Everything You Need To Know To Find The Best Antioxidant Graphite Electrode

For more information, please visit our website.

Abstract

In recent years, the growing research interests in the applications of plant and fruit extracts (synthetic/stabilization materials for the nanomaterials, medicinal applications, functional foods, and nutraceuticals) have led to the development of new analytical techniques to be utilized for identifying numerous properties of these extracts. One of the main properties essential for the applicability of these plant extracts is the antioxidant capacity (AOC) that is conventionally determined by spectrophotometric techniques. Nowadays, electrochemical methodologies are emerging as alternative tools for quantifying this particular property of the extract. These methodologies address numerous drawbacks of the conventional spectroscopic approach, such as the utilization of expensive and hazardous solvents, extensive sample pre-treatment requirements, long reaction times, low sensitivity, etc. The electrochemical methodologies discussed in this review include cyclic voltammetry (CV), square wave voltammetry (SWV), differential pulse voltammetry (DPV), and chronoamperometry (CAP). This review presents a critical comparison between both the conventional and electrochemical approaches for the quantification of the parameter of AOC and discusses the numerous applications of the obtained bioextracts based on the AOC parameter.

Keywords:

antioxidant capacity, antioxidants, plant extract, spectrophotometric technique, cyclic voltammetry

1. Introduction

During the metabolic conversions of food into the energy, excess amounts of the free radicals are constantly generated in the human body. These unstable and highly reactive free radical species are a constant threat to the constituents of the cells (particularly the genetic materials) and destroy the constituents via numerous destructive mechanisms [1]. Therefore, the modulation of the concentration of these free radical species is an essential task [2]. Though oxidative metabolism plays an essential role in the survival of our cells, the production of free radicals (along with some other reactive oxygen species) during this process still causes various oxidative changes [3]. However, we are not defenseless against these free radicals and their relentless attack. This is attributed to the presence of certain molecules in our bodies called antioxidants. Antioxidants operate in two ways in order to address the issue of the oxidative stress in the human body: (1) the antioxidants scavenge/quench these reactive oxygen species (ROS) or reactive nitrogen species (RNS) by chain-breaking mechanisms (these antioxidants are called primary antioxidants) [4]; and (2) the antioxidants that suppress the generation of the oxidation promoters (such as singlet oxygen, metal ions, and pro-oxidation enzymes, etc.) by chelation mechanisms (these antioxidants are called secondary or preventive antioxidants) [5]. The most familiar out of thousands of such antioxidant substances are Vitamin C, Vitamin E, beta carotenes, many related carotenoids, and minerals like selenium and manganese, etc. [6]. Antioxidants hold great interest for pharmacists and biochemists because of their capability to moderate the damages caused by RNS, ROS, or even chlorine-like reactive species [7]. The importance of antioxidants is highlighted in the food preservation applications as presented in .

Open in a separate window

Growing interest has been recently observed in developing cost-effective and reliable techniques for screening and quantifying the antioxidants present in various biological/biogenic samples. The quantification of the antioxidant potential is done by using the parameter of the antioxidant capacity (AOC), which represents the concentration or number of moles of the specific free radical species scavenged by a particular antioxidant present in the sample [9]. The AOC parameter is a thermodynamic parameter and is found to be connected with the equilibrium constant of the process (scavenging reaction of antioxidant) [10]. Though a number of methodologies have been reported for estimating the AOC of the antioxidants, identifying the direct-action mechanism of antioxidants still remains a challenge in the field of free radical chemistry [10]. Antioxidant behavior is quantified using the two broad-term methodologies that reflect the focus on activity in foods (first category) and bioactivity in humans (second category). The category focusing on food systems involves the assessment of antioxidant efficacy, especially in fruits, vegetables and beverages so that a clear observation of their dietary burden and in vivo activity could be reported. As far as the antioxidant status in humans is concerned, there has been reported an obvious observation regarding an imbalance between the reactive oxygen species and defense/repair mechanisms in vivo [3,11]. Understanding both quantification methodologies (usually termed as antioxidant assays) is essential in developing a clear picture of the working of antioxidants.

The antioxidant assays can be broadly categorized into two classes of direct antioxidant assays and indirect antioxidant assays [12]. The first category of direct assays is the competitive technique, and the added probe, as well as the antioxidants present in the sample both competes with each other to attack the free radical/free radical initiator. This makes these direct assays a little less accurate technique to detect the AOC [11]. The indirect antioxidant assays are non-competitive tests where an artificial probe (an oxidant) is added to the sample to observe the impact of the antioxidant action on this probe. The attack of antioxidant molecules generates structural changes in the added probe, which is measured by spectroscopic, fluorescence, electrochemical, or other methodologies. The indirect assays are more common and provide more accurate results in comparison to the direct assays owing to their non-competitive nature [12]. shows the schematics of the mechanism of direct or indirect assays used in this regard. Furthermore, it should also be mentioned here that the free radicals are scavenged by utilizing two mechanisms, including the electron transfer (ET) and hydrogen atom transfer (HAT) mechanism. In the ET mechanism, the compound of interest is reduced by the transference of one or more electrons from antioxidants to the unstable species, while in the HAT mechanism, the compound of interest is reduced by using the transference of the hydrogen atom to the probe [12].

