This study provides the first report on the intrinsic electrochemical behavior of mitragynine, the principal alkaloid of Mitragyna speciosa (kratom). Voltammetric analysis revealed that mitragynine undergoes a pH-dependent anodic oxidation at relatively low potentials (Epa = 530 mV vs Ag/AgCl at pH 7), with a Nernstian slope of 59 mV per pH unit, suggesting a proton-coupled electron transfer mechanism involving the indole moiety. Under the studied experimental conditions, the mitragynine anodic feature is irreversible and diffusion-limited. Based on these electrochemical findings, a sensitive HPLC method with dual-channel coulometric detection (HPLC-ECD) was developed for the determination of mitragynine without requiring any prior derivatization steps. Under optimized conditions at +700 mV vs Pd/H2, the method achieved a superior limit of detection (LOD) of 1.8 ng mL-1 and a limit of quantification (LOQ) of 5.5 ng mL-1, maintaining excellent linearity (R2 = 0.999) across the concentration range of 20-200 ng mL-1. The utility of the dual-channel configuration was demonstrated through the adaptive optimization of selectivity: while +700 mV ensured maximum sensitivity for kratom plant extracts (yielding 13.5 mg g-1), a lower potential of +300 mV effectively suppressed co-eluting endogenous interfering species in complex human plasma matrices. The use of dual-channel detection enabled not only the selectivity optimization, but also confirmation of analyte identity based on the consistency of signal ratios at different detection potentials. The proposed approach represents a simple, cost-effective, and reliable strategy for the determination of mitragynine and highlights the potential of electrochemical detection for the analysis of electroactive alkaloids in complex matrices.

In this work, we demonstrate a sensitive high-performance liquid chromatography (HPLC) method for the determination of piperazine antihistamine drugs employing innovative electrochemical detection based on a spark-generated nickel oxide nanoparticle-modified carbon fiber microelectrode built into a miniaturized electrochemical detector. The direct carbon fiber-to-nickel plate electrode spark discharge, carried at 0.8 kV DC, with the nickel electrode connected to the negative pole of the high-voltage power supply, provides extremely fast (1 s) in situ tailoring of the carbon fiber microelectrode surface by nickel oxide nanoparticles. It has been found that nickel oxide nanoparticles exhibit an electrocatalytic effect toward the piperazine moiety electrooxidation process, as confirmed by voltammetric experiments, revealing the shift in the peak potential from 1.25 to 1.09 V versus Ag/AgCl. Cetirizine, cyclizine, chlorcyclizine, flunarizine, meclizine, and buclizine were selected as sample piperazine antihistamine drugs, while diclofenac served as an internal standard. The isocratic reversed-phase separation of the above set was achieved within 15 min using an ARION-CN 3 mu m column with a binary mobile phase consisting of 50 mM phosphate buffer (pH 3) and methanol (45/55, v/v). The limits of detection (LOD) were within the range of 3.8-120 nM (for cyclizine and buclizine) at E = +1500 mV (vs Ag/AgCl), while the response was linear within the concentration range measured up to 5 mu mol L-1. The method was successfully applied to the determination of piperazine antihistamine drugs in spiked plasma samples.

Electrochemical techniques are rarely applied for the evaluation of total antioxidant capacity of beers due to severe electrode inactivation (fouling) by adsorbed oxidation products of the species present in the samples. In this work, we show that microdialysis online-coupled with amperometry is suitable for beer analysis, permitting also an assay automation. The working electrode of the amperometric detector employs a carbon fiber microelectrode that is regenerated between the analyses by short (1 sec) rapid (50 Hz) sinusoidal potential cycling between 0 and 2.9 V vs Ag/AgCl. This step, along with the sample cleanup by microdialysis effectively removes electrode fouling. The suitability of this approach for beer analysis was studied using eight different beers produced by international manufacturers. The results were compared with standard Kaneda diphenylpicrylhydrazil (DPPH) assay. Steady state currents, obtained at 600 mV vs Ag/AgCl correlate well with the results obtained with Kaneda method and thus can be used for quantification of beers' antioxidant capacities. HPLC-ED of microdialysates with and without DPPH treatment were employed to get an insight into DPPH reactivity with beer components.

