Antioxidants: Terminology, methods, and future considerations

Unreliable terminology and incompatible units of antioxidant activity/concentration expression lead to the failure of antioxidant clinical trials, ambiguity of conclusions about the effect of a chosen therapy in medicine and evaluation of food quality, diet, difficulties using information in monitoring the training process in sports, etc. Many different terms (antiradical activity, antioxidant activity, antioxidant capacity, antioxidant power, antioxidant ability) and methods: Trolox equivalent capacity assay (TEAC), Ferric Reducing Antioxidant Power assay (FRAP), Cupric Reducing Antioxidant Capacity assay (CUPRAC), antioxidative activity assay (ABTS), the oxygen radical absorbance capacity (ORAC), and different options of electrochemical ones) proposed for the determination of antioxidants are described. Possible approaches to the development of this field of science and practice are considered. © 2019 by the authors. Licensee MDPI, Basel, Switzerland

Interplay between antioxidant activity, health and disease

The article discusses the relationship between oxidative stress (OS) and pathological conditions, the possibilities and benefits of estimating OS considering the integral antioxidant activity (AOA) as an OS criterion, and using a simple accessible hybrid potentiometric method (HPM) with a mediator system for AOA monitoring. The results of AOA of blood serum in healthy volunteers and patients with various diseases are presented. Preliminary reference values are found. The lower levels of AOA of blood serum in patients with different diseases in comparison with the control group are observed. The potential mechanisms of changes in the AOA levels and it’s clinical significance are discussed from the position of biointerfaces interplay. With AOA equal or greater than 1.40 mmol-eq l–1 the person is healthy, the range from 0.95 to 1.40 mmol-eq l–1 indicates that the patient is at risk and needs to undergo a further medical examination. When AOA blood serum is below 0.95 mmol-eq l–1, detailed diagnostics and relevant treatment are required. The findings allow suggesting that the approach determine antioxidant/oxidant activity of biological fluids holds considerable promise for monitoring OS; it opens up new opportunities in expanding the use of analytical chemistry in medicine. © 2019 by the authors

Silver chloride/ferricyanide-based quasi-reference electrode for potentiometric sensing applications

Processes' occurring at the Ag/AgCl/Cl-, ([Fe(CN)6]3-/4-) ions interface study results are presented. Conditions are selected for the mixed salts' precipitate formation on the silver surface. It has been shown that the potential of a silver screen-printed electrode (AgSPE) coated with amixed precipitate containing silver chloride/ferricyanide is stable in the presence of [Fe(CN)6]3-/4-. The electrode can serve as a quasi-reference electrode (QRE) in electrochemical measurements in media containing ions [Fe(CN)6]3-/4-. The electrode is formed during polarization of AgSPE (0.325 V vs. Ag/AgCl/KCl, 3.5M) in a solution containing chloride-and ferri/ferrocyanides ions. The results of the obtained QRE study by potentiometry, scanning electronmicroscopy and cyclic voltammetry are presented. The proposed QRE was used in a sensor system to evaluate the antioxidant activity (AOA) of solutions by hybrid potentiometricmethod (HPM). The results of AOA assessment of fruit juices and biofluids obtained using new QRE and commercial Ag/AgCl RE with separated spaces do not differ. © 2020 by the authors.Russian Foundation for Basic Research, RFBR: 18-33-00215Funding: The reported study was funded by RFBR according to the research project № 18-33-00215

Disposable potentiometric sensory system for skin antioxidant activity evaluation

The skin is a natural barrier between the external and internal environment. Its protective functions and the relationship of its state with the state of health of the organism as a whole are very important. It is known that oxidant stress (OS) is a common indicator of health status. This paper describes a new sensory system for monitoring OS of the skin using antioxidant activity (AOA) as its criteria. The contact hybrid potentiometric method (CHPM) and new electrochemical measuring scheme were used. A new sensory system, including disposable modified screen-printed carbon and silver electrodes covered by membrane impregnated by mediator, was developed. Its informative ability was demonstrated in the evaluation of the impact of fasting, consumption of food and food enriched by vitamins (antioxidants) on skin AOA. This device consisting of a sensory system and potentiometric analyzer can be used in on-site and in situ formats. © 2019 by the authors. Licensee MDPI, Basel, Switzerland

Electrochemical hybrid methods and sensors for antioxidant/oxidant activity monitoring and their use as a diagnostic tool of oxidative stress: Future perspectives and challenges

The terminology used in electrochemical methods which are used to generate the measured signal in antioxidant/oxidant activity (AOA/OA) sensors is briefly considered. The review presents a hybrid version of electrochemical methods for the determination of AOA/OA. Invasive electrochemical methods/sensors for AOA/OA of blood/serum/plasma, and non-invasive ones for semen, sweat, saliva and skin determination are described. AOA/OA sensors application in health estimation, cosmetology, food and nutrients is presented. Attention is paid to widely described approaches and technologies used in chemical/biochemical sensors. It will be considered as base/prototypes for developing sensors of the kind for AOA/OA determination. Prospects for the development of wearable, written sensors and biosensors are considered. Miniature and wireless sensors will allow for the monitoring of the patient’s state, both at the bedside and far beyond the hospital. The development of wearable self-powered written and printed sensors is an important step towards personalized medicine. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Bi-enzyme sensor based on thick-film carbon electrode modified with electropolymerized tyramine

