Tamoxifen (TAM), a selective oestrogen receptor modulator prescribed for the treatment of breast cancer and endocrine disorders, was investigated in an acidic aqueous-alcoholic unbuffered environment in terms of its oxidation, protonation/deprotonation, electron transfer, and interaction with the charged phase interface. Advanced analysis of TAM oxidation was performed experimentally using a screen-printed boron-doped electrode (SP/BDDE). The study highlights not only the influence of electrochemical pre-treatment of the SP/BDDE surface (O-termination), pH and ethanol content on the irreversible electron transfer in the TAM molecule, but also the ability of differential pulse and elimination voltammetric analysis (DPV and EVLS) to recognize the nature of the ongoing electrochemical processes. The optimal protocol for the most efficient heterogeneous electron transfer for TAM (O-terminated boron-doped diamond) and the mechanism of its oxidation (pH 2, EtOH 7.7% vol.) was proposed. The effect of ethanol content on the oxidation voltammetric signal of TAM has been discussed through the physicochemical properties of aqueous-alcoholic solutions, such as their permittivity, ionic mobility, and viscosity. We identified a distinct rate-determining step for the oxidation process of TAM on carbon and diamond, and we proposed the oxidation mechanism of TAM using quantum chemical (DFT) calculations. The study also shows that the combination of an unbuffered acidic medium and an oxygen-terminated, highly boron-doped diamond electrode represents a robust and practical approach for electrochemical analysis of TAM in real urine samples, offering excellent antifouling properties, stable oxidation peak potentials, and reliable detection of trace-level TAM.

The three-electrode screen printed boron-doped diamond (SP/BDD) sensor fabricated by chemical vapor deposition (CVD) on an alumina substrate allowed testing its different electroanalytical performance in terms of: (1) the termination of the SP/BDDE surface (H- or O-termination), (2) the immersion mode of the SP/BDDE in the analyte solution (B - bulk mode) and in the drop experiment (D - drop mode), and (3) the presence and absence of atmospheric oxygen. The electrochemical methods (CV - cyclic voltammetry, EVLS - elimination voltammetry with linear scan, EIS - electrochemical impedance spectroscopy) were complemented by methods that characterize the surface properties of the electrode material (Raman spectroscopy, SEM - scanning electron microscopy, XPS - X-ray photoelectron spectroscopy, OCA - optical contact angle). Mott-Schottky analysis demonstrated a higher density of free charge carriers and a closer localization of the valence band (VB) to Fermi level for Htermination than for O-termination of surfaces, therefore further experiments were performed on the H-terminated SP/BDDE. The B and D mode tests in the absence and presence of atmospheric oxygen using the redox probe [Fe(CN)6]3- /4- demonstrated a significant difference in both the heterogeneous charge transfer rate constants and charge transfer resistance values. The H-terminated SP/BDDE surface and the D-mode experiment in the presence of O2 provided the best electroanalytical performance of this screen-printed diamond electrode. The O2-enhanced surface electron transfer was discussed in the context of the electrochemical potential model of boron-doped diamond and the used electrolyte.

Pencil leads can be considered well-defined and cheap graphite electrodes for a wide range of electrochemistry applications. These electrodes display many intriguing properties; however, the origin of these properties is not clear. Using various analytical approaches applied to two different commercially available Tombow (TO) and Staedtler (ST) pencils we reveal a causal relationship between the unique properties of pencils and their graphite/polysiloxane composite. We explore the impact of chloroform etching on chemical composition changes, thermal stability and electrochemical parameters of pencils. Using a combination of X-ray photoelectron spectroscopy (XPS) and gas chromatography-mass spectrometry (GC-MS/MS) various polydimethylsiloxanes in composites are revealed. The polysiloxane species leave into the chloroform solvent during the etching resulting in a significant decrease of their content within the electrodes. Differential scanning calorimetry (DSC) data, corroborated by gravimetric measurements, provide additional proof of the presence of composite structures in ST and TO pencils, showing glass transition temperatures at around 76 degrees C and 81 degrees C. The main difference between the TO and ST electrodes is the content and composition of the polysiloxanes within the graphite matrix. ST composites have significantly higher polymer content (- 30 %) with traces of Na and S impurities compared to TO ones (- 14 %) free of contaminations. Furthermore, mainly cyclic nanostructures appear in chloroform extracts of ST composites whereas rather chain-like clusters are liberated out of the TO counterparts. Complementary electrochemical experiments using cyclic voltammetry (CV), impedance spectroscopy (EIS) and the less known elimination voltammetry with linear scan (EVLS) reflect the performance superiority of TO electrodes with much lower polysiloxane content and free of impurities. High conductivity, low capacitive current along with favoured charge carrier transfer all promise a wide range of technological applications for the TO pencil material.

