Chemometrically assisted differential pulse voltammetry for simultaneous and interference-free quantification of gallic and caffeic acids

This article explores the application of chemometric tools including multivariate curve resolution with alternating least squares for the simultaneous determination of gallic and caffeic acids on the surface of a glassy carbon electrode without additional modification. Gallic and caffeic acids are primary polyphenols, the most abundant in red wines produced in Argentina, and are often used as quality markers for them. These polyphenols significantly contribute to the organoleptic properties of wines from this origin, but their electrochemical signals overlap significantly, making simultaneous quantification challenging without additional experiments such as electrode modification or alternative analytical techniques beyond differential pulse voltammetry. This study successfully quantified these compounds in complex mixtures by generating second-order data from differential pulse voltammetry experiments conducted at various potential steps and subsequently applying multivariate curve resolution with alternating least squares. The use of constraints during optimization prevented rotational ambiguities common in this modeling, leading to unique results in validation samples. The limits of detection (LOD) found for gallic and caffeic acids were 1.6 and 7.6 mg L-1, which are in excellent agreement with the expected concentrations of these compounds in red wines. The concentration ranges analyzed showed a linear dependency (between the LOD and 300 mg L-1) with the signals estimated by the model for both analytes. Advantages such as simplicity, low cost, and high speed, as well as not requiring electrode modification, combined with excellent results obtained for real samples, make it a promising alternative for polyphenol analysis in the wine industry.

Phytochemical Composition and Antioxidant Activity of Various Extracts of Fibre Hemp (Cannabis sativa L.) Cultivated in Lithuania

The phytochemistry of fibre hemp (Cannabis sativa L., cv. Futura 75 and Felina 32) cultivated in Lithuania was investigated. The soil characteristics (conductivity, pH and major elements) of the cultivation field were determined. The chemical composition of hemp extracts and essential oils (EOs) from different plant parts was determined by the HPLC/DAD/TOF and GC/MS techniques. Among the major constituents, β-caryophyllene (≤46.64%) and its oxide (≤14.53%), α-pinene (≤20.25%) or α-humulene (≤11.48) were determined in EOs. Cannabidiol (CBD) was a predominant compound (≤64.56%) among the volatile constituents of the methanolic extracts of hemp leaves and inflorescences. Appreciable quantities of 2-monolinolein (11.31%), methyl eicosatetraenoate (9.70%) and γ-sitosterol (8.99%) were detected in hemp seed extracts. The octadecenyl ester of hexadecenoic acid (≤31.27%), friedelan-3-one (≤21.49%), dihydrobenzofuran (≤17.07%) and γ-sitosterol (14.03%) were major constituents of the methanolic extracts of hemp roots, collected during various growth stages. The CBD quantity was the highest in hemp flower extracts in pentane (32.73%). The amounts of cannabidiolic acid (CBDA) were up to 24.21% in hemp leaf extracts. The total content of tetrahydrocannabinol (THC) isomers was the highest in hemp flower pentane extracts (≤22.43%). The total phenolic content (TPC) varied from 187.9 to 924.7 (average means, mg/L of gallic acid equivalent (GAE)) in aqueous unshelled hemp seed and flower extracts, respectively. The TPC was determined to be up to 321.0 (mg/L GAE) in root extracts. The antioxidant activity (AA) of hemp extracts and Eos was tested by the spectrophotometric DPPH^● scavenging activity method. The highest AA was recorded for hemp leaf EOs (from 15.034 to 35.036 mmol/L, TROLOX equivalent). In the case of roots, the highest AA (1.556 mmol/L, TROLOX) was found in the extracts of roots collected at the seed maturation stage. The electrochemical (cyclic and square wave voltammetry) assays correlated with the TPC. The hydrogen-peroxide-scavenging activity of extracts was independent of the TPC.

Elucidating the complete oxidation mechanism of betanidin in an aqueous solution

An important point to take advantage of the use of antioxidants in industrial applications in a more efficient way is to know in depth their oxidation mechanism. This is not always a simple task and requires an in-depth study that is often insufficient to precisely describe all the structures and processes involved. This is the case of betanidin, a natural pigment employed in the drug, food, and cosmetic industries. In the present work, we seek to unravel the complete oxidation mechanism of betanidin with the use of computational techniques, supported by experimental data. For this aim, the pK_as and oxidation potentials of the reactions involved at different pHs were analyzed using density functional theory (DFT) with the B3LYP/6-31+G(d,p)/SMD approach. Moreover, the decomposition mechanism of the intermediate products (decarboxylation reactions) was studied deeply. The analysis of DFT results allowed the proposal of a tentative mechanism that was put to test using the digital simulations of cyclic voltammetry by comparing the results of these simulations with an experimental case. Based on the rigorous experimental analysis, DFT, and simulations of cyclic voltammetry, the complete mechanism of the oxidation of betanidin in an aqueous medium was proposed. The dimerization of the oxidation products was also considered to explain the voltammetric response of betanidin.

