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Drop deposition of a carboxylated multi-walled carbon nanotube suspension and layer-by-layer electrochemical deposition of silver and palladium were used to modify thick-film carbon electrodes. The modified electrodes exhibited a pronounced catalytic activity in the electrochemical oxidation of glucose in a neutral medium. The results can be used to develop an enzymefree electrocatalytic sensor for quantitative determination of glucose.

The results of studying electrochemical conversions of 7-R-3-X-1,2,4-triazolo[5,1-c][1,2,4]-triazin-4-ones (Triazavirin® and its derivatives) using cyclic voltammetry, chronoamperometry, and ESR spectroscopy are presented. The derivatives of 7-R-3-X-1,2,4-triazolo[5,1-c][1,2,4]-triazin-4-ones were found to be capable of electrochemical reducing in the potential range from −0.16 to −0.68 V (vs Ag/AgCl) in a Britton—Robinson buffer at pH 2–12. At the potentials of the first electroreduction step at pH 2–6, the main process of transformations is the four-electron scheme of reduction of the nitro group of Triazavirin®. The adduct of the radical substances with the spin probe N-(1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl)-2-methylpropanamide was registered after the preliminary generation of the Triazavirin® radical anion at −0.5 V vs Ag/AgCl in an acidic medium. A linear dependence of the current of the first electroreduction step of the Triazavirin® on the concentration was obtained at pH 2 (linearity range from 10−4 to 1.6 · 10−2 mol L−1, I = −1.93 · C, R2 = 0.9977).

We studied the effect of the mutual arrangement of substituents in an aminobenzoic acid molecule on the analytical characteristics of the determination of bacteria Staphylococcus aureus using a label-free electrochemical immunosensor. An immunosensor based on m-aminobenzoic acid demonstrated the best performance. This is apparently due to the best combination of the density of the coating formed by electrochemical deposition and the availability of carboxyl functional groups for carbodiimide cross-linking with an immunoreceptor.

The new Russian drug Triazid® is a highly effective antiviral agent. The active substance of the drug (5-methyl-6-nitro-7-oxo-4,7-dihydro-1,2,4-triazolo[1,5-a] pyrimidinide l-arginine monohydrate, triazid) belongs to azoloazine series compounds. Using cyclic voltammetry, it was found that the electrochemical activity of triazid (TD) in Britton–Robinson aqueous buffer solutions (BRB) on a glassy carbon electrode is due to the electroreduction of the nitro group bound with the conjugated aromatic system. The maximum current of triazid reduction in the potential range 0.0‒(‒1.25) V was used as an analytical signal. A method was developed for the quantitative determination of triazid by direct cathodic square-wave voltammetry. The linearity range of the calibration graph using the developed method in a BRB solution (pH 4) is 10–500 mg/L (R2 = 0.9998) with the limit of detection for TD 1.9 mg/L. The developed method was used to analyze the Triazid pharmaceutical substance for the concentration of TD. The results obtained satisfy the stated requirements of the draft pharmacopoeial article of the company “TRIAZID substance” for the content of TD found by HPLC.

In this article, an original method is proposed for address and covalent immobilization of anti-E. coli antibodies on a screen-printed electrode (SPE). The method is based on a copper-catalyzed “click” reaction between a polyvinylbenzylazide (PVBA) film electrochemically deposited on the electrode surface and acetylene fragments of propargyl-N-hydroxysuccinimide ester. The products of electrochemical oxidation of copper particles incorporated in the polymer film on the electrode were first used for catalysis of the click reaction. This approach allowed us to reduce the immobilization time from a few hours for conventional methods to just 30 min, and to prevent denaturation of the immunoreceptor. The modified electrodes were characterized by scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). Based on the results obtained, a label-free impedimetric immunosensor for E. coli detection was developed. The detection limit of the immunosensor was estimated as 6.3 CFU/ml, with a linear range of 103–106 CFU/ml. The immunosensor demonstrated good stability during 30 days of storage in phosphate buffer solution (PBS, pH 7) and selectivity toward excess Staphylococcus aureus bacteria.