The article presents an open personal approach to the future of elimination voltammetry with linear scan (EVLS) in the context of the expected development and research of new materials and new technologies. With this development, the application capability of EVLS will be expanded, the basis of which is a mathematical apparatus that allows some current components to be eliminated from the overall voltammetric record, while others to be preserved. Although EVLS played a provisional and unquestionable role in electroanalytical applications, where it helped to obtain lower limits of detection (LOD) values of various organic and inorganic substances and to reveal electrode processes hidden in voltammetric signals, its further development indicates promising use in other research areas as well. The aim of the communication is a more precise but also more general definition of the elimination method, its specific paradigms, prospects and goals for the future.

The article presents an open personal approach to the future of elimination voltammetry with linear scan (EVLS) in the context of the expected development and research of new materials and new technologies. With this development, the application capability of EVLS will be expanded, the basis of which is a mathematical apparatus that allows some current components to be eliminated from the overall voltammetric record, while others to be preserved. Although EVLS played a provisional and unquestionable role in electroanalytical applications, where it helped to obtain lower limits of detection (LOD) values of various organic and inorganic substances and to reveal electrode processes hidden in voltammetric signals, its further development indicates promising use in other research areas as well. The aim of the communication is a more precise but also more general definition of the elimination method, its specific paradigms, prospects and goals for the future.

We report the design, synthesis, electrochemical, UV-vis, fluorescence, and computational study of nine π-linked donor-acceptor (D-π-A) chromophores. The series of novel compounds comprises a terphenyl, terthiophene, or 2,5-diphenyl thiophene linker, with one electron-donating group (methyl or p-N,N-diethyl) and one electron-withdrawing group (nitrone, formyl, or dicyanovinyl) at opposite ends of the molecule. The HOMO-LUMO gaps were determined via cyclic voltammetry and found to correspond well to DFT-calculated values. Furthermore, the influence of the π-linker character and substituent on the HOMO-LUMO gap was analysed and interpreted in terms of MO composition via DFT.

Electrochemical methods have undergone many important developments initiated by efforts to increase the sensitivity, selectivity, and stability (S&S&S) and to understand the underlying electrode processes. To meet these goals, there are hardware solutions with applications of new technologies and materials, and software solutions utilizing existing instrumentation. The article presents a viable, easy-to-implement software solution to the main shortcomings of linear sweep voltammetry (low sensitivity, high share of the capacitive component of the current, and the problem of overlapping signals) in the form of elimination voltammetry with a linear scan (EVLS). Based on the different dependences of the individual currents that make up the total voltammetric current (diffusion, charging, kinetics, and various irreversible components) on the scan rate, EVLS can eliminate or preserve particular current components. Increased sensitivity and selectivity can be expected especially in the case of a fully adsorbed electroactive particle subjected to an irreversible electrode process when the elimination of kinetic and capacitive current while preserving the diffusion current provides a theoretically confirmed peak-counter peak signal. EVLS is applicable for testing changes in the polarized electrode/electrolyte interface using EVLS functions eliminating the diffusion component of the current and preserving the kinetic and capacitive currents, which should ensure zero current conduction for a purely diffusion-controlled Nernstian-type reversible process. We point out the strengths, opportunities, and weaknesses of EVLS. Nevertheless, EVLS offers a new tool that contributes to a better understanding of electrochemical processes and their mechanisms.

The article contributes to understanding the microscopic details of the electrochemical interphase and the electron transfer mechanism in a real experiment. The dynamic responses of the electrical phase interface were monitored via cyclic voltammetry of [Fe(CN)6]3-/4- on a pencil graphite electrode in an aqueous KCl solution, and the CV recordings were evaluated using elimination linear scan voltammetry (EVLS). The voltammetric data processing EVLS software, derived primarily for the reversible system, is capable of describing and distinguishing the diffusion, capacitance, and kinetic contributions of these current components. Based on the EVLS functions which eliminate the diffusion and charging current components or the diffusion and kinetic current components, we performed a detailed analysis of the state where the electronics meet the ionics. An emphasis was placed on the relationship between the dynamic response of the electric double layer and (a) the material of the graphite electrode, (b) the concentration of the [Fe(CN)6]3-/4-, (c) the concentration of the KCl, and (d) the presence or absence of oxygen in the solution. Our new approach to evaluating voltammetric data is characterized by experiments that promote understanding the electrochemical processes, influenced especially by changes in the electrical double layer. The results open a perspective of further research to facilitate a rational clarification and optimization of electrode processes.