Bi-enzyme sensor based on thick-film epoxy-carbon electrode modified with polytyramine has been developed and examined for the determination of peroxidase substrates and cholinesterase inhibitors. Polytyramine was obtained on the electrode surface by repeated scanning of the potential from +600 to +1800 mV vs. Ag/AgCl in tyramine solution. The enzymes were immobilized in the polytyramine matrix by cross-linking with glutaraldehyde. The biosensor developed provides a reliable and inexpensive way for preliminary testing of common environmental pollutants with a single sensor in accordance with assumed toxic effect by the choice of appropriate substrate and measurement conditions. The bi-enzyme sensor makes it possible to determine substituted phenols and aromatic amines in the micromolar range of their concentrations and anticholinesterase pesticides with detection limits of 0.1 (Coumaphos) and 0.03 μmol l-1 (Chloropyrifos-methyl). © 2004 Published by Elsevier B.V

Electrochemical sensor based on a carbon veil modified by phytosynthesized gold nanoparticles for determination of ascorbic acid

An original voltammetric sensor (Au-gr/CVE) based on a carbon veil (CV) and phytosynthesized gold nanoparticles (Au-gr) was developed for ascorbic acid (AA) determination. Extract from strawberry leaves was used as source of antioxidants (reducers) for Au-gr phytosynthesis. The sensor was characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy and electrochemical methods. Optimal parameters of AA determination were chosen. The sensor exhibits a linear response to AA in a wide concentration range (1 µM–5.75 mM) and a limit of detection of 0.05 µM. The developed sensor demonstrated a high intra-day repeatability of 1 µM AA response (RSD = 1.4%) and its stability during six weeks, selectivity of AA determination toward glucose, sucrose, fructose, citric, tartaric and malic acids. The proposed sensor based on Au-gr provides a higher sensitivity and a lower limit of AA detection in comparison with the sensor based on gold nanoparticles synthesized by the Turkevich method. The sensor was successfully applied for the determination of AA content in fruit juices without samples preparation. The recovery of 99%–111% and RSD no more than 6.8% confirm the good reproducibility of the juice analysis results. A good agreement with the potentiometric titration data was obtained. A correlation (r = 0.9867) between the results of AA determination obtained on the developed sensor and integral antioxidant activity of fruit juices was observed. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.Russian Foundation for Basic Research, RFBR: 17-03-00679This research was partially funded by RFBR grant number 17-03-00679

The effect of the antioxidant activity of plant extracts on the properties of gold nanoparticles

Synthesis of gold nanoparticles (phyto-AuNPs) with the use of leaf extracts (phytosynthesis) is based on the concept of Green Chemistry. The present study is conducted to discuss how antioxidant activity (AOA) of extracts from plant leaves impacts on the kinetics of phytosynthesis, the size of the formed nanoparticles, and the stability of their nanosuspensions. Results show that the formation rate of phyto-AuNPs suspensions accelerate due to the increase in the AOA of the extracts. Accompanying the use of transmission electron microscopy (TEM), UV-Vis-spectrophotometry and dynamic light scattering (DLS), it also has been found that higher AOA of the extracts leads to a decrease in the size of phyto-AuNPs, an increase in the fraction of small (d ≤ 5 nm), and a decrease in the fraction of large (d ≥ 31–50 nm) phyto-AuNPs, as well as an increase in the zeta potential in absolute value. Phyto-AuNPs suspensions synthesized with the use of extracts are more resistant to destabilizing electrolytes and ultrasound, as compared to suspensions synthesized using sodium citrate. Thus, the AOA of the extract is an important parameter for controlling phytosynthesis and predicting the properties of phyto-AuNPs. The proposed approach can be applied to the targeted selection of plant extract that will be used for synthesizing nanoparticles with desired properties. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.Russian Foundation for Basic Research, RFBR: 17-03-00679_АFunding: This research was funded by the Russian Foundation for Basic Research, grant number 17-03-00679_А

Cholinesterase sensors based on thick-film graphite electrodes for the flow-injection determination of organophosphorus pesticides

Flow cholinesterase sensors based on planar thick-film graphite electrodes and immobilized preparations of acetyl- and butyrylcholinesterase with various activities were developed. Analytical parameters of the determination of organophosphorus pesticides depend on hydrophilic-hydrophobic interactions at the membrane-solution interface. When passing from the steady-state to flow-injection conditions of signal measurements, the limits of detection of coumaphos and chlorpyrifos-methyl decrease by a factor of 5-8, while the limit of detection of hydrophilic trichlorfon increases by a factor of 3-4. The cholinesterase sensors developed in his work can measure the concentrations of pesticides in a continuous mode for 8-12 h with the throughput of up to 5-6 measurements per hour