Antioxidant activity and enzymatic of lichen substances: A study based on cyclic voltammetry and theoretical

The antioxidant activity of nine lichen substances, including methylatrarate (1), methyl haematommate (2), lobaric acid (3), fumarprotocetraric acid (4), sphaerophorin (5), subsphaeric acid (6), diffractaic acid (7), barbatolic acid (8) and salazinic acid (9) has been determined through cyclic voltammetry. The compounds 1-4 presented slopes close to the Nernst constant of 0.059 V, indicating a 2H⁺/2e^- relation between protons and electrons, as long as the compounds 5, 6, 7, 8, and 9 present slopes between 0.037 V and 0.032 V, indicating a 1H⁺/2e^- relation between protons and electrons. These results show a high free radical scavenging activity by means of the release of H⁺, suggesting an important antioxidant capacity of these molecules. Theoretical calculations of hydrogen bond dissociation enthalpies (BDE), proton affinities (PA), and Proton Transfer (PT) mechanisms, at M06-2x/6-311+G(d,p) level complement the experimental results. Computations support that the best antioxidant activity is obtained for the molecules (3, 4, 5, 6, 7 and 8), that have a carboxylic acid group close to a phenolic hydroxyl group, through hydrogen atomic transfer (HAT) and sequential proton loss electron transfer (SPLET) mechanisms. Additional computations were performed for modelling binding affinity of the lichen substances with CYPs enzymes, mainly CYP1A2, CYP51, and CYP2C9*2 isoforms, showing strong affinity for all the compounds described in this study.

In Vitro Antioxidant and Prooxidant Activities of Red Raspberry (Rubus idaeus L.) Stem Extracts

Leaves and stems of red raspberry (Rubus idaeus) are used in Lithuanian folk medicine. Healing properties of raspberry are related to the content of bioactive compounds, mainly polyphenols. Extracts of raspberry leaves contained higher total phenolic content (TPC) (1290 mg/L, expressed in gallic acid equivalent) compared to that in extracts of stems or peeled bark (up to 420 mg/L and 598 mg/L, respectively). To find out whether the collection time of herbal material was critical for the properties of the extracts, the stems were collected at different times of the year. TPC in the extracts depended more on extraction conditions rather than on the sampling time. Antioxidant activity of raspberry stem and bark extracts tested by spectrophotometric (DPPH● scavenging) and electrochemical (cyclic and differential pulse voltammetry) assays correlated with TPC. DPPH radical scavenging activity values for stem, leaf, and bark extracts were as follows: ≤1.18 ± 0.07, 1.63 ± 0.10, and ≤1.90 ± 0.04 (mmol/L, TROLOX equivalent), respectively. Assessed electrochemically, hydrogen peroxide-scavenging activity of extracts was independent on TPC. The latter activity was related to the presence of some protein in the extract as revealed by gel electrophoresis. Prooxidant activity of raspberry stem extracts was dependent on solution pH and temperature.

(±)-Catechin-A Mass-Spectrometry-Based Exploration Coordination Complex Formation with FeII and FeIII

Catechin is an extensively investigated plant flavan-3-ol with a beneficial impact on human health that is often associated with antioxidant activities and iron coordination complex formation. The aim of this study was to explore these properties with FeII and FeIII using a combination of nanoelectrospray-mass spectrometry, differential pulse voltammetry, site-specific deoxyribose degradation assay, FeII autoxidation assay, and brine shrimp mortality assay. Catechin primarily favored coordination complex formation with Fe ions of the stoichiometry catechin:Fe in the ratio of 1:1 or 2:1. In the detected Fe-catechin coordination complexes, FeII prevailed. Differential pulse voltammetry, the site-specific deoxyribose degradation, and FeII autoxidation assays proved that coordination complex formation affected catechin's antioxidant effects. In situ formed Fe-catechin coordination complexes showed no toxic activities in the brine shrimp mortality assay. In summary, catechin has properties for the possible treatment of pathological processes associated with ageing and degeneration, such as Alzheimer's and Parkinson's diseases.