Open in a separate window

In this review article, critical comparisons of numerous antioxidant assays utilized for the estimation of AOC in the plant/fruit extracts are presented. The first section provides a brief description of the fundamentals of antioxidants. The second section represents a summary of the conventional methodologies, while the third section provides a detailed analysis of the electrochemical techniques utilized for the estimation of AOC in the biogenic extracts. Some peculiar case studies explaining the applicability of the electrochemical techniques for the estimation of AOC are summarized in the fourth section. The last section details the conclusion and future prospects associated with this particular research domain of electrochemical techniques used for the determination of AOC.

2. Conventional Assays/Methods for Determination of AOC

Many analytical techniques have been developed for the extraction, separation, identification and quantification of antioxidant compounds by using various conventional methodologies [13]. The classification of the numerous antioxidant assays utilized for the determination of AOC is presented in . AOC is either expressed in the form of equivalents of the selected reference antioxidants (particularly gallic acid and Trolox) in the case of radical/ROS-based scavenging assay or inhibition potential of the antioxidant against the oxidation of the added probe [14]. The oxidation of the added probe is determined by using analytical techniques such as fluorimetric, spectrophotometric, cyclic voltammetry, amperometric methods, and chemiluminescent, etc. [13]. The electrochemical methods fall under the category of non-radical/potential-based antioxidant assays and are not discussed in . The electrochemical methodologies are discussed in detail in the next section. A comparative study of different methodologies for the determination of the antioxidant activity of Venezuelan propolis, as an example, has been shown in .