The article notes the latest presented work on the electrochemical detection of insulin and presents a critical view of the research and development of its electrochemical non-enzymatic sensors. It monitors the effect of expe­rimental conditions on the insulin oxidation signal and considers the catalytic effects of nanoparticles or nanocomposites deposited on the surfaces of the electrochemical sensor.

The review presents the application potential of elimination voltammetry with linear scan (EVLS) in the electroanalysis of organic and inorganic electroactive substances. This innovative method can be considered a software tool that transforms the total current-voltage records measured at different scan rates into elimination functions providing new useful information about a given electrode process. Some EVLS functions are capable of separating overlapped voltammetric signals, identifying the adsorbed state of the analyze, and detecting the preceding chemical reaction before the electron transfer. EVLS finds use in solving the mechanisms of electrode processes and determining significant physicochemical parameters, such as charge transfer coefficients and electrochemical equilibrium protonation constants. This review selects and draws attention to certain problems that may accompany EVLS in voltammetric experiments on solid electrodes.

Pencil graphite electrodes are a simple, disposable, and low-cost alternative to screen-printed graphite electrodes. In terms of stability and sensitivity, pencil electrodes often outperform conventional carbon ones. This paper discusses and emphasizes the superior properties of polymer pencil graphite electrodes (pPeGEs), which can be exploited in the electrochemical analysis of molecules, such as chlorides, whose signals are missing on common graphite electrodes. The chemical and structural behaviour of pencil leads after exposure to acids (HF, HNO3, HClO4) or organic solvents (CH3CN, CH3Cl) was monitored via X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The electrochemical activity of pristine and treated pPeGEs was studied by the cyclic voltammetry (CV) responses of reversible redox probes [Fe(CN)6]3−/4− and [Ru(NH3)6]3+/2+. XPS proved the presence of siloxanes in the surface matrix of the pencil leads; this finding relates to the hydrophobic surface character of the electrodes. SEM then provided images of the pencil surfaces with microplates and flakes and revealed the removal of siloxanes upon chemical treatment. The CVs of non-dried and dried pPeGEs displayed surface changes in the polymer matrix, accompanied by water loss. Our study shows that the pPeGE retains the character of a stable graphite sensor when exposed to acids and organic solvents, except for HF and chloroform. The discovered effects explain the electrochemical processes occurring on pPeGEs and can contribute to their application in electrochemical sensing and energy storage.

Diabetes mellitus represents one of the most widespread diseases in civilization nowadays. Since the costs for treating and diagnosing of diabetes represent several billions of dollars per year, a cheap, fast, and simple sensor for diabetes diagnosis is needed. Electrochemical insulin sensors can be considered as a novel approach for diabetes diagnosis. In this study, carbon electrode with electrodeposited NiO nanoparticles was selected as a suitable electrode material for insulin determination. The morphology and surface composition were studied by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, and X-ray photoelectron spectroscopy (XPS). For a better understanding of insulin determination on NiO-modified electrodes, the mechanism of electrochemical reaction and the kinetic parameters were studied. They were calculated from both voltammetric and amperometric measurements. The modified carbon electrode displayed a wide linear range from 600 nM to 10 µM, a low limit of detection of 19.6 nM, and a high sensitivity of 7.06 µA/µM. The electrodes were stable for 30 cycles and were able to detect insulin even in bovine blood serum. Additionally, the temperature stability of this electrode and its storage conditions were studied with appropriate outcomes. The above results show the high promise of this electrode for detecting insulin in clinical samples.

In this paper, the d(GCGAAGC) heptamer and the closely related d(GCGAGC) hexamer are examined via electrochemical (cyclic voltammetry) and spectroscopic (circular dichroism) methods. Dramatic changes in the CD spectroscopic and CV electrochemical properties, induced by the loss of only one single nucleotide (A), are detected. The CD spectra and native polyacrylamide gel electrophoresis (PAGE) confirmed structural changes taking place in the relevant chain-like oligodeoxynucleotide assemblies. Dedicated studies suggest that the heptamer (Hp) possesses a hairpin structure, whereas the hexamer (Hx) appears to be rather a duplex. Both of the structures exhibited completely different adsorption behavior at the hanging mercury drop electrode, and this factor was readily confirmed by means of elimination voltammetry with linear scan (EVLS). We established that the Hp hairpin (similar to-1300 mV), compared to the Hx duplex (similar to-1360 mV), is the thermodynamically favored electron acceptor. The adsorption isotherms were constructed based on the voltammetric peak height values, reflecting the reduction of the adenine (A) and cytosine (C) moieties as well as the oxidation of the 7,8-dihydroguanine (7,8-DHG) moieties. Finally, as revealed by the spectroscopic and electrochemical results, Hx forms a bimolecular antiparallel homo-duplex carrying both Watson-Crick base pairs (CG or GC) and mismatched edge-to-edge base pairs (GA or AG). (C) 2020 Published by Elsevier B.V.

