The performance of cathodically pretreated boron-doped diamond electrode in cationic surfactant media for enhancing the adsorptive stripping voltammetric determination of catechol-containing flavonoid quercetin in apple juice

Bioactive Flavonoids: A Comprehensive Review on Pharmacokinetics and Analytical Aspects

Flavonoids are a diversified group of natural substances which were discovered to provide a variety of health benefits in human beings. Vegetables, fruits, wine and tea are the primary flavonoid dietary sources for humans and as the flavonoids are so closely connected to human dietary items and health, it is vital to explore the structural-activity connection. The arrangement, replacement of functional groups, and total number of hydroxyl groups around flavonoid's nucleus structure affect their biological activity, metabolism, and bioavailability. Various flavonoids have been proven to have hepatoprotective properties, that help in the prevention of coronary heart disease. Similarly, these flavonoids also possess anticancer, and anti-inflammatory activities. Flavonoids have been found to have a functional and structural link with their enzyme inhibitory action, that appears to have antiviral effect through acting as antioxidants, damaging cell membranes, blocking enzymes, activating mechanisms of host self-defense, and limiting virus penetration and attaching to cells. Identification, characterization, isolation, and biological role of flavonoids, as well as their uses on health advantages, are all major topics in research and development currently. This review represents a summary of various sources of flavonoids, class, subclass, their chemical structures, biological activities, the pharmacokinetics of flavonoids and various analytical, bioanalytical and electrochemical methods for determination of flavonoids from different matrices.

A Journey from the Drops of Mercury to the Mysterious Shores of the Brain: The 100-Year Adventure of Voltammetry

Voltammetry, which is at the core of electroanalytical chemistry, is an analytical method that investigates and evaluates the current-potential relationship obtained at a given working electrode. If it is used dropping mercury as working electrode, the method is called as polarography. The current year 2022 marks the 100th anniversary of the discovery of polarography by Czech Jaroslav Heyrovský. He received the Nobel Prize in Chemistry in 1959 for this discovery and his contribution to the scientific world. A hundred years, within the endless existence of the universe is maybe nothing. A hundred years, in the history of mankind is a line, maybe a short paragraph. But, in science, a hundred years can lead to very significant advances in a field and often to the birth and establishment of an entirely new scientific discipline. Indeed, in the last hundred years, the design and use of new electrochemical devices, depending on the progress in microelectronics and computer technologies, has almost revolutionized voltammetry. Besides these developments, due to the fact that the redox (oxidation/reduction) process is very basic for living organisms; the voltammetry, especially with the beginning of the 21st century, has started to be used as a very powerful tool in neuroscience to solve the mystery of the brain (the basic problems of biomolecules with physiological and genetic importance in brain tissue). This review article is an overview of the 100-year history and fascinating development of voltammetry from Heyrovský to the present.

Electrochemistry of Flavonoids: A Comprehensive Review

Flavonoids represent a large group of aromatic amino acids that are extensively disseminated in plants. More than six thousand different flavonoids have been isolated and identified. They are important components of the human diet, presenting a broad spectrum of health benefits, including antibacterial, antiviral, antimicrobial, antineoplastic, anti-mutagenic, anti-inflammatory, anti-allergic, immunomodulatory, vasodilatory and cardioprotective properties. They are now considered indispensable compounds in the healthcare, food, pharmaceutical, cosmetic and biotechnology industries. All flavonoids are electroactive, and a relationship between their electron-transfer properties and radical-scavenging activity has been highlighted. This review seeks to provide a comprehensive overview concerning the electron-transfer reactions in flavonoids, from the point of view of their in-vitro antioxidant mode of action. Flavonoid redox behavior is related to the oxidation of the phenolic hydroxy groups present in their structures. The fundamental principles concerning the redox behavior of flavonoids will be described, and the phenol moiety oxidation pathways and the effect of substituents and experimental conditions on flavonoid electrochemical behavior will be discussed. The final sections will focus on the electroanalysis of flavonoids in natural products and their identification in highly complex matrixes, such as fruits, vegetables, beverages, food supplements, pharmaceutical compounds and human body fluids, relevant for food quality control, nutrition, and healthcare research.