Sensitive analysis of nine selected antipsychotic drugs is demonstrated by high performance liquid chromatography coupled with innovative electrochemical detector employing the non standard, 0.2 mm diameter Pentel AinStein pencil graphite as a working electrode. In addition to low cost, the detector performance is characterized by low background current, capability to operate in low ionic strength media and very fast attainment of stable baseline. These advantageous features are attributed to microelectrode character of the pencil lead surface. The concentration LODs ranged between 1.0 (olanzapine) and 7.9 nmol/L (chlorpromazine) with linearity up to 500 nmol/L . Model analyses in spiked control serum as well as the real sample analyses of human plasma samples of subjects after intoxication with olanzapine resp. promethazine are shown. The developed method could be used for forensic monitoring of commonly used APs in blood samples. According to relevant assessment tools, the whole procedure can be regarded as green.

A concentric thin layer cell accommodating a non-standard 0.2 mm diameter pencil graphite disposable working electrode is described. The cell was installed into HPLC manifold as an electrochemical detector. Trouble-free operation in mobile phases containing both low and high content of organic solvents is demonstrated by HPLC analyses of phenolic acids and tocopherol isomers. The data obtained from HPLC separation of model mixtures of gentisic, caffeic and dihydrocaffeic acids show a remarkable electrolytic efficiency exceeding 80% at 200, and 50% at 500 mu L min(-1), while for gentisic acid the limit of detection (LOD) was 0.4 nmol L-1 at 20 microliter sample loading (8 fmol on-column). Similar performances were found in non-aqueous mobile phase, where a LOD of 0.8 nmol L-1 was achieved for delta-tocopherol. The developed flow-through detector is designed to allow easy replacement of pencil graphite working electrode in a highly reproducible manner. The relative standard deviation for the HPLC analysis of tocopherol isomers was of 5.3% (n=3, C=500 nmol L-1). The combination of simple construction, excellent electrochemical performance and hydrodynamics identical to that of commercial UV-VIS HPLC detector suggests that the proposed device is a viable low-cost alternative to commercially available electrochemical detectors.

Electrochemical deposition of the material released by anodizing nickel and copper nickel alloy in pure water onto carbon fiber microelectrodes was used to assemble miniature glucose sensors. The composition and morphology of the deposits was investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The deposition of anode-derived materials proceeded by two consecutive mechanisms, which are explained in detail. The electrochemical properties of the designed electrodes were subsequently investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The different Ni- and mixed CuNi-modified microelectrodes were examined as glucose sensors and the best performing electrodes based on the alloyed deposit exhibited very high sensitivity (5720 μA mM−1 cm−2), low detection limit (0.3 μM) and ability to quantify glucose in blood serum.

A novel method of carbon fiber microelectrode activation using spark discharge was demonstrated and compared to conventional electrochemical pretreatment by potential cycling. The spark discharge was performed at 800 V between the microelectrode connected to positive pole of the power supply and platinum counter electrode. Spark discharge led both to trimming of the fiber tip into conical shape and to the modification of carbon fiber microelectrode with platinum, as proven by scanning electron microscopy and electron dispersive X-ray spectroscopy. After the characterization of electrochemical properties using ferricyanide voltammetry, the activated electrodes were used for electrochemical analysis of 8-oxo-7,8-dihydro-2'-deoxyguanosine, an oxidative stress marker. Subnanomolar detection limits (0.55 nmol L-1) in high-performance liquid chromatography were achieved for spark platinized electrodes incorporated into the flow detection cell. (C) 2016 Elsevier B.V. All rights reserved.