A Comprehensive Review on the Electrochemical Sensing of Flavonoids

Flavonoids are bioactive polyphenolic compounds, widespread in the plant kingdom. Flavonoids possess broad-spectrum pharmacological effects due to their antioxidant, anti-tumor, anti-neoplastic, anti-mutagenic, anti-microbial, anti-inflammatory, anti-allergic, immunomodulatory, and vasodilatory properties. Care must be taken, since excessive consumption of flavonoids may have adverse effects. Therefore, proper identification, quantification and quality evaluations of flavonoids in edible samples are necessary. Electroanalytical approaches have gained much interest for the analysis of redox behavior and quantification of different flavonoids. Compared to various conventional methods, electrochemical techniques for the analysis of flavonoids offer advantages of high sensitivity, selectivity, low cost, simplicity, biocompatibility, easy on-site evaluation, high accuracy, reproducibility, wide linearity of detection, and low detection limits. This review article focuses on the developments in electrochemical sensing of different flavonoids with emphasis on electrode modification strategies to boost the electrocatalytic activity and analytical efficiency.

The new rapid and accurate analytical HPLC-ECD method for the determination of rosmarinic acid in meat products

A new rapid method has been developed for the determination of low levels of rosmarinic acid extracted from rosemary (Rosmarinus officinalis L.) and has been used as an antioxidant in meat and meat products after cold storage at 4°C. The method is a high performance liquid chromatography using a coulometric electrochemical detector. It provides a significant improvement on the limit of detection, which was 0.33 ppb, while the limit of quantification was 1 ppb of rosmarinic acid. The advantage of the method also lies in the simpler and faster sample preparation, which can quantify a very low concentration of rosmarinic acid (60 ppb), and is more than 40 ppb below the limits of previously existing methods. A coulometric method is well suited for determining low analyte concentrations and is one of the most sensitive analytical approaches available today, in addition to being time efficient and cost effective. PRACTICAL APPLICATION: A new method for determining low concentrations (60 ppb) of rosmarinic acid in meat and meat products is presented. The method is user-friendly, as it does not require complex sample preparation. It is a selective, precise, and accurate method that makes it useful for routine applications in the meat and other food industries.

Effects of the Reactive Moiety of Phenolipids on Their Antioxidant Efficiency in Model Emulsified Systems

Our previous research was focused on the effects of hydrophobicity on the antioxidant (AO) efficiency of series of homologous antioxidants with the same reactive moieties. In this work we evaluate the antioxidant efficiency of hydrophobic phenolipids in 4:6 olive oil-in-water emulsions, with different phenolic moieties (derived from caffeic, 4-hydroxycinnamic, dihydrocaffeic acids, tyrosol and hydroxytyrosol), with alkyl chains of 8 and 16 carbons, and compare the antioxidant efficiency with that of the parent compounds. All catecholic phenolipids, in particular the C8 derivatives, have proven to be better antioxidants for the oxidative protection of emulsions than their parental compounds with octyl dihydrocafffeate being the most efficient (16-fold increase in relation to the control). To understand the importance of some factors on the antioxidant efficiency of compounds in emulsions, Pearson’s correlation analysis was carried out between antioxidant activity and the first anodic potential (E_pa), reducing capacity (FRAP value), DPPH radical scavenging activity (EC₅₀) and the concentration of antioxidants in each region of the emulsified system. Results confirm the importance of the effective concentration of AOs in the interfacial region (AO_I) (ρ = 0.820) and of the E_pa (ρ = −0.677) in predicting their antioxidant efficiency in olive oil-in-water emulsions.

Preparation of Graphene Oxide-Embedded Hydrogel as a Novel Sensor Platform for Antioxidant Activity Evaluation of Scutellaria baicalensis

Antioxidation is very important in medicine and food. The current evaluation technologies often have many shortcomings. In this work, an improved electrochemical sensing platform for the evaluation of antioxidant activity has been proposed. A hydrogel was prepared based on graphene oxide, zinc ions, and chitosan. Zinc ions play the role of crosslinking agents in hydrogels. The structure of chitosan can be destroyed by injecting hydrogen peroxide into the hydrogel, and the free zinc ions can diffuse to the surface of the electrode to participate in the electrochemical reaction. This electrochemical sensor can evaluate the antioxidant activity by comparing the current difference of zinc reduction before and after adding the antioxidant. With the help of graphene oxide, this hydrogel can greatly enhance the sensing effect. We conducted tests on 10 real samples. This proposed electrochemical platform has been successfully applied for evaluating the antioxidant activity of Scutellaria baicalensis, and the results were compared to those obtained from the 2,2-diphenyl-1-picrylhydrazyl-based traditional analysis technique.