Sensitive and Selective Voltammetric Sensor Based on Anionic Surfactant-Modified Screen-Printed Carbon for the Quantitative Analysis of an Anticancer Active Fused Azaisocytosine-Containing Congener

3-(4-Nitrophenyl)-8-(2,3-dimethylphenyl)-7,8-dihydroimidazo[2,1-c][1,2,4]triazin-4(6H)-one (NDIT) is one of the most promising candidates for anticancer agents. Hence, a sensitive and selective sodium dodecyl sulfate-modified screen-printed carbon sensor (SPCE/SDS) was used for its quantitative analysis. The SPCE/SDS, in contrast to the SPCE, showed excellent behavior in the electrochemical reduction of NDIT by differential-pulse adsorptive stripping voltammetry (DPAdSV). Cyclic voltammetric (CV) studies reveal an irreversible, two-stage and not purely diffusion-controlled reduction process in 0.01 M HNO₃. The sensor was characterized by CV and electrochemical impedance spectroscopy (EIS). Under the optimized conditions (t 45 s, ΔE 175 mV, ν 150 mV/s, and t_m 5 ms), the DPAdSV procedure with the SPCE/SDS presented a very wide linear range from 1 to 2000 nM and a low detection limit of 0.29 nM. A 1000-fold excess concentration of potential interferents commonly present in biological samples did not significantly alter the peak current of NDIT. The practical application of the proposed DPAdSV procedure with the SPCE/SDS was successfully checked by analyzing spiked human serum samples.

Three-dimensional MoS2-graphene aerogel nanocomposites for electrochemical sensing of quercetin

A facile and novel electrochemical sensing platform is reported for quercetin determination with MoS₂ nanoflowers-3D graphene aerogel (3D MoS₂-GA) nanocomposite as signal amplified material. The 3D MoS₂-GA nanocomposite was synthesized through a two-step hydrothermal method, in which MoS₂ nanoflowers were prepared in advance. Characterizations of 3D MoS₂-GA were performed by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The 3D MoS₂-GA-modified glassy carbon electrode (3D MoS₂-GA/GCE) was used to investigate the electrochemical behaviors of quercetin with electrochemical parameters calculated, reaction mechanism discussed, and experimental conditions optimized. Notably, the redox peak current densities of quercetin on 3D MoS₂-GA/GCE raised 5.14 and 6.40 times compared with those on a bare GCE. Furthermore, a novel electroanalytical approach was proposed for the sensitive determination of quercetin within the concentration range 0.01 ~ 5.0 μmol/L, accompanied by a low detection limit of 0.0026 μmol/L (at a working potential of 0.38 V vs. Ag/AgCl). The recovery for practical sample analysis ranges from 97.0 to 105%, and the relative standard deviation is less than 4.2%. This established method shows reliable performance in determination of quercetin in tablets and urine samples.

A Three-Reagent “Green” Paper-Based Analytical Device for Solid-Phase Spectrometric and Colorimetric Determination of Dihydroquercetin

Microfluidic paper-based analytical devices (µPADs) represent one of the promising green analytical strategies for low-cost and simple determination of various analytes. The actual task is the development of such devices for quantitation of antioxidants, e.g., flavonoids. In this paper, possibilities of a novel three-reagent µPAD including silver nitrate, 4-nitrophenyldiazonium tetrafluoroborate, and iron(III) chloride as reagents are assessed with respect to the determination of dihydroquercetin. It is shown that all the three reagents produce different colorimetric responses that can be detected by a mini-spectrophotometer–monitor calibrator or by a smartphone. The method is applicable to direct measuring high contents of dihydroquercetin (the linearity range is 0.026–1 mg mL⁻¹, and the limit of detection is 7.7 µg mL⁻¹), which is favorable for many dietary supplements. The analysis of a food supplement was possible with the relative standard deviations of 9–26%, which is satisfactory for quantitative and semiquantitative determinations. It was found that plotting a calibration graph in 3D space of the three reagents’ responses allows us to distinguish dihydroquercetin from its close structural analogue, quercetin.

