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Field-effect transistors have strong applications in biosensing field from pH and glucose monitoring to genomics, proteomics, cell signaling assays, and biomedical diagnostics in general. Notable advantages are the high sensitivity (thanks to intrinsic amplification), quick response (useful for real-time monitoring), suitability for miniaturization, and compact portable read-out systems. The initial concept of ion-sensitive field-effect transistors evolved with the emergence of novel classes of materials beyond traditional semiconductors. Recently, 2D nanomaterials are redesigning the field providing superior performances with large surface-to-volume ratio, high carrier mobility, more effective local gating, high transconductance, and operation at low voltages. Here, after a brief conceptual introduction, we review progresses and perspectives of 2D materials field-effect-transistor biosensors with special focus on opportunities, most recent applications, present challenges, and future perspectives.

Rudolf Brdička (Figure 1) was a Czech physical chemist and electrochemist, particularly known for his research on biomedical applications of polarography.

Brdička was a pupil and later a collaborator of Prof. Jaroslav Heyrovský (the inventor of the polarographic method and recipient of the Nobel Prize in 1959). Following his studies on polarography performed with Heyrovský, Brdička devoted all his scientific career to the use of polarography for different electroanalytical applications. Since at that time polarography was used for electrochemical analysis of small organic redox molecules and particularly for the detection of various inorganic cations and their complexes, Brdička studied the electrochemistry of cobalt cations (Co³⁺). While the Co³⁺ polarographic wave was following the expected redox behavior similar to other polarographic metal ion reactions, surprisingly very unusual polarographic waves were observed in the presence of some proteins. The observed phenomenon was explained as a catalytic redox process that involves complex formation between Co³⁺ cations with thiol (-SH) groups in the protein backbone. The polarographic waves were named Brdička waves. The exact mechanism, which involves two catalytic processes proceeding at different potentials, was elucidated in detail later (B. Raspor, J. Electroanal. Chem. 2001, 503, 159–162). It was shown that the electrochemical process includes the redox process of the thiol-complex of Co³⁺ and then catalytic reduction of H⁺ cations and H₂ evolution at more negative potentials, thus resulting in double polarographic waves. The observed waves were used as a very sensitive indication of proteins (note that it was a catalytic process) and the waves were specific to different kinds of proteins (note that they were dependent on the presence of thiol groups in the proteins). The Brdička waves were used in the analysis of protein-biomarkers of cancer and other health problems over several decades (Figure 2).

Presently, the polarographic analysis is not used and the Brdička waves have only historic interest. Notably, the Brdička waves originate from the redox processes of thiol groups in the peripheral lysine residues, thus is not related to the redox transformations of enzyme active centers, which are important for various biosensor and bioelectronic applications.

The author declares no conflict of interest.

Hermann Kolbe (Figure 1) was a German scientist who greatly contributed to the development of organic chemistry, transforming it to the state as we know it now. Kolbe pioneered organic synthesis from inorganic sources and introduced the term “synthesis” in the meaning how we use it in chemistry now. His name is associated with several synthetic reactions in organic chemistry, e.g., the Kolbe-Schmitt reaction in the preparation of aspirin, the Kolbe nitrile synthesis, etc. His work is particularly remembered in connection to electrolysis of carboxylic acids resulting in the synthesis of various organic compounds, known as the Kolbe reaction.

The Kolbe reaction (Figure 2), proceeding as the electrolysis, results in the oxidative decarboxylation of carboxylic acids yielding free radicals, which dimerize producing symmetrical products. For example, the Kolbe electrolysis process can proceed in an aqueous solution of sodium acetate (Figure 2). The acetate ions get decomposed and form methyl radicals. These combine with other free methyl radicals, which leads to the generation of ethane. In general, Kolbe's electrolysis method uses sodium salts of fatty acids to form the corresponding alkanes as products (D. Klüh, W. Waldmüller, M. Gaderer, Clean. Technol. 2021, 3, 1–18). A similar electrochemical synthesis can be used to produce more sophisticated products (Figure 2B). If the initial mixture includes two different acids, the reaction results in three different products from the cross-reaction of two different free radicals. The Kolbe electrolytic decarboxylation of 1,2-dicarboxylic acids results in the formation of double or triple chemical bonds (Figure 3). When carboxylic groups are located at a longer distance in a molecule, the electrolytic decarboxylation may result in the intramolecular radical cyclization of the reaction product.

It should be noted that the Kolbe electrolysis reaction may result in the formation of numerous byproducts (Figure 4). The formation of side products depends on the ease of the follow-up oxidation, which leads to carbenium ions, and their subsequent rearrangements. The exact mechanism and kinetics study of the electrochemical Kolbe process have been investigated confirming the complexity of the electrochemical reaction (A.K. Vijh, B.E. Conway, Chem. Rev. 1967, 67, 6, 623-664).

The author declares no conflict of interest.

Julius Tafel (Figure 1) was a Swiss chemist and electrochemist. Tafel started his scientific career working on the field of organic chemistry with Hermann Emil Fischer, but soon changed his interests to electrochemistry after his work with Wilhelm Ostwald.