Responses of a hot spring cyanobacterium under ultraviolet and photosynthetically active radiation: photosynthetic performance, antioxidative enzymes, mycosporine-like amino acid profiling and its antioxidative potentials

This study summarizes the response of a hot spring cyanobacterium Fischerella sp. strain HKAR-14, under simulated light conditions of ultraviolet radiation (UVR), photosynthetically active radiation (PAR), PAR + UV-A (PA) and PAR + UV-A + UV-B (PAB). Exposure to UVR caused a decline in growth and Chl a while total carotene content increased under PA and PAB. Maximum photochemical efficiency of photosystem II (F v /F m) and relative electron transport rate decreased significantly in PA and PAB exposure. Higher non-photochemical quenching and lower photochemical quenching values were observed in UVR-exposed samples as compared to the control. Levels of intracellular reactive oxygen species (ROS) increased significantly in PAB and PA. Fluorescence microscopic images showed an increase in green fluorescence, indicating the generation of ROS in UVR. The antioxidant machinery including superoxide dismutase, catalase and peroxidase showed an increase of 1.76-fold and 2.5-fold superoxide dismutase, 2.4-fold and 3.7-fold catalase, 1.83-fold and 2.5-fold peroxidase activities under PA and PAB, respectively. High-performance liquid chromatography equipped with photodiode array detector, electrospray ionization mass spectrometry, Fourier-transform infrared spectroscopy and nuclear magnetic resonance spectroscopy analyses reveal the occurrence of a single mycosporine-like amino acid, shinorine (λ max 332.3 ± 2 nm, m/z 333.1), with a retention time of 1.157 min. The electrochemical characterization of shinorine was determined by cyclic voltammetry. The shinorine molecule possesses electrochemical activity and represents diffusion-controlled process in 0.1 M (pH 7.0) phosphate buffer. An antioxidant assay of shinorine showed its efficient activity as antioxidant which increased in a dose-dependent manner.

In Vitro Evaluation of Pro- and Antioxidant Effects of Flavonoid Tricetin in Comparison to Myricetin

Flavonoids are rather common plant phenolic constituents that are known for potent antioxidant effects and can be beneficial for human health. Flavonoids with a pyrogallol moiety are highly efficient reducing agents with possible pro- and antioxidant effects, depending on the reaction milieu. Therefore, the redox properties of myricetin and tricetin were investigated by differential pulse voltammetry and deoxyribose degradation assay. Tricetin proved to be a good antioxidant but only showed negligible pro-oxidant activity in one of the deoxyribose degradation assay variants. Compared to tricetin, myricetin showed pro- and antioxidant effects. The more efficient reducing properties of myricetin are probably caused by the positive mesomeric effect of the enolic 3-hydroxy group on ring C. It is evident that the antioxidant properties of structurally similar flavonoids can be converted to apparent pro-oxidant effects by relatively small structural changes, such as hydroxylation. Since reactive oxygen species (ROS) often serve as secondary messengers in pathological and physiological processes in animal and plant cells, the pro- and antioxidant properties of flavonoids are an important part of controlling mechanisms of tissue signal cascades.

Electrochemical behaviour of piperine. Comparison with control antioxidants

Piperine, as the most abundant alkaloid in pepper, gained a lot of attention for possible antioxidant and therapeutic properties. Electrochemical techniques were applied to widely evaluate the redox behavior of piperine by comparison to that of well-known antioxidants: ascorbic acid, protocatechuic acid, syringic acid, tyrosine and capsaicin used as controls. Also, electrochemistry was involved in an innovative way to investigate the potential antioxidant properties of piperine combined with different in vitro peroxidation and reducing assays: (i) 1,1-diphenyl-2-picryl-hydrazyl free radical (DPPH) scavenging; (ii) 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) scavenging; (iii) ferric ions (Fe³⁺) reducing power; (iv) hydrogen peroxide (H₂O₂) scavenging. Results show that piperine readily reacts with highly oxidizing radicals and bind redox-active metal ions in a similar manner as antioxidants used as model.

