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Electrochemical evaluation of the antioxidant capacity of natural compounds on glassy carbon electrode modified with guanine-, polythionine-, and nitrogen-doped graphene

  • Yafen Fu , Zongyi You , Aiping Xiao , Liangliang Liu EMAIL logo and Weien Zhou EMAIL logo
Published/Copyright: August 19, 2020
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Open Chemistry
From the journal Open Chemistry Volume 18 Issue 1

Abstract

An electrochemical sensor based on guanine-, polythionine-, and nitrogen-doped graphene modified glassy carbon electrode (G/PTH/NG/GCE) was fabricated and applied for antioxidant capacity evaluation of natural compounds and complexes in electrochemical method since natural sources of active compounds exhibited various antioxidant activities. When the antioxidants existed in the system, the generated hydroxyl radicals were scavenged and the damage to guanine immobilized on the electrode was reduced less resulting in the oxidation peak current increased in square wave voltammetry. After the modifications of polythionine- and nitrogen-doped graphene, the oxidation peak current was improved. The effects of pH, incubation time, and concentrations of guanine and Fe2+ ions on the performances of the electrode were investigated and optimized. The G/PTH/NG/GCE showed good linearity, reproducibility, and storage stability for antioxidant capacity evaluation of ascorbic acid at the optimum conditions. The antioxidant capacities of three flavonoids and three plant extracts were measured using the G/PTH/NG/GCE and DPPH methods. Myricetin showed the highest antioxidant capacity in both electrochemical and DPPH methods. The proposed G/PTH/NG/GCE exhibited easy fabrication procedure, rapid detection time, and low cost for the detection of antioxidant activity for various kinds of samples.

Keywords: antioxidant capacity; electrochemical sensor; flavonoids; nitrogen-doped graphene; polythionine

1 Introduction

Reactive oxygen species (ROS) were common in natural biological processes. However, when there was an accumulation of ROS induced by environment and chemicals, the oxidative stress would occur in the body and cause damage of proteins, lipids, and even cells [1]. This process could lead to various chronic diseases such as cancer, muscular dystrophy, heart disease, and atherosclerosis [2]. Antioxidant compounds existing in various fruits, vegetables, and beverages could react with ROS by scavenging free radicals, transition metal chelation, and singlet oxygen quenching [3]. Flavonoids and polyphenols compounds were the major class of antioxidants that came from diet possessing many activities in anti-inflammation and anti-virus [4]. Accordingly, efficient detection of antioxidant capacity for various samples could promote the discovery of new antioxidants and related researches. Different methods were proposed as tools for the analysis of antioxidant capacity for compounds or complex mixtures such as absorption spectroscopy and high-performance liquid chromatography [5]. However, the main problem of spectroscopic methods was the color of samples that would interfere with the reaction system and affect the precision of detection [6]. As an easy, sensitive, and fast technique, the electrochemical method could become a suitable option for evaluating the antioxidant content and the reducing ability of mixture [7]. Based on a specific redox reaction, there was no need for difficult sample preparation, and the disturbance in the detection was much less [8].

Graphene was a kind of carbon nanomaterial with high-specific surface area and conductivity, which has been used in electrochemical sensing [9]. Heteroatom doping and functionalization of graphene could make an improved performance in gravimetric capacitance and magnetic response by increasing conductivity, pseudo-capacitive behavior, electron transfer, and so on [10]. Recently, nitrogen-doped graphene (NG) showed advantages in electrocatalysis owing to good chemical stability and high-electrical conductivity [11]. It has attracted particular research interests in electrocatalyst [12], semiconductor [13], lithium-ion batteries [14], and other fields [15]. Therefore, it could be expected that NG could exhibit enhanced performance in chemical and electrical properties.

Thionine was a phenothiazine redox dye with good solubility in water and ethanol [16]. Polythionine (PTH) was widely applied in the modification of electrochemical sensors because of its good redox reversibility, fast charge transfer capacity, and good stability [17]. An electroactive and conductive PTH membrane could be easily accomplished on the surface of the electrode using electrochemical polymerization. This kind of fabrication showed advantages in the thickness of the film, response time, and charge transfer rate than the suspension coating method [18,19]. It has been reported that PTH-modified electrode was applied for the determination of endocrine-disrupting compounds, hydrogen peroxide, hepatitis B, and nicotinamide adenine dinucleotide [20,21,22,23].

In this study, NG and PTH membranes were consequently modified on glassy carbon electrode (PTH/NG/GCE) to combine the excellent properties of them in electrical conductivity and adsorption ability. Then, guanine was immobilized on PTH/NG/GCE (G/PTH/NG/GCE) and used as a stable G/PTH/NG/GCE electrochemical sensor in the evaluation of antioxidant capacity. During the detection procedures, the hydroxyl radicals could make damage guanine on electrode, resulting in the decrease of oxidation peak current in square wave voltammetry (SWV). When antioxidants existed in the reaction system, the number of hydroxyl radicals would decrease due to the antioxidant activity, and there would be less damages on guanine. As a result, a higher peak current in SWV was observed and the following calculation could be completed. After modification, the proposed G/PTH/NG/GCE showed higher oxidation peak current, which meant higher sensitivity for evaluation. It could be directly used in the determination of antioxidant capacity for compounds and mixture without extra processes and reagents. Therefore, this modified electrode could provide a fast and cheap sensor for the analysis of antioxidants with promising prospects in the food and chemical industries.

2 Experimental

2.1 Chemicals and apparatus

Ethylenediaminetetraacetic acid (EDTA), guanine, and thionine were purchased from Merck KGaA (Darmstadt, Germany). Ammonium hydroxide, hydrogen peroxide, sulfuric acid, potassium permanganate, urea, and nitric acid were purchased from Sinopharm Chemical Reagent (Shanghai, China). Graphite flakes were purchased from Nanjing XFNANO Materials Tech Co., Ltd (Nanjing, China). Myricetin, kaempferol, and galangin were acquired from Shanghai Yuanye Biotechnology Co., Ltd (Shanghai, China). Ultrapure water (18.25 MΩ cm) was obtained from a Millipore system (Millipore Corporation, Milford, MA, USA). All other chemicals were of analytical grade without further purification.