Open in a separate window

Table 1

ClassificationAssaysMechanism InvolvedMain ProtocolAdvantages/DisadvantagesDetection TechniqueRef. Chemical-Based Assays ROS/free radical-based scavenging assay TEAC/ABTS assayET mechanism and HAT mechanism(Mixed mechanisms)The AOC is estimated by studying the color diminishing capability of the sample. The ABTS+ radicals are colored and generated in the medium by the interaction of ABTS with K2S2O8. The antioxidant reaction with this organic radical provides the information regarding the AOC of the sampleAdvantages
Rapid.
Facile.
Natural and synthetic antioxidant samples are studied.
Assay can be applied over large pH ranges.
Hydrophilic and hydrophobic antioxidants can be studied.
Disadvantages
Precision is moderate.
The result of the assay depends upon the reaction time, which lowers the chance of comparative analysis.
The steric hindrance and molecular size of the ABTS+ radical make it difficult for the antioxidants present in the medium to interact with this radical.Calorimetry.
Spectroscopy
(decrease in the absorbance value at 743 nm).[16]ORAC assayThe radical chain-breaking potential of the antioxidants present in the sample by blocking the peroxy radical generation of AAPH. The peroxy radicals react with the fluorescent probe to generate the non-fluorescent product.Advantages
Food-based samples involving the peroxy radicals (lipid oxidation) can be properly assessed by this assay.
Rapid.
Can be automated.
Disadvantages
A competitive technique where the antioxidant and fluorescent probe compete for scavenging the peroxy radicals.
Use of fluorescent probe (ß-PE) can cause false ORAC readings.
For studying the AOC for hydrophilic and hydrophobic antioxidants, the modification of ORAC assays is required.Fluorimetry
(decrease in the fluorescence intensity
area under the curve of fluorescence intensity versus time graph).
Quantification of the non-fluorescent product generated.[17]TRAP assayThe luminol-enhanced chemiluminescence owing to the presence of the luminol-derived radicals (generated due to the AAPH thermal decomposition) is quenched by the antioxidants present in the sample.Advantages
Rapid.
Change can be visually observed.
Disadvantages
Laborious and complex methodology to perform.
The antioxidants that do not have a clear lag phase cannot be detected by using TRAP assay.
Low reproducibility.Quenching of chemiluminescence.
The duration of the lag phase is utilized for the quantification of the antioxidant.[18]ß-Carotene bleaching assayAqueous emulsion of ß-carotene and linoleic acid generates the free radical in the medium owing to the spontaneous oxidation of the fatty acids. The sample is decolorized owing to the presence of antioxidants in the medium.Advantages
Rapid.
Change can be visually observed.
Can screen both the hydrophobic and hydrophilic antioxidants.
Disadvantages
ß-Carotene is sensitive to temperature and pH change which causes the errors in reading.
Low reproducibility.Calorimetry.
Spectroscopy (decrease in the absorbance at 470 nm).[19]DPPH assayThe AOC is estimated by studying the decrease in the absorbance value at 517 nm associated with the DPPH· radical. The antioxidant reaction with this organic radical lowers the concentration of the radical in the medium that expresses itself by the reduction in the absorbance intensity at the characteristic wavelength of the radical.Advantages
Rapid.
Facile.
Low cost.
Disadvantages
Lacking standardization.
The reaction parameters (including pH, solvent, dissolved oxygen, exposure to light radiations, etc.) affect the AOC values. This kinetic fact is generally ignored.
The steric hindrance and molecular size of the DPPH· radical make it difficult for the antioxidants present in the medium to interact with this radical.
DPPH is not favorable for ranking the antioxidants owing to its dependence on numerous uncontrollable factors.Calorimetry.
Spectroscopy (decrease in the absorbance at 517 nm).[16]TOSC assayThe oxidation of KMBA results in the formation of ethylene by ROS. Presence of antioxidants inhibits the formation of ethylene.Advantages
Rapid.
Facile.
Low cost
Disadvantages
Low accuracy.Headspace gas chromatography for the determination of ethylene.[20] Redox potential based assay (non-radical based assays) CUPRAC assayET mechanismPotential of antioxidants to reduce the Cu2+ ions into the Cu1+ ions is utilized as a means to estimate the AOC. The Cu1+ ions form a complex with neocuproine or bathocuproine, which is detected by spectrophotometric analysis.Advantages
Rapid.
Facile.
Hydrophilic and lyophilic antioxidants can be studied.
Disadvantages
Require the specific pH.
Variation in the pH makes this method ineffective as the complex formation is hindered.
CUPRAC cannot measure the HAT mechanism and therefore, it cannot be utilized to study the intracellular fluids and the human blood plasma/serum.Calorimetry.
Spectroscopy
(appearance of absorption band at 450 nm and 490 nm in case of neocuproine and bathocuproine, respectively).[21]FRAP assayPotential of antioxidants to reduce the [Fe3+-(TPTZ)2]3+ complex into the [Fe2+-(TPTZ)2]2+ is utilized as a means to estimate the AOC.Advantages
Rapid.
Facile.
Disadvantages
Require the acidic medium.
Variation in the pH makes this method ineffective as the iron complex undergoes precipitation.
FRAP cannot measure the HAT mechanism and therefore, it cannot be utilized to study the intracellular fluids and the human blood plasma/serum.Calorimetry
Spectroscopy
(increase in the absorption values at 593 nm).[21]TPC or FCR assayThe Folin&#;Ciocalteu reagent in a strong basic environment generates the phenolate anions, which are reduced by the phenolic antioxidants present in the sample.Advantages
Rapid.
TPC/FCR assay determines the total antioxidant assay.
Operationally simple.
Reproducible.
Disadvantages
Non-phenolic compounds can interfere with the Folin&#;Ciocalteu reagent and cause error in the readings.
Strong basic environment is required.Colorimetry
Spectroscopy
(appearance of blue-colored chromophore having maximum wavelength values in the range of 620&#;765 nm).[22]Metal chelation capacityChelation effects of antioxidants for the quenching of the metal ions (ferrous ions) are used as a means to measure the AOC values.Advantage
Utilized to study the AOC value of the secondary antioxidants.
The test provides the accurate assessment of the antioxidant potential of compounds during lipid peroxidation.
Disadvantages
Standardization of the protocol is required.Spectroscopy
(decrease in absorbance intensity of Fe2+-ferrozinen at 562 nm).[23] Biochemical based assay In vivo assay LDL assayET/HAT mechanismsThe oxidation capacity of LDL lipid peroxidation is reduced owing to the presence of antioxidants.Advantage
Utilized to study the AOC value of the secondary antioxidants.
The test provides an accurate assessment of the antioxidant potential of compounds during lipid peroxidation.
Endogenous antioxidants can be studied by using the LDL assay.
Disadvantages
Standardization of the protocol is required.
Mechanism involved in the quenching requires further exploration.Chemiluminescent (radiations emitted by the oxidative products).
Spectroscopy (formation of diene conjugates at 234 nm).[24]TBARS assayThe unsaturated lipid degradation results in the formation of MDA in the presence of ROS, and this MDA interact with TBA to produce [MDA-(TBA)2] complex. The presence of antioxidants reduces the formation of this complex, which is used as a tool to measure the AOC of the antioxidant.Advantage
Utilized to study the AOC value of the secondary antioxidants.
The test provides an accurate assessment of the antioxidant potential of compounds during lipid peroxidation.
Disadvantages
It is a pH-dependent procedure.
Acidic pH and temperature values of 100 °C are essential for carrying out the reaction.Calorimetry
Spectroscopy
(Reduction in the [MDA-(TBA)2] complex presents the absorption band at 532 nm).[25]CAA assayWithin the cell medium, the DCFH-DA is deacylated by cellular enzymes to generate DCFH within the cells. The peroxy radicals formed from AAPH easily cross the cellular membrane and convert the DCFH into fluorescent DCF. The presence of the antioxidants reduces the DCF generation.Advantage
The performance of the test within the cell medium makes this test the best representative test for studying antioxidants present within the cell.
Dietary supplements, food samples, beverages, and other biogenic samples can be studied by using this assay.
Disadvantages
It is a complex test with intercalated mechanisms.
The results acquired from the CAA assay do not correlate well with the chemical assays.Fluorimetry
(Decrease in the fluorescence intensity).[26]Open in a separate window