Transition metal nanoparticles can be considered as promising low cost materials for insulin oxidation catalysis. Combination of transition metal nanoparticles with multi walled carbon nanotubes (MWCNTs), which enlarges the active surface area of the electrode, can improve the analytical characteristics of electrodes. Two types of screen printed carbon electrodes (SPCFs) modified by the combination of copper (CuNPs) or cobalt (CoNPs) nanoparticles with chitosan and MWCNTs were prepared. In the effort to find the most suitable modification for electrochemical insulin determination, stability, analytical characteristics and selectivity of both electrode types were compared. The results proved better stability and analytical characteristics with no significant influence of interferences on insulin determination at CoNPs/chitosan-MWCNTs/SPCE. These electrodes provided low limit of detection (25 nM), high sensitivity (0.031 mA mu M-1) and wide linear range (0.05 mu M to 5 mu M) for insulin determination. The stability of the CoNPs/ chitosan-MWCNTs/SPCE was very high, with only 1.7% decrease of maximal current value after 50 measurements. Therefore, CoNPs/chitosan-MWCNTs/SPCE can be considered as the suitable modification for new electrochemical sensor for insulin determination, with better analytical characteristics in comparison with SPCE modification using MWCNTs, chitosan and CuNPs described in this study.

Due to the simplicity of the redox reactions of Fe2+/3+ ions, ferrocenes are popular for electrochemical measurements. While ferrocene itself is rather nonpolar for direct use in aqueous solvents, it’s many derivatives (aminoferrocene, ferroceneboronic acid etc.) are soluble enough to be of use in aqueous solvents. However, improved solubility is not the only change to the ferrocene properties, due to introduction of protonizable group, pH sensitive behavior is expected. The pH-dependent behavior is not completely straightforward as one could expect, electrochemical behavior drastically differs depending on the presence of buffer in solution (figure). On the other hand, not only pH affects the electrochemical behavior, also the electrochemical processes can affect pH-dependent behavior - the pKa of the reduced/oxidized molecules. So, deducing the mechanism behind the effect of buffer hints at very interesting interplay between the pH sensitive group and the redox state Fe2+/3+ ions, where one could be controlled by the state of the other, leading to the possibility of pH/redox switches.

It is empirically expected that if two galvanic cells are inserted in a single electrolyte cell, their output parameters will not be enhanced due to the internal (electrolyte) short circuit. Our previous concept of a galvanic cell with a galvanic short circuit (GSC) in a thin-layer arrangement disproved this expectation. Here, the system with GSC was studied in depth, described and an empiric model was derived and confirmed experimentally. The crucial influence of the solution and the charge transfer resistances on the output performance of the cell were observed. A set of galvanic cells was prepared using screen-printing. The panel of galvanic cells was designed and prepared in a way which enabled simple modular combination of more than one GSC. Thus, multiples of the voltage of the initial single cell can be obtained, while using only scissors for cutting the desired shape of the cell. The output characteristics under load were determined in a bulk measurement. Although, the enhancement of the output voltage in such experimental setup was not as pronounced as in a thin layer, the cell with GSC performed better in comparison with a single galvanic cell. (C) 2019 Elsevier Ltd. All rights reserved.