The electrochemical measurements were accomplished on a CHI 660E Electrochemical Workstation (Shanghai Chenhua Instrument Company, Shanghai, China) using a standard three-electrode system. The characterizations of materials were conducted by transmission electron microscope (TEM) and scanning electron microscope (SEM). The observations were completed by a JSM 6610Lv scanning electron microscopy (JEOL, Tokyo, Japan) operated at 25 kV and a JEM-2100 electron microscope (JEOL, Akishima, Japan).

2.2 Preparation of NG

Graphene oxide (GO) was prepared according to the Hummers method with some modifications [24]. About 2.0 g of graphite flakes and 8.0 g of KMnO4 were added into 100 mL of ice-cold concentrated H2SO4. The mixture was kept in an ice bath and stirred for 3 h. Then, 400 mL of water containing 20 mL of 30% H2O2 was slowly added. The produced golden mixture was filtrated and the solid was washed with water and 5% HCl three times, respectively. The finally obtained solid was dried in vacuum at 60°C for 24 h.

For the preparation of NG, 0.1 g of GO was dispersed into 100 mL of water. About 2.0 g of urea was added into GO solution as the source of nitrogen and ultrasonically treated for 2 h. Then, the solution was transferred into a stainless-steel autoclave for the reaction at 180°C for 12 h. Finally, the products were washed with lots of water and dried in vacuum at 50°C for 24 h.

2.3 Fabrication of G/PTH/NG/GCE

The G/PTH/NG/GCE was fabricated by depositions of NG, guanine, and electropolymerization of thionine. Specifically, NG suspension (0.5 mg mL−1) was sonicated for 30 min prior to use. After sonication, 6 µL of NG suspension was dropped on the surface of a GCE and dried at room temperature. Then, the electrode was immersed into 4.4 mol L−1 acetic acid containing 0.5 mmol L−1 thionine solution and scanned from −0.4 to 1.4 V by cyclic voltammetry for 30 loops with the scanning rate at 80 mV s−1. The electrode was washed with water, dried in air, and marked as PTH/NG/GCE. Finally, 6 µL of guanine solution (1.0 mg mL−1) was dropped on to the surface of PTH/NG/GCE. After drying, the G/PTH/NG/GCE was obtained and kept for further use.

2.4 Electrochemical determination of antioxidant capacity

The antioxidant capacity assay was measured as our previous report with some modifications [25]. First, a certain concentration of test sample was mixed with freshly prepared Fe2+–EDTA solution in phosphate buffer saline (PBS, 50 mmol L−1, pH 3.5). Then, the fabricated G/PTH/NG/GCE was immersed into the mixture for 240 s. After incubation, the G/PTH/NG/GCE was washed with water and immersed in PBS (50 mmol L−1, pH 1.5) to perform the SWV. The oxidation peak of guanine in SWV was recorded, and the measurements were completed three times consecutively to obtain the average values. The same amount of PBS was used as a control instead of samples. The antioxidant capacity of the sample was calculated using the following equation:

(1)Antioxidantcapacity%=(Is–Ic)/(I0–Ic)×100

where I0 is the oxidation peak current before damage in Fenton solution, and Ic and Is are the oxidation peak currents of control and samples after damage in Fenton solution, respectively.

2.5 DPPH radical scavenging assay

The DPPH radical scavenging assay was measured according to Sridhar’s report with some modifications [26]. Briefly, 2 mL of DPPH solution (0.05 mg/mL in methanol) and 1.0 mL of the sample with different concentrations were mixed. Then, the mixture was kept in dark for 5 min and transferred into a UV2700 UV-Vis Spectrophotometer (Shimadzu, Kyoto, Japan) for testing the absorbance at 517 nm. The same amount of water was used as a control. The scavenging rate of DPPH radicals was calculated by the following equation:

(2)DPPHradicalscavengingrate%=(1−As/A0)×100%

where As and A0 are the absorbance of sample and control, respectively. The DPPH radical scavenging rate of sample was expressed as the concentration of sample needed to scavenge 50% of DPPH (EC50).

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

3 Results and discussions

3.1 Characterization of NG

TEM and SEM were used to characterize the morphology of prepared NG. The images of NG are shown in Figure 1. In the TEM image of NG (Figure 1a), the typical characteristics of NG such as smooth surface, sheet-like structure, and wrinkled edge could be found [27]. It could be seen the structure of NG was not influenced during nitrogen-doped procedure. The NG agglomerated into particles with nanoscale could be found in SEM image (Figure 1b). This kind of agglomeration might occur in the drying process of samples. The elemental mapping images of NG verified the existence of carbon and nitrogen elements in the materials (Figure 1c–e). In elemental mapping images, the green spots denoting nitrogen atoms and red spots denoting carbon atoms were well distributed over the NG [28]. It demonstrated that nitrogen atoms were uniformly doped in graphene, and the atomic percentage of nitrogen to carbon was about 5.59%. These characterization images could be considered as evidences that NG was successfully prepared in this research.

Figure 1 (a) TEM, (b) SEM, and (c–e) elemental mapping images of NG.
Figure 1

(a) TEM, (b) SEM, and (c–e) elemental mapping images of NG.

3.2 Fabrication of G/PTH/NG/GCE

The cyclic voltammograms for the electrochemical polymerization process of PTH on electrode with consecutive cyclic scanning are shown in Figure 2a. With the increasing scanning times, a pair of redox peaks could be found from 0 to 0.3 V, and the peak currents increased as well. Finally, a thin blue membrane could be observed on the surface of electrode, which showed that thionine was successfully polymerized on the electrode [29]. The experimental results showed that a stable and uniform blue PTH membrane could be obtained with a satisfied response through 30 loops consecutive cyclic scanning in 4.4 mol L−1 acetic acid containing 0.5 mmol L−1 thionine solution with a scanning potential range from −0.4 to 1.4 V and a scanning rate at 80 mV s−1. The oxidation peak current of modified electrodes during the fabrication process was compared (Figure 2b). After the modifications of NG and PTH, the oxidation peak currents of electrodes were increased. Moreover, when NG and PTH were both modified on the electrode, the oxidation peak current was higher than that of electrode modified using any single material. It could be assumed that good electrical conductivities and electrocatalytic properties of NG and PTH played important roles in this sensor [30].

Figure 2 (a) The cyclic voltammograms for electrochemical polymerization of PTH on the electrode. (b) The oxidation peak current of several modified electrodes using SWV detection.
Figure 2

(a) The cyclic voltammograms for electrochemical polymerization of PTH on the electrode. (b) The oxidation peak current of several modified electrodes using SWV detection.