Miniature Surface Enhanced Raman Scattering (SERS) sensors were fabricated by coating the carbon fiber microelectrodes with copper nanowires. The coating procedure, based on anodizing the copper wire in ultrapure water followed by cathodic deposition of the anode-derived material onto carbon fiber electrodes, provides a “clean” copper nanowire network. The developed miniature (10 µm in diameter and 2 mm in length) and nanoscopically rough SERS substrates are applicable in drug sensing, as shown by the detection and resolving of a range of seized designer drugs in trace amounts (microliter volumes of 10−10–10–12 M solutions). The copper nanowire modified carbon microfiber substrates could also find further applications in biomedical and environmental sensing.

Electrochemical oxidation of four brominated phenols (2-bromophenol, 3-bromophenol, 4-bromophenol, and pentabromophenol) was studied using voltammetry on glassy carbon electrode and controlled potential electrolysis on platinum gauze electrode in 90% (v/v) short-chain primary alcohols. Oxidation products obtained by controlled potential electrolysis were analysed using gas chromatography with mass spectrometry. Several oxidation products were identified and their structures were proposed.

A facile technique enabling CFMEs to be modified with nanostructured metal layers was developed. In this contribution, the approach is demonstrated for copper and bismuth. Copper modified CFMEs were employed in HPLC electrochemical detection of carbohydrates while bismuth coated CFME was used to detect nonsteroidal antiandrogens nilutamide and flutamide.

We report that highly effective electrode modification can be achieved by sparking process between a flat electrode substrate and a tip counter electrode. The concept is introduced by the development of Bi2O3-modified graphite screen printed electrodes (SPEs). SPEs were sparked with a bismuth wire at 1.2 kV under atmospheric conditions. The effect of polarity on the morphology of the sensing surface, bismuth loading and the sensitivity of the resulting sensors for the simultaneous anodic stripping voltammetric determination of Cd(II) and Pb(II) was investigated. Compared with electroplated and various bismuth precursors bulk-modified SPEs, the developed sparked electrodes exhibited considerably lower limit of detection (0.2 μg L− 1, S/N = 3) for each target ion. Therefore, sparking technique offers a facile and green approach for the development of highly sensitive bismuth-based electrodes, and a wide-scope of applicability in the development of metal-modified sensing surfaces.

We report a novel method for fabricating nanostructured copper-coated carbon cylindrical fiber microelectrodes and show the high efficiency of these electrodes in carbohydrate non-enzymatic and label-free amperometric sensing in both batch and flow-detection arrangements.

Electrochemically pretreated carbon fiber microelectrode was used to develop a simple, fast and sensitive HPLC-ECD method for the determination of brominated phenols. In addition to simple mono-, di- and tri-bromophenols (4-bromophenol, 2,4-dibromophenol, 2,6-dibromophenol, 2,4,6-tri-bromophenol) the possibility of electrochemical detection of 3,3',5,5'-tetrabromobisphenol A in oxidation mode is reported for the first time. The isocratic separation was achieved within 14 min using ternary mobile phase consisting of 50mM-phosphate buffer (pH 3.5), acetonitrile and methanol (35/15/50, v/v), and detection potential of E=+1450 mV (vs. Ag/AgCl). The carbon fiber microelectrode permitted to use high anodic potentials (up to +1800 mV vs. Ag/AgCl), the optimum analytical response was achieved at +1450 mV vs. Ag/AgCl. The limits of detection (LOD) for the studied analytes were within the range of 1.8-56.6 ng mL(-1). The developed method was applied to determination of brominated phenols in spiked water samples. Furthermore, after simple extraction with methyl tert-butyl ether, it was possible to quantify tetrabromobisphenol A (TBBA) in a piece of CRT monitor plastic casing. The found amount of TBBA was 10.22 mg kg(-1) (±0.43).

We demonstrate the electrochemical pretreatment of carbon fiber microelectrodes (CFEs) based on the sinusoidal-wave potential cycling (50 Hz) of CFEs prior to amperometric analysis. The surface modification and morphology of CFEs after pretreatment was studied using Raman spectroscopy and scanning electron microscopy. The pretreated CFEs were extensively tested for sensing of bioactive compounds (dopamine, nitric oxide, ascorbate and nitrite) by constant current amperometry at 830 mV vs. Ag/AgCl.