This article presents the results of a study of electrochemical transformations in aqueous and aprotic media of 5-methyl-6-nitro-7-oxo-4,7-dihydro-1,2,4-triazolo[1,5-a]pyrimidinide l-arginine monohydrate (1a, Triazid) obtained by electrochemical methods and ESR spectroscopy. The effect of pH on the current and the reduction potential of 1a in an aqueous Britton–Robinson buffer solution was studied. It was found that 1a is irreversibly reduced in aqueous acidic media on a glassy carbon electrode in one stage with the participation of six electrons and the formation of 5-methyl-6-amino-7-oxo-1,2,4-triazolo[1,5-a]pyrimidin. The electroreduction of 1a in DMF on a background of tetrabutylammonium salts proceeds in two stages, controlled by the kinetics of second-order reactions. In the first stage, the reduction of 1a is accompanied by protonation by the initial compound of the basic intermediate products formed in the electrode reaction (self-protonation mechanism). The second quasi-reversible stage of the electroreduction 1a corresponds to the formation of a dianion radical upon the reduction of the heterocyclic anion 5-methyl-6-nitro-7-oxo-4,7-dihydro-1,2,4-triazolo[1,5-a]pyrimidin, which is formed upon the potentials of the first peak. The ESR spectrum of the radical dianion was recorded upon electroreduction of Triazid in the presence of Bu4NOH. The effect of the formation of ion pairs on the reversibility of the second peak of the 1a transformation is shown. A change in the rate and regioselectivity of the protonation of the dianion radical in the presence of Na+ and Li+ ions is assumed. The results of studying the electroreduction of 1a by ESR spectroscopy with a TEMPO trap make it possible to assume the simultaneous formation of both a nitroxyl radical and a radical with the spin density localized on the nitrogen at the 4 position of the six-membered ring. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Funding: This work was supported by the Russian Foundation for Basic Research (RFBR, project No. 19-29-08015 mk).

In this work, a screen-printed electrode (SPE) modified by 1,3/1,4-diazines was prepared for the rapid and sensitive determination of nitrobenzene (NB). The obtained results indicated enhanced cathodic currents of direct NB reduction into hydroxylaminophenol on the diazine-modified SPEs. The enhanced effect was most likely due to the combination of complexation and collisional processes of diazines towards nitroaromatic compounds and also the diazine-modified electrodes’ increased electroconductivity. The best electrochemical responses were obtained in square wave voltammetry mode by using the carbazolyl substituted diazines as a component of the sensitive layer, which was assembled by co-electropolymerization with the unsubstituted carbazole on the electrode during 5 cycles. The low detection limit estimated as 0.107 μM and wide linear range (1–1000 µM) enables NB in water and food samples to be determined. The developed modified electrode was applied in the analysis of commercial honey samples.

In this article, we have shown the results of E. coli bacteria determination in water using the enzyme free electrochemical immunosensor with covalent via click chemistry immobilized receptor layer in comparison with dropwise immobilized antibodies. Covalent immobilization was realized by creating precursors on the surface of the working electrode with the reaction of azide-alkyne copper-catalysed cycloaddition approach. The detection limit of the immunosensor with covalent and dropwise immobilization of antibodies was estimated as 6.6 CFU/ml and 11.2 CFU/ml, respectively, a linear range was 103-106 CFU/ml. Moreover, the sensor with "click"immobilized antibodies showed good stability for 30 days when stored in a phosphate buffer, while the sensor with dropwise immobilized receptor layer was stable for 3 days. © Published under licence by IOP Publishing Ltd. This work was supported by the Presidential Grants Fund of the Russian Federation (grant MK-567.2020.3).

A convenient transition-metal-free approach, based on nucleophilic substitution of hydrogen (SNH), for consecutive regioselective C–H functionalization of 1,3,7-triazapyrene scaffolds with carboranyllithium and phenyllithium is reported. The theoretical calculations disclosed highlight key features in the regioselectivity and mechanism of the investigated SNH transformations. The novel 1,3,7-triazapyrene-based ortho-carboranes obtained have large potential in the field of molecular electronics as organic luminophores, which are characterized by the aggregation-induced emission and dual-emission effects.

In this work, the surface of the platinum disk electrode was modified with p–aminobenzoic acid by the method of oxidative electrografting (OEG) and the mechanism was proposed. The density of carboxyl groups immobilized in this way was estimated using differential pulse voltammetry. In addition, the effect of preliminary modification of the electrode surface by carboxylated carbon nanotubes (c–MWCNTs) on the density of carboxyl groups was studied. Modification allowed consequently improving the analytical characteristics of the working electrode.