3.3 Optimization of fabrication and detection parameters

3.3.1 Effect of pH

The effect of pH on the oxidation peak of G/PTH/NG/GCE was studied. As shown in Figure 3a, with the growth of pH values (from 1.5 to 7.0), the peak currents decreased gradually and finally reached to a stable status. As a result, the peak current reached the maximum when pH was 1.5. Therefore, pH 1.5 was selected for electrolyte used in this study.

Figure 3 Effects of (a) pH, (b) guanine concentration, (c) Fe2+ concentration, and (d) incubation time on the oxidant peak currents of G/PTH/NG/GCE.
Figure 3

Effects of (a) pH, (b) guanine concentration, (c) Fe2+ concentration, and (d) incubation time on the oxidant peak currents of G/PTH/NG/GCE.

3.3.2 Effect of guanine concentration

The amount of guanine on the modified electrode could influence the voltammetric response (Figure 3b). As a result, the peak currents increased with guanine concentrations from 0.2 to 1.4 mg mL−1 and reached a stable state when guanine concentration was higher than 1.0 mg mL−1. This behavior might be due to the fact that when the concentration of guanine reached a certain amount, the guanines adsorbed on the surface of electrode were saturated. More guanines could not be immobilized and were washed off during the washing procedure. Therefore, 1.0 mg mL−1 of guanine solution was selected as the optimum concentration of guanine solution for modification of the electrode.

3.3.3 Effect of Fe2+ concentration

It was reported that the concentration of Fe2+ affected the generation of hydroxyl radical and following damage to guanine [31]. The effect of Fe2+ concentration on the oxidation peak current of G/PTH/NG/GCE was studied. As shown in Figure 3c, the peak currents decreased with increasing of Fe2+ concentration. When the Fe2+ concentration was higher than about 2.5 mmol L−1, there was only a little change of peak current and the damage to guanine reached the maximum. Therefore, 2.5 mmol L−1 of Fe2+ ions was chosen as the optimum Fe2+ concentration for this study.

3.3.4 Effect of incubation time

The effect of incubation time on oxidation peak current of G/PTH/NG/GCE was also investigated (Figure 3d). As the incubation time increased to 240 s, the peak currents decreased gradually. When the incubation time was longer than 240 s, the peak current remained constant, which showed the 240 s of incubation was enough for the damage to guanine. Therefore, 240 s incubation time was chosen as the optimum incubation time.

3.4 Determination of antioxidant capacity using G/PTH/NG/GCE

The fabricated G/PTH/NG/GCE was used in the detection of antioxidant capacity for investigated samples. As a well-known antioxidant, ascorbic acid was selected for the determination of antioxidant capacity using G/PTH/NG/GCE [32]. The prepared G/PTH/NG/GCE was processed with Fenton solution containing different concentrations of ascorbic acid and then detected using SWV. It was observed that the peak current increased with the growing concentration of ascorbic acid (Figure 4). A good linear relationship between peak current and ascorbic acid concentration could be found when the ascorbic acid concentration ranged from 0.5 to 3.0 mg L−1. The corresponding linear function was fitted as Ip (μA) = −7.7526 × CAA − 2.7169 with a correlation coefficient at 0.999. The detection limit was 0.21 mg L−1 (S/N = 3).

Figure 4 (a) The oxidation peak current of G/PTH/NG/GCE after incubation in Fenton solution with different concentration of ascorbic acid (a–i): 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mg L−1. (b) The calibration plot of oxidant peak current against the concentration of ascorbic acid.
Figure 4

(a) The oxidation peak current of G/PTH/NG/GCE after incubation in Fenton solution with different concentration of ascorbic acid (a–i): 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mg L−1. (b) The calibration plot of oxidant peak current against the concentration of ascorbic acid.

To further evaluate the validity of determination for antioxidant capacity using G/PTH/NG/GCE, the samples containing ascorbic acid were measured by the standard addition method [33]. The analytical results are listed in Table 1. It could be found the ascorbic acid could be detected with a recovery rate of 98.7–101.5%, and the relative standard deviation (RSD) was less than 1.6%. These results illustrated that the determination of antioxidant capacity using G/PTH/NG/GCE was effective with good linearity and high accuracy.

Table 1

Determination of ascorbic acid at different addition (n = 5)

SamplesConcentrations (mg L−1)Added (mg L−1)Found (mg L−1)Recovery (%)RSD (%)
Ascorbic acid2.00.252.28101.50.91
2.00.52.4798.70.47
2.01.02.9899.21.59

3.5 The performances of G/PTH/NG/GCE

In order to evaluate the selectivity of G/PTH/NG/GCE in determination of antioxidant capacities, some samples or substances were used for this comparison. Figure 5 shows the peak currents for these samples. It could be seen that the currents of substances, such as potassium chloride, sodium chloride, calcium chloride, and aluminum nitrate, were very low, which mean that these samples did not show antioxidant activities. However, the currents of ascorbic acid and galangin were relatively higher, which illustrated that these samples have antioxidant capacities. The antioxidant capacities of ascorbic acid and galangin would be verified in the following study. This comparison showed that the determination of antioxidant capacity using G/PTH/NG/GCE has a good selectivity of antioxidants and showed a certain anti-interference ability.

Figure 5 The selectivity of G/PTH/NG/GCE in the detection of samples (KCl, NaCl, CaCl2, Al(NO3)3 and galangin: 5.0 μmol L−1).
Figure 5

The selectivity of G/PTH/NG/GCE in the detection of samples (KCl, NaCl, CaCl2, Al(NO3)3 and galangin: 5.0 μmol L−1).

The reproducibility and stability of G/PTH/NG/GCE were evaluated as well. Several electrodes were used for parallel measurements to evaluate the reproducibility of test. The results showed a good reproducibility in 10 times tests with the RSD value at 3.1% (shown in Figure 6a). After one week of storage at 25°C for a certain electrode, the peak current reduced about 5.2% (shown in Figure 6b). These results demonstrated that the modified electrode was stable for determination at least for one week.

Figure 6 The (a) reproducibility and (b) storage stability of G/PTH/NG/GCE in the detection of ascorbic acid.
Figure 6

The (a) reproducibility and (b) storage stability of G/PTH/NG/GCE in the detection of ascorbic acid.