Advanced spectrometric methods have also been widely documented for analyzing the antioxidant properties of reported natural antioxidants found in the biogenic samples. However, these methodologies exhibit strong dependence on parameters such as optimization of temperature, time, concentration, and other physicochemical characterization of antioxidant substances, which is needed to be crucially considered before experimentation [27]. High-performance liquid chromatography has been primarily used for the separation, identification, and quantification of nutraceutical components for many years. However, mass spectrometry, when coupled with liquid chromatography, has recently been introduced as a detection method for many such investigations with even lower detection limits [28]. The summary of some case studies documenting the use of conventional and electrochemical detection methodologies used for the identification of AOC in natural and synthetic antioxidants, along with their applications, are provided in .

Table 2

Detection MethodSamples Analysed for Antioxidant CapacityNature of the SampleDetection AssayApplication of AntioxidantRef.Spectroscopic methodsRed propolis extractsNaturalMultivariate analysis (determining phenolic compounds and their bioactive properties)
DPPH assay
The reducing power assay
β-carotene assaySeveral types of biological activities were documented attributed to its chemical constituents[29]Aqueous and methanolic extracts from A. fragrans leaves (Two dihydrochalcone glycosides named confusoside and vacciniifolin)Natural ExtractDPPH assay
ABTS assay
FRAP assayA bioactive functional plant and the leaves of A. fragrans could be used as a potential source of natural antioxidants for food and pharmaceutical applications[30]Co (II) and Fe (II) complexes of Schiff baseSyntheticIn vitro assays
FRAP assay
CUPRAC assay
ABTS assay
DPPH assay
Enzyme inhibition studies.Antioxidant activities and enzyme inhibition determination were carried out. The best inhibitor complex for CA I and II isoenzymes was the Fe(II) complex with Ki values of 52.83&#; ± &#;11.52 and 63.34&#; ± &#;8.88, respectively[31]Caffeic acid grafted chitosanSyntheticDPPH assayA novel dual-functional stabilizer for food-grade emulsions and additive antioxidant property[32]γ-alumina nanoparticlesSyntheticDPPH assay
Computational methods (DFT-D studies)Radical scavenging performance and antioxidant activity[33]Chromatographic MethodsAchillea Arabica
Contents of phenolics, including caffeoylquinic acid derivatives, quercetin, luteolin, apigenin and some of their derivativesNaturalHPLC-PDA fingerprint analysis
ABTS assay
FRAP assays.It can be incorporated in food and pharmaceutical applications[34]Essential antioxidant algal pigments present in marine algae (S. vulgare, C. barbata brown, and F. petiolata green algae species)NaturalCUPRAC assay
ABTS assay
HPLCPotentially rich sources of valuable nutrients and the favorable choice in many different industrial fields: hydrocolloid extraction, fertilizers, extracts for cosmetics and pharmaceuticals, biofuels, bioplastics, and wastewater treatment[35]Flavone triglycosides identified in sugarcane molasses (Chrysoeriol-C-glycosides such as scoparinNaturalABTS assay
CAA assays
LCMSsCan be used as potential source of polyphenols that can be beneficial to health[36]Mulberry leaf (Prenylated phenolics a, moracin N)NaturalDPPH assay
CAA assay
HPLC
UHPLCMultiple health benefits[37]Electrochemical methodsCitrus peel residues (flavedo)NaturalCUPRAC assay
DPPH assayFood, pharmaceutical and cosmetic industries[38]Plant tea samples such as green tea, lime, and coral mossNaturalVoltammetryNutritional science[39]quinones: Mansanone-D, Mansanone-H, Sissoidenone and 4-methoxy dalbergione isolated from Dalbergia sissoidesNaturalCVAntioxidant activity and strong interaction with DNA[40]Open in a separate window

4. Applications of the Electrochemical Methods for the AOC

Different case studies utilizing the electrochemical methods as a means for the determination of AOC are discussed in the subsection below [69,70,71,72]. These particular case studies are classified on the basis of the origin and utility of the biomass extract for this review.