Study of Antioxidant Properties of Agents from the Perspective of Their Action Mechanisms

The creation and analysis of a large variety of existing methods for the evaluation of integrated antioxidant properties are quite relevant in connection with a range of biological mechanisms of the antioxidants (AO) action. In this work, the existing methods are correlated with mechanisms of antioxidant action. It is shown that the results obtained by various methods are mainly incomparable. This can be connected with the implementation of various mechanisms of antioxidant action in methods. The analysis of the literature data presented in this review indicates the difficulty of creating a universal method and the feasibility of using integrated approaches based on the use of several methods that implement and combine various mechanisms of the chemical conversion of antioxidants. This review describes methods for studying the chelating ability of antioxidants, except for methods based on electron and hydrogen atom transfer reactions, which are currently not widely covered in modern literature. With the description of each mechanism, special attention is paid to electrochemical methods, as the interaction of active oxygen metabolites of radical and non-radical nature with antioxidants has an electron/proton/donor-acceptor nature, which corresponds to the nature of electrochemical methods and suggests that they can be used to study the interaction.

Electrochemical Methods and (Bio) Sensors for Rosmarinic Acid Investigation

Rosmarinic acid (RA) is an important bioactive phenolic acid with significant biochemical activities, including the antioxidant one. It is widely found in plants of the families Lamiaceae and Boraginaceae and has many uses in the food, pharmaceutical and cosmetics industries. RA is an electroactive species owing to the presence of the two catechol groups in its structure. Due to their inherent characteristics, such as sensitivity, selectivity, ease of operation and not too high costs, electrochemical methods of analysis are interesting tools for the assessment of redox-active compounds. Moreover, there is a good correlation between the redox potential of the analyte and its capability to donate electrons and, consequently, its antioxidant activity. Therefore, this paper presents a detailed overview of the electrochemical (bio)sensors and methods, in both stationary and dynamic systems, applied for RA investigation under different aspects. These comprise its antioxidant activity, its interaction with biological important molecules and the quantification of RA or total polyphenolic content in different samples.

Antioxidant and Antibacterial Properties of Carbosilane Dendrimers Functionalized with Polyphenolic Moieties

A new family of polyphenolic carbosilane dendrimers functionalized with ferulic, caffeic, and gallic acids has been obtained through a straightforward amidation reaction. Their antioxidant activity has been studied by different techniques such as DPPH (2,2′-diphenyl-1-picrylhydrazyl) radical scavenging assay, FRAP assay (ferric reducing antioxidant power), and cyclic voltammetry. The antioxidant analysis showed that polyphenolic dendrimers exhibited higher activities than free polyphenols in all cases. The first-generation dendrimer decorated with gallic acid stood out as the best antioxidant compound, displaying a correlation between the number of hydroxyl groups in the polyphenol structure and the antioxidant activity of the compounds. Moreover, the antibacterial capacity of these new systems has been screened against Gram-positive (+) and Gram-negative (−) bacteria, and we observed that polyphenolic dendrimers functionalized with caffeic and gallic acids were capable of decreasing bacterial growth. In contrast, ferulic carbosilane dendrimers and free polyphenols showed no effect, establishing a correlation between antioxidant activity and antibacterial capacity. Finally, a viability assay in human skin fibroblasts cells (HFF-1) allowed for corroborating the nontoxicity of the polyphenolic dendrimers at their active antibacterial concentration.

Natural phenolic antioxidants electrochemistry: Towards a new food science methodology

Natural phenolic compounds are abundant in the vegetable kingdom, occurring mainly as secondary metabolites in a wide variety of chemical structures. Around 10,000 different plant phenolic derivatives have been isolated and identified. This review provides an exhaustive overview concerning the electron transfer reactions in natural polyphenols, from the point of view of their in vitro antioxidant and/or pro-oxidant mode of action, as well as their identification in highly complex matrixes, for example, fruits, vegetables, wine, food supplements, relevant for food quality control, nutrition, and health research. The accurate assessment of polyphenols' redox behavior is essential, and the application of the electrochemical methods in routine quality control of natural products and foods, where the polyphenols antioxidant activity needs to be quantified in vitro, is of the utmost importance. The phenol moiety oxidation pathways and the effect of substituents and experimental conditions on their electrochemical behavior will be reviewed. The fundamental principles concerning the redox behavior of natural polyphenols, specifically flavonoids and other benzopyran derivatives, phenolic acids and ester derivatives, quinones, lignins, tannins, lignans, essential oils, stilbenes, curcuminoids, and chalcones, will be described. The final sections will focus on the electroanalysis of phenolic antioxidants in natural products and the electroanalytical evaluation of in vitro total antioxidant capacity.