3.6 Comparison of antioxidant capacity using G/PTH/NG/GCE and DPPH method

The antioxidant capacities of three flavonoids were analyzed by fabricated G/PTH/NG/GCE. The concentration of three flavonoids was unified at 6.0 μmol L−1, and the results of the three flavonoids and ascorbic acid are listed in Table 2. Ascorbic acid displayed the highest antioxidant capacity (80.4 ± 2.5%), then followed by myricetin (73.1 ± 3.7%), kaempferol (63.2 ± 1.2%), and galangin (60.7 ± 3.1%). Simultaneously, the antioxidant capacities of myricetin, kaempferol, and galangin were detected by the DPPH assay, and the activities were presented as EC50 values in Table 2. For the DPPH assay, the lower EC50 values mean higher antioxidant capacities of the sample [34]. The rank of three flavonoids from high antioxidant activity to low was myricetin, kaempferol, and galangin. These results were in accordance with that obtained through the G/PTH/NG/GCE method.

Table 2

Determination of antioxidant capacities of myricetin, kaempferol, and galangin compared to ascorbic acid using G/PTH/NG/GCE and DPPH method

FlavonoidsAntioxidant capacity
G/PTH/NG/GCE (%)DPPH method (µmol L−1)
Myricetin73.1 ± 3.710.5 ± 0.4
Kaempferol63.2 ± 1.223.5 ± 1.8
Galangin60.7 ± 3.128.4 ± 2.4
Ascorbic acid80.4 ± 2.55.5 ± 0.5

As reported in references, the antioxidant activities of flavonoids were affected by structure and substituent groups [35,36,37]. The main structure of myricetin, kaempferol, and galangin was the same with different hydroxyl groups on B ring (Figure 7). The number of hydroxyl groups on B ring mostly affected the antioxidant activities of flavonoids, and more hydroxyl groups on B ring showed higher antioxidant activities [38]. According to this principle, myricetin should have the best antioxidant ability among these three flavonoids, while galangin would have the lowest antioxidant capacity. The antioxidant capacities detected by G/PTH/NG/GCE and DPPH methods confirmed this principle, which showed that this kind of detection followed reported principle and could be considered effective in the detection of antioxidant capacities.

Figure 7 The chemical structures of myricetin, kaempferol, and galangin.
Figure 7

The chemical structures of myricetin, kaempferol, and galangin.

3.7 Antioxidant capacities of real samples

In order to explore the application of G/PTH/NG/GCE for real samples, the prepared G/PTH/NG/GCE was utilized to evaluate the antioxidant capacities of fruit juices and plant extracts. Three kinds of fruit juices including grape juice, guava juice, and orange juice were bought from the local supermarket. Three plant extracts were made in our labs including jute leaves extract, ramie leaves extract, and hemp leaves extract. All of the samples were filtered to remove insoluble substances and set the concentration at 2.0 mg L−1. As shown in Figure 8, all of the three juices showed antioxidant capacities from 56% to 68%, whereas the plant extracts showed relatively higher antioxidant capacities (from 85% to 92%) than those of juices. It might be due to fact that the plant extracts contain more active compounds and antioxidants. Based on these results, the fabricated G/PTH/NG/GCE showed predictable ability in determination of antioxidant capacities of real samples.

Figure 8 Determination of antioxidant capacities of fruit juices and plant extracts using G/PTH/NG/GCE.
Figure 8

Determination of antioxidant capacities of fruit juices and plant extracts using G/PTH/NG/GCE.

4 Conclusions

In this research, the G/PTH/NG/GCE was fabricated and used for the electrochemical determination of antioxidant capacity. After the modification, the related oxidation peak current on G/PTH/NG/GCE was improved. The factors including pH, guanine concentration, Fe2+ concentration, and incubation time were optimized for the optimum performance of the modified electrode. The G/PTH/NG/GCE showed good linearity and selectivity, satisfied reproducibility, and storage stability. Three flavonoids were selected as the samples for the antioxidant capacity tests using G/PTH/NG/GCE and DPPH methods, and the results have a good agreement. Some fruit juices and plant extracts were also tested as real samples. The proposed G/PTH/NG/GCE provided to be a rapid and efficient sensor for the evaluation of antioxidants in food and natural chemical fields.

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Acknowledgments

This work was supported by the Central Public-interest Scientific Institution Basal Research Fund (No. Y2019PT22-02 and 1610242020005), Open project of key laboratory of biology and processing for bast fiber crops, MARA and National Agricultural Science and Technology Innovation Project (Characteristic fruit and vegetable innovation team, ASTIP-IBFC05).

  1. Conflict of Interest: The authors declare no conflict of interest.

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Received: 2020-05-29
Revised: 2020-07-13
Accepted: 2020-07-16
Published Online: 2020-08-19