Literature data on the modern state and prospects for the development of research in the area of receptor layer immobilization in electrochemical immunosensors were examined and systematized. The first section of the review is devoted to structural features and functions of biological immunoreceptors, viz., antibodies or their fragments. The main methods of antibody immobilization on the working electrode surface are considered in detail in the second section. The prospects for the development of electrochemical immunosensors are discussed in the third section of the review.

Kinetic features of the reactions of K4[Fe(CN)6] with radicals initiated by water-soluble azo-initiator 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) at 37 °C were studied using the potentiometric method. Potassium ferrocyanide was shown to be a radical acceptor, whereas K3[Fe(CN)6] formed by the oxidation with the radicals in combination with K4[Fe(CN)6] is an electrochemical system, the study of which makes it possible to determine kinetic characteristics of radical reactions. The rate constants for the reactions of peroxide radicals RO2 · with K4[Fe(CN)6] were calculated.

We present the results of synthesis and study of the electrocatalytic activity of gold and silver nanoparticles of different composition (individual metals, core–shell particles, nanoalloys, and particles synthesized electrochemically), immobilized on the surface of a glassy carbon electrode, with respect to cholesterol. A surfactant (cetyltrimethylammonium bromide) is selected to create an aqueous–organic emulsion of cholesterol. It is demonstrated that nanoparticles with a gold core and a silver shell with the regression equation of I = 1.4 × 10–5 c chol + 5.8 × 10–5 (R 2 = 0.97) and silver nanoparticles synthesized electrochemically with the regression equation of I = 1.0 × 10–5 c chol + 3.0 × 10–4 (R 2 = 0.95) possess optimal electrocatalytic characteristics.

A novel N,N ’ ,N ” ,O-tetradentate Schiff base ligand was obtained by the condensation of salicylaldehyde and 5-(1-hydrazono-ethyl)-3-(trifluoromethyl)pyrazole. Five-coordinated dinuclear copper(ii) complex with this ligand was characterized by XRD analysis, magnetometry and voltammetry.

A series of novel D–A and D–π–A push-pull systems based on a pyrazine and quinoxaline acceptor, bearing various electron-donating triphenylamine and carbazole moieties, are compared. A significant difference in electrochemical and photophysical properties was found depending on molecular structure. The compounds have strong solvatochromic properties. Quinoxaline-containing systems exhibit delayed fluorescence (DF) in thermal vacuum deposition films. Despite the low quantum yield of fluorescence in the solid state (less than 10%), organic light-emitting diodes with sufficiently high efficiency (4.2 cd/A) have been fabricated on the basis of this push-pull systems. The best results were obtained for compounds exhibiting DF. The possible channel for increasing the efficiency of OLED can be associated with the “hot excitons” mechanism.

A simple and inexpensive immunosensor is reported for the rapid determination of Staphylococcus aureus B-1266 that uses Fe3O4–SiO2–NH2 nanoparticles as the direct signal label. The electrochemical immunoassay procedure includes the incubation of bacteria with excess magnetite nanoparticles, the magnetic separation of the free nanoparticles, a labeled immunocomplex formation on the surface of a planar electrode, and the electrochemical response from the magnetite nanoparticles in the immunocomplex. The electrochemical immunosensor allows for the selective and accurate detection of S. aureus from 10 to 105 CFU mL−1 with a relative standard deviation lower than 10%. The limit of detection was 8.7 CFU mL−1.

The determination of cholesterol concentration in the blood is an important characteristic of the state of health. Modern methods for the detection of cholesterol involve the use of enzymes or expensive and complicated equipment. In this study, an enzymeless electrochemical method for the quantitative determination of cholesterol using cobalt (II) chloride as an electrocatalyst has been proposed. An analytical signal was peak current of cobalt (II) chloride faradaic oxidation on a platinum disk electrode in acetonitrile media in the presence of cholesterol via linear sweep voltammetry. A possible path of the electrocatalytic oxidation of cholesterol on the electrode surface was proposed. The catalytic constant kcat and the Michaelis constant Km calculated for the electrocatalytic oxidation process of cholesterol on the electrode surface are comparable in magnitude to the natural enzyme cholesterol oxidase, indicating the high efficiency of CoCl2 as an electrocatalyst for oxidation and, therefore, the determination of cholesterol. The highest value and the best reproducibility of analytical signal are reached at a CoCl2 concentration of 25 mM and a scan rate of 100 mV·s− 1. The linear region of the calibration curve “current-cholesterol concentration” obtained using a developed method in a solution of 0.1 M LiClO4 and 25 mM CoCl2 in acetonitrile is 25–200 μM (R2 = 0.971) with a detection limit of 2 μM.