Organic-Inorganic Composite Nanorods as an Excellent Mimicking Peroxidases for Colorimetric Detection and Evaluation of Antioxidant

N,N'-Dicarboxy methyl perylene diimide-coated CeO₂ nanorods (PDI/CeO₂ NR) were synthesized via an ultrasonic-assisted method. PDI/CeO₂ exhibits the superb mimic peroxidase functions confirmed by the catalyzed oxidation of TMB by hydrogen peroxide in less than 60 s along with color transformation from colorless to blue. The catalytic mechanism was confirmed to be an electronic transfer mechanism by fluorescence experiment. Thus, a visual colorimetric sensor was constructed for hydrogen peroxide detection with a low detection limit (LOD = 2.23 μM). Comparing the inhibition effects of l-cysteine (Cys), ascorbic acid (AA) and glutathione (GSH) on the catalytic oxidation of TMB, it can be found that they possessed different types of inhibition on the oxidation of TMB by H₂O₂ using PDI/CeO₂, and AA has better antioxidant effect, followed by Cys and GSH. On the basis of the excellent antioxidant effect of AA, a low-cost colorimetric sensor was also used to detect AA, and a lower LOD value (0.68 μM) was obtained in the linear range of 5.0-30 μM.

Identification of functional compounds in baru (Dipteryx alata Vog.) nuts: Nutritional value, volatile and phenolic composition, antioxidant activity and antiproliferative effect

This work aimed to contribute to the nutritional and functional characterization of roasted baru nuts, a seed widely consumed and produced in Brazil. Baru nut was characterized in terms of its nutritional value and volatile composition (SPME-GC-MS analysis). The ultrasound assisted extraction was used to extract free and bound phenolic compounds that were identified by LC-DAD-ESI-MS/MS method. Bioactivity assays were carried out to evaluate the antioxidant activity (ORAC and HOSC assay) and anticancer effect (inhibition of HT29 cell growth and targeting of cancer stemness) of baru nut extracts and phenolic compounds. Results showed that baru is a good source of protein and monounsaturated fatty acids, specifically oleic acid (47.20 g/100 g). The predominant volatile compounds are hexanal (71.18%) and 2,5-dimethyl-pyrazine (9.43%). The main phenolic compounds identified were gallic acid and its derivatives, such as gallic acid esters and gallotannins. Among all, gallic acid and methyl gallate seemed to be the main compounds responsible for the high antioxidant activity. The antiproliferative effect evaluated of baru extracts in HT29 cell line showed ability to impair cell growth in both monolayer and spheroid cultures and to reduce ALDH+ population. These results supply new information about the functional compounds presents in baru nut, which are important sources of natural antioxidants and antiproliferative compounds.

Electrochemical Sensors Modified with Combinations of Sulfur Containing Phthalocyanines and Capped Gold Nanoparticles: A Study of the Influence of the Nature of the Interaction between Sensing Materials

Voltametric sensors formed by the combination of a sulfur-substituted zinc phthalocyanine (ZnPc^RS) and gold nanoparticles capped with tetraoctylammonium bromide (AuNP^tOcBr) have been developed. The influence of the nature of the interaction between both components in the response towards catechol has been evaluated. Electrodes modified with a mixture of nanoparticles and phthalocyanine (AuNP^tOcBr/ZnPc^RS) show an increase in the intensity of the peak associated with the reduction of catechol. Electrodes modified with a covalent adduct-both component are linked through a thioether bond-(AuNP^tOcBr-S-ZnPc^R), show an increase in the intensity of the oxidation peak. Voltammograms registered at increasing scan rates show that charge transfer coefficients are different in both types of electrodes confirming that the kinetics of the electrochemical reaction is influenced by the nature of the interaction between both electrocatalytic materials. The limits of detection attained are 0.9 × 10⁻⁶ mol∙L⁻¹ for the electrode modified with the mixture AuNP^tOcBr/ZnPc^RS and 1.3 × 10⁻⁷ mol∙L⁻¹ for the electrode modified with the covalent adduct AuNP^tOcBr-S-ZnPc^R. These results indicate that the establishment of covalent bonds between nanoparticles and phthalocyanines can be a good strategy to obtain sensors with enhanced performance, improving the charge transfer rate and the detection limits of voltammetric sensors.

Polarography as a technique of choice for the evaluation of total antioxidant activity: The case study of selected Coprinus Comatus extracts and quinic acid, their antidiabetic ingredient

This study was focused on in vitro screening of the total antioxidant activity of the selected extracts of the mushroom Coprinus comatus and quinic acid, one of their antidiabetic ingredients, by an uncommon electrochemical assay. Indeed, direct current (DC) polarographic HydroxoPerhydroxo Mercury(II) Complex (HPMC) assay based on decrease of anodic limiting current originating from HPMC formation in alkaline solutions of hydrogen peroxide at potential of mercury dissolution, observed upon gradual addition of antioxidants, was applied herein for the estimation of the natural products' antioxidativity. Quinic acid was found to exhibit most promising antioxidant potential (4.0 ± 0.2%µL^-1) being ≈ 2-fold more active than the screened C. comatus extract samples. Actually, such a finding puts some light on the antioxidativity of cyclic polyols, well understimated class of organic compounds, compared to aromatic (poly)phenolics. As a low cost, easy-to handle and accurate this polarographic assay may be thoroughly recommended for much broader use. [Formula: see text].