© 2020 Yafen Fu et al., published by De Gruyter

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

Articles in the same Issue

  1. Regular Articles
  2. Electrochemical antioxidant screening and evaluation based on guanine and chitosan immobilized MoS2 nanosheet modified glassy carbon electrode (guanine/CS/MoS2/GCE)
  3. Kinetic models of the extraction of vanillic acid from pumpkin seeds
  4. On the maximum ABC index of bipartite graphs without pendent vertices
  5. Estimation of the total antioxidant potential in the meat samples using thin-layer chromatography
  6. Molecular dynamics simulation of sI methane hydrate under compression and tension
  7. Spatial distribution and potential ecological risk assessment of some trace elements in sediments and grey mangrove (Avicennia marina) along the Arabian Gulf coast, Saudi Arabia
  8. Amino-functionalized graphene oxide for Cr(VI), Cu(II), Pb(II) and Cd(II) removal from industrial wastewater
  9. Chemical composition and in vitro activity of Origanum vulgare L., Satureja hortensis L., Thymus serpyllum L. and Thymus vulgaris L. essential oils towards oral isolates of Candida albicans and Candida glabrata
  10. Effect of excess Fluoride consumption on Urine-Serum Fluorides, Dental state and Thyroid Hormones among children in “Talab Sarai” Punjab Pakistan
  11. Design, Synthesis and Characterization of Novel Isoxazole Tagged Indole Hybrid Compounds
  12. Comparison of kinetic and enzymatic properties of intracellular phosphoserine aminotransferases from alkaliphilic and neutralophilic bacteria
  13. Green Organic Solvent-Free Oxidation of Alkylarenes with tert-Butyl Hydroperoxide Catalyzed by Water-Soluble Copper Complex
  14. Ducrosia ismaelis Asch. essential oil: chemical composition profile and anticancer, antimicrobial and antioxidant potential assessment
  15. DFT calculations as an efficient tool for prediction of Raman and infra-red spectra and activities of newly synthesized cathinones
  16. Influence of Chemical Osmosis on Solute Transport and Fluid Velocity in Clay Soils
  17. A New fatty acid and some triterpenoids from propolis of Nkambe (North-West Region, Cameroon) and evaluation of the antiradical scavenging activity of their extracts
  18. Antiplasmodial Activity of Stigmastane Steroids from Dryobalanops oblongifolia Stem Bark
  19. Rapid identification of direct-acting pancreatic protectants from Cyclocarya paliurus leaves tea by the method of serum pharmacochemistry combined with target cell extraction
  20. Immobilization of Pseudomonas aeruginosa static biomass on eggshell powder for on-line preconcentration and determination of Cr (VI)
  21. Assessment of methyl 2-({[(4,6-dimethoxypyrimidin-2-yl)carbamoyl] sulfamoyl}methyl)benzoate through biotic and abiotic degradation modes
  22. Stability of natural polyphenol fisetin in eye drops Stability of fisetin in eye drops
  23. Production of a bioflocculant by using activated sludge and its application in Pb(II) removal from aqueous solution
  24. Molecular Properties of Carbon Crystal Cubic Structures
  25. Synthesis and characterization of calcium carbonate whisker from yellow phosphorus slag
  26. Study on the interaction between catechin and cholesterol by the density functional theory
  27. Analysis of some pharmaceuticals in the presence of their synthetic impurities by applying hybrid micelle liquid chromatography
  28. Two mixed-ligand coordination polymers based on 2,5-thiophenedicarboxylic acid and flexible N-donor ligands: the protective effect on periodontitis via reducing the release of IL-1β and TNF-α
  29. Incorporation of silver stearate nanoparticles in methacrylate polymeric monoliths for hemeprotein isolation
  30. Development of ultrasound-assisted dispersive solid-phase microextraction based on mesoporous carbon coated with silica@iron oxide nanocomposite for preconcentration of Te and Tl in natural water systems
  31. N,N′-Bis[2-hydroxynaphthylidene]/[2-methoxybenzylidene]amino]oxamides and their divalent manganese complexes: Isolation, spectral characterization, morphology, antibacterial and cytotoxicity against leukemia cells
  32. Determination of the content of selected trace elements in Polish commercial fruit juices and health risk assessment
  33. Diorganotin(iv) benzyldithiocarbamate complexes: synthesis, characterization, and thermal and cytotoxicity study
  34. Keratin 17 is induced in prurigo nodularis lesions
  35. Anticancer, antioxidant, and acute toxicity studies of a Saudi polyherbal formulation, PHF5
  36. LaCoO3 perovskite-type catalysts in syngas conversion
  37. Comparative studies of two vegetal extracts from Stokesia laevis and Geranium pratense: polyphenol profile, cytotoxic effect and antiproliferative activity
  38. Fragmentation pattern of certain isatin–indole antiproliferative conjugates with application to identify their in vitro metabolic profiles in rat liver microsomes by liquid chromatography tandem mass spectrometry
  39. Investigation of polyphenol profile, antioxidant activity and hepatoprotective potential of Aconogonon alpinum (All.) Schur roots
  40. Lead discovery of a guanidinyl tryptophan derivative on amyloid cascade inhibition
  41. Physicochemical evaluation of the fruit pulp of Opuntia spp growing in the Mediterranean area under hard climate conditions
  42. Electronic structural properties of amino/hydroxyl functionalized imidazolium-based bromide ionic liquids
  43. New Schiff bases of 2-(quinolin-8-yloxy)acetohydrazide and their Cu(ii), and Zn(ii) metal complexes: their in vitro antimicrobial potentials and in silico physicochemical and pharmacokinetics properties
  44. Treatment of adhesions after Achilles tendon injury using focused ultrasound with targeted bFGF plasmid-loaded cationic microbubbles
  45. Synthesis of orotic acid derivatives and their effects on stem cell proliferation
  46. Chirality of β2-agonists. An overview of pharmacological activity, stereoselective analysis, and synthesis
  47. Fe3O4@urea/HITh-SO3H as an efficient and reusable catalyst for the solvent-free synthesis of 7-aryl-8H-benzo[h]indeno[1,2-b]quinoline-8-one and indeno[2′,1′:5,6]pyrido[2,3-d]pyrimidine derivatives
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  49. Enhancement of thermal properties of bio-based microcapsules intended for textile applications
  50. Exploring the effect of khat (Catha edulis) chewing on the pharmacokinetics of the antiplatelet drug clopidogrel in rats using the newly developed LC-MS/MS technique
  51. A green strategy for obtaining anthraquinones from Rheum tanguticum by subcritical water
  52. Cadmium (Cd) chloride affects the nutrient uptake and Cd-resistant bacterium reduces the adsorption of Cd in muskmelon plants
  53. Removal of H2S by vermicompost biofilter and analysis on bacterial community
  54. Structural cytotoxicity relationship of 2-phenoxy(thiomethyl)pyridotriazolopyrimidines: Quantum chemical calculations and statistical analysis
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  58. Synthesis and characterization of a novel chitosan-grafted-polyorthoethylaniline biocomposite and utilization for dye removal from water
  59. Optimisation of urine sample preparation for shotgun proteomics
  60. DFT investigations on arylsulphonyl pyrazole derivatives as potential ligands of selected kinases
  61. Treatment of Parkinson’s disease using focused ultrasound with GDNF retrovirus-loaded microbubbles to open the blood–brain barrier
  62. New derivatives of a natural nordentatin
  63. Fluorescence biomarkers of malignant melanoma detectable in urine
  64. Study of the remediation effects of passivation materials on Pb-contaminated soil
  65. Saliva proteomic analysis reveals possible biomarkers of renal cell carcinoma
  66. Withania frutescens: Chemical characterization, analgesic, anti-inflammatory, and healing activities
  67. Design, synthesis and pharmacological profile of (−)-verbenone hydrazones
  68. Synthesis of magnesium carbonate hydrate from natural talc
  69. Stability-indicating HPLC-DAD assay for simultaneous quantification of hydrocortisone 21 acetate, dexamethasone, and fluocinolone acetonide in cosmetics
  70. A novel lactose biosensor based on electrochemically synthesized 3,4-ethylenedioxythiophene/thiophene (EDOT/Th) copolymer
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https://doi.org/10.1515/chem-2020-0157
Keywords for this article
antioxidant capacity; electrochemical sensor; flavonoids; nitrogen-doped graphene; polythionine
Creative Commons
BY 4.0