4.1. AOC Determination of the Fruit&#;s Extract

A literature survey indicated that among the utilized electrochemical techniques, CV is the most abundantly exploited electrochemical technique used for the determination of the AOC in the fruit&#;s extract [73]. Antioxidants having moderately low-molecular weight have been particularly documented to be determined with quite an ease via this technique [74]. Zeliǹska et al. [75] examined 11 randomly selected samples of apple cultivars and used the techniques of CV for documenting the AOC, reduction, and chelating potential of the apple&#;s (peel and flesh) extract. The parameter of first anodic peak potential (Epa) was utilized to investigate the reduction potential of the phytochemicals present in the extract. The Epa values of 0.815 V, 0.759 V, 0.618 V, 0.571 V, 0.515 V, 0.512 V, 0.511 V, 0.391 V, 0.390 V, 0.339 V and 0.334 V were recorded for the phytochemicals of phloretin, phloridzin, cyanidin 3-galactoside, catechin, quercetin 3-rhamnoside, quercetin 3-arabinoside, quercetin 3-glucoside, chlorogenic acid, rutin, epicatechin, and quercetin, respectively. The higher Epa values are attributed to the lower reducing activity of the phytochemicals and these results are translated into the high to moderate antioxidant power. Area under the current wave was also utilized to detect the order of the AOC potential. Comparison and correlation studies revealed that the results acquired from CV technique were found to be most compatible with Ferric reducing antioxidant power (FRAP) assay among all other spectroscopic and chromatographic techniques. The literature survey further indicated that the peak current, the area under the current curve, and peak potential are the most important parameters for studying the AOC of the extract [76,77]. The peak potential of the acquired phytochemicals correlates with the type of the reducing agent utilized [78]. The lower oxidation potential values were found to be associated with the enhanced strength of particular phytochemicals for the electron donation and, therefore, directly provide information regarding the AOC of the molecule [79]. The Randles&#;Sevcik equation (discussed in the above equation) also provides a means to access the antioxidant dose present in the extract via the value of the peak current (Ip).

Jara-Palacios et al. [80] utilized the AOC values of the grape pomace (a solid residue composed of approximately 20% of total solid weight of the fruit of the grape, acquired as waste from the grape juice or wine industries) by using the CV technique and thiobarbituric acid reactive substances (TBARS) assay. The TBARS assay is a measure of inhibition of lipid peroxidation (the series of the particular chemical reactions responsible for the oxidative degradation of the lipids molecules). This inhibition is done owing to the presence of the antioxidants in the medium as peroxide molecules are scavenged by these molecules. Apart from identifying the AOC via CV, the electrochemical data was also used to study the by-products of these antioxidant reactions [80]. De Macêdo et al. [73] synthesized a laccase-based biosensor (LLB) and used it for the detection of antioxidants in the extracts of acerola, cranberry, camu-camu, and açai red fruits. The electroanalytical DPV, CV and SWV techniques were also used for this purpose and the parameter of the electrochemical index (EI, presented in Equation (1)) was evaluated for studying the antioxidants:

El=Ipa1Epa1+Ipa2Epa2+&#;+IpanEpan

(1)

where the Ipa1 to Ipan and Epa1 to Epan represent the anodic current and anodic potential of each successive anodic peak from 1 to n. The LLB and EI both represented the same order for the AOC potential in different varieties of the red fruits, i.e., camu-camu > cranberry > açai = acerola. However, the order of AOC potential documented by the DPPH assay did not relate to the LLB and electrochemical techniques, indicating that further research is still needed in this domain to fully comprehend the usefulness of the electrochemical techniques for measuring the AOC values [73]. The electrochemical techniques were also utilized for investigating the AOC values in the extracts of the following edible nutraceuticals, including apricot, black currant, grape pomace (by CV) [64], carrots, persimmons, pumpkins (by DPV), [46], Vaccinium meridionale swart/Agraz (by CV) [81], hops (Barth-Haas, Hallertau, Columbus, and Cascade) and malted barley samples (Chateau Cristal, Pilsen, Chateau Munich, and Wheat), sour cherry, pomegranate, plum, prickly pear (cactus fruit), date, and nectarine (by CV) [82], etc.