Guanine, having lower one-electron oxidation potential than other nucleobases, is of relevance to oxidative degradation of nucleic acids in mutagenesis, carcinogenesis, and aging. Here we compare oxidation potentials of guanine (G), guanosine (Guo), deoxyguanosine (dGuo), guanosine -5′- monophosphate (GMP) and 2′- deoxyguanosine -5′- monophosphate (dGMP) obtained by theoretical and experimental methods. Structures of G species were optimized and the identities of minima were verified by vibration frequency calculations. Redox equilibria were modelled in terms of corresponding thermochemical cycles. The changes in free energy were calculated at DFT level using the two different functionals: (i) general purpose B3LYP functional, and (ii) more specific ωB97X-D functional (both with 6-31 + G(d) basis set). Experimental oxidation potentials of all G analogues were measured voltammetrically on a polymer pencil graphite electrode (pPeGE) providing the best results from all carbon electrodes used (glassy carbon electrode, basal and edge plane pyrolytic graphite electrodes). The oxidation process is strongly dependent on the pH value and with increasing pH a linear shift of G oxidation peaks (Epa) towards negative potentials is observed. The theoretically and experimentally obtained oxidation potentials were compared for the pH 5. Anodic peak potentials increase in the order G « dGMP ≤ GMP < dGuo ≤ Guo and correlate with the calculated thermodynamic redox potentials as well as with NBO charges in purine moiety. The oxidation of deoxy analogues was predicted theoretically to occur at lower potentials than that of corresponding parent compounds and this fact was experimentally verified. The assumption that due to negatively charged phosphate group of GMP or dGMP their oxidation potentials could be observed at lower positive potential has not been confirmed and the significant difference (more than 200 mV) between the oxidation potentials of G nucleobase and its nucleosides and nucleotides is discussed. Moreover, conformity of theoretical and experimental data for radicals (cation, neutral) indicates that while the deprotonation process of G differs from its analogues, the oxidation process of all species takes place on imidazole ring.

Some selenium compounds are potent inhibitors of the cell growth with remarkable tumor specificity and their use in attempts to treat cancer has a relatively long history dating back to at least 1912 when selenite was reportedly used to cure a tongue cancer. The function of selenium relates to its role as an antioxidant. It is as a constituent part of glutathione peroxidase important in the detoxification of peroxides, which leads to a reduction in the level of reactive oxygen species in cells and tissues. A remarkable feature of selenium consists of its ability to oxidize thiols under reducing conditions and one mode of action recently suggested is the oxidation of thiol groups of metallothionein. Metallothionein II (MT) is a cytosolic, ubiquitous, low-molecular-weight protein present in various tissues of mammals and non-mammals. A high content of thiol groups (–SH) of MTs can bind mineral micronutrients and xenobiotic heavy metals. In this study, we investigated the electrode processes of MT at a mercury electrode in the presence of sodium selenite (Na2SeO3) by means of the Brdicka reaction in the differential pulse voltammetric mode. The interaction between MT and Na2SeO3 were analyzed via the hydrogen evolution catalytic signals Cat2. It was found that with the increasing selenite concentration, cobalt in MT is replaced by selenium. When concentration of Na2SeO3 increases above the MT binding capacity (MT cannot bind more Se), only selenite ions can be responsible for Cat2 signals. We have answered the questions: (i) how selenite participates in the Brdicka reaction, (ii) which competitive behavior of selenium against cobalt should be expected and (iii) what is the sequence of reaction processes in the modified Brdicka reaction. A new interpretation leading to complete description of the mechanism is presented. Our results can be helpful in biochemical and clinical studies involving selenium compounds as potential chemotherapeutics.

Conformational transitions of nucleic acid fragments are strongly dependent on nucleotide sequences. The C-rich ODNs can adopt different secondary structures which are held together by hemiprotonated and intercalated cytosine base pairs (CC*). These structures, so called i-motifs, were investigated in DNA nonamers with 6C by both electrochemical (voltammetry and electrophoresis) and spectral (absorption UV-Vis spectra and circular diehroism) methods. The application EVLS (elimination voltammetry with linear scan) procedure to the reduction signals of cytosine (C) and adenine (A) on a hanging mercury drop electrode revealed different conformations of studied nonamers.

Diabetes mellitus can be considered one of the most widespread diseases globally. Hence, the diabetes research is currently focused on developing an effective, low-cost sensor having high stability and suitable analytical characteristics. Screen printed carbon electrodes (SPCEs) embody ideal candidates for insulin determination due to the small area of the working electrode eliminating the solution volume required for the given purpose. Modification of SPCEs by using nanoparticles resulted in an increase of the working electrode surface area and formation of a higher number of active species. The aim of this paper is to examine the impact of a chitosan membrane on the electrochemical determination of insulin on NiO nanoparticles (NiONPs) and multi-walled nanotube (MWCNTs) modified SPCE (NiONPs/MWCNTs/SPCE). This study is primarily conceived to compare the analytical characteristics and stability of NiONPs/chitosan-MWCNTs/SPCE and NiONPs/MWCNTs/SPCE. An electrode modified with chitosan displays a wider linear range, one of 0.25 μM - 5 μM (R2 0.997); a lower limit of detection, 94 nM; a high sensitivity (0.021 μA/μM) and better stability than that of an electrode without chitosan. According to these characteristics, the polymer is considered a necessary compound of the electrochemical insulin sensor, improving the sensor's analytical characteristics.