Articles in the same Issue

  1. Regular Articles
  2. Electrochemical antioxidant screening and evaluation based on guanine and chitosan immobilized MoS2 nanosheet modified glassy carbon electrode (guanine/CS/MoS2/GCE)
  3. Kinetic models of the extraction of vanillic acid from pumpkin seeds
  4. On the maximum ABC index of bipartite graphs without pendent vertices
  5. Estimation of the total antioxidant potential in the meat samples using thin-layer chromatography
  6. Molecular dynamics simulation of sI methane hydrate under compression and tension
  7. Spatial distribution and potential ecological risk assessment of some trace elements in sediments and grey mangrove (Avicennia marina) along the Arabian Gulf coast, Saudi Arabia
  8. Amino-functionalized graphene oxide for Cr(VI), Cu(II), Pb(II) and Cd(II) removal from industrial wastewater
  9. Chemical composition and in vitro activity of Origanum vulgare L., Satureja hortensis L., Thymus serpyllum L. and Thymus vulgaris L. essential oils towards oral isolates of Candida albicans and Candida glabrata
  10. Effect of excess Fluoride consumption on Urine-Serum Fluorides, Dental state and Thyroid Hormones among children in “Talab Sarai” Punjab Pakistan
  11. Design, Synthesis and Characterization of Novel Isoxazole Tagged Indole Hybrid Compounds
  12. Comparison of kinetic and enzymatic properties of intracellular phosphoserine aminotransferases from alkaliphilic and neutralophilic bacteria
  13. Green Organic Solvent-Free Oxidation of Alkylarenes with tert-Butyl Hydroperoxide Catalyzed by Water-Soluble Copper Complex
  14. Ducrosia ismaelis Asch. essential oil: chemical composition profile and anticancer, antimicrobial and antioxidant potential assessment
  15. DFT calculations as an efficient tool for prediction of Raman and infra-red spectra and activities of newly synthesized cathinones
  16. Influence of Chemical Osmosis on Solute Transport and Fluid Velocity in Clay Soils
  17. A New fatty acid and some triterpenoids from propolis of Nkambe (North-West Region, Cameroon) and evaluation of the antiradical scavenging activity of their extracts
  18. Antiplasmodial Activity of Stigmastane Steroids from Dryobalanops oblongifolia Stem Bark
  19. Rapid identification of direct-acting pancreatic protectants from Cyclocarya paliurus leaves tea by the method of serum pharmacochemistry combined with target cell extraction
  20. Immobilization of Pseudomonas aeruginosa static biomass on eggshell powder for on-line preconcentration and determination of Cr (VI)
  21. Assessment of methyl 2-({[(4,6-dimethoxypyrimidin-2-yl)carbamoyl] sulfamoyl}methyl)benzoate through biotic and abiotic degradation modes
  22. Stability of natural polyphenol fisetin in eye drops Stability of fisetin in eye drops
  23. Production of a bioflocculant by using activated sludge and its application in Pb(II) removal from aqueous solution
  24. Molecular Properties of Carbon Crystal Cubic Structures
  25. Synthesis and characterization of calcium carbonate whisker from yellow phosphorus slag
  26. Study on the interaction between catechin and cholesterol by the density functional theory
  27. Analysis of some pharmaceuticals in the presence of their synthetic impurities by applying hybrid micelle liquid chromatography
  28. Two mixed-ligand coordination polymers based on 2,5-thiophenedicarboxylic acid and flexible N-donor ligands: the protective effect on periodontitis via reducing the release of IL-1β and TNF-α
  29. Incorporation of silver stearate nanoparticles in methacrylate polymeric monoliths for hemeprotein isolation
  30. Development of ultrasound-assisted dispersive solid-phase microextraction based on mesoporous carbon coated with silica@iron oxide nanocomposite for preconcentration of Te and Tl in natural water systems
  31. N,N′-Bis[2-hydroxynaphthylidene]/[2-methoxybenzylidene]amino]oxamides and their divalent manganese complexes: Isolation, spectral characterization, morphology, antibacterial and cytotoxicity against leukemia cells
  32. Determination of the content of selected trace elements in Polish commercial fruit juices and health risk assessment
  33. Diorganotin(iv) benzyldithiocarbamate complexes: synthesis, characterization, and thermal and cytotoxicity study
  34. Keratin 17 is induced in prurigo nodularis lesions
  35. Anticancer, antioxidant, and acute toxicity studies of a Saudi polyherbal formulation, PHF5
  36. LaCoO3 perovskite-type catalysts in syngas conversion
  37. Comparative studies of two vegetal extracts from Stokesia laevis and Geranium pratense: polyphenol profile, cytotoxic effect and antiproliferative activity
  38. Fragmentation pattern of certain isatin–indole antiproliferative conjugates with application to identify their in vitro metabolic profiles in rat liver microsomes by liquid chromatography tandem mass spectrometry
  39. Investigation of polyphenol profile, antioxidant activity and hepatoprotective potential of Aconogonon alpinum (All.) Schur roots
  40. Lead discovery of a guanidinyl tryptophan derivative on amyloid cascade inhibition
  41. Physicochemical evaluation of the fruit pulp of Opuntia spp growing in the Mediterranean area under hard climate conditions
  42. Electronic structural properties of amino/hydroxyl functionalized imidazolium-based bromide ionic liquids
  43. New Schiff bases of 2-(quinolin-8-yloxy)acetohydrazide and their Cu(ii), and Zn(ii) metal complexes: their in vitro antimicrobial potentials and in silico physicochemical and pharmacokinetics properties
  44. Treatment of adhesions after Achilles tendon injury using focused ultrasound with targeted bFGF plasmid-loaded cationic microbubbles
  45. Synthesis of orotic acid derivatives and their effects on stem cell proliferation
  46. Chirality of β2-agonists. An overview of pharmacological activity, stereoselective analysis, and synthesis
  47. Fe3O4@urea/HITh-SO3H as an efficient and reusable catalyst for the solvent-free synthesis of 7-aryl-8H-benzo[h]indeno[1,2-b]quinoline-8-one and indeno[2′,1′:5,6]pyrido[2,3-d]pyrimidine derivatives
  48. Adsorption kinetic characteristics of molybdenum in yellow-brown soil in response to pH and phosphate
  49. Enhancement of thermal properties of bio-based microcapsules intended for textile applications