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.

First electrochemical evaluation of favipiravir used as an antiviral option in the treatment of COVID-19: A study of its enhanced voltammetric determination in cationic surfactant media using a boron-doped diamond electrode

Favipiravir, a promising antiviral agent, is undergoing clinical trials for the potential treatment of the novel coronavirus disease 2019 (COVID-19). This is the first report for the electrochemical activity of favipiravir and its electroanalytical sensing. For this purpose, the effect of cationic surfactant, CTAB was demonstrated on the enhanced accumulation of favipiravir at the surface of cathodically pretreated boron-doped diamond (CPT-BDD) electrode. At first, the electrochemical properties of favipiravir were investigated in the surfactant-free solutions by the means of cyclic voltammetry. The compound presented a single oxidation step which is irreversible and adsorption controlled. A systematic study of various operational conditions, such as electrode pretreatment, pH of the supporting electrolyte, concentration of CTAB, accumulation variables, and instrumental parameters on the adsorptive stripping response, was examined using square-wave voltammetry. An oxidation signal at around +1.21 V in Britton-Robinson buffer at pH 8.0 containing 6 × 10^-4 M CTAB allowed to the adsorptive stripping voltammetric determination of favipiravir (after 60 s accumulation step at open-circuit condition). The process could be used in the concentration range with two linear segments of 0.01-0.1 μg mL^-1 (6.4 × 10^-8-6.4 × 10^-7 M) and 0.1-20.0 μg mL^-1 (6.4 × 10^-7-1.3 × 10^-4 M). The limit of detection values were found to be 0.0028 μg mL^-1 (1.8 × 10^-8 M), and 0.023 μg mL^-1 (1.5 × 10^-7 M) for the first and second segments of calibration graph, respectively. The feasibility of developed methodology was tested to the analysis of the commercial tablet formulations and model human urine samples.

Electrosynthesis of three-dimensional nanoporous nickel on screen-printed electrode used for the determination of narirutin in citrus wastewater

In this study, an electrochemical sensor was designed for the detection of narirutin using three-dimensional nanostructured porous nickel on screen-printed electrode (3DnpNi/SPE). The modified electrode was successfully synthesized by the dynamic hydrogen bubble template method. The 3DnpNi/SPE was characterized by spectroscopic, microscopic, and electrochemical methods. The results showed that the 3DnpNi/SPE presents good electrocatalytic activity for the oxidation of narirutin. The quantification of narirutin was conducted by differential pulse voltammetry, which showed a wide concentration range (1.0 × 10^-7 - 1.0 × 10^-5 mol L^-1), with low detection limit (3.9 × 10^-8 mol L^-1), and excellent sensitivity (0.31 A L mol^-1). The proposed electrode was applied toward the determination of narirutin in yellow water sample from the citrus industry, where it presented a good degree of accuracy. The 3DnpNi/SPE showed repeatability, long-term stability, and selectivity. The results obtained showed agreement with those obtained by HPLC/DAD method. Chemical compounds studied in this article.

A Review on Recent Advances in the Applications of Boron-Doped Diamond Electrochemical Sensors in Food Analysis

The usage of boron-doped diamond (BDD) material has found to be very attractive in modern electroanalytical methods and received massive consideration as perspective electrochemical sensor due to its outstanding (electro)chemical properties. These generally known facilities include large potential window, low background currents, ability to withstand extreme potentials and strong tendency to resist fouling compared to conventional carbon-based electrodes. As evidence of superiority of this material, couple of reviews describing the overview of various applications of BDD electrodes in the field of analytical and material chemistry has been reported in scientific literature during last decade. However, herein proposed review predominantly focuses on the most recent developments (from 2009 to 2020) dealing with the application of BDD as an advanced and environmental-friendly sensor platform in food analysis. The main method characteristics of analysis of various organic food components with different chemical properties, including additives, flavor and aroma components, phenolic compounds, flavonoids and pesticides in food matrices are described in more details. The importance of BDD surface termination, presence of sp² content and boron doping level on electrochemical sensing is discussed. Apart from this, a special attention is paid to the evaluation of main analytical characteristics of the BDD electrochemical sensor in single- and multi-analyte detection mode in food analysis. The recent achievements in the utilizing of BDD electrodes in amperometric detection coupled to flow injection analysis, batch injection analysis, and high-performance liquid chromatography are also commented. Moreover, actual trends in sample preparation techniques prior to electrochemical sensing in food analysis are referred.