Laser-Cut Polymer Tape Templates for Scalable Filtration Fabrication of User-Designed and Carbon-Nanomaterial-Based Electrochemical Sensors

We report here a simple filtration method for the scalable fabrication of user-designed and carbon-nanomaterial-based electrode arrays using laser-cut poly(vinyl chloride) (PVC) tape templates. This method can produce electrode arrays with high uniformity and low resistance from the dilute dispersions of single-walled carbon nanotubes (SWNTs) and graphene nanoplatelets (GNPs). For these two carbon arrays, the SWNT array is demonstrated to possess several interesting properties, e.g., good mechanical properties, excellent flexibility, and favorable electrochemical behavior. Moreover, its porous structure enables the construction of a paperlike solid-state electrochemical sensor using Nafion electrolytes, which is suitable for the on-site monitoring of trace phenol pollutants in electrolyte-free water. Besides, an electrochemically addressable 36-zone sensor was constructed by this method. With the aid of an inexpensive 3D printer, the addressable sensor can achieve the semiautomatic and high-throughput evaluation of antioxidant capacity on a series of vegetables and fruits using a single-channel electrochemical analyzer.

Metal Oxide Nanoparticle Based Electrochemical Sensor for Total Antioxidant Capacity (TAC) Detection in Wine Samples

A single-use electrochemical screen-printed electrode is reported based on biomimetic properties of nanoceria particles (CeNPs). The developed tool showed an easy approach compared to the classical spectrophotometric methods reported in literature in terms of ease of use, cost, portability, and unnecessary secondary reagents. The sensor allowed the detection of the total antioxidant capacity (TAC) in wine samples. The sensor has been optimized and characterized electrochemically and then tested with antioxidant compounds occurred in wine samples. The electrochemical CeNPs modified sensor has been used for detection of TAC in white and red commercial wines and the data compared to the 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid (ABTS)-based spectrophotometric method. Finally, the obtained results have demonstrated that the proposed sensor was suitable for the simple and quick evaluation of TAC in beverage samples.

A different approach for the analysis of grapes: Using the skin as sensing element

In this work, an alternative method to monitor the phenolic maturity of grapes was developed. In this approach, the skins of grapes were used to cover the surface of carbon paste electrodes and the voltammetric signals obtained with the skin-modified sensors were used to obtain information about the phenolic content of the skins. These sensors could easily detect differences in the phenolic composition of different Spanish varieties of grapes (Mencía, Prieto Picudo and Juan García). Moreover, sensors were able to monitor changes in the phenolic content throughout the ripening process from véraison until harvest. Using PLS-1 (Partial Least Squares), correlations were established between the voltammetric signals registered with the skin-modified sensors and the phenolic content measured by classical methods (Glories or Total Polyphenol Index). PLS-1 models provided additional information about Brix degree, density or sugar content, which usually used to establish the harvesting date. The quality of the correlations was influenced by the maturation process and the structural and mechanical skin properties. Thus the skin sensors fabricated with Juan García and Prieto Picudo grapes (that showed faster polyphenolic maturation and a higher amount of extractable polyphenols than Mencía), showed good correlations and therefore could be used to monitor the ripening.

Antioxidant Phytochemicals in Fresh Produce: Exploitation of Genotype Variation and Advancements in Analytical Protocols