  50. Exploring the effect of khat (Catha edulis) chewing on the pharmacokinetics of the antiplatelet drug clopidogrel in rats using the newly developed LC-MS/MS technique
  51. A green strategy for obtaining anthraquinones from Rheum tanguticum by subcritical water
  52. Cadmium (Cd) chloride affects the nutrient uptake and Cd-resistant bacterium reduces the adsorption of Cd in muskmelon plants
  53. Removal of H2S by vermicompost biofilter and analysis on bacterial community
  54. Structural cytotoxicity relationship of 2-phenoxy(thiomethyl)pyridotriazolopyrimidines: Quantum chemical calculations and statistical analysis
  55. A self-breaking supramolecular plugging system as lost circulation material in oilfield
  56. Synthesis, characterization, and pharmacological evaluation of thiourea derivatives
  57. Application of drug–metal ion interaction principle in conductometric determination of imatinib, sorafenib, gefitinib and bosutinib
  58. Synthesis and characterization of a novel chitosan-grafted-polyorthoethylaniline biocomposite and utilization for dye removal from water
  59. Optimisation of urine sample preparation for shotgun proteomics
  60. DFT investigations on arylsulphonyl pyrazole derivatives as potential ligands of selected kinases
  61. Treatment of Parkinson’s disease using focused ultrasound with GDNF retrovirus-loaded microbubbles to open the blood–brain barrier
  62. New derivatives of a natural nordentatin
  63. Fluorescence biomarkers of malignant melanoma detectable in urine
  64. Study of the remediation effects of passivation materials on Pb-contaminated soil
  65. Saliva proteomic analysis reveals possible biomarkers of renal cell carcinoma
  66. Withania frutescens: Chemical characterization, analgesic, anti-inflammatory, and healing activities
  67. Design, synthesis and pharmacological profile of (−)-verbenone hydrazones
  68. Synthesis of magnesium carbonate hydrate from natural talc
  69. Stability-indicating HPLC-DAD assay for simultaneous quantification of hydrocortisone 21 acetate, dexamethasone, and fluocinolone acetonide in cosmetics
  70. A novel lactose biosensor based on electrochemically synthesized 3,4-ethylenedioxythiophene/thiophene (EDOT/Th) copolymer
  71. Citrullus colocynthis (L.) Schrad: Chemical characterization, scavenging and cytotoxic activities
  72. Development and validation of a high performance liquid chromatography/diode array detection method for estrogen determination: Application to residual analysis in meat products
  73. PCSK9 concentrations in different stages of subclinical atherosclerosis and their relationship with inflammation
  74. Development of trace analysis for alkyl methanesulfonates in the delgocitinib drug substance using GC-FID and liquid–liquid extraction with ionic liquid
  75. Electrochemical evaluation of the antioxidant capacity of natural compounds on glassy carbon electrode modified with guanine-, polythionine-, and nitrogen-doped graphene
  76. A Dy(iii)–organic framework as a fluorescent probe for highly selective detection of picric acid and treatment activity on human lung cancer cells
  77. A Zn(ii)–organic cage with semirigid ligand for solvent-free cyanosilylation and inhibitory effect on ovarian cancer cell migration and invasion ability via regulating mi-RNA16 expression
  78. Polyphenol content and antioxidant activities of Prunus padus L. and Prunus serotina L. leaves: Electrochemical and spectrophotometric approach and their antimicrobial properties
  79. The combined use of GC, PDSC and FT-IR techniques to characterize fat extracted from commercial complete dry pet food for adult cats
  80. MALDI-TOF MS profiling in the discovery and identification of salivary proteomic patterns of temporomandibular joint disorders
  81. Concentrations of dioxins, furans and dioxin-like PCBs in natural animal feed additives
  82. Structure and some physicochemical and functional properties of water treated under ammonia with low-temperature low-pressure glow plasma of low frequency
  83. Mesoscale nanoparticles encapsulated with emodin for targeting antifibrosis in animal models
  84. Amine-functionalized magnetic activated carbon as an adsorbent for preconcentration and determination of acidic drugs in environmental water samples using HPLC-DAD
  85. Antioxidant activity as a response to cadmium pollution in three durum wheat genotypes differing in salt-tolerance
  86. A promising naphthoquinone [8-hydroxy-2-(2-thienylcarbonyl)naphtho[2,3-b]thiophene-4,9-dione] exerts anti-colorectal cancer activity through ferroptosis and inhibition of MAPK signaling pathway based on RNA sequencing
  87. Synthesis and efficacy of herbicidal ionic liquids with chlorsulfuron as the anion
  88. Effect of isovalent substitution on the crystal structure and properties of two-slab indates BaLa2−xSmxIn2O7
  89. Synthesis, spectral and thermo-kinetics explorations of Schiff-base derived metal complexes
  90. An improved reduction method for phase stability testing in the single-phase region
  91. Comparative analysis of chemical composition of some commercially important fishes with an emphasis on various Malaysian diets
  92. Development of a solventless stir bar sorptive extraction/thermal desorption large volume injection capillary gas chromatographic-mass spectrometric method for ultra-trace determination of pyrethroids pesticides in river and tap water samples
  93. A turbidity sensor development based on NL-PI observers: Experimental application to the control of a Sinaloa’s River Spirulina maxima cultivation
  94. Deep desulfurization of sintering flue gas in iron and steel works based on low-temperature oxidation
  95. Investigations of metallic elements and phenolics in Chinese medicinal plants
  96. Influence of site-classification approach on geochemical background values
  97. Effects of ageing on the surface characteristics and Cu(ii) adsorption behaviour of rice husk biochar in soil
  98. Adsorption and sugarcane-bagasse-derived activated carbon-based mitigation of 1-[2-(2-chloroethoxy)phenyl]sulfonyl-3-(4-methoxy-6-methyl-1,3,5-triazin-2-yl) urea-contaminated soils
  99. Antimicrobial and antifungal activities of bifunctional cooper(ii) complexes with non-steroidal anti-inflammatory drugs, flufenamic, mefenamic and tolfenamic acids and 1,10-phenanthroline
  100. Application of selenium and silicon to alleviate short-term drought stress in French marigold (Tagetes patula L.) as a model plant species