Then, Tafel's work was concentrated on the electrochemistry of organic compounds and relation between rates of electrochemical reactions and applied overpotentials. Tafel's name is presently associated with many electrochemical terms: Tafel equation, Tafel slope, Tafel rearrangement, and Tafel mechanism of hydrogen evolution.

The Tafel equation and the corresponding Tafel plot (Figure 2) in electrochemical kinetics are relating the rate of an electrochemical reaction (in terms of the current density [i] to the overpotential [η] applied). The Tafel equation was first deduced experimentally and was later shown to have a theoretical justification. Indeed, it represents a simplified version of the theoretically derived Butler–Volmer equation (Figure 2) when the overpotentials are rather high (|η| > 0.1 V; Tafel region). For a large overpotential (anodic or cathodic), one part of the Butler–Volmer equation becomes negligible while the second part can be transformed to the Tafel equation. The Tafel slope (A) shows how much the overpotential needs to be increased to increase the reaction rate (which is current in electrochemistry) by 10-fold. In a simple case of a one-electron transfer electrochemical reaction, the Tafel slope is determined by the symmetry factors (α_a and α_c), which are usually ca. 0.5, translating to a Tafel slope (A) of 120 mV. The Tafel equation, empirically derived from his experiments with electrochemical evolution of H₂, laid the background for a new scientific area of electrochemical kinetics. Tafel is also credited for the discovery of the catalytic mechanism of hydrogen evolution (the Tafel mechanism), construction of a new kind of hydrogen coulometer used in his study of H₂ evolution. Also, he demonstrated that hydrocarbons with isomerized structures can be generated upon electrochemical reduction of the respective acetoacetic esters (named Tafel rearrangement) (Figure 3). This was an important method for the synthesis of certain hydrocarbons from alkylated ethyl acetoacetate, a reaction accompanied by the rearrangement reaction of the alkyl group.

The author declares no conflict of interest.

The mechanism of electrochemical reduction of a series of six cone-calix[4]arene-bis-nosylates (4-nitrophenylsulfonate aryl esters) was investigated on mercury electrodes using DC-polarography and cyclic voltammetry (CV) combined with in situ electron paramagnetic resonance (EPR)-spectroelectrochemistry in aprotic dimethylformamide. Model compounds – expected fragments and products - were studied for comparison. The experimental results are supported by quantum chemical calculations. All calix[4]arene-bis-nosylates are reduced in a first reversible step to bis-(radical anion) by two simultaneous one-electron transfers. Each of the two electrons is unpaired and separately localized on two nosylate groups.

In the second reduction step next 2×2 electrons are transferred and both sulfonate ester groups are cleaved to two 4-nitro-benzenesulfinate ions and a calixarene bis-phenolate (95%). This electroreductive generation of arylsulfinate anions is a significant finding from the electrosynthetic point of view. Activated arylsulfinates, the synthesis of which is generally difficult, can be easily prepared by electrochemical reduction of the nosyl esters.

The interest in electrochemical processes to produce ammonia has increased in recent years. The motivation for this increase is the attempt to reduce the carbon emissions associated with its production, since ammonia is responsible for 1.8% of the global CO₂ emissions. Moreover, green ammonia is also seen as a possible transportation fuel in various renewable energy transition scenarios. Several electrochemical processes are being investigated such as N₂, NO₃^–, or NO conversion. Since nitrates are an attractive source of nitrogen, due to their role as water contaminants and facility to break N-O bonds, this mini review is focused on the electrocatalytic synthesis of ammonia from NO₃⁻ reduction. Here, we summarized the important work on reaction mechanisms and electrocatalysts for this reaction.

Electrochemical oxidation of the new psychoactive substance 3-fluorophenmetrazine (FPM) was studied in phosphate buffers by cyclic voltammetry and differential pulse voltammetry (DPV) on a glassy carbon electrode. The redox potential of FPM in buffered solution strongly depends on pH. Cyclic voltammetry behavior shows the partial influence of adsorption on the electrode process not allowing detailed analysis of the individual steps of the reaction scheme, it means the involvement of electron transfer (E) and chemical reaction (C). Nevertheless, the irreversible shape of the cyclic voltammogram is explained by the participation of hydroxylation nucleophilic addition of water (hydroxylation) after two-electron/two-proton oxidation of molecule at the tetrahydro-1,4-oxazine ring. The suggested mechanism leading to hydroxylated derivative 2-(3-fluorophenyl)-3-methyl-5-hydroxymorfolin is supported by the calculated highest occupied molecular orbital spatial distribution and atomic charges calculations for electrochemically formed radical cation. Infrared spectroelectrochemistry performed during oxidation in acetonitrile/water also supported the formation of this product.

The analytical method of FPM determination on glassy carbon electrode was developed using DPV with an attained limit of detection = 4.7 μmol/L in phosphate buffer of pH 9. The linear range of the calibration curve is from 7.0 to 107.00 μmol/L, correlation coefficient (r) = 0.9988.