4.2. AOC Determination of the Plant&#;s Extract

Similar to the trend observed in the case of the fruit extracts, the technique of CV was found to be most effectively utilized for investigating the antioxidants of the plant&#;s extract as well [83]. However, the utilization of other techniques of DPV and SWV was found to be more abundant in comparison to the fruit extracts [84]. Khiya et al. [83] determined the AOC, TPC, condensed tannin content (TCT), and TAC of the Salvia officinalis by using the following assays/techniques of FRAP, DPPH, phosphomolybdate reduction, and CV. The extract acquired from the Boulemane region exhibited the highest value of total phenolic content (176 mgGAE/g), while the extract acquired from Khenifra region exhibited the highest TCT values (162.53 mgEC/g). In the case of TAC (highest value: 108 mgGAE/g), the aqueous fraction of the S. officinalis leaves acquired from the Boulemane region exhibited the best results. However, the CV results exhibited different results in comparison to other assays, as the best AOC results (288.8 mgGAE/g) were found in the case of the hydro-methanolic extract of the particular plant. Interestingly, the authors also highlighted the fact that the CV can only be utilized to specify the generalized type of phytochemicals; the technique of HPLC is still required for the documentation/characterization of individual molecules present in the understudy extract [60]. The authors also performed the correlation studies and concluded that the phenolic components (rather than flavonoid compounds) were responsible for AOC. This study highlights that every assay/technique has its limitation and the selection of the type of extracting medium (such as aqueous, methanoic, ethanoic or hydro-methanoic, etc.) should be considered beforehand for selecting the technique used for measuring AOC.

Amamra et al. [85] investigated the AOC values for Thymus vulgaris by utilizing spectroscopic assays and electrochemical techniques. The authors investigated butanol, ethyl acetate, aqueous, chloroform, petroleum ether, and methanolic extracts and revealed that all the extracts contained phenolics, flavonoids and tannins components. In terms of CV, the methanolic extract exhibited the best results with a correlation coefficient of 0.964 and 0.989 for gallic acid equivalent (GAE) and ascorbic acid equivalent, respectively, with the other assays (DPHH, Iron chelation assay, and 2,2-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid)-radical cation (ABTS+) assay) for phenolic contents. Chochevska et al. [86] presented a comparison of the AOC values for the homemade-fruit vinegar (HMV) and commercial vinegar by using the technique of CV and exhibited that all the samples of HMV (acquired from the fermentation of the fruits of blueberry (Vaccinium myrtillus); apple (Malus domestica), raspberry (Rubus idaeus), rose hip (Rosa canina), blackberry (Rubus fruticosus), and persimmon (Diospyros kaki)) exhibited at least ten times more AOC values in comparison to the commercial vinegar samples. Fuentes et al. [87] used the technique of the chronoamperometry and chronopotensiometry techniques as a means to detect the AOC in the extracts of Aliso (Alnusacuminate) plant. summarizes the critical remarks observed to be associated with the electrochemical techniques utilized for the detection of AOC.