Electrooxidation of tetracycline antibiotic demeclocycline at unmodified boron-doped diamond electrode and its enhancement determination in surfactant-containing media

In this paper, for the first time, the study of voltammetric determination of tetracycline antibiotic demeclocycline was conducted. The oxidation of compound was investigated using a commercially available boron-doped diamond electrode pretreated electrochemically (anodic and subsequent cathodic). Addition of anionic surfactant, sodium dodecylsulfate (SDS) and cationic surfactant, cetyltrimethylammonium bromide (CTAB) to the demeclocycline-containing electrolyte solution at pH 2.0 and 9.0, respectively, was found to improve the sensitivity of the stripping voltammetric measurements. Employing square-wave stripping mode (after 30 s accumulation at open-circuit condition) in Britton-Robinson buffer, the limits of detection were found to be 1.17 μg mL^-1 (2.3 × 10^-6 M) for 4 × 10^-4 SDS-containing buffer solution at pH 2, and 0.24 μg mL^-1 (4.8 × 10^-7 M) for 1 × 10^-4 CTAB-containing buffer solution at pH 9.0. The feasibility of the developed approach for the quantification of demeclocycline was tested in urine samples.

Ultra-sensitive amperometric determination of quercetin by using a glassy carbon electrode modified with a nanocomposite prepared from aminated graphene quantum dots, thiolated β-cyclodextrin and gold nanoparticles

Thiolated β-cyclodextrin functionalized gold nanoparticles (Au-β-CDs) with layered wrinkled flower structure were prepared. Au-β-CDs were electrostatically combined with protonated aminated graphene quantum dots (NH₂-GQDs) to form a nanocomposite with better supramolecular recognition, conductivity, catalysis and dispersion properties. For constructing a quercetin (QU) sensor, the nanocomposites were one-step electrodeposited by a cyclic voltammetry (CV) method onto a glassy carbon electrode to form a stable film. Under optimized conditions, the sensor showed a wide linear response range of 1-210 nM, with a lower detection limit of 285 pM. At the same time, flavonoids with similar structures hardly interfere with the determination of QU. The sensor has been used to determine QU in honey, tea, honeysuckle and human serum with satisfactory results. Graphical abstractSchematic representation of the fabrication of an ultrasensitive quercetin electrochemical sensor based on aminated graphene quantum dots, thiolated β-cyclodextrin and gold nanoparticles (NH₂-GQDs/Au-β-CD/GCE).

High-throughput mass spectrometry scheme for screening and quantification of flavonoids in antioxidant nutraceuticals

Antioxidant nutraceuticals functional characteristic science is a challenging field for combining sensitivity and comprehensiveness. A untargeted screening and quantification method based on ultra-high performance liquid chromatography coupled to Quadrupole-Orbitrap high resolution mass spectrometry has been developed for determination of multiple classes of flavonoids in eight-three nutraceuticals samples. The data acquisition is based on a non-target approach of sequential full scan and variable data independent acquisition of twenty consecutive fragmentation events. The flavonoids include flavanols, flavones, flavanones, anthocyanidins, flavonols and isoflavones. A processing strategy is introduced to implementing filtering methods based on data feature extraction, common ion selection, shoulder peak removal, response threshold adjustment, mass shift and characteristic structural fragments evaluation. Confirmation is based on both accurate mass and isotopic assignment of standards, and further quantification is achieved by fragmentation. This scheme allows in depth characterization of flavonoids with the entire fragments.