  101. Screening and analysis of xanthine oxidase inhibitors in jute leaves and their protective effects against hydrogen peroxide-induced oxidative stress in cells
  102. Synthesis and physicochemical studies of a series of mixed-ligand transition metal complexes and their molecular docking investigations against Coronavirus main protease
  103. A study of in vitro metabolism and cytotoxicity of mephedrone and methoxetamine in human and pig liver models using GC/MS and LC/MS analyses
  104. A new phenyl alkyl ester and a new combretin triterpene derivative from Combretum fragrans F. Hoffm (Combretaceae) and antiproliferative activity
  105. Erratum
  106. Erratum to: A one-step incubation ELISA kit for rapid determination of dibutyl phthalate in water, beverage and liquor
  107. Review Articles
  108. Sinoporphyrin sodium, a novel sensitizer for photodynamic and sonodynamic therapy
  109. Natural products isolated from Casimiroa
  110. Plant description, phytochemical constituents and bioactivities of Syzygium genus: A review
  111. Evaluation of elastomeric heat shielding materials as insulators for solid propellant rocket motors: A short review
  112. Special Issue on Applied Biochemistry and Biotechnology 2019
  113. An overview of Monascus fermentation processes for monacolin K production
  114. Study on online soft sensor method of total sugar content in chlorotetracycline fermentation tank
  115. Studies on the Anti-Gouty Arthritis and Anti-hyperuricemia Properties of Astilbin in Animal Models
  116. Effects of organic fertilizer on water use, photosynthetic characteristics, and fruit quality of pear jujube in northern Shaanxi
  117. Characteristics of the root exudate release system of typical plants in plateau lakeside wetland under phosphorus stress conditions
  118. Characterization of soil water by the means of hydrogen and oxygen isotope ratio at dry-wet season under different soil layers in the dry-hot valley of Jinsha River
  119. Composition and diurnal variation of floral scent emission in Rosa rugosa Thunb. and Tulipa gesneriana L.
  120. Preparation of a novel ginkgolide B niosomal composite drug
  121. The degradation, biodegradability and toxicity evaluation of sulfamethazine antibiotics by gamma radiation
  122. Special issue on Monitoring, Risk Assessment and Sustainable Management for the Exposure to Environmental Toxins
  123. Insight into the cadmium and zinc binding potential of humic acids derived from composts by EEM spectra combined with PARAFAC analysis
  124. Source apportionment of soil contamination based on multivariate receptor and robust geostatistics in a typical rural–urban area, Wuhan city, middle China
  125. Special Issue on 13th JCC 2018
  126. The Role of H2C2O4 and Na2CO3 as Precipitating Agents on The Physichochemical Properties and Photocatalytic Activity of Bismuth Oxide
  127. Preparation of magnetite-silica–cetyltrimethylammonium for phenol removal based on adsolubilization
  128. Topical Issue on Agriculture
  129. Size-dependent growth kinetics of struvite crystals in wastewater with calcium ions
  130. The effect of silica-calcite sedimentary rock contained in the chicken broiler diet on the overall quality of chicken muscles
  131. Physicochemical properties of selected herbicidal products containing nicosulfuron as an active ingredient
  132. Lycopene in tomatoes and tomato products
  133. Fluorescence in the assessment of the share of a key component in the mixing of feed
  134. Sulfur application alleviates chromium stress in maize and wheat
  135. Effectiveness of removal of sulphur compounds from the air after 3 years of biofiltration with a mixture of compost soil, peat, coconut fibre and oak bark
  136. Special Issue on the 4th Green Chemistry 2018
  137. Study and fire test of banana fibre reinforced composites with flame retardance properties
  138. Special Issue on the International conference CosCI 2018
  139. Disintegration, In vitro Dissolution, and Drug Release Kinetics Profiles of k-Carrageenan-based Nutraceutical Hard-shell Capsules Containing Salicylamide
  140. Synthesis of amorphous aluminosilicate from impure Indonesian kaolin
  141. Special Issue on the International Conf on Science, Applied Science, Teaching and Education 2019
  142. Functionalization of Congo red dye as a light harvester on solar cell
  143. The effect of nitrite food preservatives added to se’i meat on the expression of wild-type p53 protein
  144. Biocompatibility and osteoconductivity of scaffold porous composite collagen–hydroxyapatite based coral for bone regeneration
  145. Special Issue on the Joint Science Congress of Materials and Polymers (ISCMP 2019)
  146. Effect of natural boron mineral use on the essential oil ratio and components of Musk Sage (Salvia sclarea L.)
  147. A theoretical and experimental study of the adsorptive removal of hexavalent chromium ions using graphene oxide as an adsorbent
  148. A study on the bacterial adhesion of Streptococcus mutans in various dental ceramics: In vitro study
  149. 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
  150. Special Issue on Chemistry Today for Tomorrow 2019
  151. Diabetes mellitus type 2: Exploratory data analysis based on clinical reading
  152. Multivariate analysis for the classification of copper–lead and copper–zinc glasses
  153. Special Issue on Advances in Chemistry and Polymers
  154. The spatial and temporal distribution of cationic and anionic radicals in early embryo implantation
  155. Special Issue on 3rd IC3PE 2020
  156. Magnetic iron oxide/clay nanocomposites for adsorption and catalytic oxidation in water treatment applications
  157. Special Issue on IC3PE 2018/2019 Conference
  158. Exergy analysis of conventional and hydrothermal liquefaction–esterification processes of microalgae for biodiesel production
  159. Advancing biodiesel production from microalgae Spirulina sp. by a simultaneous extraction–transesterification process using palm oil as a co-solvent of methanol
  160. Topical Issue on Applications of Mathematics in Chemistry
  161. Omega and the related counting polynomials of some chemical structures
  162. M-polynomial and topological indices of zigzag edge coronoid fused by starphene
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