Table 3

Studied ExtractExtract TypeConventional TechniquesElectrochemical TechniquesElectrodesParameter IdentifiedRemarksRef.Zespri® Sweet Green kiwifruit (Actinidia chinensis x Actinidia deliciosa)Flesh and skin extract of the kiwi fruitFor phenolic contents:
TPC analysis
TFC analysis
TAC analysis
ABTS assay
DPPH assay
FRAP assayCV coupled with the HPLCReference Electrode:
Ag/AgCl electrode
Working Electrode:
Glassy carbon electrodeIdentification of phenolic contents by comparing them with the phenolic standards.
Identified phenolic components:
Catechin
Gallic acid
Protocatechuic acid
Vanillic acid
Caffeic acid
Syringic acid
Epicatechin
Epigallocatechin gallate
p-Coumaric acid
Rutin
Ferulic acid
Epicatechin gallate
Quercetin
trans-Cinnamic acid
The reducing potential and AOC of these compounds were studied by using the parameters of Epa, Ipa and Q500.Among the fourteen identified phenolic components, CV was able to identify the nine antioxidant molecules present in the extract.
Moreover, CV was identified to be the better technique in comparison to the spectroscopic and antioxidants assays as it records the total AOC values in comparison to the particular mechanism (scavenging, ability to reduce metal, or lipid peroxidation inhibition.
One drawback that was observed in case of CV technique was that the ascorbic acid could not be studied through this, while other techniques detected it in the phytochemical profiling.[50]Zespri® SunGold Kiwifruit (Actinidia chinensis)Hayward commercial fruitVitis vinifera
L. (Grape pomace)Grape pomace (seed, skin and stem extract)TPC analysis
TBARS assayCVThree-electrode system
Reference system:
Ag/AgCl electrodeWorking electrode:
Glassy carbon electrode
Auxiliary electrode:
Platinum wireIdentification of electrochemical phenolic compounds: (3&#;, 4&#;-dihydroxyl moiety of phenolic compounds).
The Q500 parameter was used to identify the total phenolic content of the extract.
The Ipc, Ipa, Epa, Epc, and Emid parameters are utilized to investigate the following observables:
Total AOC of the extracts
Byproducts after the antioxidant reaction
Concentration of antioxidantsElectrochemical mechanismIndividual phenols are not detected.
Correlation between the TBARS and CV was studied by Pearson&#;s correlation.[80] Greigia Sphacelata FruitORAC testDPVThree-electrode system
Reference system:
Ag/AgCl electrodeWorking electrode:
Glassy carbon electrode
Auxiliary electrode:
Platinum wireCurrent, potential and area under the current-potential curve are the main Electrochemical parameters utilized to investigate the AOC potential of the sample.Despite differences in the operating mechanism of the utilized assays of ORAC assay (fluorescent method) and DPV (an electrochemical method), Pearson&#;s correlation test revealed that both methods exhibited a correlation value of 0.969 with each other. The authors also reported that the understudy sample has very low antioxidant activity values.[88] Solanum nigrum Black nightshade fruitTPC analysis
TFC analysis
TAC analysis
DPPH assay
FRAP assayCVThree-electrode system
Reference system:
Hg/Hg2Cl2 electrode
Working electrode:
Glassy carbon electrode
Auxiliary electrode:
Platinum wireEpa and Ipa values were used to estimate the AOC values of the fruit extract. The higher Ipa values were co-related with, the higher antioxidant activity. The shape of the voltammogram (i.e., absence of the cathodic peak) was used to obtain information regarding the irreversibility of the phenolic components present in the extract.Superoxide radical scavenging assay was utilized as a means to study the AOC values by using the CV technique, indicating that CV can be used to investigate the ability of the extract to quench the superoxide radical. Ethyl acetate extract of fruit expressed better AOC and scavenging results in comparison to chloroform and n-butanol extract of the fruit.[89]Phoenix dactylifera. L.Date palm fruitTPC analysis
TFC analysis
DPPH assay
FRAP assayCVThree-electrode system
Reference system:
Hg/Hg2Cl2 electrode
Working electrode:
Glassy carbon electrode
Auxiliary electrode:
Platinum wireReduction in the Ipa values associated with the superoxide radical was used as a representation tool indicating the decrease in the concentration of radical owing to the presence of phytochemicals in the extract. The shape of voltammogram is used for studying the irreversibility of the phytochemicals involved.Authors observed that spectrophotometer suffers from the drawback that phytochemicals exhibit the peaks in the same range and, consequently, are not properly identified by spectroscopic techniques. The electrochemical method removes this problem.[62] Bunchosia glandulifera Seed, bark, leaf and root extractTPC analysis
β-Carotene test
Ascorbic acid test
DPPH assay
FRAP assay
ABTS assayCV
DPVThree-electrode system
Reference system:
Ag/AgCl electrode
Working electrode:
Glassy carbon electrode
Auxiliary electrode:
Platinum wireCurrent, potential and area under the current-potential curve are the main electrochemical parameters utilized to investigate the AOC potential of the sampleThe solvent of DMSO is noxious and its use as a solvent should not be encouraged. However, the study presented an excellent comparison between electrochemical methodologies and revealed that the CV showed better linearity in the case of the DPV. Furthermore, the authors found the AOC value of the seed extract presented the best results among all other extracts. [90]Punica granatum L.Pomegranate peelsTPC analysis
TFC analysis
TCT analysis
DPPH assay
FRAP assayCVThree-electrode system
Reference system:
Ag/AgCl electrode
Working electrode:
Glassy carbon electrode
Auxiliary electrode:Platinum wireAnodic current, anodic potential and area under the current-potential curve are the main electrochemical parameters utilized to investigate the AOC potential of the sample.The extraction of the phenolic components was performed by using the response surface methodology (RSM). The authors identified that the AOC potential of the five medicinal plants exhibited almost 36 times lower efficacy in comparison to the ascorbic acid-based test, indicating that the CV is not suitable for measuring the AOC value for the extracts containing the high molecular weight antioxidants.[91]Argania spinosa L.Argan hull Corrigiola telephiifolia Pourr. Serghina Prunus dulcis AlmondRubia tinctorum L.Madder rootOpen in a separate window

5. Conclusions

This recent literature review presents critical insights into the use of the electrochemical methodologies as a tool to detect the AOC values of the plant and fruit extracts. The techniques of CV, DPV, SWV, and AM were explored for achieving this purpose. It was found that the CV is the most extensively exploited technique among all other electrochemical techniques for the detection of the AOC in numerous extract-based samples. In most studies, the electrochemical methodologies were used as an additional tool alongside other conventional (spectroscopic and chromatographic) techniques for the AOC determination, which is a welcome approach as the literature survey indicates that a contradiction still exists regarding the documented values of correlation of these electrochemical techniques with other technique. Furthermore, the AMM technique is far less explored in comparison to all other techniques, indicating that further research is still needed to explore the full potential/advantage of the electrochemical techniques for the determination of AOC. The nature of phytochemicals was also found to be critical as the AOC values of the extract containing the high molecular weight antioxidants cannot be found by using the electrochemical techniques.