Peculiarities of electrochemical behavior of the Fe3O4 magnetic nanoparticles immobilized on the surface of a platinum electrode in aprotic organic media were investigated. Possible scheme of electrochemical behavior of nanoparticles depending on pre-electrolysis potential (–1.3,–2.5 V) was suggested. The effect of pre-electrolysis time, potential scan rate and nature of supporting electrolyte on the processes investigated was determined. A linear dependence of electrochemical oxidation signal versus the concentration of nanoparticles in modifying suspension in the concentration range of 0.05—0.5 g L–1 was observed. The results of the performed research allow using magnetite nanoparticles as a direct signal-generating label in electrochemical immunoassay.

A convenient synthetic route to novel 7-substituted benzo[f]thieno[3,2-h]quinazoline and 8-substituted benzo[g,h]dithieno[2,3-e:3′,2′-j]perimidine systems, bearing the fused pyrimidine ring, has been advanced. A commercially available 5-bromopyrimidine was used as the starting material to obtain various polycyclic systems through the sequence of nucleophilic aromatic substitution of hydrogen (the SNH reaction), Suzuki cross-coupling and oxidative photocyclization. Evidence for the structure of benzo[g,h]dithieno[2,3-e:3′,2′-j]perimidines has been obtained by X-ray crystallographic analysis. Molecular orbital calculations (DFT), as well as redox and optical measurements for all new compounds have been performed. The data show that the reported polycyclic systems have potential for use in organic electronic applications.

A potentiometric method was proposed for the determination of rate constants for peroxyl radical generation using the reduced form of the metal in the complex. The kinetics of the thermal decomposition of 2,2′-azobis(2-methylpropionamidine)dihydrochloride was studied. The requirements for an electron donor (reactant) were formulated. The iron(ii) complex with EDTA, the iron(ii) complex with o-phenanthroline, and potassium hexacyanoferrate(ii) were used as reactants. The rate for peroxyl radical generation and the generation rate constant (the latter turned out to be (0.92±0.06)•10–6 s–1) were calculated. The studies were carried out for different temperatures and concentrations of the initiator and complex.

To study acidity of polyfluorosalicylic acids, their ionization constants (pKa) were determined. A series of metal salts and heteroligand complexes was obtained from polyfluorosalicylic acids containing biogenic metal ions and coligands, such as triphenylphosphine, pyridine, 2,2′-bipyridine and 1,10-phenanthroline. The structures of synthesized salts and complexes were determined by elemental analysis, HRMS and IR spectroscopy. X-Ray diffraction analysis was used to confirm the structure of the heteroligand metal complexes based on polyfluorosalicyic acids and bipyridine-type coligands. In the heteroligand complexes, it was possible to achieve different ratios of polyfluorosalicylate and coligand, depending on the metal cation. The antimycotic and antibacterial activities of polyfluorosalicylic acids were studied and their metal complexes were investigated. The metal heteroligand complexes were found to exhibit high anti-gonorrhea activity. In addition, cobalt(II) 2,3,4,5-tetrafluorosalicylato-κ2O-bis(2,2′-bipyridine-κ2N) dihydrate was found to show a pronounced activity against Trichophyton rubrum, T. interdigitale, Epidermophyton floccosum strains.

Biocatalytic cascades involving more than one or two enzyme-catalyzed step are inefficient inside alginate hydrogel prepared on an electrode surface. The problem originates from slow diffusion of intermediate products through the hydrogel from one enzyme to another. However, enzyme activity can be improved by surface immobilization. We demonstrate that a complex cascade of four consecutive biocatalytic reactions can be designed, with the enzymes immobilized in an LBL-assembled polymeric layer at the alginate-modified electrode surface. The product, hydrogen peroxide, then induces dissolution of iron-cross-linked alginate, which results in release process of entrapped biomolecular species, here fluorescently marked oligonucleotides, denoted F-DNA. The enzymatic cascade can be viewed as a biocomputing network of concatenated AND gates, activated by combinations of four chemical input signals, which trigger the release of F-DNA. The reactions, and diffusion/release processes were investigated by means of theoretical modeling. A bottleneck reaction step associated with one of the enzymes was observed. The developed system provides a model for biochemical actuation triggered by a biocomputing network of reactions. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim National Science Foundation, NSF: CBET-1403208 Russian Science Foundation, RSF: 17-13-01096 This work was supported by National Science Foundation, USA, (award CBET-1403208) and by Russian Science Foundation (project no. 17-13-01096).

