,5E)-2,5-dibenzylidenecyclopentanone and (2E,5E)-bis[(4-dimethylamino)benzylidene]cyclopentanone: Experimental and theoretical study”
  86. Erratum to “Modified TDAE petroleum plasticiser”
  87. Corrigendum
  88. Corrigendum to “A nitric oxide-releasing prodrug promotes apoptosis in human renal carcinoma cells: Involvement of reactive oxygen species”
  89. Special Issue on 3rd IC3PE 2020
  90. Visible light-responsive photocatalyst of SnO2/rGO prepared using Pometia pinnata leaf extract
  91. Antihyperglycemic activity of Centella asiatica (L.) Urb. leaf ethanol extract SNEDDS in zebrafish (Danio rerio)
  92. Selection of oil extraction process from Chlorella species of microalgae by using multi-criteria decision analysis technique for biodiesel production
  93. Special Issue on the 14th Joint Conference of Chemistry (14JCC)
  94. Synthesis and in vitro cytotoxicity evaluation of isatin-pyrrole derivatives against HepG2 cell line
  95. CO2 gas separation using mixed matrix membranes based on polyethersulfone/MIL-100(Al)
  96. Effect of synthesis and activation methods on the character of CoMo/ultrastable Y-zeolite catalysts
  97. Special Issue on Electrochemical Amplified Sensors
  98. Enhancement of graphene oxide through β-cyclodextrin composite to sensitive analysis of an antidepressant: Sulpiride
  99. Investigation of the spectroelectrochemical behavior of quercetin isolated from Zanthoxylum bungeanum
  100. An electrochemical sensor for high sensitive determination of lysozyme based on the aptamer competition approach
  101. An improved non-enzymatic electrochemical sensor amplified with CuO nanostructures for sensitive determination of uric acid
  102. Special Issue on Applied Biochemistry and Biotechnology 2020
  103. Fast discrimination of avocado oil for different extracted methods using headspace-gas chromatography-ion mobility spectroscopy with PCA based on volatile organic compounds
  104. Effect of alkali bases on the synthesis of ZnO quantum dots
  105. Quality evaluation of Cabernet Sauvignon wines in different vintages by 1H nuclear magnetic resonance-based metabolomics
  106. Special Issue on the Joint Science Congress of Materials and Polymers (ISCMP 2019)
  107. Diatomaceous Earth: Characterization, thermal modification, and application
  108. Electrochemical determination of atenolol and propranolol using a carbon paste sensor modified with natural ilmenite
  109. Special Issue on the Conference of Energy, Fuels, Environment 2020
  110. Assessment of the mercury contamination of landfilled and recovered foundry waste – a case study
  111. Primary energy consumption in selected EU Countries compared to global trends
  112. Modified TDAE petroleum plasticiser
  113. Use of glycerol waste in lactic acid bacteria metabolism for the production of lactic acid: State of the art in Poland
  114. Topical Issue on Applications of Mathematics in Chemistry
  115. Theoretical study of energy, inertia and nullity of phenylene and anthracene
  116. Banhatti, revan and hyper-indices of silicon carbide Si2C3-III[n,m]
  117. Topical Issue on Agriculture
  118. Occurrence of mycotoxins in selected agricultural and commercial products available in eastern Poland
  119. Special Issue on Ethnobotanical, Phytochemical and Biological Investigation of Medicinal Plants
  120. Acute and repeated dose 60-day oral toxicity assessment of chemically characterized Berberis hispanica Boiss. and Reut in Wistar rats
  121. Phytochemical profile, in vitro antioxidant, and anti-protein denaturation activities of Curcuma longa L. rhizome and leaves
  122. Antiplasmodial potential of Eucalyptus obliqua leaf methanolic extract against Plasmodium vivax: An in vitro study
  123. Prunus padus L. bark as a functional promoting component in functional herbal infusions – cyclooxygenase-2 inhibitory, antioxidant, and antimicrobial effects
  124. Molecular and docking studies of tetramethoxy hydroxyflavone compound from Artemisia absinthium against carcinogens found in cigarette smoke
  125. Special Issue on the Joint Science Congress of Materials and Polymers (ISCMP 2020)
  126. Preparation of cypress (Cupressus sempervirens L.) essential oil loaded poly(lactic acid) nanofibers
  127. Influence of mica mineral on flame retardancy and mechanical properties of intumescent flame retardant polypropylene composites
  128. Production and characterization of thermoplastic elastomer foams based on the styrene–ethylene–butylene–styrene (SEBS) rubber and thermoplastic material
  129. Special Issue on Applied Chemistry in Agriculture and Food Science
  130. Impact of essential oils on the development of pathogens of the Fusarium genus and germination parameters of selected crops
  131. Yield, volume, quality, and reduction of biotic stress influenced by titanium application in oilseed rape, winter wheat, and maize cultivations
  132. Influence of potato variety on polyphenol profile composition and glycoalcaloid contents of potato juice
  133. Carryover effect of direct-fed microbial supplementation and early weaning on the growth performance and carcass characteristics of growing Najdi lambs
  134. Special Issue on Applied Biochemistry and Biotechnology (ABB 2021)
  135. The electrochemical redox mechanism and antioxidant activity of polyphenolic compounds based on inlaid multi-walled carbon nanotubes-modified graphite electrode
  136. Study of an adsorption method for trace mercury based on Bacillus subtilis
  137. Special Issue on The 1st Malaysia International Conference on Nanotechnology & Catalysis (MICNC2021)
  138. Mitigating membrane biofouling in biofuel cell system – A review
  139. Mechanical properties of polymeric biomaterials: Modified ePTFE using gamma irradiation
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