Horticultural commodities (fruit and vegetables) are the major dietary source of several bioactive compounds of high nutraceutical value for humans, including polyphenols, carotenoids and vitamins. The aim of the current review was dual. Firstly, toward the eventual enhancement of horticultural crops with bio-functional compounds, the natural genetic variation in antioxidants found in different species and cultivars/genotypes is underlined. Notably, some landraces and/or traditional cultivars have been characterized by substantially higher phytochemical content, i.e., small tomato of Santorini island (cv. "Tomataki Santorinis") possesses appreciably high amounts of ascorbic acid (AsA). The systematic screening of key bioactive compounds in a wide range of germplasm for the identification of promising genotypes and the restoration of key gene fractions from wild species and landraces may help in reducing the loss of agro-biodiversity, creating a healthier "gene pool" as the basis of future adaptation. Toward this direction, large scale comparative studies in different cultivars/genotypes of a given species provide useful insights about the ones of higher nutritional value. Secondly, the advancements in the employment of analytical techniques to determine the antioxidant potential through a convenient, easy and fast way are outlined. Such analytical techniques include electron paramagnetic resonance (EPR) and infrared (IR) spectroscopy, electrochemical, and chemometric methods, flow injection analysis (FIA), optical sensors, and high resolution screening (HRS). Taking into consideration that fruits and vegetables are complex mixtures of water- and lipid-soluble antioxidants, the exploitation of chemometrics to develop "omics" platforms (i.e., metabolomics, foodomics) is a promising tool for researchers to decode and/or predict antioxidant activity of fresh produce. For industry, the use of optical sensors and IR spectroscopy is recommended to estimate the antioxidant activity rapidly and at low cost, although legislation does not allow its correlation with health claims.

Nanomaterial-Based Sensing and Biosensing of Phenolic Compounds and Related Antioxidant Capacity in Food

Polyphenolic compounds (PCs) have received exceptional attention at the end of the past millennium and as much at the beginning of the new one. Undoubtedly, these compounds in foodstuffs provide added value for their well-known health benefits, for their technological role and also marketing. Many efforts have been made to provide simple, effective and user friendly analytical methods for the determination and antioxidant capacity (AOC) evaluation of food polyphenols. In a parallel track, over the last twenty years, nanomaterials (NMs) have made their entry in the analytical chemistry domain; NMs have, in fact, opened new paths for the development of analytical methods with the common aim to improve analytical performance and sustainability, becoming new tools in quality assurance of food and beverages. The aim of this review is to provide information on the most recent developments of new NMs-based tools and strategies for total polyphenols (TP) determination and AOC evaluation in food. In this review optical, electrochemical and bioelectrochemical approaches have been reviewed. The use of nanoparticles, quantum dots, carbon nanomaterials and hybrid materials for the detection of polyphenols is the main subject of the works reported. However, particular attention has been paid to the success of the application in real samples, in addition to the NMs. In particular, the discussion has been focused on methods/devices presenting, in the opinion of the authors, clear advancement in the fields, in terms of simplicity, rapidity and usability. This review aims to demonstrate how the NM-based approaches represent valid alternatives to classical methods for polyphenols analysis, and are mature to be integrated for the rapid quality assessment of food quality in lab or directly in the field.

Determination of Phenolic Acids in Sugarcane Vinasse by HPLC with Pulse Amperometry

A reversed-phase liquid chromatographic separation with pulsed amperometric detection of phenolic acids at a glassy carbon electrode is described. Chromatographic separation was carried out in isocratic conditions using 0.20 mol·L^-1 acetic acid (pH 5.0)/water (80 : 20, v/v) as mobile phase under constant working potential mode of 0.80 V. Chromatographic peaks presented high resolution and separation. Calibration curves exhibited excellent correlation coefficients, above 0.995. Linear ranges of the analytes, in mg L^-1, were of 0.018-18 (gallic acid), 0.146-19 (vanillic acid), 0.13-17 (caffeic acid), 0.016-16 (ferulic acid), and 0.008-17 (p-coumaric acid), respectively. Limits of detection ranged from 1.6 to 97 μg·L^-1 and precision varied in 1.73-3.78% interval. Concentrations of 19 ± 0.51 mg·L^-1 and 7.8 ± 2.5 mg·L^-1 were found for vanillic and caffeic acids, respectively, in a sugarcane vinasse sample. Gallic, ferulic, and p-coumaric acids were not detected. Recovery results demonstrated that the proposed method is accurate, and it can be used to detect and quantify phenolic acids in sugarcane vinasse without any influence of interferents.

Influence of hydroxyl substitution on flavanone antioxidants properties

The aim of our study was to determine the effect of the position of the hydroxyl group on the antioxidant properties of flavonoid derivatives. For this purpose, we performed electrochemical analysis and quantum-mechanical calculations to describe the mechanisms of electrochemical oxidation, and we selected the two methods of ABTS (2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) and DPPH (2,2-diphenyl-1-picryl-hydrazyl-hydrate), which allowed us to determine the ability to scavenge free radicals. On the basis of the research, we found that the derivatives of flavonoids, which have a hydroxyl group substituted at the R-3 position on the C ring, have outstanding antioxidant activity. Flavone, which had an OH group substituted at the R-6 and R-7 position on the ring A, showed similar antioxidant activity to flavone without -OH groups in the structure and slightly higher activity than the di-substituted flavone on the ring A.