6. Future Perspectives

Innovation in every field needs a multiprobe approach. The use of simulations and data-driven techniques can speed up the development in the investigation of the antioxidant potential of plant/fruit extracts. The density functional theory can be used to study the antioxidant potential of molecules in plant/fruit extracts [92,93,94]. The information from the theoretical analysis can help in selecting methods for experimental work. Recently, the use of machine learning has been increasing in biological and material science [95,96]. The machine learning models also can be used to predict the antioxidant potential of plants/fruits. It will shorten the discovery of efficient antioxidants; it is easy and more cost-effective way.

Additional reading:
Development prospect of industrial graphite products

Zhongsheng Product Page

10 Questions You Should Know About Stainless Steel Alloys
Grounding Wire Clip Essentials: Secure Your Electrical Safety
4 Tips to Select the Perfect Flexible Graphite Wire Custom
How to Choose the Right Graphite Packing Rope?

Following future perspectives can be observed regarding the use of the electrochemical methodologies for AOC values.

• The electrochemical techniques should be applied in conjugation with the chromatographic technique as the electrochemical techniques were still not able to identify the phytochemicals present in the extract.

• During the study of the correlation of the electrochemical techniques with the conventional methodologies, the fact that the electrochemical techniques are used to measure the TAC rather than measuring the particular antioxidants for the particular reaction should not be ignored. The cumulative antioxidant potential should be considered to be a preferable approach in determining the selective antioxidants via particular reactions.

• The flow injection analysis alongside the stationary electrochemical techniques should also be performed for studying the AOC potential of the extract.

Acknowledgments

This work was supported by Al Bilad Bank Scholarly Chair for Food Security in Saudi Arabia, the Deanship of Scientific Research, Vice-Presidency for Graduate Studies and Scientific Research, King Faisal University, Saudi Arabia [Project No. CHAIR69].

Funding Statement

This work was supported by Al Bilad Bank Scholarly Chair for Food Security in Saudi Arabia, the Deanship of Scientific Research, Vice-Presidency for Graduate Studies and Scientific Research, King Faisal University, Saudi Arabia [Project No. CHAIR69].

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Publisher&#;s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Introduction:

In the realm of industrial applications, graphite electrodes play a vital role in various processes such as steelmaking and electric arc furnaces. However, their susceptibility to oxidation can significantly impact their performance and lifespan. In this article, we will explore key strategies to effectively prevent graphite electrode oxidation, ensuring optimal efficiency and durability in your operations.

1. Material Selection: The Foundation of Oxidation Resistance

Choosing high-quality graphite materials with excellent oxidation resistance is paramount. Look for keywords such as "high purity," "low impurity content," and "fine grain structure" when selecting graphite electrodes. These attributes ensure enhanced resistance to oxidation and prolonged electrode life.

1. Surface Coatings: Shielding Against Oxidation

Applying protective coatings on graphite electrodes creates a physical barrier, preventing direct contact with oxygen and other reactive substances. Consider utilizing advanced coatings such as silicon carbide, resin-bonded graphite, or anti-oxidation coatings. These coatings act as a shield, reducing oxidation and promoting a longer electrode lifespan.

3.Proper Handling and Storage: Preserving Integrity

Proper handling and storage practices are crucial in preventing premature oxidation. Ensure that graphite electrodes are stored in a controlled environment with controlled humidity levels. Avoid exposure to moisture, extreme temperatures, and corrosive substances. Implement strict protocols for transportation, avoiding any potential damage or contamination that could accelerate oxidation.

4.Optimized Operating Parameters: Mitigating Oxidation Risks

Fine-tuning your operational parameters can significantly reduce oxidation risks. Maintain stable operating conditions such as electrode current density, power input, and process parameters. Avoid unnecessary power fluctuations, overloading, or sudden changes in voltage, which can generate excessive heat and accelerate electrode oxidation.

5.Regular Maintenance and Inspection: Proactive Care

Implementing a proactive maintenance and inspection regime is essential for identifying early signs of oxidation and taking necessary preventive measures. Regularly monitor electrode performance, including surface condition, dimensions, and electrical resistance. Schedule periodic cleaning and reconditioning to remove surface impurities and extend electrode lifespan.

6.Collaboration with Experts: Accessing Specialized Knowledge

Engage with experienced suppliers and industry experts who possess extensive knowledge of graphite electrodes. Seek their guidance on material selection, coating options, maintenance techniques, and best practices for preventing oxidation. Their expertise can help optimize your operations and minimize oxidation-related challenges.

Conclusion:

Preventing graphite electrode oxidation is a multifaceted endeavor, encompassing material selection, protective coatings, proper handling, optimized operation, regular maintenance, and expert collaboration. By implementing these strategies, you can enhance the oxidation resistance of graphite electrodes, ensuring their longevity and maximizing operational efficiency. Embrace the power of proactive prevention and unlock the true potential of your graphite electrode applications.

Want more information on Antioxidant Graphite Electrode? Feel free to contact us.