The electrochemical behavior of tolterodine, an antimuscarinic drug used to treat urge incontinence and overactive bladder, was investigated using cyclic and differential pulse voltammetry at glassy carbon electrode. Electrooxidation of tolterodine proceeds as a complex two-step pH-dependent process. Controlled potential electrolysis of tolterodine solutions was performed at platinum gauze electrode in methanolic, aqueous-methanolic and acetonitrile media. Electrolyzed solutions were analyzed using liquid chromatography with electrospray ionization quadrupole time-of-flight mass spectrometry. 5-Hydroxymethyl tolterodine, the main biologically active metabolite of tolterodine, was identified among monomeric oxidation products. Dimeric products, formed by oxidative coupling of phenoxy radicals, were found in all electrolyzed solutions. The mechanism of the electrochemical oxidation of tolterodine has been proposed.

Carbohydrates are considered as difficult to analyse species, since they lack suitable chromophores, fluorophores or substituents responsible for effective light absorption or emission, conventional methods, such as spectrophotometric or fluorimetric, fail to provide a satisfactory analytical sensitivity. A considerable number of saccharides are however electroactive species, although they require very high overpotentials on common electrodes. A nonenzymatic glucose sensor based on carbon fiber microelectrode coated by copper wire-like nanostructures was prepared and its properties tested on glucose using amperometry.

Electrochemical oxidation of the isoquinoline alkaloid berberine in aqueous medium was studied by cyclic and differential pulse voltammetry at a glassy carbon electrode (GCE). Two anodic peaks of the quaternary form of berberine were observed at + 1.2 V and + 1.4 V (vs. SCE) in acidic and neutral solutions. When the anodic polarization exceeded the value of + 1.1 V, the redox active film is formed on the GCE surface. The formation of adsorbed film was well-documented by quasireversible redox couple at + 0.25 V which was studied in redox cycling experiments. In alkaline medium, a new anodic peak at + 0.5 V appeared due to oxidation of berberine pseudobase to 8-oxoberberine. Solutions of berberine at different pH were subjected to controlled potential electrolysis on platinum gauze electrode and analyzed using liquid chromatography (HPLC) equipped with electrospray ionization/quadrupole time-of-flight mass spectrometry. The main water soluble monomeric product of berberine oxidation under physiological-near experimental conditions, OP1, was identified as demethyleneberberine cation (2,3-dihydroxy-9,10-dimethoxy-5,6-dihydroisoquinolino[3,2-a]isoquinolin-7-ium).

Derivatives of quinoxalin-2-one are interesting compounds with potential pharmacological activity. From this point of view, understanding of their electrochemical behavior is of great importance. In the present paper, a mechanism of electrochemical reduction of quinoxalin-2-one derivatives at mercury dropping electrode was proposed. Pyrazine ring was found to be the main electroactive center undergoing a pH-dependent two-electron reduction process. The molecule protonization of nitrogen in the position 4 precedes the electron acceptance forming a semiquinone radical intermediate which is relatively stable in acidic solutions. Its further reduction is manifested by separated current signal. A positive mesomeric effect of the nonprotonized amino group in the position 7 of the derivative III accelerates the semiquinone reduction yielding a single current wave. The suggested reaction mechanism was verified by means of direct current polarography, differential pulse, cyclic and elimination voltammetry, and coulometry with subsequent GC/MS analysis. The understanding of the mechanism was applied in developing of analytical method for the determination of the studied compounds.

The paper focuses on the analysis and detection of electroactive compounds using high-performance liquid chromatography (HPLC) combined with electrochemical detection (EC). The fabrication and utilization of electrochemically treated carbon fiber microelectrodes (CFMs) as highly sensitive amperometric detectors in HPLC are described. The applied pretreatment procedure is beneficial for analytical characteristics of the sensor as demonstrated by analysis of the model set of phenolic acids. The combination of CFM with separation power of HPLC technique allows for improved detection limits due to unique electrochemical properties of carbon fibers. The CFM proved to be a promising tool for amperometric detection in liquid chromatography.