This paper reports a novel enzyme-free electrochemical immunoassay for Escherichia coli (ATCC 25922) that involves the formation of an immunocomplex between antibodies immobilized on the surface of a platinum electrode and bacteria-labelled magnetite nanoparticles. Magnetite nanoparticles modified with chitosan were used as the signal label in the immunoassay. The main novelty is the collection of an electrochemical signal from a magnetic nanolabel included within the immunocomplex on the electrode surface. The developed immunosensor is highly specific, enabling the determination of E. coli from 10 to 105 colony-forming units (CFU)/mL with the relative standard deviation below 10%. The limit of detection for the immunosensor was 9.3 CFU/mL. Bacteria were determined in model mixtures and real samples. The recovery was between 95 and 99%, comparable with results from enzyme-linked immunosorbent assays.

Reaction of quinoxaline or its 6,7-difluoroderivative with C-nucleophiles in nitrogen atmosphere afforded along with the products of hydrogen substitution in the heterocyclic ring the corresponding bisquinoxalines. An ESR signal of the cation-radical of the diprotic salt of quinoxaline was detected in the DMSO solution of quinoxaline and dimedone.

An electrochemical immunoassay method is developed for determining the strain of Escherichia coli ATCC 25922 with the use of electroactive nanocomposite particles based on magnetite Fe3O4 coated by SiO2 modified with ferrocene as a signal label. A sensitive, direct, and easily measured analytical response is obtained by cyclic voltammetry. The developed method enables the determination of bacterial cells at a minimum concentration of 2.3 × 103 CFU/mL. The analytical range is 2.3 × 103–2.3 × 107 CFU/mL; the detection limit is 2.5 × 102 CFU/mL. The required sample volume is 0.5 mL, and the analysis time is 40 min.

Methods are developed for the non-enzymatic chronoamperometric determination of urea in blood serum and creatinine in model solutions imitating the composition of blood serum. The selectivity of determination is ensured using columns packed with an anion-exchange resin for urea or synthesized molecularly imprinted polymers for creatinine.

An electrochemical method for determining viral antigens was developed in relation to the measles virus antigen. Using inverse adsorption voltammetry and the synthesized conjugates of antibodies with the Fe3O4-SiO2 nanocomposite particles as the signal label, a sensitive, easily measurable analytical signal was obtained.

The nanocomposites including magnetite nanoparticles, polymer coating and electrochemically active molecules were synthesized and investigated: poly(vinylbenzyl) chloride coating modified by quinoline; coating based on polypyrrole, silica coating modified by ferrocene derivative. All samples were investigated by the cyclic voltammetry method.

Electrochemical immunoassay involving two magnetic steps-separation of unbound nanoparticles from bacteria/antigen-Fe3O4 nanoparticles conjugates and concentrating of the conjugates on solid substrate is proposed. Carbon screen printed electrodes covered antibodies specific to Salmonella typhimurium (strain SL 7207) serves as the mentioned substrate. Immunocomplex formed is being solved in the mixture of acids (0.36 M H2SO4 and 0.28 M HNO3) is analyzed for Fe3+ concentration. The last is determined by stripping voltammetric method. It provides information on quantity of conjugates localized on the substrate and the last one is proportional to the pathogen concentration in the analyte. Detection limit is for Salmonella typhimurium 8.18 CFU/ml and for antigen Salmonella typhimurium 1.51 × 10-7 mg/ml (1.51 CFU/ml).

Analytical chemistry is now developing mainly in two areas: automation and the creation of complexes that allow, on the one hand, for simultaneously analyzing a large number of samples without the participation of an operator, and on the other, the development of portable miniature devices for personalized medicine and the monitoring of a human habitat. The sensor devices, the great majority of which are biosensors and chemical sensors, perform the role of the latter. That last line is considered in the proposed review. Attention is paid to transducers, receptors, techniques of immobilization of the receptor layer on the transducer surface, processes of signal generation and detection, and methods for increasing sensitivity and accuracy. The features of sensors based on synthetic receptors and additional components (aptamers, molecular imprinted polymers, biomimetics) are discussed. Examples of bio- and chemical sensors' application are given. Miniaturization paths, new power supply means, and wearable and printed sensors are described. Progress in this area opens a revolutionary era in the development of methods of on-site and in-situ monitoring, that is, paving the way from the "test-tube to the smartphone".